There has been a good amount of interest lately with IJ's restored thread and chich's conversion of his knee mill. First off, why drive the knee, does it make a more ridgid machine and is it more accurate?

Is the basic asian 9X49 mill capable of being ridgid and accurate with the quill driven? What are the advantages and disadvantages?

Also when driving the knee, you also get an extra 8 ro 9 inches of programmable "Z" axis. With chich's conversion, an air cyclinder is used to assist movement of the knee where as it can be offset close to a balanced state where the servo is not lifting that much weight.

If the knee is driven and the servo is assisted by gas springs or an air cyclinder, will the Vertical hardened ways hold up? Getting the weight distrubution right could be tricky because of where the 150 lb table is positioned on the y-axis (front and to the rear) and of cource the positioning of the Xtravel where it is all the way to the left and to the right, this would place a lateral torque on the verticle knee ways.

I for one, would like to cnc an Asian 9X49, and drive the knee, but I need to address these type of dynamics concerning driving the knee. Also, are the Gib adjustments set up in order to support driving the knee. I looked at a couple of online manuels and only see an upper gib adjustment and in chich's conversion he mad a mod to the lower portion of the knee concerning adding a gib adjustment.

On the other hand, is there a problem with these mill as you approach the 5 inches quill limit with runout and are these mills ridgid with the quill extended in the 4 to 5 inches region, where they are able to take heafty cuts in the 30 to 40 ipm rate cutting 6061.

How much more ridgid are they with the quill locked and the knee driven. The machines which have converted and driven the knees, how are they performing and holding up?

Your thoughts,

Ron

Ron,

A lot of people here will tell you its better to drive the quill then the knee. Even IJ made a quill drive after he did the knee drive. Some will say that driving the knee is better because of greater travel.
I did a conversion on a 10x50 sized knee mill with the knee as the z axis. This machine has been running this way for about 2 years now. So far I cannot see any excess wear the slidway or have noticed any increase in slop. The knee is counter balanced so the servo does not need a lot of grut. It runs at about 135ipm max but I plan to increase that up to 500ipm soon. As far as positioning goes I can hold about +- 0.0002 in repeatability all day long.
The reason I went to a driven knee was that I thought the knee would hold its rigidity better over the full length of travel. So far so good.

Trevor

when I was calling around asking about controls and whatnot I talked to the guys at Centroid. Their kits automate the quill on bridgeports. When asked why, the reply was the accuracy of the overseas copies are poor in the Z ways. When the movement was compounded over a certain amount of distance it would up bieng quite a bit at the ends of the Z travel. That is the only valid reason I could understand. I also brought up the deal about the quil hanging out 5" can't be good for accuracy. They claimed on a bridgeport it was not a problem in the slightest.... maybe they were just trying to sell their product, but I'm not sold on automating the quill. My overall impression was they are not big fans of most anything other than a bridgeport. I have the same machine as Chich, and I am still planning on automating the knee. I already have the air cylinder etc.
My machine came with some sort of certificate of accuracy, which may not be worth much more than the paper it's written on..but according to this paper it wasn't too bad. I think a disassembly and cleaning will do wonders for accuracy..because my machine was no where near clean when it was assembled.

Trevor,
May be you could give me an idea as to the rigidity of the machine converted with the knee. The work that I would be using the mill for is cutting 6061 and any feel for feed rates on 6061 would be appreciated. I had talked to a vender of a square column type cnc system and he spoke of a lack of rigidity with the knee type of machines as far as taking healthy cuts such as .500 inch full with cuts at 40 ipm with .125 inch depths (4 flute carbide at say 4000 rpm). His thoughts were that the 9X49 were not ridgid enough for this task. I think that was his honest opinion, and I have never had opertunity to cut on anything but my shopmaster 3 in one. So, your thoughts about how much material removal rates that you would be comfortable with your machine would provide another calibration point.

Also, what kind of counterbalance system did you use on your 10 X 50. So far I have seen where the air cyclinders and gas springs have been used successfully. If you feel up to it please feel free to post some pictures of your build. We follow a good number of builds, but once there finished, rarely do we get to see how the machines are doing a couple of years later. All and all that is the true test of whether the engineering paid off, or what needed to be tweeked to get the bugs out.

Also where did you buy your 10 X 50? and any info about your configuration that you can share such which servo's (or steppers) driver's and control software (Mach 2)

Finally, it sounds like you are really pleased with your choice to drive the knee and 135 ipm is fast enough for production, but 500 ipm is flying, what type of changes will that take to diliver that type of speed to the knee.

Turbostang, pointed out that some info that was relayed to him concerned the possible trueness of the knee in some asian mills, and it's apparent that yours is dead on, that's where this thread may be an important point of calibration. IJ's thread is not complete and I could not determine whether on not the knee had a counterbalance system or what was the problem which did not allow the knee to be successfully driven, or if it was a manufactoring tolerence of the ways (or gibs).

Last off, some two years ago why did you choose to drive the knee (looks like the right choice for you), was it for the extra travel or increased ridgidity.

Again please feel free to post you build pictures or pictures of your machine, I need to see some good examples. Turbostang, same for you, I want to follow your build, it looks like a good machine!!!

Thanks

Ron

I am getting ready to move so I may not be able to start on the coversion till the end of July. bummer.

The only thing I ask is What makes you want the 500 IPM? You can't really do anything to speak of in Z at that rate - but a couple hundred IPM would be good I'd think. Now with 20+ inches in Z, it would be a bummer to watch that thing rapid back and forth that slow :D

I have made several cuts on aluminum on my machine for the same reason you ask about. I had a 1" end mill cutting 1/4" deep with the power feed on a pretty good rate of feed and it handled it just fine and would easily take more. It had a hint of vibration but I think that was primarily from the sorry vise I have and the head wasn't trammed in.

I have big plans for my machine as well, but I can't mess with it for a couple of months.

QUESTION: why would you create a 500" per minute capable machine when it is all but impossible to bury a cutter into/thru most any material at that rate of travel? Yes, it makes for impressive jogs but, really, is it needed?

We have a factory built Bridgeport mill with a BMDC based servo controlled system. Essentially the same system was used on the mid-90's lathes, VMC's, mills and surface grinders that BPT made. Although it CAN run at huge jog speeds and rapid travels, we learned that higher accuracies were achieveable with much more "reasonable" feed rates. This is especially true any time you have direction reversals in the cutter path or when trying to do circular path milling.

When it comes to roughing, pretty much anything will work. However, when we do finish/smoothing/spring passes, we really slow things down - this pretty much eliminates follower error in the servo drives.

Although this may not be the case for steppers, none the less, the faster you try to move/do things, the more power has to be used/applied. Thus, a very stiff power supply will be essential for the rapid accelerations needed to achieve 500"/min infeeds. Although a 500"/minute machine will be impressive to watch, you still have inertia and mass to deal with. Don't stand in the way in any direction when something like that is operating.

Once you get it a huge mass of a milling head moving at 500"/min, something has to slow/stop it eventually and that will probably have to be the servo or stepper - I hardly think that way/guide friction should be enhanced to do the stopping as this will have to be overcome to get it to move that fast in the first place.

Yes, watching the machine jog end for end at a slow speed is as bad if not worse than watchine paint dry. HOwever, realistic expectations can result in acceptable performance without the cost penalties usually associated with designing, tuning and building lazer fast servo/stepper systems.

NC Cams,
Thanks for you input.

But, what are you feelings about cnc'ing the knee on an Asian mill? Would there be advantage with the 10 X 54 with box ways(I think) over 9 X 49 with dove tails and if you followed IJ's thread (which is partial) what was the problem which resulted with cnc'ing quill as primary?

Add some meat to this post.

Ron

what was the problem which resulted with cnc'ing quill as primary?


IIR IJ found that the inertia/weight problem of operating the knee required a much larger force than he anticipated.
I have two Excello's that are similar to the B.P. in size and one has powered knee with air cyl. counter-balance system.
The quill is still the Z, personally I prefer it that way as in order to get the performance I have right now, would require a large change to the knee operator.
When you take the weight of the knee, the table, and if you have a part loaded that weighs 70~100lb and you want to peck drill! You can imagine the inertia of operating this load in high rate accel/decel in both directions.
If you power the knee and Z the quill, you can do a similar function I do when the knee is required to move down or up, which usually calls for a tool change also, in the case of my mill, I have no auto tool changer, just quick-change, so the M6 will be similar to a M0, I tool change lower the knee and press cycle start, the only proviso is with this process, you need to have an acurate display of knee movement, or a means of touching off again.
Al.

I am not in favor of moving the knee as the Z axis of a CNC mill.

The mass that you're trying to move is not in consequential. Even if you counter weight it, you're now DOUBLING the inertia of the system because BOTH the knee and the counterweight add up to inertia that has to be controlled. Eventually, friction comes into play and that too increases as you add more inertia.

It has been proven time and time again that reducing weight is better than increasing power when it comes to trying to make automobiles perform better. Since the mill and the auto follow the same laws of physics and inertia, the same logic should prevail.

It is hard enough to drive a spindle quill precisely with a Z axis drive and these have low mass and friction. I don't see how adding mass and inertia and friction (there is a bunch in the knee), will make it easier than driving the quill.

I do hope that folks who have done Z axis motion with the knee will chime in. Maybe they have found the holy grail on how to do it. I still contend that controlling anything with less mass and/or inertia will be easier that something with more of the same.

Some good info and theory in this thread.

I can certainly understand moving the knee in a peck cycle, like mentioned above, bieng a problem. However, if you aren't using a counter balance but an air cylinder instead...you aren't moving the weight of the counterweight AND you are only moving a small amount of the weight of the knee.

Cheech mentions that he can move his knee up or down with his finger tips with the assist of the air cylinder. I am more concerned with the rigidity of the quill when it hangs out 5". :eek:

For the records, I just went out in the garage and put a test indicator in the mill vise, extended the quill out the full amount, grabbed the back of the quill, pulled it towards the indicator and got a rather easy .004" movement. Keep in mind that is just me pulling on the quill and not the forces of a heavy cut pushing on it. Next, I pulled the quill back in the head and extended it only 1".. repeated the test and only got about .0025 with the same amount of pulling effort. That alone would heavily sway my opinion on weather or not to automate the quill or the kneed.

However, if you aren't using a counter balance but an air cylinder instead...you aren't moving the weight of the counterweight AND you are only moving a small amount of the weight of the knee.

I am more concerned with the rigidity of the quill when it hangs out 5".

The effect of the counter weight is to support or keep the weight of the knee etc in equilibrium, the required force to move the supported weight at the accelerated rate is the job of the servo to overcome the inertia required, this would be similar/same to a horizontal X or Y axis inertia demands .
With a C.B. system, you can only set an estimated weight, as the load will alter according to the part load weight. Even in a system I have, which is a constant force counter-balanced air system.

I have never experienced a problem with the quill extension, these mills were originally designed for manual milling using the quill, so I assume they have been engineered to also suit the conditions demanded by CNC.
Al.

I remember seeing a thread with a video of a knee Z axis cnc doing G83 peck cycles on a something like a 6 X 36 X 1 inch aluminum plate and then tapping the 50 to 60 1/4 inch holes, but can not locate the thread or the video..... maybe someone remembers and can post a link.

It impressed me.

And like Al says, some of you guy that have cnc'ed the knee for "Z" sing in.

Ron

I remember seeing a thread with a video of a knee Z axis cnc doing G83 peck cycles on a something like a 6 X 36 X 1 inch aluminum plate and then tapping the 50 to 60 1/4 inch holes, but can not locate the thread or the video..... maybe someone remembers and can post a link.

It impressed me.

And like Al says, some of you guy that have cnc'ed the knee for "Z" sing in.

Ron

Yeah, I remember the same video - and I didn't realize what I was watching until you mentioned it. I think it was on a 6x36 conversion and that person had posted in Chich's thread (I think, that would have been how I found the vid most likely.)

Ron,

Tried to reply to you on Friday, but while I was in the middle typing the reply I got the dreaded Blue Screen Of Death! This lead to a hard drive crash followed by a trip to the local data recovery service. Hopefully all the data is okay and I will have it back soon.

As far as feed rates, DOC, and RPM go, you should have no problem matching those performance specs you posted. I have been able to cut 6061 with a .500" 2 flute end mill running at 4000rpm, cutting full slot, with a DOC of .250" and travel speed of 40ipm that made the machine laugh. Had a 3", 6 flute carbide face mill running at 3000rpm, full slot, a DOC of .125" and 35ipm travels with no problems at all.

The counter balance system is a pneumatic system that uses two air cylinders and an adjustable relieving type regulator. This way I can dial in the amount of balance based on the table load. I agree with NC Cams that you have increase in inertia, but that increase can be overcome with changing the acceleration rates.

The machine came from a used equipment reseller, with a cost of about 1/4 of a new mill. The ways and spindle were in good shape, but quill feed was toast. Currently the mill uses PMDC servos and runs off of Mach3.

Right now the mill has a max speed of 135ipm. Soon I will be replacing the PMDC servos with AC servos. This will allow me to reach my goal of 500ipm. I know that is fast for this type of mill and there is no cutting operation I can think of that needs 500ipm. The intention of that speed is to reduce the rapid times, especially during tool changing (z axis to zero), and reduce probing cycle times. The AC servos will help a lot, due to their increased inertia and torque capabilities.
The main reasons for me to drive the knee instead of the quill, was the increase in z axis travel, and the ability to run the mill with the head at any position I needed. I can run this machine with the head at a 45 degree angle for a special set-up/part and then return it to 0 without affecting the coordinate system.

Later this week when I get my computer back I will see if I have any pictures I can post.

Ps: Thanks for the info on the VFD thread.:)

Trevor

Trevor,
Thanks for the input and I look forward to the PICs. Change of subject, the way I read the VFD is that any of the Hitachi Drives, even the one that are not specified (larger units)for single phase phase operation can be used for single phase if they are upsized accordenly. Is that how you read the document?

Your input is valuable since 1) you are in the knee drive Z axis configuration and 2) you have a machine with a 2 year history within this configuration.

Hopefully, someothers will also chime in.

Ron

Trevor,
Thanks for the input and I look forward to the PICs. Change of subject, the way I read the VFD is that any of the Hitachi Drives, even the one that are not specified (larger units)for single phase phase operation can be used for single phase if they are upsized accordenly. Is that how you read the document?

Your input is valuable since 1) you are in the knee drive Z axis configuration and 2) you have a machine with a 2 year history within this configuration.

Hopefully, someothers will also chime in.

Ron

Ron111, I'm not sure if this is what you're looking for but I thought it might help.
http://www.cnczone.com/forums/showthread.php?t=18178&page=12

http://www.flyingcritters.com/video/Tap.wmv
Take care.

Endo47,

That's the thread and the video, thanks!! I look at so many good projects that I forget where some of the good stuff is!!


Ron

I have been musing over the knee/quill dilemma for some time myself - and so read the threads above with great interest.

Why don't we face it - neither option is good - for all the reasons put forward.

Commercial NC mills are bed mills with the head moving for Z - for good reason. This is the best option.

Our problem is that there is not an ideal small bed mill available for low cost conversion - if there is tell me!!!!

The Rong Fu dovetail Z slide is in effect a small bed mill - and it is low cost - but I think it is a bit light for serious work in steel. I have been searching the web for weeks and I cannot find an ideal bed mill to convert. There are rugged bed mills out there - but all the ones I can find are just too big and heavy and expensive.

My take on the quill/knee discussion is that your choice depends on what you are trying to achieve. As several posters have suggested, moving the entire knee, table and workpiece doesn't make sense if what you want to do is drilling or tapping. Particularly if that is what you want to do that fast. A much better plan of action for that would be to have a 4th axis which actuates the quill. The quill probably has a lousy geometry / lube setup for handling this sort of service all day, but for drilled and tapped holes the accuracy requirement usually isn't that high to begin with. Anything really accurate is more than likely going to be bored after drilling the rough hole first. Feed rates during boring would be much lower.

If one does that (4th axis) there is the additional complication of unlocking the quill before using the 4th axis and locking it afterward to restore rigidity and a good Z reference prior to doing heavier work. With a "real" cnc machine, all motions run on linear ways and there is no need to "lock" anything in order to maintain rigidity. The 5Bears website has the best example I have seen of building a custom small CNC with all the right features for high speed motion. Then you find out how much is sucks having to buy expensive collets for the high speed spindle and how limited that spindle is in terms of too size. A"real" high speed spindle will cost more than what most of us have invested in our entire mill and toling...

On the other hand, for most types of machining, I think the actual feed rates needed for the knee would be much lower than the 500ipm being bandied about. Frankly, I think that anything significantly over 100ipm should have linear ways and full time pressurized lubrication to both the ways and the screws if it is going to last. Many applications would be speed limited by the spindle RPM and finally the rigidity of the spindle and turret.

One of the limitations of actuating the knee is the lead of typical ballscrews. If one was to replace the ballscrew with a plannetary roller screw with a 1mm pitch, it would eliminate the need for super powerful servos or a huge reduction ratio from servo to leadscrew. Of course these screws are not cheap, but they are a much better solution for high thrust/load applications. Servos need to rotate at a minimum % of their rated power in the application and anyone targeting 500ipm is going to have their servos rotating very slowly indeed during many typical machining operations. This would not make for a happy (or well behaved) machine.

Something that I haven't seen discussed is the (unsuitability) of the default leadscrew / nut / thrust bushing on the knee when being rotated at CNC like speeds. These are all high friction components that would wear out really quick and replacing them with ball screws with their inherent low friction would render the machine essentially unuseable for manual work without the counterbalances discussed and a brake / clutch to prevent movement in response to applied load.

peace
Keith

Surely running the heavy knee assembly up and down is a bad idea - especially with a dovetail slide - it is going to wear in the middle of travel, rack around and loose any accuracy quite quickly. Using the quill makes more sense to me - especially if travels are small - most milling jobs don't need more than 50mm or so of NC travel. But contamination of quill also worries me - anyone seen a good design quill cover?

Mr Keen:

Basd on your concern, every knee mill should be worn out as you describe. I think the fact that they are not has more to do with how they were operated (by hand) than with the design.

Substituting your hands with a servo motor cuts off the feedback that one would recieve if the ways and leadscrew were running dry. Not working the machine by hand is a great oportunity for serious neglect and unrealistic expectations (anyone tried to achieve 500ips while cranking on the knee handle ?).

Clearly, there is no 1 solution that fixes every problem. I do not have to drill a whole lot, and if I did I would use the quill if it were a lot of holes. I would anyway worry too much about blunting a drill and breaking it if I was drilling hundreds of holes and couldn't "feel" the drill losing its edge.

The sort of stuff I am interested in CNC'ing are mould /pattern shapes which generally do have more than a few inches Z height, but then I'm not expecting blazing speed and will more than likely be fitting a central lubrication system that is fully automatic as long as the spindle is running.

Keith

Hi Kieth - My associates and I have been using and reconditioning manual machine tools for over 30 years - and we agree that heavily loaded dovetail slideways are a cost saving and not ideal design. If you carefully check out moderately used dovetail slides - it is dissapointing to see how much they wear. Box slides are much better. I have a dovetail slide light duty CNC mill (Syil X3) - (for cost reasons). If I adjust the slide up neatly to do accurate work the stepper is on the point of slipping - sometimes it does. I could replace the slide with linear ball ways - but the whole machine is really too light and so it would probably be misdirected effort..... So I am trying to decide which way forward - without spending US $27,000 on a Haas TM1 !

Hi all, if you were using a right angle drive to the spindle, or had the head turned at 90 degree, and were using the knee movement to do a facing operation on a job held to the angle plate, then you would want rapid Z and also Y axis movement, with the spindle locked up tight.
This is similar to a horizontal boring mill where the head, which is counter weighted, allows you to rapid rise and fall, on the Z axis, when you are drilling and facing jobs that are angle plate mounted.
Turn a horizontal boring mill into the vertical plane and you have a vertical mill like a Bridgeport, but with all the weight of the table and slides now affected by gravity.
As far as using the spindle in the Z axis locked, and fast feeding on the knee, it doesn't make sense to try and shift such a large inertial type load, even with counterbalancing.
Bridgeport don't have it, or any of the Chinese mills that I'm aware of.
Unless anyone can give a sound reason for premertually wearing out the knee worm drive mechanism unduly, I'd say forget it, why swim against the tide?
I would agree that you CAN counterbalance any weight to "neutral boyancy", but inertia is a factor of natural occurance.
An alternative machine design is where the whole head assembly moves in the Z axis on the column, and is counterweighted, and also has a spindle for shorter Z axis movement.
In this design the table and crosslide are at floor level and only move in the X and Y planes.
The rule of length to depth applies here most profoundly.
It is most apparent in the Taiwanese Mill Drills, where the whole head assembly is held in the vertical plane by a 4" diam round column, pretty pathetic for rigidity, when any significant Z axis pressure is exerted.
The later designs with a V slide column also fail in some designs by having the ratio of column height to column depth less than 4:1, which is the same proportions for the stickout of a boring bar in a lathe, exceed the stick out of the boring bar diam to length by more than 4:1 and you get chatter and deflection.
If you look at the proportions of the Bridgeport body, which is about 2-1/2:1you get a very rigid vertical column which will resist any pressure, but no-one in their right mind would consider raising and lowering the full table weight as a design feature just because the column would take it.
The other factor is lubrication.
Unless you supply and retrieve lubricant, and resupply it to a working surface you will get metal to metal contact and scoring or fretting will occur, leading to seize up.
None of the mills made have this feature, except for the one shot, feeds all, once a day lubricator manually operated.
Even the table on Bridgeports dont have continuous lube feed, and under CNC rapid slide movement, leads to rapid wear.
Ian

Yes I agree with that Ian - I think the little Rong Fu type Mill Drills with the slidway Z axis - which is really a small 'Bed' mill would be ideal - if only the proportions were better. If only the vertical colum and length of slide were beefed up. I thought about making a linear ball way Z head for one of these - any thoughts?

Does anyone know the thickness of the casting at the V ways on the column ? Would there be enough meat to slice off the dovetails and mount a pair of good linear ways ? One would need a good friend with a big VMC to do an operation like this (couldn't be commercially justified) but if that problem could be resolved it may be possible. Of course when I worked at refineries, I saw all kinds of portable machining equipment that just got clamped onto whatever the job was (like a big flange) and skimmed it true.

I was thinking of removing the worm drive for the knee and driving the Z axis screw via a toothed belt. I recently built a specialized test fixture using 2 planetary roller screws preloaded with no backlash. The screws were short, but no mater how short the screw one still needs the nut + the fancy multiple bearing flanged thrust assembly. So I'm guessing that getting a unit with 16" of travel will only add a few $100 to the cost.

My ideal counterbalance would be hydraulic/nitrogen similar to what is used on motorcycles. It would be simple, robust and low maintenance. I don't expect high rates on the Z axis and will accept the limitations that go with what I have. If I go into busines, I will just buy a Haas VMC if I need a fast reliable piece of equipment to churn out daily production. The monthly payment on the Hass Toolroom mill is not too hard to make...

Keith

The whole problem to start with is cost.
No matter how a design is envisaged, if no-one is making a machine to it, then it's not available, that is unless you have access to spare parts from various machine to put together the ideal machine configuration.
When I worked in UK, one of the factories had a machine, huge by most standards, called a floor borer.
It was essentially a horizontal boring mill.
It consisted of a large cast iron flat bed with tee slots etc in it, and a seperate column, not attached to it, that had the X, Y, And Z axis slides on it.
The jobs were just bolted onto the cast iron bed, which remained static, and the column mounted slides presented the cutter head to the work.
The whole column traversed in the X axis plane, and carried a Z axis slide on the side of it with the Y axis movement being the spindle.
The operater rode the Z axis slide and controlled the Y axis spindle from it.
These machines were massively constructed, and on the scale of the cutter dimensions to the slide dimensions would be by comparison to a Bridgeport mill pretty tiny.
That is it would be like using 2" diam facing cutters and 1/4" end mills in a Bridgeport with spindle movement only.
So with that knowledge I dreamed up a design configuration whereby the table of the mill is as per a bed mill with only a table X and Y slide on it and a column of ample proportions carrying the Z axis Head, but without a quill.
Due to the weight of the spindle drive, which comprises a gear box and drive motor, these would be seperate, and carried on seperate Z axis slide linear bearings on the back of the machine.
The front spindle Z axis head would be driven up and down by a ball screw and motor, without a seperate quill, as there is only the weight of the spindle, bearings, head casting and whatever tooling is attached to move.
The gearbox and motor on the back Z axis would also be driven up and down by a ball screw and motor, that is synchronised to the spindle drive, so that they both move up and down together, but without the combined weight.
The drive to the front spindle would be by toothed belt from the back motor/gearbox assembly through the column, to allow flexibility.
One of the big advantages with this set-up is no splined drive shaft to the spindle, which means a larger diam and shorter spindle assembly.
The prime object is to get the weight on the spindle head to a minimum so that real Z axis movement is totally feasible.

I'm available for consultation on this design from 8Am to 12 midnight AEST, if any of the Chinese designers want to give it a thought.

Getting away from the knee design makes for a bench type configuration, that brings the profile down and so makes the whole machine lighter, while still maintaining rigidity.
An alternative drive would be to have the motor/gearbox assembly fixed on top of the column, and the drive taken down by a vertical shaft behind the Z axis head, but this would also add a splined shaft.
Either way the reduction in Z axis weight is advantageous.
Ian.

Kieth - rather than machining off the verical slide dovetails could'nt you just bolt a couple of stand off rails - perhaps with one large steel plate over them (you know to suit the slidway say about 8" wide and 25" high by say 3/4" thick) - and bolt the ball rails to that. - This might even add to needed ridgidity. Keen as.

Keen

Steel is a bad material because it rings. Cast iron is much better at supressing vibration, which is why it is popular for machine building. Another factor is that one wants to reduce overhang as much as possible. The more compact things can be kept, the more rigid everything is.

I was thinking that if one had a knee mill which had ways that were worn out, one could machine the dovetails off (or at least enough to allow the carriage of the linear way access) and then mount linear ways. There are some 1950's vintage turret mills on ebay for $700. If one could find the linear ways on ebay for the right price, the ways may cost less than $400. Machining the knee and table is much less challenging than the column.

For the knee, it would get 4 carriages on the back face for the Z axis. The rails mount to the column. On the top face of the knee, it gets 2 rails for the Y axis. The saddle gets 4 carriages underneath to ride on the Y axis rails on the top of the knee and 4 carriages on top to carry the table. The table gets 2 rails underneath for the X axis.

This configuration wil have low drag, near to zero backlash and will be capable of much higher speeds than would be the case of iron on iron (sliding vs rolling). Best of all it would be much less maintenance intensive, primarily to keep contamination out. About as close to useless for manual work since accurate and zero backlash leadscrews are required and servos would be required to hold position due to the low drag.

But I do think it would be possible to build a nice stiff NC machine this way for a very reasonable amount of money. Some machine builders set up their tables in such a way that there was a very small clearance between the fixed and moving cast iron surfaces. They then introduced oil into this gap under pressure (the oil is caught and recirculated) and that way added substantially greater damping than can be achieved with linear ways alone. Add a new/homebuilt spindle with decent size angular contact bearings, a slick preload system, (no quill) that is good for 4000-6000rpm (with a nice size flywheel built in) and one could do some good work with this system.

The cast iron in a lot of those older turret mills was good stuff and potentially a lot better than the knock offs being built today. Best of all, it would have all the good NEW stuff on it that the knock offs don't have (a good spindle, linear ways etc).

I have a few diferent diverging interests that I am looking at. The first involves building a large gantry style machine which doesn't have to be very accurate for milling structural insulated panels (SIP's) for construction projects. The second involves a much smaller (24x36") machine with very high rigidity for carving soapstone for high end masonry heaters. Getting a good finish with this machine and dealing with abrasive dust would be a big issue. Potentially milling the workpiece inside a waterbath with recirculating coolant may be needed. The third application is 5 axis milling of turbine impellers which would be an even smaller machine with a trunnion system. I would like to be making steel dies for doing the investment casting, followed by machining the actual castings. I can't see myself getting into casting the high temperature alloys, but who knows.

The reality is that no turbine engine aimed at the consumer market has made it out for decades. There are a handfull of companies who are quite happy with the present $250k+ per motor situation. There are a few companies making copies of the solar T32 which they are hoping to sell for $45K + but nothing has progressed beyond the show and tell phase yet. I'm hoping to add a recuperator and better compressor to the T32 and come up with a more reasonable fuel consumption which would make more aviation applications possible.

In the meanwhile, I have my garden tractor to fix and money to earn at my "regular job", so that I have funds for this "hobby"..

Keith

Impressive plans! - yeah , one thing that puts me off the Mill Drill 'knock offs' etc is the crap cast iron - certainly not hardened Meehanite. Anyone out there done big mileage on one who can report on wear?

Hi all, I love the way Keitholivier comes back down to earth on the garden tractor. Great guy.
I would agree with him about the casting bit, leave the exotic metals to the experts, just too much outlay to cast up, when you can get a quote from someone who's tooled up and ready to go, a bit like trying to reinvent the wheel.
I think with the design of a mill, we must go back to basics and redefine our needs.
It would seem that a lot of design parameters and ideal requirements are based on existing machines.
This is something like planning for the Monaco Grand Prix in formular 1, and looking at a VW as a starting point.
To really support a spindle under side loads requires that both X and Y axis are well supported, and this is catered for by the type of design that some high tech jig borers use, specifically the gantry type configuration as used by table routers.
There are no mills out there that have this design to modify, and to find one you'd have to look very far and wide, and then it would be a working machine that would cost a bomb to get before you even think about trying to convert to CNC.
I think there is a formular that states that to remove X amount of material off of a specific area of metal in the shortest time requires a mass of material to resist defelection to zero.
In real terms this means that your Taiwanese light duty mills will remove at their best wack a certain amount of material in a time period, using any cutter configuration, without going crazy and vibrating the nuts off the operator.
So if the Taiwanese mill weighs 1000 lb then it should remove at it's best rate X amount of steel per minute without having a seizure, and pro rata for any other material.
Try to exceed this material removal rate means that the machine must be made of heavier construction, which takes into account that the design of the various parts are already state of the art as far as deflective load resistance design is concerned.
Given that the machine has a design, that is as good as you're going to get, then you can permutate the material for a machine constructed from cast iron, fabricated from steel or cast out of alluminium sections to reduce the weight but increase the rigidity.
This leaves just the ideal type of layout.
Turret type mill, bed type mill with gantry like a planer or........what else have we?
There is no reason why a machine cannot be built by fabrication from steel or alluminium.
To build with these materials does not require any large moulding or casting facilities, yet they are plentifully available as off cuts in sections that can easily be handled in the average home workshop with ordinary metal working tools.
Try to do this with a raw casting weighing a ton and you'll soon need a sophisticated machine to work it.
Most of the machine requirements, once the carcass is made, are catered for by linear bearings, for slide ways, so it doesn't matter if the material is not of bearing quality like a lathe bed made from cast iron.
To the average person with little machine rebuilding or fitting knowledge, this is a daunting task, yet most can follow a plan, and weld this bit to that, and end up with a machine that will do the job, provided the design is worked out before hand and the knowledge to achieve it is available.
The problem starts when the design is at fault, due to lack of foresight, or lack of proportion.
I think the starting point in any requirement would be to decide on the maximum dimension for the work surface with the job size in mind.
This will limit the size that can be bolted to it.
For example if a table top with a size of 12" X 12" was decided on, with an X-Y travel of 12" x 12", then this would hold about 100% of the milling vices within the range of 8" wide jaws with an opening size of 8" max.
If this table is circular and rotated, then we have a very versatile machine that will handle a vast number of jobs within the 12" X 12" size.
All of this can be fabricated from steel or alluminium, and can be worked by the average home worker.
The problem starts to get out of hand when you want a "fitz all" situation.
Big jobs on small machines rarely work, they can, but cause problems all the way down the line.
Ian.

Ian from Oz... It wouldn't be polite to use your forum name...

I believe winter has arrived down south. I have 2 sisters in Auckland and 1 cousin "up north" in Oz, so my family is spread out like the users of this forum...

Ian, the problem with a lot of the materials you mention, is that they have virtually no long term stability. Any hot/cold rolled material has residual stresses that are worsened by welding and any of these will distort with time (ageing) or if anything happens to the surface (shot blasting, rust conversion etc).

Looking at a few websites, it seems that one can buy 4x8ft granite slabs that have been ground and polished for $2500 That seems about the cheapest, most rigid and stable surface of that size for the money, ever. Smaller plates (2x3 ft) are down in the less than $400 range.

For my gantry, I would seriously consider building the thing from surface plates. Yes, it will be heavy, but I could use the surface plate for other things like vacuum bagging composites with a perfect finish and flatness when I'm not needing to run the gantry mill.

The comments regarding "converting" the WW2 iron is not intended to be about building some sort of "ultimate" mill at all. Most of us here do not have a lot of money, or need to spend what we have on more important things like the mortage, but would like a machine that has the capability of cutting steel within a reasonable work envelope and both accurate and rigid enough to et a nice finish on those slow finishing cuts. No 500ipm screamers here.

My comments on the problems associated with expecations of high feed rates with iron on iron surfaces is that even if one gets it to work, it won't last long. But at lower speeds (which is actually quite acceptable to the majority of us not competing for bragging rights) and with a little help (balancing / counterpoise) this kind of setup can be made to work with a manageable maintenance overhead.

In my case, maybe the best way to attack this plan is to look at cncing a mill smaller than my manual 9x42. This way, I may be able to mill a lot of the individual components to take the linear ways right on the 9x42. Maybe buying an incomplete Industrial Hobbies mill is the best way to go, since that eliminates the knee and I could machine the column on my manual mill. I could look carefully at the head/spindle since I want something without the rotation (fixed perpendicular) and without the quill and associated feed mechanism. I also don't want any gears in the head at all. Maybe someone has a used one or IH could sell me a worn out copy that they have done production on as a base...

Anyway, too late to think straight so I had better call it a night..

Greetings.. Keith

Hi Keith, 'handle for short would do, I see that you are out to populate the world, my family is spread out too, some in UK, others in South Africa and Namibia, and the rest of us in OZ.
I agree with the choice of materials being a "use what you can if you can get it" case, there must be a hundred different ways to do it, and not all of them ideal.
When I worked in industry during the last century AD, about 1981, the firm I worked for, R.V. Dorman and co, now extinct, made machining centres for the car industry, namely Ford Australia and Gen Motors Holden.
The machines were linear transfer machines and had about 20 stations that did the various milling and drilling operations for components.
Complete components came off the end of these machines at about 3 minute intervals.
All of these machines were fabricated from steel in module form, and were bolted together in a long line straddling a moving beam.
A long story short, the construction of the modules never caused a problem, such as moving about and warping or twisting at a later date.
The slides, that were mounted side by side on the box sections were made from case hardened alloy steel, 4"x2"x4' long, and were surface ground.
The box sections were machined, and finally ground for the slide mounting.
Usually a box construction is inherently stable and tends to resist movement once it is finally machined.
During machining a lot of stresses are released and so a final machining operation is used to offset this.
I read horror stories of warping and twisting with welded structures, but mostly these are urban myths, and if you were to actually use sound principles of construction and a bit of logic, no problems occur.
To be successfull in fabrication means you must know what you are working with and allow for occurences.
I built a lathe from mild steel in 1966, and have had no problems with the welded bed construction or the headstock.
The problem arises when you use methods that overstep the bounds of logic and expect nature to be scaled without consequences.
For example if you welded two pieces of steel angle together, back to back to form a "T" shaped section, with the welding at intervals along the seams, then it is obvious that if the angle sections were 2"x2"X1/4" and 12" long no apparent warping or missallignment would be apparent, and no subsequent machining would come amiss if it was machined all over to clean up.
However if the same angles were 20' long and construction was attempted, then you would drive yourself crazy trying to get the material to behave and not twist and warp, and that is before you even tried to machine it, yet some folk think that it can be done, and drive themselves crazy attempting it.
As I said previously, if you were to build a machine out of fabricated sections, you must know the design limitations otherwise you end up with twisted scrap, but it doesn't mean it can't be done.
CNC'ing a machine as it exists is a short cut to the end, and you will have to put up with the limitations of it's design when you require rigidity to maintain machine tolerances.
I would like to see some alternative designs for the ideal machine layout, as regards to carcase design and construction, material choice, and work area sizes, disregarding costs initially.
Once you have established the shape of the machine you can then apply logic for material requirements and a cost structure.
The bottom line is you only get what you are willing to pay for.
Ian.

seems like everyone is pretty quick to pick linear bearings as the best choice ....they may be in many instances but its always struck me that the massive reduction in bearing area doesn't seem to enter into discussions. i came across the following which i thought discusses the issue well and also points out that bolted on ways are going to impede the vibration dampening ability of the machine - one of the advantages in using old iron in the first place. if you did move the knee, why not just redo the dovetail ways? based on what that article had to say, that trade off might be more favourable....or do the guys who have built both have a different opinion?

http://www.desktopcnc.com/swarf/taper_50.htm

Many years ago I had a series II NC bridgeport with a boss C6 option this machine had an air assisted knee and a about a five inch diameter quill which had an air over hydraulic feed system with I think about an eight position turret depth stop which provided drilling cycles tapping cycles and feed to depth for milling. It also had programmable canned cycles on the knee which was classed as the Z axis for drilling and peck drilling and three axis linear feed for contour milling it was a great machine it had the large 42 size stepper motors to drive all the axis and the table had 30" x 15" travel with about 16" on the knee like I said a great machine "! BUT BOY was it frightening to see it doing peck drilling on the knee using a 1/8" dia drill and considering you are asking about using a 48" x 9" turret type mill without such a well supported knee you may finish up with a lot of wear you would be better to look at using 4 axis control using the knee for normal contour z axis movement and the quill for doing smaller drilling type jobs and high speed moves hope this gives you another view to it all but my own view is go with the quill it's easier on the nerves. Colin

Mcgyver.. As I said before, this is not about building the ultimate mill, nor about achieving the fastest material removal rates or taking the deepest cuts or having the highest feed rates.

What I have been suggesting are a few proposals for how to get to a low cost cnc mill that has a reasonable work envelope (not mini mill size) and also a reasonable maintenance schedule. I know of no practical way to provide continuous lubrication on either a desktop nor a knee mill, since there is neither an effective way to redirect the oil, nor any suitable way to keep contamination out of those bearing surfaces.

There is no point (in my mind at least) in spending $3-6K on doing a cnc conversion on a mill and then be continuously adjusting the gibs to try to keep table clearances to a reasonable level. Trying to reduce clearances will also play havoc with the tuning parameters of the drives.

If one can set aside the macho "my cut is bigger than yours, and so is my feedrate and so is my spindle motor, and so is my penis etc etc", then one might be able to get some common sense into discussions like this. I know and have already commented on machine builders combining linear ways with close tolerance clearances between iron ways with forced lubrication which provides one with the best of both systems (no backlash and massive damping) . I think we all know that there is a lot of very capable equipment out there and the Haas equipment in my mind is very affordable when one takes the capabilities into account, but my budget just doesn't stretch to a $70k machining center right now.

So, knowing that, and knowing that I have to lower my expectations (a lot !)in terms of throughput (= slower speeds feeds etc), the question is what is the best that one can do for an accurate repeatable system that one doesn't constantly have to fiddle with to keep working. After all, we would like to spend time making things on these machines and not just spend time fixing the machine itself (although that s obviously another potential hobby)..

Keith

Keith, i don't think that one is universally the right way to go over another or that you're going about it wrong, just thought the article did a good job of discussing the merits of each and the respective trade offs. the machine and your objectives of course are the final arbitrator. the objections you raise, ie constant lubrication required etc i think have merit, but don't they also apply to the x/y dovetails which will likely see a lot more movement than the Z? can't prove but it seems logical that as the amount and speed of movement goes up, the need for lubrication goes up...for example if there was lots of very rapid traverse movement it would be a different environment than if it was running at slower speeds, similar to manual. I suppose one could put a small gear pump on for constant supply, but i think you are right; if it's that rapid a speed you want the linear ways would be better.

i do like the idea of knee movement for the Z because despite the challenges.... if based on nothing more than it being poor practice to be hogging away with the quill fully extended :D

Again, there are some good valid points of argument here - but in my case I don't see the benifit of using the quill, even considering the "wear" of the ways and dovetail in the Z axis. Remember, and some of you may have missed this, but my particular Grizzly G3617 knee mill has about .004 of slack, clearance,slop or whatever you want to call it - and it is brand new. What I mean is that you can put a test indicator on the quill and with it fully extended you can grab the back of the quill and pull it towards you and make th indicator move about .0045. If you fully retract the quill you can get about .003.
I know it's a cheapo import mill, and I wish I would have done quite a bit more research first - but this won't work for my purpouses. I'm going to sell the the ol' clunker and get a Tormach (I think).

Mr Turbo

You can most likely get rid of your slop by having the quill hardchromed, measuring the bore of the quill housing to find the exact size and then having the quill ground to match. I didn't find this service very expensive (last time I did it was a long time ago, but money is still money..).

Something to check out would be to make sure that the quill bore is round and without taper. Also, if you dismantle the qill, you may want to check the fit of the bearing outer race in the quill and the fit of the spindle in the inner race of the bearing. Any slop in any of these locations cannot be removed without adding material (hardchroming). I personally suspect that the first place to look is the fit of the outer bearing race in the quill since it is a fairly sizeable socket, then the next suspect is the spindle in the inner race.

Looseness in those locations causes all sorts of machining maladies and reduces rigidity dramatically. If those issues get fixed ad you have the right preload on the bearings, your machining experiene would be transformed...

Keith

keith, I had already thought of having that done but I am not sure that I want to spend more money on this particular machine. There are other things besides that - The belt changing is a major PITA. Again, it's my own fault for not researching a bit better than I did and I accept that. I may still keep this thing around for those quicky manual jobs that you always seem to run into...

Hello everyone!
I have seen my name mentioned a few times in this thread so I guess I'd better add my coments to it.
I have converted a cheap chineese mill and chose to convert the knee. My machine is identical to Turbo's - it's just got a different name and spindle tapers, but I would say their made in the same factories. I added a 100mm dia (4") pneumatic ram under the knee supplied by a vented presicision regulator. The knee travels so smoothly that I can raise and lower the knee with my finger tips on the graduated dial. Believe it or not there is less force on my Z ballscrew that what there is on my X axis with the table fully traversed to one side! The ram add's no added mass or inertia to the system and I have tuned my servo's to high accelleration speeds without any problem.
In my case I am extreemy happy with how it works.

Here is my thread if any one is interested:
http://www.cnczone.com/forums/showthread.php?t=25895

And here is a small clip of the mill moving through full interpolated travell on all 3 axis's with my pendant:
http://www.youtube.com/watch?v=hhvd6rLctOI

My reason for converting the knee over the quill is as Turbo states above. (Remember this doesn't apply to everyone out there with a knee mill) The quill on these cheap machines simply isn't acurate. My thought's - Don't use it then. The rigidity in the knee is the most solidest thing on the entire machine - use it!

My MAIN reason for converting the knee on my mill is beacuse my machine (and turbo's) has a horizontal spindle as well as a verticle one. The horizontal spindle is built into the colum and is super rigid. As a hobby mill I don't know what sort of jobs I want to mill on it in the future so I made sure I converted it in such a way that it can do absolutely everything I want it to do.


My 2 cent's on the whole knee and quill bit: No one is right and no one is wrong. I think it comes down to what you need your machine to do. I wanted to utilise both spindles on my mill so I did the knee. For vertical milling I simply lock the quill and away I go.

As far as wear rates on the slides go and WHAT? - Cutting them off to put linear slides on!!!!!!!! My goodness what next!
I use a good old oil can with a medium/heavy weight Hydraulic oil. I don't detect any discolouration in the oil when it eventually runs out so I would suspect very little wear in my slides. Look after the machine it will look after you. The duty on a well used hobby machine is so rediciously low that in 50 years time you MIGHT consider re-scraping the machine true again.
If I am wrong on this one - they are so affordable - get another one!

I hope the last coment didn't sound rough. It's meant to highlight the whole hobby bit and not to complicate something that has proven itself over 50 years. That's my 2 cents.



Cheers
Chich

Hi all, there should be no problem as regards to continuous lube supply, recovery and resupply.
Options would be:-
1- Continuously supply oil with a pump of some sort so that it weeps out of the bearing surfaces when the machine is working, which will indicate that you're getting enough.
2- Make sure that the waste oil drains into the coolant sump in the base of the machine if it has one, to ensure that you aren't standing in a puddle of it and make recovery easier.
3- The oil can be skimmed off of the coolant surface as it won't mix with the coolant, and can be filtered and re-used.
4- Option 2 is to discard all lube oil after skimming, which is a bit expensive but guarantees purity.
The lube system needs to be supplying oil only when the slides are working, so the pump could be a cam driven piston pump, driven off of the X Y and Z axis screws.
A similar system to this is used by the live steam model railroad locomotives, whereby a lube oil pump is driven from the axle and supplies oil to the inlet to the cylinders.
A lot depends on whether or not lube is a problem, which can only be ascertained down the track when a slide gets scored up or just wears out prematurely.
Whichever way is contemplated, oil is the cheapest form of machine maintenance known and will extend a machine's life enormously, CNC or not.
Ian.

Hi all, the problem with driving the knee is not a problem when it is counterbalanced, as far as acceleration is concerned.
If the ramp up and down is taken into consideration then the inertia will not be apparent, and the knee can be driven quite nicely and without undue loading on the respective motor.
It would become a problem if the lube is not factored into the final equation, and was just used as per normal manufacturers design arrangements, and that would go for all slides, especially if they were adjusted so close as to give minimum clearance for deflection prevention.
Can anyone say for certain whether or not adding Graphite or Molybdenum compounds to the lube would help, especially in the case of constant short slide movement or long rapid slide travel?
Ian.

Re: moly and graphite: Both of these are known friction MODIFIERS but, used improperly or if used in/with the wrong carrier solvent/liquid, they can do more harm than good.

Moly is usually used in the form of a moly disulphide compound (MoS2). It is used in this form quite extensively as a roller bearing and/or a ball joint lube. Under high load, low magnitude relative motion, I can pervent siezure by facilitating the sliding of ground/smooth surfaces over one another.

Moly is essentially a metallic substance, hence it is a hard "foreign" particle unless it is properly sized and suspended in a carrier lubricant. We have and did misuse it as an "assembly lube" for some bushed applications. The thinking that this superiour load and friction fighting fortified lubricant would work like a champ. What a nighmare.

The moly imbedded itself into the soft bearing alloy and actually caused the bushings to sieze faster than if we'd have not used anthing but oil which is/what was recommened in the first place. What didn't sieze actually started to abrade the shaft. From that fiasco, I've learned to NOT slather moly grease willy-nilly onto anything that moves.

Graphite is effectively finely ground carbon. Not unlike moly, it is a metallic substance that is both slippery and, depending on the size, it can also be quite ABRASIVE. It will do a fine job of acting as a dry film lubricant. It can also serve as a lapping compound if it is prepared or used improperly. One version helps, the other will facilitate wear rather than preventing it as intended

Both moly and graphitic compounds have been used as grease and paste supplements for assebling auto engines, especially flat tappet camshafts. These are notorious for encountering short term rubbing failures until/unless a lube film is built and maintained. The problem with moly/graphite lubes is that they tend to get filtered out via the filters in auto engines - yes, the metallic particls are "debris" as far as the filter is concerned.

Frankly, the moly and graphite "debris particles" can and do have a detrimental affect on some sliding as well as some rolling element bearings - recall that the stuff is actually finely milled METAL and needs to be used/considered carefully and properly when use as a lubricant is contemplated.

Either can and has done more harm than good in some situations, in spite of the good intent of the user.

BTW: I pumped some MoS2 grease into my gibbs on a mill and, initially it worked great. After time, it attracted dirt and crap like you can't believe and, now, I can't get the stuff out of the lube feed tubes. The ways were scheduled to used/accept "way oil". Sadly, I wish I'd not experimented and "used something WAY better" (no pun intended).

Hi Nc, "all that glitters is not good for you", and just when it seemed there was a cure all to our problems.
Oh well, I was going to add some of the Black Magic to the lube oil that drips into the bronze bearings on my old lathe headstock, but I think I'll give it a miss. Too uncertain for the results.
There was a test on Moly done in the '50's last century, whereby a car engine had some moly added to the sump oil and was allowed to run for an hour or so.
Then the oil was drained from the sump and the car was driven for a distance of about 100 miles, no oil in the sump, and then the engine was stripped down and there was no wear apparent on any bearing surface. Big selling point at the time.

I would sympathise with you on the using grease in the ways of the mill, as when it gets squeezed out of the slides it can't go anywhere and just stays on the machine gathering up the swarf in a gooey mess, unlike oil which when it evacuates the slideways just gets added to the coolant and ends up in the sump as a top layer for the skimmer to remove.

I used to hate those old lathes with the massive iron headstocks and gears lubricated with occasional daubings of grease.
They always had a queer smell, like someone had added garlic or Permatex gasket compound to the grease, YUKKKK, takes me back a bit, everything was black and dirty on those heartbreakers.

There used to be a warning in one of the tech journals about adding graphite to lube oil, as it would not stay in suspension, and would eventually dump out on the walls of oilways and block them, so causing oil starvation, especially where overhead camshafts were concerned.

On the subject of lubricants, we had a German crankshaft grinder in the '50's that had a recommended head bearing mix of 50:50 SAE 30 engine oil and diesel fuel. If that mix wasn't used the bearing, being a hydraulic adjusted bronze bearing, would get hot and seize the shaft, which took 2 hours to cool down, before it could be started again.
The first time it happened there was a right old panic in the ranks as the machine had been entrusted to the chargehand, a German fellow, highly qualified in the old school, to get it up and running to supplement the Churchill grinder next to it.
It eventually worked out right, when the manual, in German, was fully read and the lube requirements understood.

Which brings us back to the original topic of driving the knee, oil it must be and plenty of it.
I suppose there is a formular that states that for every sq metre of bearing surface traversed, multiplied by the load in Kilograms, there should be so many litres of oil delivered, at a viscosity determined by the Dupont flask calibration, and having a pressure of X kilograms/sq cm to keep them seperated.
This was known in the 1800's as Brunel's lubrisocity theorum for moving metal, and was used to calculate the required amount of blubber and tallow mix for getting The Great Eastern down the wood ramps and into the water.

I don't really know, I just pump my oil can a few times untill the oil squeezes out of the bearing and thing go quiet for a while, then do it again a while later.
Ian.

nice input to start with.

while my mind is boggling still i can't add to this but another question/wannabe suggestion perhaps.
Would driving the knee on a "old"deckel design mill solve/lighten up the weight considerations AND maintain most of the rigidity.

Deckel used to use the complete spindle assembly for an X-axis leaving only the Y and Z movement to the knee wich looks like a massice weightsaving even the though the knees on kneemills are hollow.

On the link you can see a clear picture of a beautiful deckel fp1 displaying the "bolt-on" horizontal and tilting table.While the X movement is madeby a slide on top of the "pilar" moving the entire spindle.

http://www.arnold-richter-werkzeugmaschinen.de/index.php?id=29&bild=FP1%20EAE.jpg&gal=5&count=2

correct me if ..uhmm WHEN you have time to do so.

I reckon if you knew someone with one of these machines they would have an armed guard round it to keep the sticky fingers off.
It would be the height of sacriligeous vandelism to contemplate probing into the innards to see if it can be "converted" to CNC, though it would be pretty accurate, as long as the large weight factor of the combined Z Y axis is catered for.
Like all conventional factory machinery, the slides and the attendent metal mass were not designed to having to move metal mass at CNC speeds during a machining cycle, apart from the lubrication problems.

On the topic of the knee drive, I would assume that with a counterbalanced knee, the Z axis would be catered for quite well provided the ramp up and down were taken into the equation and the lubrication was stepped up.
If the lube was a continuous supply the bearings of the slides would not "bottom out" so to speak but would have a cushion of oil to ride on for every movement, X Y or Z.
Neglect this factor and you've got trouble.
With all that oil being pumped, a means of recovery and filtration would ensure that the bearing surfaces stayed seperated, which is a similar requirement for a car's engine or an air bearing on the end mill sharpening fixture on a tool & cutter grinder, and it runs very smooth on air alone, with no wear.
They are also both plain metal bearings, not linear bearings.
It would almost be the case for CNC conversion, that the lube must be supplied in large amounts and the coolant in an air/mist form so that the coolant, such as it was that got into the lube would be filtered out and discarded.
The other option is to run on a total cutting oil supply for the slides and cutting tools, with the necessary metal partical filtration.
Ian.

thx but i would not even dream about thinking of converting a deckel like that to cnc ,but what appealed to me is a seemingly much lighter Z-axis combined with the rigidity of a bridgeportdesign.

My guess is that the weightsaving will make for much lower wear on the Z-slide.Wich seems to be one of the major concers of the people trying to drive the knee on their machine.And i won't go into the difference in stopping or reversing direction and the forces related with this cause i lack the knowhow but i can imagine a big difference.

there are asian deckelclones and even more eastern european ones wich can be comeby in the pressence of luck.

Thx fo reading.

i hope yall just been looking for a new mill ...

If any1 around let us/me know what you think.


THX

http://www.youtube.com/watch?v=rgt8EB9kEb4

Pimpbike,
Good to hear from you. I had started this thread because I have an interest in driving the knee on some type of 9X 49 mill with dovetail ways. Any details that you could give on your friend's mill as far as the make (dove tail vs. boxways) and whether he uses a gas springs vs. an air accumulator, and of course whether servos vs. stepper and the size and pulley ratios. Did he also cnc the quill for drilling/tapping operations?

Also, I's looking at the possibility of using a VFD with the stock 3 phase non vector rated motor, but will only operating the frequency from say 45 to say 75 hz in order to fill in spendle speed gaps which occur with step pully arrangements. So any experience, from anyone doing such is also appreciated. (I not looking for all the reasons to use vector rated motors and long lists of evils associated with cooling problem of lower temperature rated insulation on motor windings and the effects of improper cooling caused by decreased motor speed and air flow, ect., ect.) I want to hear from the guys who are actually doing so. Where the rubber meets the road, so to say!!

Thanks,

Ron

Thanks for any info

I scanned through the thread, didn't read entirely so sorry if I'm repeating.
There are a bunch of production machines with a knee drive. For example the older Abene and the Deckel FP-series have Z on the knee. Non of them can offer the best, either you get speed but may have sloppyness with the quill or a sturdy but slower operational speed.
It's like choosing cutting tool; knife or an axe? Well, it depends what you want to cut. ;)

He's got a 5 hp VFD on his 3 hp, 3 phase motor running on single phase. The Ratio is 2:1 and the Z is a box way the X,Y is dovetail. The drive motors are DC servos running on Geckos Not sure of the specs of the motors. There are 2 air actuators not sure of the rating I thought he said something like 200lbs each but I will ask. My opinion is that driving the knee is the way to go. I have a Quill Z feed CNC mill myself that I will be doing a retrofit on. I know from experience that when the quill is fully extended you are way more susceptible to vibrations. The added amount of travel you get when driving the knee is also very valuable. When you have multiple tools at different lengths 6" of travel is not always enough and having to crank the table down and make sure you compensate in you program or offset can be a pain to deal with.

Thanks for the info pimpbike, did he use the 3 phase motor that came on his mill or did he go with a vector rated motor and is his mill a imported 10 X 54 type (since it has the square boxways on the knee). I saw one air cyclinder on one side of the knee, so I guess the second would be opposite. Think out loud, I would suppect that it would be handy for the quill to also be cnc'ed for drilling and tapping operations, but then maybe with good counter weighting (preloading), sufficient knee rapids maynot be a problem.

What machine do you have? and how soon are you planning to convert the knee?

thanks,

Ron

It is an import mill. The machine in the video would have no problem tapping the problem is the Mach software at this time as far as I'm told. It is the 3 phase motor it came with on a VFD not a vector drive. I have many machines but I'm retrofitting a Hurco tm-5 at the moment and I have a Excello that I just bought from Ebay with 4th axis that will get retrofitted and I have a millport CNC knee mill that I bought from EBay earlier this year that has the Z on the quill.
http://www.cnczone.com/forums/showthread.php?t=40638
,
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=170153272544&ru=http%3A%2F%2Fsearch.ebay.com%3A80%2Fsearch%2Fsearch.dll%3Ffrom%3DR40%26_trksid%3Dm37%26satitle%3D170153272544%26category0%3D%26fvi%3D1
,
http://www.cnczone.com/forums/showthread.php?t=37642

Good to know that the existing 3 phase motors probely will do ok if they are not pushed too much. Thanks for the links, looks like your plate is full. This gives me a couple of more good threads to follow.

Take care,

Ron

Ron:

Merry Christmas and Happy New Year. Peace. :rainfro:

Link to my completed conversion --

http://www.cnczone.com/forums/showthread.php?t=8749

I still feel really new to this whole game. But here are some observations that might help. If this has been covered, I appologize. I read about 3 pages and cut to the end.

The knee driven conversions get done faster simply owing to the fact that one does not have build or add on much.

Accuracy is really subjective. +/- .005 could be as good as you need. Some guys are freaking out as they read this now at how "bad" that is.

I didn't believe it until I checked it out; your machine grows and shrinks a lot as it warms up and cools down. Turns out the vise does too. So, there's a lot going on that can affect accuracy and repeatability.

Tool deflection is more of a problem sometimes than the machine slop.

I've not had any issues with the quill hanging out 5". Ever.

The bigger issue with the Z conversion is driving the quil off one bolt that was clearly never intended to do so. Your drive mechanism needs to be a close to it as possible. Mines a little farther out than I would like. But, it's pretty good still. I've had no problems.

I have had issues at the ends of the bed travel. Even with the gibs tight, it gets a little sloppy.

After doing a "Z" conversion I might be tempted to try the knee. But I would just get a big motor. The counter balancing by air or springs or whatever is just one more set of moving things to worry about. I've seen two conversions that just went with the big motor and the guys seem to be doing fine. The motors were not that big by the way.

Oh, I agree, 500 ipm is interesting but not needed. 60 ipm gets you over the slow as paint drying rapids issue. Depending on what you are doing you could want to slow it down to 10 ipm to get a good surface. Fast rapids make a difference at high part counts. More $$ / day right?

Finding the right center of gravity on the knee seems impossible as the table is moving all over the place. So there's, what, 400# that could be anywhere at any time.

Hope that helps.

Best,
John

Just my past experience with the knee on mills, do this on a manual mill.

Clamp a bearing or something down on the mill table.

Run the table down about 6" or so.

Indicate the bearing in, and lock the x and y on the table.


Now run the table up 5 or 6 inches, and check the bearing. Is it still zero or the same all around as it was when the table was down low?

Run it back down and check it again. Is it still the same? In 25 years of machining I have only seen a few mills that would pass this test. Maybe I've always been working on abused junk, too. :)

Also the first cnc knee mill I ran was unpredictable, from one day to the next it would develop glitches. A program that ran good yesterday, or all week, might run through the table today??????

Every morning before running a program I would back the table down and run the program close to the part to confirm it wasn't lost.

It was nice being able to just run a program simple to do a 1" depth. Set the Z depth on the prog. to -.100 and then crank the table up after every pass on inconsistent castings. (I would give it +2.00 on the Z clearance though).

But there are good and bad on both sides.

Hope this helps,

JAck

any thoughts about moving the X-axis to the column like deckel and maho did on some of their older manual mills would solve the weight issue or am i overlooking something.

Thx in advance.

Hi Jackall re post 59, whenever I've had to use a knee type I religiously lock the knee for all machining ops.

This means that the table will always be in a fixed position after it is positioned.

I rarely use the knee to increase the depth of Z travel while the machine is running, except of course on a horizontal mill, and I can't remember when I last used one of them, and even then the lock is half on to make sure the table is pulled up tight to the slides.

The fact is you need to have the knee slack enough to be able to fast traverse up or down, or you end up wearing the drive mechanism out if the knee is tightened up to get the slack out.

Anybody who machines with the knee in the unlocked position is asking for innacurate results.

I tried your method on my Ajax turret mill, (similar to Bridgeport), with the quill at it's uppermost position and locked.

By this I mean if the quill is in the top most position and the knee is raised and lowered for clocking, then you must lock the knee at each position before testing the bearing bore, if you don't the knee will just be in a position that it happens to get to, due to the slackness required for traversing, when down or up, but the lock will pull it against the opposite dovetail etc.

Some years ago I worked for a firm that bought a new Mass horizontal mill with the vertical bolt on head.

From the beginning it never gave accurate results, untill they stopped using GREASE in the knee slides and used oil as the manufacturers stated.

When they used the grease the slides had to be slacked off to get the drive motor to raise and lower the knee, and this led to all sorts of problems, especially in winter.

Which leads on to anyone who is going to use the knee for Z axis travel must decide if the tightness of the knee slides can be overcome when positioning and fast traversing without affecting the positional accuracy due to the slackness required.

Here's a test for you, position the table in it's central position in the X axis and mid way in the Y axis, and lock both X and Y.

Now position a dial indicator to read at the end of the table, ( I swung the head around for this test).

Try to lift the table ends by placing a short plank of wood under each end alternately, and using a piece of wood as a lever to exert some pressure.

This will give you some idea of the amount of movement you will have when the knee is allowed to be left unlocked.

Try this method with the knee locked and unlocked.
Ian.

Yeah, that is what I've run into. I've been repairing the bearing fit in a gear box or end plate of a rotary blower. After indicating the locating register or dowels, bore it out for a sleeve.

When the sleeve arrives, you find out they want a different style seal in the end cap also.

This means you must crank the table down, maybe 1/2", and use different boring bar or head.

After moving the table down with all 3 axis' locked you re-indicate the dowels or locator register and find out the table moved .002 and you have a tolerance of .0004 . Time to relocate the part, and that doesn't count having to tram the head to the face now because it has moved, too. You have to keep the quill as short as possible for ridgity, yet at the same time allow enough for the stroke. Some of the knees on these mill even have 2 locks.

I have seen some of the bridgeports with air gap ways for smoother travel They were even more sloppy than the standard ones.

It gets real frustrating, especially when they expect you to do a job like this in 3 hrs total. :)

Some of the machines in this area are so worn that if you try checking the play on the ends of the table it would read .020+ with everything locked. The gibs are bad or the dovetail is so worn in one area.

I have checked some new Birmingham, Kent, and Supermax mills that had a quite a bit of play in the gibs and dovetails. The shops in this area tell guys applying for jobs " Anybody can make parts on a perfect machine, you have to know how to sweet talk this kind of junk". They don't know how true the word "Junk" is.

Jackal

I take the angle of if it woks.......Use it. For example, Look at any "real" CNC like a vertical machining center. X and Y are rigid in a base and Z is a big heavy head that slides up and down a column. It simply has a counterweight. Works perfect. That's another reason I "Z" my knee. Quill's deflect too much. Adjust the Z knee properly (just like a real CNC machine is adjusted) and anjoy the rigidity. I'd also like to say that knee mills are good......BUT...... there's a reason real CNC manufacturers don't design them in a knee mill configuration.

My 2 cents
Cheers
Chich

Hi all, I think there are two possibilities that can drive you, one is an old mill that is well past it's use by date, and a new mill that is just not "bang for the buck".

The moment you put the dollar value as the first priority, then you get what we had in the 70's the "Taiwanese Terrors", those horible little bench mills with round columns that couldn't mill to save their lives, but didn't cost much as they were glorified drills anyway.

I deeply sympathise with anyone who does not understand the phrase that "you only get what you pay for" in it's fullness, and are wowed by the shine of a first time novice buyer's latest aquisition.

If you don't know what machinery is then you will fall into the trap everytime you go to an auction and grab the "cheapies".

The bottom line is if you bought a Bridgeport, then you only get what you pay for, that is if it's worn out but looks a million dollars, don't expect it to perform like a new machine with all the certificates of trueness that the makers build into a new machine.

This goes even more so for a new Chinese piece of crap, or for that matter any other country that produces cheap machinery, and if the buyer bought strictly on the price, more fool them.

I would go so far as to state that I'd rather buy a cheap worn out Bridgeport and rebuild it, than a new foreign piece of crap that no matter how you adjust it just won't "cut the muster" no matter how you struggle with it.

OK, so it's going to cost you a bundle, if you dive into the realms of Bridgeport rebuilding, but at the end of the day you get more "bang for your buck" than any new cheap forign junk, and will outlast you given proper care and maintainence.

I bought a cheap Bridgeport, A$2,800 in '97, last century, part of our factorie's down sizing plan, and lived with the worn out bits that in the end didn't really affect the accuracy of the work I was doing and could be relied on to get things square and flat, which is all I asked of it anyway.

Second hand machinery is not for the novice or newbie, unless you have a fair degree of machine knowledge, or someone else's you can tap into, and can do the allignment checks that will determine if you have a "treasure" or a boat anchor.

As far as "sweat talking" old junk, I worked for some of the places that had "old junk" as part of their main armoury, only for a short time at that, at the very least it wasn't a well paying situation, and the work they turnd out was never of the highest standard too.
Ian.

That is really different. This is one of the better paying shops around here, between $16 to $20 an hour. The shops with all of the new equipment usually pay from $8 to $11 an hour. The claim is: with the paying for new machines, there isn't any money left over for wages ;) . Besides that, it is their belief that anyone can make parts on good equipment. If you go in one of those shops you see high schools kids out for the summer, cutting keyways and such. The foreman will tell you "On this good equipment it's easy to be a boss, tell 'em to dial in .500 and it will take .500, no problem."


If I have something that needs real rigid machining I'll take it to the Haas VF3 or the Mori-Seiki. Just the little stuff on the Bridgeport Boss, 10-32 tapping , 1/4-20 and a lot of small aluminum stuff.

JAckal

Ha Ha, I get your drift, bit like going to a society wedding in a $2 suit from the pawnbrokers.

It comes down to how intelligent the business executive is when he tools up to compete on a world platform, some don't know what day it is let alone the time.

You have to be pretty desperate or cluless to take wages as low as that.

I worked a few places like that when I finished my apprenticeship, but then I was still learning and learned fast.
Ian.