Question on Z-axis retrofitting

[Neil_J]

Hi,

I have a stupid question... I tried to search through the forums but I didn't find what I needed...

I am currently shopping for a new mill to retrofit... The problem I see with CNCing a Bridgeport-type knee mill, is only having 4-5 inches of quill movement. I will going between large and small tools (i.e. 1/8" end mill to large drill bits and chucks).. I can see large tools eating up my quill depth which would require repositioning... Is this correct? I have used benchtop mills but never a knee mill.

And are most bridgeport retrofits on the Z-axis quill only? Is there anyway to accurately control the knee movement up and down?

[Al_The_Man]

I personally would not recommend the knee as a controlled axis, too much mass etc for rapid moves, I have an Excello with Z quill and a stepper motor operating a Ballscrew type jack under the knee with air counter balance cylinders, which is semi manual operated, i.e. push-button, it has a graduated indicator for close positioning.
Al.

[Neil_J]

Originally Posted by Al_The_Man I personally would not recommend the knee as a controlled axis, too much mass etc for rapid moves, I have an Excello with Z quill and a stepper motor operating a Ballscrew type jack under the knee with air counter balance cylinders, which is semi manual operated, i.e. push-button, it has a graduated indicator for close positioning. Al.

So automating the knee can be done? I wouldn't use it for rapid movement, just for repositioning between tool changes.

Do most knee mills have graduated handwheels on the knee? I was under the impression you lost your Z reference after moving it.

[Al_The_Man]

Most have a hand crank, this one was converted, you do not lose Z zero ref as the reference is at the quill extreme up position, so the table is just moving away, the only thing it will affect is the Z work co-ordinate, so some way has to be established to overcome this, you could either move the knee a known number of steps and reset the tool offset or I just usually re-zero the Z part zero, but I'm not a machinist!
Al.

[HuFlungDung]

Neil,
Available quill travel is a problem alright. It is tough to beat the convenience of a VMC bed mill for that very reason.

However, it is often possible to purchase toolholders that will extend the reach of small tools. And, it is also possible to purchase morse taper adapters to fit your spindle. Then, you also purchase what are known as 'drill drivers' which are a simple morse taper socket with a split cut lengthwise. A person can then buy one of these drill drivers for each size of straight drill shank that you will use. The advantage of this method, versus using a chuck to hold larger drills, is that the overall length of the toolholder plus drill will be 2 or 3 inches shorter than when using an arbor mounted drill chuck.

[Neil_J]

Originally Posted by HuFlungDung Neil, Available quill travel is a problem alright. It is tough to beat the convenience of a VMC bed mill for that very reason. However, it is often possible to purchase toolholders that will extend the reach of small tools. And, it is also possible to purchase morse taper adapters to fit your spindle. Then, you also purchase what are known as 'drill drivers' which are a simple morse taper socket with a split cut lengthwise. A person can then buy one of these drill drivers for each size of straight drill shank that you will use. The advantage of this method, versus using a chuck to hold larger drills, is that the overall length of the toolholder plus drill will be 2 or 3 inches shorter than when using an arbor mounted drill chuck.

I've heard of those, but would like to avoid them if possible...

HuFlungDung, in your opinion, what type of setup should I buy/build? Here are my requirements...

* Material cut: aluminum, steel, plastic, wood, (i.e. most everything)
* Material shape: Mostly flat stock that will be engraved/cut, but also the ability to hold blocks and odd shapes, ranging from very small to big.
* Work performed: Milling/drilling (straight from CAM or thru Mach3 conversational), engraving (text or PCB routing), 3d contouring, possibly digitizing using a probe, possibly mounting a plasma/laser if using a Gantry type)
* Machine construction: Very heavy cast iron/steel if possible. Must be very rigid (the main reason I would consider retrofitting a larger machine)
* CNC specs, after retrofitting: ballscrews, Geckos, servos, 100+IPM rapids, 0.0002" limit/home switches, auto coolant/spindle, etc... 3-axis to start, but will be adding a rotary table later on
* Accuracy: 0.003 or better
* Table size: X=20"+, Y=8"+, Z=10-20" (without losing z position!)
* CNC engraving (10K+ RPM) is a must
* Budget: $3000-6000 total, over 1-2 years time
* Experience/knowledge: very high in electronics/mechanical areas (but pretty new to machining)


If I could find one, a used gantry or small VMC would be perfect... However, modifying a manual mill or starting from scratch might be all my budget can handle.

[NC Cams]

It would seem that you really want a high speed engraver of some sort, especially with an RPM spec of 10K as a "must".

A Bridgeport is not going to run that sort of spindle RPM and if it could, not for long.
The generic bearings used to make the 1119238 oem spindle bearings used in most BPT milling machines are rated at only 9500 max and that is in an un-preloaded "stock" condition.

The oem bearings are custom preloaded at about 60 lbs which makes them stiffer for less tool deflection BUT it also reduces the allowable peak speed. Good engough for a mill that runs at the design speed but definitely not at 10K.

Simply put, a Bridgeport mill is NOT what one would/should consider a high speed machining center. Other (and surely more costly) machines will do the high spindle speeds that you want but these generally must operate at lower infeed rates. Othere than the spindle speed requirement, you could retrofit the BPT to do MOST of what you want.

Your 0.003 tolerance should be easily held with rolled thread ballscrews, good servo tuning and properly adjusted gibs. However, don't plan on doing fast inches/min. while trying to hold tenths in accuracy and repeatability. Used iron with work gibs/non-roller bearing ways won't facilitate it - simply too much slop and/or friction.

We hold error to within 0.0001 as compared to CNC ground part on our BPT EZTRAK but we take very light cuts (0.002/0.005 and travel at 1-2 inches/minute (this reduces effects of table friction). Moreover, we've got a pristine machine with chrome ways, several thousand dollars worth of ball screw and custom ball screw bearing work/rework and several weeks of maching tuning.

Result: we can cut the pattern we want but it takes us nearly 2 hours on a well tuned up BPT. The neighbor does the same thing with comparable accuracy and in only ~30 minutes with his HAAS machining center.

The key to NOT being disappointed with the results of a project are to embark upon it with realistic expectations while concurrently considering what the machine was designed and intended to do in the first place.

Hope this helps....

[HuFlungDung]

What he said

Good reply NC Cams.

If Andrew McCalip happens to come along here, maybe he can relate what he's had to spend to get his mill retrofit going. I think what he's done should be fairly representative of the least one would spend, starting with a decent used machine.

Every retro I've done, I've spent anywhere from $10k up to $30K, plus the cost of the used iron, which sometimes is not all that much

[Neil_J]

Originally Posted by NC Cams It would seem that you really want a high speed engraver of some sort, especially with an RPM spec of 10K as a "must". A Bridgeport is not going to run that sort of spindle RPM and if it could, not for long. The generic bearings used to make the 1119238 oem spindle bearings used in most BPT milling machines are rated at only 9500 max and that is in an un-preloaded "stock" condition.

Yep, sorry, I should have clarified that. I will either be retrofitting the spindle/motor to something high-speed/high torque (3-phase with VFD), OR use the mill as-is to build a seperate gantry-type machine out of steel. Right now I would seriously consider either.

Originally Posted by NC Cams Your 0.003 tolerance should be easily held with rolled thread ballscrews, good servo tuning and properly adjusted gibs.

Good, that's probably all I can afford Would a rolled double-circuit preloaded ballscrew have zero backlash? Same question for single-circuit preloaded?

Originally Posted by NC Cams However, don't plan on doing fast inches/min. while trying to hold tenths in accuracy and repeatability. Used iron with work gibs/non-roller bearing ways won't facilitate it - simply too much slop and/or friction.

Yea, I would be using the high feed rate for rapids and rough cutting only.

Originally Posted by NC Cams The key to NOT being disappointed with the results of a project are to embark upon it with realistic expectations while concurrently considering what the machine was designed and intended to do in the first place.

True. My reason for wanting to start with a decent knee mill is rigidity. (My CNC adventure started earlier this year with a Sherline 2000-CNC.. Taking a 3/8" cut at more than 0.05" or so will knock the whole thing out of tram. When it is cutting, you can feel the entire head vibrating, and see the effects in the cut).

I would start building a machine from scratch today if I had a few hundred/thousand pounds of steel and a steel furnace. I can't imagine building anything in my garage that would be accurate, and rigid, and cheap.

Any suggestions?? Wait for a used VMC?

[jdelaney44]

I would wait for / go find a used Series II with a dead BOSS or other controller and replace it with a PC based controller and the appropriate stepper / servo motor drives. If you want to see my retrofit story it's on this board. Pretty sure I'm at about $8K including the machine.

JDs BP J-Head CNC Conversion Blog

Now you said you just want the big Z travel for the ability to change tools? Is that right? I think if you use tool holders for your drill bits you'll avoid the problem. Also, use shorter bits. Also, you'll probably end up doing a lot of "drilling" with center cutting end mills. I have had times when I've had to assemble the drill rig a piece at a time. First the tapered spindle, then the chuck, then the drill........ So yeah, it would be great to have 20 inches of travel and not lose reference.

You could put a DRO on the knee and hand crank it. Or fab up some kind of optical home finding "switch" or something. If the machine you get has a DRO you could move the DRO to the knee. OBTW, the knee on mine has a dial indicator. But I have not found it to be as reliable as I would like.

[Neil_J]

Originally Posted by jdelaney44 I would wait for / go find a used Series II with a dead BOSS or other controller and replace it with a PC based controller and the appropriate stepper / servo motor drives.

There is a dealer near Daytona Beach that has a few of those, used... I will have to stop by and check them out. What are the main things to look for on the bridgeports?? I know a lot of older ones are worn out, but how do you tell that?

Originally Posted by jdelaney44 If you want to see my retrofit story it's on this board. Pretty sure I'm at about $8K including the machine.

If I'm patient and do it all myself could I do it all for $5k? If it costed more that would be OK, it would just take longer.

Originally Posted by jdelaney44 Now you said you just want the big Z travel for the ability to change tools? Is that right? I think if you use tool holders for your drill bits you'll avoid the problem. Also, use shorter bits. Also, you'll probably end up doing a lot of "drilling" with center cutting end mills. I have had times when I've had to assemble the drill rig a piece at a time. First the tapered spindle, then the chuck, then the drill........ So yeah, it would be great to have 20 inches of travel and not lose reference.

That and also the ability to go between large pieces sitting in a vice and small plate material clamped to a sacrifial plate. Using 0.0002" limit/home switches and a Tormach R-8 fixed-Z tooling kit, and an electronic edgefinder, I would basically like to take my 3d part from SurfCAM and have the machine start doing its thing... I want any cranking/locking to be minimal.

At this point though, it doesn't look like I could ever get a Bridgeport to dance at 10,000 RPM, so I don't know what I'm going to do. Can you think of any way to attach one of these to the side of the Bridgeport spindle? (middle) It would do 30K+ and has a built-in pneumatic tool release.

[Hood]

Originally Posted by Neil_J If I'm patient and do it all myself could I do it all for $5k? If it costed more that would be OK, it would just take longer.

What it will cost will depend on what Iron you have in the first place. I have done a manual Bridgeport and am now in the process of Retro fitting a series 1 CNC.
For the manual I needed ball screws, motors and all the rest, the CNC already has the ballscrews and motors although I have opted to change out for modern motors.
My costs are:
Series1 CNC = £400
916oz/in steppers = £270
Gecko G202 = £230
PMDX 122 breakout board = £45
Nemicon MPG = £45
Limit switches (Industrial Hobbies optical)= £85
power supply (could be salvaged from original controls) = £50
Various bits and bobs (switches,wires, materials for brackets, etc) £200
17inch Touch screen = £260
Keyboard with touchpad =£40
Computer = £150
Mach3 software = £85
Time = free
Sold parts of the CNC that I didnt need and got £300 back
total = £1500 = approx $2600

Obviously it will depend on how much you get your mill for, whether you decide to use the old motors and any of the power components of the mill. Also some of the bits are not a necessity such as touch screen and MPG, so costs could also be reduced there.
Hope this gives you some encouragement as it is very rewarding to DIY and another advantage is, if in the future you have problems you should be able to fix it because you built it in the first place.
Hood