hey guys,

i'm looking at getting a stepper for the top of the column to adjust the head. i need to know what size stepper i will need and also if i will need to counter balance the weight of the head. if i do counter balance it whats a good way to do that that won't interfere with anything else?

I'm doing the same thing to a very similiar mill. I'm going to use a 1303oz-in stepper from MCG IH34114. I also picked up a custom built gas spring to fit inside the column, the spring has only 8% change in force over the length of the travel.

Jay

Nice gas spring. Where did you get it?

I'm planning a 1700 oz in stepper from hobbycnc. The head is mighty heavy. I think some form of counterweight or gas spring is going to be essential to good performance. OTOH, Aaron is using 648 oz in servos in his conversion kit. Have to look at what the belt ratios are for it though.

Best,

BW

I had the spring custom made from Easy-Lift (http://www.easyliftsprings.com/products/gas_springs.html) . It took about 6 weeks, but I had time to wait.

Jay

Jesus, I would have thought this was enough to rip the thing apart. Are gas springs really necessary. This machine www.tormach.com has a 640 oz and no gas spring. The total machine weight is 1,100 lbs. What size machine are you guys talking about.

Regards
Phil (chair)

Nice gas spring. Where did you get it?

I'm planning a 1700 oz in stepper from hobbycnc. The head is mighty heavy. ......

Best,

BW

Hi All,

I'm not certain I'm interpretting the thread properly (I can't seem to visualize how a gas spring inside the column would address the problem I perceive).

For clarity, I see a problem withe the weight of the head and motor cantilevered off of the column putting a lot of stress on the ways, thus creating a lot of wear. Why is this a problem? Because a designed for manual use, the head was never intended to be moved up and down the column with the frequency that the full-Z motion CNC conversion will bring about; the quill is simply not used.

I have been working on the design of a simple fabricated structure to be bolted on the back of the column that hangs over top of the head. Attached to the structure in the back is a pair of adjustable gas springs (http://www.ameritoolmfg.com/adjustablegassprings.asp -- 1165-14 is my current estimate). Connecting the ends of the pistons to the head is a pair of lifting roller chains attached to brackets on the head. At this point I am still scrounging for parts (chain and rollers being the critical missing pieces).

The idea here is to get the weight of the head in near "neutral-buyancy", and take as much of the "canted stress" off of the ways as possible.

What do you think?

Soryr if this is a bit to far off thread topic.

David

Jesus, I would have thought this was enough to rip the thing apart. Are gas springs really necessary. This machine www.tormach.com has a 640 oz and no gas spring. The total machine weight is 1,100 lbs. What size machine are you guys talking about.

Regards
Phil (chair)

It's a bit larger than the Tormach dimensionally, but roughly the same. The difference in cost for the extra torque is low and you don't have to use it!

With that said, Tormach has potentially a number of advantages over these IH machines. First, they have PTFE ways, which will substantially reduce their friction. The IH's are known for having pretty rough ways so it may pay to have some extra torque on tap to overcome that. Second, Tormach was engineered for the carefully tuned setup they have. They built a number of prototypes before they came up with a formula they were happy with. Again, too much torque is not a problem if you choose not to use it. Too little is. Also remember how narrow the torque curve is for these step motors and how fast it falls off. I would say that if you had servos, 600 oz in would be no prob. I like a little extra insurance for step motors.

The head of these mills is over 200 lbs and that's going to be moving up and down, potentially fairly rapidly. Just going to have to see how well it performs.

Best,

BW

David,
Your observations regarding the cantilever stresses is bang on, and it's something I've been wrestling with. I like your roller chain approach. Are you going to mount the chains at the center of gravity for the head? that would put them out past the motor.

As for the Tormach, I recognize it as a well engineered machine, also note that they do not have a gearbox on the head so it's somewhat less heavy than a typical IH style.

As this is my first conversion I've often wondered if I'm trying to solve a non-issue by counter-balancing the head.

Jay

Jay,

Yes, on the CG. I have the head off right now, and was going to use the bathroom scales to calculate the CG.

Your question of solving a nonexistent problem is certainly not far from my mind as well. Am I obsessing over a non-issue??? I know that some of these machines have been running in CNC mode for some time now, and would really like to hear from some of the owners and their take on this. I wonder if anyone has inspected the ways or noticed any wear after one, two, or three years of CNC service.

In conversations with Aaron (not specifically about this), he said he has his z-axis gib "tightened down quite a bit and is rather stiff" (paraphrased). He was referring to the ability of the z-axis servo to handle the load. But the implication was also that the gib must be tightened to keep head good and square, and the mating ways in full contact along the length of the ways. Anything less will introduce a degree of heel-toe contact, concentrated points of wear.

Obviously, I feel this is a problem. Be clear I am a novice. The analysis is my own (I'm fully willing to be challenged and corrected). But I have been around machinery all my life, and I have seen wear on mating surfaces that has amazed me. Wear in the ways will make maintaining accuracy difficult.
However, the severity of the problem will only be evident after hours (500, 1000, 3000, more???) of use. I'm not an industrial user and the machine will likely not see heavy use. That said, I'm not fond of saying "I should have..." either. I plan on using the machine for at least 20 years (hopefully more if my body will cooperate). I'd like it to stay in good shape over that time.

With respect to the original issue raised in this thread, the sizing of motor would become secondary by such a counter balance. Implemented perfectly, the servo or stepper would only need to overcome the static friction of the z-axis system. Thus, the x, y and z motors could all be the same.

I'd like to hear what other folks think. Sorry to have hijacked the thread.

Thanks,

David

For typical operations such as straight cutting, slotting, drilling, etc, the Z axis moves so minimally compared to the other two axes that it probably wouldn't matter.

On the otherhand, if you frequently make 3D parts where the Z is constantly bobbing up and down, I would convert both the column AND the quill to CNC.

One could argue that it'd be simpler and cost less to just do the quill, but IME, the column leadscrew that comes with the machine is a piece of crap with an enormous amount of backlash. You want to replace that thing no matter what, so you might as well spend a little bit more to get that thing motorized.

Likewise, it'd probably cost the same to convert the quill as it would be to add a counterweight/gas piston assembly. It'd also be prettier. I've seen those X3 guys add weird cable pulley systems and it looks like a circus rig. :rolleyes:

:cheers:

Hi Guys,

My TORMACH PCNC1100 is using a 1200 oz stepper driving the ballscrew direct on the Z axis and 640 oz direct drive on the X and Y. The column is bigger, the head is bigger and everything else is bigger. The X, Y, and Z travel is less than my IH mill.

The ways on my IH mill were real rough, the head leaks oil, and the motor has been temperamental. Other than that, I still use it at least once or twice a week to make something.

Get the larger motors for direct drive. The 640 oz motors would work fine with 2:1 belt drive reduction (5tpi ballscrew). I believe the IH kit is using 2:1 belt drive reduction.

Mike

Mike,
Can you tell us about the column, does it has any sort of counterbalance? The big worry is: Will the head spin down on it's own when the power's off. Also do you notice any difference in Z-cuts between cutting up and cutting down. I'm wondering if the extra head weight biases cuts.

Jay

Mike,
Can you tell us about the column, does it has any sort of counterbalance? The big worry is: Will the head spin down on it's own when the power's off. Also do you notice any difference in Z-cuts between cutting up and cutting down. I'm wondering if the extra head weight biases cuts.

Jay


Hi Jay,

There is no counterbalance or shock to offset head weight. I believe the head assembly is lighter than the IH head as there is no gearset and the head is hollow inside.

I don't have my machine online yet to tell if the Z axis spins down. I just mounted the machine on its stand Saturday. I hope to clean it up and run it for the first time this weekend.

When I get a chance to, I'll take pictures of some of the features that others may wish to incorporate into machine builds such as the method used for making adjustments to the column and spindle for alignment.

Mike

Section 9.5.4 rev. B2 of the operating manual says that if the z axis falls under it own weight with the power off (but everything wired up) then the gib needs adjusting. It also says you should not electrically disconnect the stepper without supporting the head.

Regards
Phil (chair)



Mike,
Can you tell us about the column, does it has any sort of counterbalance? The big worry is: Will the head spin down on it's own when the power's off. Also do you notice any difference in Z-cuts between cutting up and cutting down. I'm wondering if the extra head weight biases cuts.

Jay

Hi Mike,

I was of the impression from the documentation that all steppers where 640 oz. Did yours come with the 1200 or have you fitted it yourself. Strangely I can find no reference in any of the documentation to the actual size of stepper motors used other than one vague reference to 640 oz in the design analysis document. The part number for the x and y steppers is even missing from the exploded view parts list.

The part number on the expoded view for the z motor is 30358 however the part number on the size 34 machine tool motor data sheet for a 640 oz is 30155 and for the 1200 oz is 30156. I'm confused (chair)

regards
Phil


Hi Guys,

My TORMACH PCNC1100 is using a 1200 oz stepper driving the ballscrew direct on the Z axis and 640 oz direct drive on the X and Y. The column is bigger, the head is bigger and everything else is bigger. The X, Y, and Z travel is less than my IH mill.

The ways on my IH mill were real rough, the head leaks oil, and the motor has been temperamental. Other than that, I still use it at least once or twice a week to make something.

Get the larger motors for direct drive. The 640 oz motors would work fine with 2:1 belt drive reduction (5tpi ballscrew). I believe the IH kit is using 2:1 belt drive reduction.

Mike

No the Z is definately larger than X and Y on the Tormach. I am pretty sure it is around 1000oz

Hi Phil,

Take a careful look at this picture. The Z axis motor is a double stack motor compared to the X axis motor in the picture.

The motor is an STM34-1200-MTM

1208 oz holding torque.

Mike

Hi Mike,

I don't doubt what you see. I just can rationalize it with the documentation part numbers. Kinda irrelevant anyway, but interesting.

Phil


Hi Phil,

Take a careful look at this picture. The Z axis motor is a double stack motor compared to the X axis motor in the picture.

The motor is an STM34-1200-MTM

1208 oz holding torque.

Mike

Hmm.. the head on a IH style weighs in at approx 215 lbs (without motor), what would the Tormach head weigh. I would guess around 100-125 lbs ?

The Z stepper provide a 'lockup' torque while electrically connected. This is what holds the head from dropping while powered off - must be some special circuitry to do that.

So we would have to emulate that circuitry and tighten the gibs to support a 'light' head.

Hi Jay,

There is no special circuitry, when the machine is powered down friction between the dovetails in the column and the head carriage as well as the double nut on the ballscrew (preload) keeps the head from spinning down.

If you were using ball bearing linear ways, the motor is usually fitted with a brake to hold position. The brake normally is on until you apply power to the brake to release it.


Mike

Mike,
I was under the impression that when the windings of a stepper were shorted together (without any power applied of course) that they electrically 'lock' the stepper or make it very hard to turn. I'm not an expert on this, however.

Jay,

Yes, on the CG. I have the head off right now, and was going to use the bathroom scales to calculate the CG.......


I know this thread is pretty old, but the issue (problem?) still exists so I thought I'd throw out my solution for what it's worth.

I bought an IH mill with the intention of converting to CNC, mostly because I wanted a CNC mill to play with at the end of the day, but almost as important, the fun of converting it from scratch.

Most of the conversion went well but I had purposely avoided the Z axis along the way, knowing "one day" I'd have to deal with it. As others have discovered, using the quill is all but useless for CNC applications due to the "slop" (and limited 4" of travel). I had decided early on that moving the whole head was the only way to go, but at 200+ lbs and that twisting moment, how to do it? Enter the gas spring. Not my idea, and I'm not trying to take credit for it but man does it work great!!!. I put one on each side of the head, anchored at the top of the column, I get 14" of Z travel which is more than enough for my application, and I can drive the whole head assembly up and down with a 120 oz-in DC servo motor and a Duracell 9 volt battery. The head literally "floats" up and down with no effort, and best of all, the gas springs pull the Head into the column so there's no twisting moment associated with 200 lbs of gearbox and motor hanging 12" away from the column. I have the gas springs that work in tension and have attached them so one end is at the top of the column and the other end is on the head roughly at the Center of Gravity so they support the weight of the head and also counteract the twisting moment at the column ways.

I know some think of gas springs or wires and pulleys as "________" (insert colloquial for sub-standard engineering), but my engineering instincts tell me that there's a lot less stress on all the load bearing surfaces and the ball screw so I feel a lot better, plus I don't need a 1,200 oz-in motor just to overcome the weight of the head and the friction caused by running the gibbs super tight to stop the head falling without power.

Linear ways would be nice on the Z axis like the high end VMCs but as we are stuck with a dovetail and gibb arrangement I think the gas springs are a good solution for the cost. If you're less than impressed with the Z axis on your IH mill (manual or CNC) consider some gas springs, they make a huge difference!!

I put one on each side of the head, anchored at the top of the column, I get 14" of Z travel which is more than enough for my application
How about a pic of the springs and mounting?

How about a pic of the springs and mounting?

Here's a picture.... There's one on the other side just like it.

where did you get the gas springs and what is the part number? i like your solution. can that go through the full range of travel?

I'll second that Runner.
Any more detail would be great. What sort of travel.
I notice a tube connected. How does it all work.
Only seen gas springs holding up the hood on a car :)

where did you get the gas springs and what is the part number? i like your solution. can that go through the full range of travel?

The springs can be found here http://www.mcmaster.com part number 9502T8. I used the 100 lb springs but I think the 80 lb units would have been better. At the bottom of the stroke the motor is actually having to push the head down, if I lost power, the head would drift up (which is not a bad thing I guess). I think 80 lb springs would be just about perfect.

Travel is about 14" which is more than enough for my needs, I think stock is about 20" and the Tormach has 16" of Z travel. The loss of a few inches is more than made up for by the reduced stress on all the components. My wife is looking over my shoulder and says "it's not the size that matters, it's how you use it" whatever that means? :)

Attachment is easy, just drill and tap two 8mm holes on each side and that's it. I made an "educated" guess at where the Center of Gravity was, but anywhere on the head will work better than the stock setup (just don't drill into the gearbox itself :) ) You could go with a higher rated spring and mount it farther out on the head which would give you more travel and also pull the head into the column more.

One other tip, it's best to mount the springs upside down as the oil inside helps lubricate and seal at the rod/cylinder interface.

I'll second that Runner.
Any more detail would be great. What sort of travel.
I notice a tube connected. How does it all work.
Only seen gas springs holding up the hood on a car :)

See previous post for detail on the gas springs, as far as the "tube" you saw in the photo.......

The tube you see is not part of the gas spring setup, it's part of the one-shot oil lube system that I installed at the same time. I had to drill through the gibb to make sure it had some oil. On the stock setup, there is nowhere to lube the gibb/dovetail friction surface, which in my opinion is the hardest working part of the Z axis.

So Max,
This indead looks like the ticket!! How long has your Unit been up and running and how has it performed. We, that have been kicking the tires, so to speak want to hear what our dream machine will do. Throw us a couple of bones (in other words what have you doing with the machine and what kind of cut rates can we expect (I cut mostly 6061) ? I know some of the Guys are toggling, like me, between the Tormach and the IH. So, if you can take a minute and tell us your experiences with this beauty we would sure appreciate it.

Thanks for sharing the gas spring system,

Ron

you say that the had wants to drift up when its low and i assume down when its high on the column. does this affect the accuracy and repeatability of the cuts? does this change the backlash on the z axis?

you say that the had wants to drift up when its low and i assume down when its high on the column. does this affect the accuracy and repeatability of the cuts? does this change the backlash on the z axis?

When the head is at the top, the spring force is a little bit less than that needed to hold it up and when its at the bottom it's slightly more. I positioned the springs so that the head would be neutrally balanced where I do most of my cutting. But even at the ends of travel, the force required to move the head is minimal. I only have a 24v DC servo motor rated for 120 oz-in and it's more than enough to move the head through the full range of movement and it's a direct drive on the ballscrew.

As far as backlash, I'm using a zero backlash ground ballscrew so it's not an issue, but if you were using the standard bevel gears and screw setup, you would have to compensate for backlash. The Z axis doesn't have a very good scale on it or a dial on the handle so you would need some type of DRO on the column if you wanted to make accurate height adjustments.

So Max,
This indead looks like the ticket!! How long has your Unit been up and running and how has it performed. We, that have been kicking the tires, so to speak want to hear what our dream machine will do. Throw us a couple of bones (in other words what have you doing with the machine and what kind of cut rates can we expect (I cut mostly 6061) ? I know some of the Guys are toggling, like me, between the Tormach and the IH. So, if you can take a minute and tell us your experiences with this beauty we would sure appreciate it.

Thanks for sharing the gas spring system,

Ron


I don't want to hijack this thread so I'm going to start a new one and post some pictures in the gallery that should answer some of your questions. This is my first CNC project and I have not seen the Tormach in the flesh, but from what I have learned about the IH mill through the conversion and what others have written about the Tormach I would guess that I now have a machine that performs close to the Tormach but has cost me as much as just buying a Tormach and is still inferior in many ways. I wanted to do a CNC conversion to gain experience and for the challenge, if all I wanted was a machine to cut parts, I would probably have gone for the Tormach.

I might add the IH mill has alot going for it. Size, no ptfe and the ability to fix some of the problems with it. I know some have complained about the spindle play I dont have any noticable play till it gets extended more than 2 inches.
Randy

Max711
You may already know, but I am mostly posting for the benefit of others.

"When the head is at the top, the spring force is a little bit less than that needed to hold it up and when its at the bottom it's slightly more."

Just to clarify; the spring force is actually not changing - they are constant - the reason for the difference top to bottom is because they are mounted on that angle, when the head is down the springs are closer to vertical allowing them to pull closer to full strength, when the Z is up the springs are rotated towards horizantal effectively transferring the pull force inwards towards the column instead of vertically.

There is nothing wrong with this, just adding some insight.

Max711
You may already know, but I am mostly posting for the benefit of others.

"When the head is at the top, the spring force is a little bit less than that needed to hold it up and when its at the bottom it's slightly more."

Just to clarify; the spring force is actually not changing - they are constant - the reason for the difference top to bottom is because they are mounted on that angle, when the head is down the springs are closer to vertical allowing them to pull closer to full strength, when the Z is up the springs are rotated towards horizantal effectively transferring the pull force inwards towards the column instead of vertically.

There is nothing wrong with this, just adding some insight.

Your are right, the angle of the struts does change the force (approx 30% difference top to bottom), but by far the biggest change is in the spring itself. Gas struts are not constant force, unfortunately. According to the manufacturer the force at the end of the stroke is approximately 60% greater than at the start. So in my case, 100 lbs at the top and 160 lbs at the bottom. Manufacturers refer to this increase in force as "progressivity". I have seen units as low as 8%, which would be nice, but they didn't come in a traction type. They are mounted at an angle for 2 reasons, first is to pull the head tight into the column dovetail ways, relieving the stress on the gibb and second to maximise the stroke. With a 12" gas strut I can get 14" of travel.

I am just finishing the CNC Conversion of my IH Mill. I have counterweighted the Z axis with just over 225 Lbs and it does not stick. The reason most of you will have the problem with the head tilting, is that the lifting point is not at the Center of gravity of the head stock. Instead of mounting the pulleys as close to the column as possible, I cantalerered pulleys so the lifting force is centered on the Head of the and now the Z axis is not cocking as it is raised ir lowered.

David,
Your observations regarding the cantilever stresses is bang on, and it's something I've been wrestling with. I like your roller chain approach. Are you going to mount the chains at the center of gravity for the head? that would put them out past the motor.

As for the Tormach, I recognize it as a well engineered machine, also note that they do not have a gearbox on the head so it's somewhat less heavy than a typical IH style.

As this is my first conversion I've often wondered if I'm trying to solve a non-issue by counter-balancing the head.

Jay


Jay and David,

Here is what I did on my RF-45 for the counterbalance:

http://ImageEvent.com/tppjr/rf45dovetailmill/zaxiscounterweight

Working out pretty good. I did swap out the gas spring for a 150lb unit instead of the 200. I just don't think that my head is as heavy as the IH one. I have not sent the 200 spring back to McMaster Carr if either of you or anyone else is interested.

Let me know if you have any questions.

Thomas