[Jay Kyle]

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.

[MAX711]

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!!

[Greolt]

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?

[MAX711]

How about a pic of the springs and mounting?

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

[Runner4404spd]

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?

[Greolt]

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

[MAX711]

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.

[MAX711]

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.

[Ron111]

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

[Runner4404spd]

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?

[MAX711]

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.

[MAX711]

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.

[Randall]

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

[DareBee]

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]

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.

[Capteod]

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.

[TPPJR]

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/rf45dove...scounterweight

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