what size stepper to move head?

[Runner4404spd]

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?

[Jay Kyle]

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

[BobWarfield]

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

[Jay Kyle]

I had the spring custom made from Easy-Lift . It took about 6 weeks, but I had time to wait.

Jay

[phil burman]

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

Originally Posted by BobWarfield 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

[haysys]

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

[BobWarfield]

Originally Posted by phil burman 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

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

[Jay Kyle]

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

[haysys]

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

[Zumba]

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.

[MikeAber]

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

[Jay Kyle]

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