[mrquacker]

So the Z axis along with the head, spindle, and motor, weigh a bunch. That's extra effort for the Z motor to move against gravity. Would hanging a counterweight provide any benefits?

[GLCarlson]

So the Z axis along with the head, spindle, and motor, weigh a bunch. That's extra effort for the Z motor to move against gravity. Would hanging a counterweight provide any benefits?

I'm not a mechanical engineer, or a machine designer, so take this with a grain of salt. Seems to me that a problem with doing that is that a sufficiently large counterweight would cause the head/counterweight/cable to act as if it were two weights separated by a spring. When one is aiming for both precision and accuracy in the range of thousandths of an inch, extra bounciness is probably a bad thing. Counterweights work well in elevators and (presumably) minimize power use. In a machine tool, the power saving is likely outweighed by the additional cost and complexity, if not the effect on accuracy and precision.

[LeeWay]

I don't own a Tormach, but some form of Z axis support can equalize the load on the Z motor. Meaning it takes some of the weight off lifting every time, but adds some of that to lowering each time. From a mechanical standpoint, I think that is a better situation for all the parts involved.

On the mill I built, I chose to go with a gas strut that is rated at probably 1/3rd the weight of Z. This has proved to be sufficient and effective.

[GJeff]

I've wanted to counterbalance my Z axis as well, but like GLCarlson said it's going to induce a bouncing effect due to the elasticity and lack of compressive strength of whatever you're hanging it with. It seems like it'd hurt more than it helps - at least the way I'm picturing it with a weight, pulley and steel cable, but I'd be interested to hear differing opinions. I suppose a constant force spring or something similar would work better in theory, but whether or not they make them strong enough and durable enough to be of any real assistance I also don't know.

[philbur]

With the current design the head moves as instructed by the control software, what benefit were you looking for. If it ain't broke, don't fix it.

Phil

So the Z axis along with the head, spindle, and motor, weigh a bunch. That's extra effort for the Z motor to move against gravity. Would hanging a counterweight provide any benefits?

[eltejano]

I wonder how that would affect the squareness of the head in relation to the table.

And i dont know if i agree with if it aint broke dont fix it. Innovation calls for this kind of thinking.

[LeeWay]

I think the main reason they didn't design something in is only because the Z motor has no trouble lifting the mass without assist. If the machine is capable of doing that, then there really is no need. An assist will not hurt anything, but the type you use can be very simple or overly complicated. If the Z has any slop that would be apparent by assisting the lifting, then it isn't designed correctly and won't last long anyway.
I think I paid about $6 for my gas spring. I never cared for the weights and pulley systems. One guy is using a soda bottle in the design. Don't recall just where I saw that though.

I just bought a Torus from Novakon and it actually has two gas springs.
Even the lowly X2 I had had a coil spring assist and it was manual.

[mrquacker]

I am referring to this build thread as a source of inspiration to this question:

http://www.cnczone.com/forums/vertic...ml#post1243090

I just figured out what I am going to use as a counterweight. 4" PVC pipe filled with concrete.

I calculated that a 4" pipe that is 24" long weighs around 25 lbs.
take a 10' section, and cut it into 5 equal parts, fill with concrete, and hang along a steel "pipe" or square section.

The Tormach head I weighed around 115 lbs, so with the aluminum slabs its sitting on its pretty close to a 1:1 weight to counterweight ratio. Close enough for me. 3/16 rubber coated steel cable will hold it all along 400 lbs pulleys.

I bolted the head on loosely, and it felt like i was bolting it on a brick wall. The frame is that solid. Could resonate to high heaven for all I know yet, but it feels solid as hell to me.

Pics later.

[Winks]

If your Z has zero backlash, it would not cause problems. It wouldn't help anything, but it wouldn't add any new problems.
If you do not have zero backlash in the system, then you would have new problems.

[BAMCNC.COM]

Tormach has ballscrews with about .001" backlash.

Is there a size/part # on the Torus springs?

[philbur]

The weight of the head effectively applies preload to the ballscrew, greatly reducing issues related to backlash. You need some very good reasons why you would spend time and effort to remove that preload.

I'm all for inovation but you have to follow a logical process.

What are the issues with the current system ?
1) ?
2) ?
3) ....

What are the alternatives to elimnate these issues. ?
1) ?
2) ?
3) .....

What are the pros and cons or each alternative ?
Pros
1) ?
2) ?
3) .....

Cons
1) ?
2) ?
3) .....

Make a choice!

Because somebody else has used a balancing system is not good enough reason for you to use it.

Phil

[nateman_doo]

I am guessing since they designed the system, they would know best. Since i was designing my system from scratch, I know I would benefit from the counter balance. I firmly believe in the counterbalance system. My X & Y have zero backlash because they aren't fighting gravity. There would be some backlash without it in my system.

I don't think there is any kind of spring effect, if the load is the same on each end the motor will have almost no stress on it. At 120ipm max, my motors can't "slack" the cable enough lifting the Z up to allow the counterbalance to "fall" there is zero harm in my backlash system. It could also help to dampen the head a bit (speculation)

[twocik]

There will always be room for improvement, just be careful which can of worms you open... .

[bozidar22]

Not a tormach but,I use an 85 pound counter weight to balance a 230 pound head with out any problems. A number 35 chain connects them together.


Being an auto mechanic 25 years and getting smacked in the head too many times I don't trust gas springs.

[David Bord]

I asked Tormach this question and they said the added mass would actually make the machine perform worse.

David

[nateman_doo]

how would the machine have "added mass"? If the counterweight is done like mine (cable & pulley 1:1) then there is no additional mass. It is more "reduction" in mass by alleviating some weight.

My take on this, is that you never KNOW until you DO. Everyone here could have different theory's about how its going to perform. Until someone actually does it, you just don't know for sure.

Here is what my machine does without the counterbalance:


The head falls down the minute the steppers are turned off. Why? Because Tormach has dovetails which have friction, and i use linear rails which are VERY low friction. So the head just falls down. Not the case with some added weight.

[Fastest1]

The head falls after power is removed due to the ballscrews pitch. Not the weight or the dovetails.

IMO the only advantage a counterweight would have is to decrease electrical consumption due to less load.

[philbur]

You don't need to counter balance to avoid the fall. The Tormach design doesn't fall because it has a brake that is electrically activated when you turn off the stepper. This way you avoid the negative aspects of the counter balance.

If you are able to do the engineering you can avoid having to build it to find out if the concept is a good one. As twocik pointed out there are several cans of worms and if you don't have a good grasp of the engineering principles envolved you are likely to have them all open at the same time.

Phil

My take on this, is that you never KNOW until you DO. Everyone here could have different theory's about how its going to perform. Until someone actually does it, you just don't know for sure.

The head falls down the minute the steppers are turned off. Why? Because Tormach has dovetails which have friction, and i use linear rails which are VERY low friction. So the head just falls down. Not the case with some added weight.

[bill south]

I have a RF 45 and have used a counter weight system for years. I have not noticed any springing and it improved my z axis movement tremendously. However, that being said, I don't own a Tormach machine so I really can't speak to that. When making your decision, do what you think is best. If it's not then correct it. Hell, that's part of the fun!!!!
Have a good day.

Bill

[gerald wubs]

Hi All,
First post - looks like a great forum here!

I have a Xenetech engraver which has been modded to G-code. I've been thinking about this topic for a while, and I would like to take the vertical weight load off my Z axis stepper. One thing stops me from considering a weight however - It is the fact that a counter weight increases the mass of the portion of the machine that is moved by my Z & X axis. On feed moves, this in no problem, but on rapids I'm sure it would be bad. Think of the difference in power required to get a large truck to accelerate and stop compared to a small car or motorcycle. I'm talking about on the level, not going up a hill. I have worked with large milling machines that DO have counterweighted Z axis, so it is done, but I feel on a lighter machine where rigidity, and stepper/servo power are lower, it might cause problems.
I am thinking of ways to integrate a constant force spring instead. On a lighter machine that seems the better approach IMOHP. Also, I'm guessing it would not necessarily be a good idea to reduce the "weight" of the Z axis to zero. Maybe having the ballscrew, and nut "preloaded to one side is beneficial for accuracy and repeatability.
Have a great day guys
Gerald