[cwpeters]

I emailed keling to ask for help in getting a quote. Here's what I wrote.

I have an older cnc from company no longer around. It was made in 1992 from a company called Digital Tool. I have the original owners manual with a few machine specs. I'm getting alot of backlash, causing longer 3d projects to start cutting to deep. I have my backlash compensation set to .0013 on all axis. I notice the biggest problem on my Z axis. Some of my programs have 350,000 z moves. By the end of the program I'm a quarter of a inch to deep. It has a rack and pinion system. If I change this over to ballscrews with this help with my backlash problem? Is it worth trying it?

The emailed me back a suggested I write my problem here to get other peoples thoughts. I thank everyone who can give me any advice, tips ect. I'd also be interested in ordering ballsrews ect from anyone with extra laying around. Thank you
chris

[ger21]

By the end of the program I'm a quarter of a inch to deep.

Backlash won't cause that. Your Z axis is loosing steps. You probably need to decrease the acceleration, and possibly the feedrate as well.

[cwpeters]

Would increasing the size of my stepper motors help me? They are nema34 420oz. Put on the machine in 1992. I can push on the machine while powered up and move it.
chris

[ger21]

That would depend on the drives, for one thing.
Also, large steppers may have less torque at faster speeds than the smaller ones, so it could actually perform worse.

[cwpeters]

I was figuring on replacing the drivers with the steppers. I cant really machine these parts any slower. I've tried all different speeds and spindle speeds or in my case router speed. Some programs run for 3 to 4 hours.

[cjjonesarmory]

I've had good luck with these motors. I don't know much about your setup, and can't tell you for sure these would work, but its what I have used before. I have Gecko G203V drives.

Page Title

The 570 oz-in motor at the top of the page is the motor I'm referring to. Here's a link to the PDF with the specs:

http://www.kelinginc.net/KL23H2100-50-4B.pdf

[cwpeters]

Its a gantry system. with Nema 34 -420oz stepper motors on all axis. My table size is 6 foot by 8 foot but the cutting area is more like 5 by 7 foot. Most of the 3d artwork I've tried to run is about 8in by 10in. or smaller. I dont have any problem with etching programs but with most etchings in wood if it gets a little deeper towards the end of the program its not real noticable.
I guess my main question is would it be worth the money to invest in a ballscrew setup and get rid of the rack and pinion. I've looked at the machine and dont think it would be real difficult to change it. But would it be worth it? Is the problem I'm having with my parts getting moire than a 1/4of an inch to deep towards the end of long programs cuased by backlash or is it being caused by my stepper motor loosing steps. If I bought a dual shaft stepper motor could I hook it to compensate for lost steps like servo motors do?

[cjjonesarmory]

In my opinion, in general, rack and pinion setups are superior to ball screws, especially when there is a large travel. Without knowing more about your setup I would make an educated guess that the problems you are having are related to missing steps, not backlash issues. Backlash will be consistent whereas you describe the problem you are having as getting worse as time progresses in a given toolpath.

I must have skipped over the size designation in your previous post. The motors I pointed out were NEMA 23 and it looks like you need NEMA 34. On your particular machine, I'm not sure what would be involved in mounting a NEMA 23 motor in place of your NEMA 34's. There would be several advantages to using a NEMA 23 in place of a NEMA 34 provided that a large NEMA 23 has sufficient torque and the shaft size is of adequate size. A NEMA 23 will have a lower inertial mass and, in general, contribute to being able to run at higher acceleration values.

Also contributing to acceleration values is the mass of the pinion, or if you use ball screws, the weight of the ball screw. As you can imagine rotating an entire ball screw will be much more difficult than rotation only a pinion. This may or may not be offset by the potential mechanical advantage provided by a ball screw versus a rack and pinion setup.

I would advise you to continue to use your rack and pinion and attempt to adjust the amount of backlash. First check to see if there is any built in adjustment for the position of either the rack or the position of the pinion. Moving the pinion closer to the rack, or vice-versa, will take up some of the backlash. Once you have checked and adjusted the rack and pinion, check to see if your couplers, timing belts, or other mechanisms have gained some slack over time. Replace those components if necessary.

The backlash adjustments will most likely not cure missing steps. For that you will need to check that your wiring is in good shape and that you are not getting any EMI (Electromagnetic Interference) on the signal wires.

Reducing the acceleration values on the Motor Tuning page in Mach3 (if that's what you are using) will help with missing steps if it's a motor torque issue.

Finally, if everything else checks out, consider replacing the motors and/or drives with updated components.

You can contact me directly if you need any help with custom parts or in choosing appropriate components for your system.

[cjjonesarmory]

As a follow up point, I re-read your earlier post where you mentioned that you can push on the machine while powered up and move it. I don't know HOW hard you have to push, but on my own gantry machine I would have to put a very significant effort into pushing in order to move the gantry.

I think this points towards motor issues. Perhaps your motors have become weak over time, or some other components are failing. It would still be a good idea to follow the suggestions regarding backlash from my previous post, but I'm fairly sure you have a motor/driver problem.

I'm not familiar with the machine you are using, but you could also consider changing the gear ratio instead of changing the motors and drivers. It may be cheaper. There are many possibilities depending on how the parts are coupled. Just an idea I thought I'd throw out there.

[cjjonesarmory]

You can call me the "absent minded professor" if you like, but something else just struck me from your post. You only mention the depth of cut as the issue, meaning the Z Axis primarily. Depending on your other components, you may be able to simply upgrade the Z Axis motor. Also, check for binding on the drive components of the Z.

I run an 8 lb router motor with an additional 7lbs of weight in the moving components of my own gantry (totaling about 15lbs) and I am easily able to move the Z axis with a 166 oz-in motor. I settled on a 280 oz-in motor in the end because I have plenty of them on hand, but the 166 oz-in motor worked fine. The drive component is a 3/4" dia by 6 TPI leadscrew with anti-backlash plastic nut. My point being a 420 oz in motor should be able to lift and move a significant amount of weight without missing steps, but the critical thing being the acceleration, or the "thrashing back and forth" of the motor. I'd start with the Z and go from there.

[Bear5k]

A thought not covered by the above: changing from R&P to ball screw will not be an easy task. You can go from a lead screw to a ball screw, but R&P to a screw may be a hill too far while keeping the expense reasonable. R&P has less mechanical overhead than a screw, so there may not be the right clearances available to make the conversion.

[ikhan42]

Now I'm really confused lol I know I'm making a habit of it but hey that's how we learn right ?

I have read in numerous books and also discussions that the z axis should really be ball and screw with zero backlash nut and that the rack and pinion is ideal for long runs and not short distances, is this not true?

ikhan42

In my opinion, in general, rack and pinion setups are superior to ball screws, especially when there is a large travel. Without knowing more about your setup I would make an educated guess that the problems you are having are related to missing steps, not backlash issues. Backlash will be consistent whereas you describe the problem you are having as getting worse as time progresses in a given toolpath.

I must have skipped over the size designation in your previous post. The motors I pointed out were NEMA 23 and it looks like you need NEMA 34. On your particular machine, I'm not sure what would be involved in mounting a NEMA 23 motor in place of your NEMA 34's. There would be several advantages to using a NEMA 23 in place of a NEMA 34 provided that a large NEMA 23 has sufficient torque and the shaft size is of adequate size. A NEMA 23 will have a lower inertial mass and, in general, contribute to being able to run at higher acceleration values.

Also contributing to acceleration values is the mass of the pinion, or if you use ball screws, the weight of the ball screw. As you can imagine rotating an entire ball screw will be much more difficult than rotation only a pinion. This may or may not be offset by the potential mechanical advantage provided by a ball screw versus a rack and pinion setup.

I would advise you to continue to use your rack and pinion and attempt to adjust the amount of backlash. First check to see if there is any built in adjustment for the position of either the rack or the position of the pinion. Moving the pinion closer to the rack, or vice-versa, will take up some of the backlash. Once you have checked and adjusted the rack and pinion, check to see if your couplers, timing belts, or other mechanisms have gained some slack over time. Replace those components if necessary.

The backlash adjustments will most likely not cure missing steps. For that you will need to check that your wiring is in good shape and that you are not getting any EMI (Electromagnetic Interference) on the signal wires.

Reducing the acceleration values on the Motor Tuning page in Mach3 (if that's what you are using) will help with missing steps if it's a motor torque issue.

Finally, if everything else checks out, consider replacing the motors and/or drives with updated components.

You can contact me directly if you need any help with custom parts or in choosing appropriate components for your system.

[cjjonesarmory]

ikahn42:

I think your statement is correct for the most part. Using a ball screw or an ACME screw with some type of backlash adjustment system for the Z Axis is a good idea in most cases.

I think what needs to be cleared up is that my previous posts were regarding rack and pinion versus leadscrew systems used in all axes. If I were to design a new Z Axis I would almost certainly use a leadscrew setup because it would be the easiest to implement for that purpose, but I don't believe that a rack and pinion is necessarily a BAD idea on short spans either, at least in terms of performance. A rack and pinion will have the same performance characteristics on a short span as it would on a long span since all that is turning is the pinion in either case. A ball screw or other leadscrew is different, however, since the longer the span, the larger the rotating weight. Z Axes are typically fairly short and the mechanical advantage of leadscrews, compared to rack and pinion systems, is usually a benefit for the Z Axis. There are obviously going to be pros and cons of both systems in terms of installation, but that depends on the overall design.

When considering different options for the design of your system it all depends on how everything will work TOGETHER. It also depends on what you are trying to accomplish in terms of the type of material (steel versus wood or plastic) you are cutting, the method of cutting (plasma, router, laser, etc), and the performance you are seeking.

If anyone ever tells you a certain method of doing something is "bad" or "wrong", then their advice should be evaluated carefully. What is wrong for one application may be just right in another.

My first gantry build was a leadscrew system with ACME screws and plastic anti-backlash nuts. My latest build was using a rack and pinion. You can see the rack and pinion gantry here:

YouTube - ‪cjamasterworks's Channel‬‏

[ikhan42]

cjjonesarmory,

Thanks for the clarification I like your video's especially the flange one.

The more I read the more I am thinking about chickening out building one LOL

regards
ikhan42

ikahn42:

I think your statement is correct for the most part. Using a ball screw or an ACME screw with some type of backlash adjustment system for the Z Axis is a good idea in most cases.

I think what needs to be cleared up is that my previous posts were regarding rack and pinion versus leadscrew systems used in all axes. If I were to design a new Z Axis I would almost certainly use a leadscrew setup because it would be the easiest to implement for that purpose, but I don't believe that a rack and pinion is necessarily a BAD idea on short spans either, at least in terms of performance. A rack and pinion will have the same performance characteristics on a short span as it would on a long span since all that is turning is the pinion in either case. A ball screw or other leadscrew is different, however, since the longer the span, the larger the rotating weight. Z Axes are typically fairly short and the mechanical advantage of leadscrews, compared to rack and pinion systems, is usually a benefit for the Z Axis. There are obviously going to be pros and cons of both systems in terms of installation, but that depends on the overall design.

When considering different options for the design of your system it all depends on how everything will work TOGETHER. It also depends on what you are trying to accomplish in terms of the type of material (steel versus wood or plastic) you are cutting, the method of cutting (plasma, router, laser, etc), and the performance you are seeking.

If anyone ever tells you a certain method of doing something is "bad" or "wrong", then their advice should be evaluated carefully. What is wrong for one application may be just right in another.

My first gantry build was a leadscrew system with ACME screws and plastic anti-backlash nuts. My latest build was using a rack and pinion. You can see the rack and pinion gantry here:

YouTube - ‪cjamasterworks's Channel‬‏