Rack and Pinion and changing horses in mid stream

Rack and Pinion and changing horses in mid stream

Postby Mr.Chips » Mon Oct 21, 2019 2:34 am

I built my CNC about 11years ago, linear rail bearings on all axis, two NSK 20mm ball screws driving the Y axin, two start acme thread driving the X axis, and recently added a 5 start acme thread to the Z axis. Running Mach3 and Vectric VcarvePro 8.5 controlling. My X is 48” and the Y is 36”.

I’ve made hundreds of things on it, but I turn 80 this coming February and life is getting too short to stick with the X and Y axis running at 70 IPM tops.

So I’m converting to rack and pinion on the X and Y axis, heard a lot about it and the speeds that you can be achieved without loosing too much accuracy, anyway I mostly make things that I’m not selling to the government, so I’m gonna be OK.

Please suggest some suppliers and options, keeping in mind that money is an issue.

Thanks
HH
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Re: Rack and Pinion and changing horses in mid stream

Postby GEdward » Tue Oct 22, 2019 8:53 pm

I don't see any reason why you can only get 70 IPM out of a ball screw/lead screw setup unless you are under powered on your drive system. What type of drive system do you have? Is it AC servo, DC servo or stepper? I have a fair amount of experience in the machine tool business and there is no system faster and more accurate than zero backlash ball screws in humble opinion. I am thinking perhaps you might get more bang for the buck upgrading your drive system. Again, just my opinion given that there is a lot I do not know about your setup.
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Re: Rack and Pinion and changing horses in mid stream

Postby metalworkz » Wed Oct 23, 2019 4:28 am

I kind of agree with Gedward in that I feel ballscrews are probably going to be more easily adapted to your current system than the rack and pinion drives. I went with rack and pinion on my mid sized CNC router build but that was mostly because I had a source for the racks with a very long assembled length. I had to figure out the thread and then purchase the matching pinions because the pinions from the machine the rack and pinion was on could not be removed. Of course you will probably be buying the rack and pinion in a set so that part may not be needed in your case but you may still have to fabricate the drive mechanisms for the motors to mount to the drive wheels and I think all this extra work can be avoided by going with the ballscrew idea instead of rack an pinion. I do achieve good speeds with my machine using the rack and pinion and it seems to be very accurate but if I had the money and option to use ballscrews I do think I would have gone that route. The other side to the rack and pinion setup is the rack can accumulate dust and chips very easily and if not protected against the foreign matter inclusion I do think it can be minor headaches. I did not have the cash to buy ballscrews in the lengths I needed and the used stuff was not always available in lengths I thought would work in my setup so the rack and pinion has worked and is still in use. Just thinking since you currently use lead screws it may be an easier conversion as far as implementing compared to rack and pinion. I don't know anything about you machine axis sizes or general construction so adjust my thoughts to your own best use. Oh I am only 70 now but I think I can relate to what you are saying about wanting more speed. I do however find that if I plan things well the increase in speed does not always equal getting it all done quicker.
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Re: Rack and Pinion and changing horses in mid stream

Postby TReischl » Sat Oct 26, 2019 2:00 pm

If you are set on going to R&P a good source for parts to make the conversion easier is CNC Router Parts.
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Re: Rack and Pinion and changing horses in mid stream

Postby ger21 » Sat Oct 26, 2019 3:18 pm

CNC Router Parts is now known as Avid CNC.
Almost everyone building a DIY R&P machine uses their drives.
Gerry - http://www.thecncwoodworker.com
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Re: Rack and Pinion and changing horses in mid stream

Postby Mr.Chips » Sat Oct 26, 2019 4:25 pm

TR and Ger, I have one ball screw that is going bad, and don’t have the source for them that I had 11 years ago, and I see lots of serious users that are using R&P that are good results from them.
However I’m still in the research phase as to the best way to adapt them to my machine.

Did you both post photos if your builds and if so can you share the name of those so I could view them.

Thank you both

HH
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Re: Rack and Pinion and changing horses in mid stream

Postby IslaWW » Sat Oct 26, 2019 5:39 pm

Chips...
It is fairly common to achieve 3-400 ipm with 16/20 dia by 05 pitch ballscrews with current NEMA23 ~400ozin+ motors, double that with NEMA34's on 10 pitch versions. The FK and BK style bearing blocks are now readily available along with ballnut mounts. And it seems much better pricing than in yesteryear.

OF course, if you are into the upgrade, prices are getting very attractive on the small servos and closed loop products. Seems the mfgrs are lowering pricing in anticipation of EtherCat products that are entering the market.

"Almost everyone building a DIY R&P machine uses their drives."

I like the idea behind those reduction units, just wish they had more attractive reduction numbers
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Re: Rack and Pinion and changing horses in mid stream

Postby Mr.Chips » Sun Oct 27, 2019 12:35 am

Thanks Gary,
I did some air cuts on my CNC and changed the motor tuning on the Y axis (two ball screws, nema 23’s) to 300 IPM and it consistently ran fine, but around 400 IPM it would loose steps.

I set the X axis ( two start acme rod, nema 23) to 187 IOM and it would not loose any steps. For some reason the X axis cannot be set higher than 187IPM???? And the Y axis would allow to 500 or more.

I tried a 0.250 EM cutting at 187 IPM cutting a groove in MDF 0.100” deep in the X axis and no steps lost.

The reason I wanted to switch to rack and pinion is that one of the ball nuts had started to drop fine pieces of what looks like black plastic, along with some noise, so I figured I would switch to RP to gain some speed. But air testing today gave consistent runs of 300 IPM. I had always been very consecutive and seldom cut above 50IPM.

The ball screws look fine, maybe I just need to get some new nuts of if possible rebuild these. Please comment on this.

Thanks
HH
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Re: Rack and Pinion and changing horses in mid stream

Postby IslaWW » Sun Oct 27, 2019 1:13 pm

The product I was referring to are the more than common metric ballscrews sold, well almost anywhere, on the net. They use ball bearings and can be lubed. different than ACME or Lead screws.
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Re: Rack and Pinion and changing horses in mid stream

Postby GEdward » Sun Oct 27, 2019 8:57 pm

Gary is right in that there is a difference between ball screws and ACME lead screw and the difference is huge under load.

Mr.Chips wrote: Y axis (two ball screws, nema 23’s) to 300 IPM
[quote=
"Mr.Chips"]X axis ( two start acme rod, nema 23) to 187 IOM[/quote]

I assume the Y axis is pushing both the x axis carriage which also carries the Z axis assembly and does so with less effort than X axis drive can push the Z axis assembly around. Also, even though a 2 start thread is twice as fast as a single start thread, it takes twice the power (roughly) to turn the thread at any given rpm and acceleration loads are even higher due to the inherent friction of an ACME nut vs. a ball nut.

If it is possible you might try gearing the X axis down if the drive assembly is belt/gear driven. I suspect your drive motor is capable of far more rpm than you are getting out of it and giving the motor a little more mechanical advantage might get you over the hump so to speak.

One thing you might check is to make sure your X axis stepper drive output amperage is set correctly for the motor it is driving. If the NEMA motor is rated at 3.5 amps and the driver is set to a lower output you will experience the kinds of problems you are having as well.
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