Ball Screws vs Rack and Pinion

[leversole]

Searched around and couldn't find exactly the answer I was looking for...

I have a home built CNC router aprox 22" X 29" loosely based on OX components...have rebuilt the thing several times making improvements along the way...I designed and built a nice Z axis with linear rails and an acme screw, rebuilt the X gantry with linear rails also...still a belt drive machine, but I have VERY good repeatability with the belts (less than .0015") on all axis...anyhow, thinking about building a stronger machine like a minature version of one of the high end machines...will probably be 24" by 40" or so...

There are kits on EBAY complete with linear rails and ball screws and double nuts in the sizes I need, but I prefer the simple design of the frame and such using rack and pinion. A LOT of high end machines use rack and pinion. Just want the best setup in the size I want.

I would either build or buy a 4 to 1 drive system like CNCRP sells...so what I am looking for is opinions on this type of rack and pinion versus the chinese ball screws as far as accuracy and repeatability. I will be reusing my nema 23 motors. RIght now I cut wood around 50 ipm and have rapids of 160 or so, whcih is okay for the work I do,

Thanks
Leslie

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[louieatienza]

Searched around and couldn't find exactly the answer I was looking for...

I have a home built CNC router aprox 22" X 29" loosely based on OX components...have rebuilt the thing several times making improvements along the way...I designed and built a nice Z axis with linear rails and an acme screw, rebuilt the X gantry with linear rails also...still a belt drive machine, but I have VERY good repeatability with the belts (less than .0015") on all axis...anyhow, thinking about building a stronger machine like a minature version of one of the high end machines...will probably be 24" by 40" or so...

There are kits on EBAY complete with linear rails and ball screws and double nuts in the sizes I need, but I prefer the simple design of the frame and such using rack and pinion. A LOT of high end machines use rack and pinion. Just want the best setup in the size I want.

I would either build or buy a 4 to 1 drive system like CNCRP sells...so what I am looking for is opinions on this type of rack and pinion versus the chinese ball screws as far as accuracy and repeatability. I will be reusing my nema 23 motors. RIght now I cut wood around 50 ipm and have rapids of 160 or so, whcih is okay for the work I do,

Thanks
Leslie

Those high end machines have a master and slave motor each with their own pinion, which either work in tandem for maximum power and acceleration, or opposed to each other for electronic preloading. A lower-end R&P uses a sprung split pinion for preload. The hobby-level machines use a spring load mechanism because of the difficulty in mounting the rack and keeping a constant "center distance" with the pinion. When you "mash" the pinion into the rack you bottom the pinion out at the root, causing a "cogging" effect and prematurely wearing the system down. Also, commercial R&P have an automatic oiler, which aside from the obvious, takes up "clearance" in between teeth, because like anything, R&P are made to a tolerance.

[RCaffin]

The difference between rack&pinion and ball-screw is a bit like the difference between 1 mm and 1 micron.

Cheers
Roger

[ger21]

A LOT of high end machines use rack and pinion. Just want the best setup in the size I want.

No truly high end machine uses spring loaded drives like the CNCRP drives.

I'm using helical rack and pinion from Automation Overstock, with the pinion mounted to a 10:1 gearbox, with AC servos.

On a small machine like that, I think 2010 ballscrews would be a better choice.

[wizard]

For most uses, in the machine sizes indicated, ball screws are a better choice.

Some reasons why:
1. An overall easier install especially when zeroing out backlash is considered!
2. More mounting options. Racks have to be bolted to something for their entire length.
3. Reduced need for gearing. If you need any gear reduction at all with ball screws it can often be handled with a simple one stage belt reduction. Rack and pinion often requires 20:1 or more (specifics of the machine here).
4. Ball screws give you a reduced issue with contamination. Not the word reduced, both systems require maintenance to maintain operational performance. A Rack drive though can pick up chips readily that can significantly jam the teeth. Depending upon usage abd design ball screws will likely have less trouble.


Now all of the above considered don't dismiss alternatives to ball screws. On a small machine though it might be hard to justify a rack and pinion solution.

An aside, you appear to be happy with a belt drive solution, is there a reason for your desire to "upgrade". A well engineered belt drive can be "good enough" for a wood working machine. This makes me wonder if the new machine will be expected to work materials other than wood. It probably doesn't matter here as ball screws would be the right answer.

[leversole]

Thanks all...

The complaints I have with the machine are few...I don't have enough bracing in the sub frame, the double layer MDF table flexes too much. I use the Openbuild V slot with the wheels, would like to upgrade to linear rail. I have some cool 3D printed belt tension adjusters that allow some pretty good repeatability, though probably not long length accuracy.

The new machine plans include a welded steel frame (ground flat and square), a heavy and thick aluminum gantry...size will only increase a small amount, plus, I like the ShopSabre design with the linear rails and ballscrews under the table. Also, I want a table with a slot to work on the ends of boards (think dovetails!)...

Leslie