[billwann]

I am working on my second cnc router , this one will be a 4x8 all steel table . I am having trouble deciding on the x axis drive . I know that I want to to use rack and pinions but i am not sure how to drive them . I know it is common to use two sets of belt reduction units on each side slaved in the software but that looks like trouble waiting to happen . I was thinking of using one central mounted belt drive with a 1 inch shaft connecting both sides of the rack and pinion spur gear . has anyone tried this ? I cant seem to find any info on this setup , it would seem to be a simple idea but I am sure there are problem with it , such as the shaft twisting .

[BanduraMaker]

No experience with this one but, the Romaxx wd-1 uses a single motor and one or two timing belts to slave two small ball screws on each side of the long axis.

Otherwise, keep in mind that the longer the belt, the more slop you're going to have. What works in a 25" wide machine might not work in a 48" machine without major changes.

[jsheerin]

You'll also have to deal with the critical speed of the shaft. You've basically just removed one of the big advantages of rack and pinion over screws - higher speed capability without having to worry about screw resonance. To get around that you could make the shaft a hollow tube, but that's possibly adding weight and size and is definitely increasing the inertia of the drive system meaning it takes more torque to accelerate at a given rate which leaves less force left to cut with.

Another downside is you can't support the bed anywhere between the ends as the shaft would have to travel with the gantry, at least if you put it under the table. A better solution might be to put it across the gantry. Then you could support it with bearings at shorter intervals to avoid some of what I talked about above. However I'd recommend you don't do any of this and just put a motor on each side. It works fine.

[BanduraMaker]

Yea, I just re-read your original post and didn't fully understand what you were describing at first.

I don't have a rack and pinion machine so I can't speak from experience but, it kind of sounds like your plan combines the downsides of R&P with Ballscrew without any of their respective advantages.

p.s. I know nothing.

[Phife]

There really isnt much problem having to motors slaved together, you must always home a machine when its powered up and by using 2 limit switches on both X sides you can ensure that its perfectly square and ready to go.

I wouldnt worry too much about it and go with the proven method.

[PaulRowntree]

My machine has a single gantry drive, so I know nothing about slaving motors.

What I don't get is how homing 2 sides independently is supposed to work with an extremely rigid gantry (which we are all trying so hard to make). The 2 homing switches would have to be aligned to an accuracy comparable to the maximum flex in the gantry. Am I overestimating the rigidity of these gantries? Or can you get by with a single switch if the gantry is stiff enough?

[jsheerin]

Yes, you're probably overestimating the stiffness for most machines people here build. In any case if the gantry is square and you home it, it should stay pretty square throughout the homing procedure - both sides are moving at the same time.

[billwann]

This is exactly what I needed to know , I new the shaft would turn into a torsion bar just not sure how much . I was going to use a precision ground a hardened 1 inch shaft supported on baring every 12 inches , what a pain in AXX .
so I will go with motors on each side . so how about belt drives or gear drives , does any one have any suggestions . I think I will so servo with this one since I have not don't that yet .I was thinking of 800 oz servos but I have no basis in reality to base that off of so if any one has experience please let me know . I was thinking of using 20 pitch 20 degree PA rack and pinion . this seems a bit small to me . does any one have any recommendations . I would like to go larger but i am not sure what size to use and still get good resolution .
my gantry will weigh around 200-300 lbs . it is made out of 1/4 wall 3x3 square tube .