rack and pinion questions

[klick0]

I recently purchased the 4'x4' machinetoolcamp plans and their X and Y axis are rack and pinion. Before this I"ve only heard about rack and pinion and done absolutly no research on the subject. I'm slightly concerned about the accuracy for rack and pinion, basically the plans denote that you just make sure it is firmly applied to the rack, as most call it a rigid machine I believe. I'm alright with this, but is this method precise? The rack and pinion they use are from mcmaster, which they only have like half a dozen pages on rack and pinion so for the most part i'm guessing they are about the same if you want to look them up.

I also saw wmberg (and others) have anti-backlash pinion gears that are basically 2 gears next to each other that have a spring which load them in opposite directions (not sure if that is the best explanation...). I would prefer to use these but machinetoolcamp plans don't use these.

I guess i'm wondering about people who have rack and pinion, would you prefer to use anti-backlash pinion gears, or does a straight pinion gear give you the tolerances you desire (and if so what are your tolerances). I think the plans say .004 accuracy in there somewhere, which really 1/100th of an inch would be good enough for me, but no more then that. I'd rather invest in anti-backlash pinion gears then the standard ones unless everyone here thinks that they will obtain the tolerances I want (1/100th of an inch, hopefully a little more).

Also as a side not they do have a belt drive reduction using gears and a belt (duh), i'm not really regarding the backlash of that stepup, i'm just assuming for this thread (for discussion focusing on rack and pinion) that there is no backlash in this part of the setup.

Thanks everyone, your help is always appreciated.

Ross

[mcmmach]

Hi, they were truthful in saying backlash would be @ .004, my machine has spring loaded pinion to rack gears and that's the amount I have. I have thought about changing to spring loaded gears but really don't need the greater accuracy versus the cost.
There is a special kind of timing belt that has zero backlash, it has half moon rounded cogs instead of the trapezoid shaped ones. I think they had them at mcmaster carr.
Dean

[ger21]

Use GT2 belts. Lots of belt info available at http://www.sdp-si.com

ShopBot uses a method of pulling the pinion into the rack, and they claim positional accuracy of .005, so that's probably what you'll have.

[DDM]

With Rack and Pinion what kind of ratio are people using to obtain decent accuracy. I figured on a 20 tooth spur gear on 20 deg 20 pitch rack and 500 count/step per revolution was around 13:1 ratio for .001 steps. That seems kinda steep to me. Anyone had any experance on these gear ratios. Just looking at the rack and pinon system it would appear that a 4:1 would work just fine but the math doesn't say so.

Carl

[dgalaxy]

what size gear rack did you get for your machine toolcamp from mcmaster carr it loks like they only sell 14.5 was it a 24dp