Re: Rack and Pinion

> Hi all,
>
> I am probably missing something very simple in this.
>
> A spur gear has a unit value as the rack with a pitch diameter
> value of 1 inch. ie: a 12 pitch rack matches a 12 tooth gear when
> the pitch diameter of the gear is 1 inch.
>
> This then means the linear movement equals Pi for one revolution of
> the gear.
>
> Pi does not fit very well into any gear ratios or stepper or encoder
> ranges.
>
> How then, does one get a 4 foot axis without some accumulated error ?
>
> I can understand accept the table and machine will probably be worse,
> but theoretically, the best I get is a little over 1 thou over 72
> inches. ( what did you say?? shut up and quit complaining over 1
> thou ?)
>
> I just don't want to use two jackshafts with 4 timing gears to get
> this theoretical accuracy.
>
> Am I missing something simple ?
>
> I've been looking at 12 and 16 pitch as the rack and gear width seem
> to be good. Higher pitch seems to be much thiner, maybe too thin ?
> I do know the larger plasma table makers use much finer pitch, but
> special order racks to get the wide face contact.
>
>
> Dave

Yes dave with DP pitch gears there will be some error.
A 12 pitch rack will be 0.2618" between teeth.

If you need a very accurate linear movemnet you use circular pitch gears and racks, CP, instead of DP
Nearest to 12 DP is 1/4" CP whose pitch is 0.250" or equal to 12.5664 DP
Next common is 1/5" CP whose pitch is 0.200" or equal to 15.7080 DP

John S.




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