Re: Need help to decide what rack and pinion to use in a 6'x10' table

Hi, folks, thanks for the replyes,
Ballendo,
I see you mention you are using steppers, in my case i want to use
servos.I dont realize if you have to take different aspects into
account in the decision of what rack and pinion if your are using
seros or steppers. To make it clear does it matter what kind of
motors are you using in the rack & pinion decision?

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "ballendo" <ballendo@...>
wrote:

>
> Hello,
>
> Snips, inserts below...
>
> >In CCED, "Hector" <hector_a_quintanilla@> wrote:
> >Hello,
> >Im confused on which combination of rack and pinion to use for a
> >6'x10' table.
> >1.- which presure angle (PA) to choose, 14.5 PA or 20 PA?
>
> I have long used and suggested--look in this group archive for my
> many previous posts-- 14.5 Pressure angle, as it will minimise
> backlash caused by changes in pinion to rack distance.
>
> Others have replied that the undercutting of teeth on small

pinions

> using this PA is not a good thing, and mention that certain

machines-

> -including Bystronic CNC laser cutters--use 20PA.
>
> I still prefer and suggest 14.5 PA. (But would use 20PA for
> a "lightly loaded" machine, where I might want to economise by

going

> with a smaller rack and DP.)
>
> >2.- after knowing what pressure angle will be used, the next step
> >is to decide the diametral pitch to use.
>
> The DP (Diametral Pitch, which is the number of teeth in a one

inch

> gear) affects cogging, strength, and cost. I use 16DP. Coarser

gears

> can carry more load, but also are more susceptible to backlash
> (especially is 20PA is used.) Smaller teeth--larger DP--will have
> less ability to endure shop swarf and dust, and a lower strength.
> They may even cost more. 16DP seems to be a good compromise for a
> 4'x8' or thereabouts size machine...
>
> >3.- In this step we already know the PA and DP we are going to

use,

> >now comes to decide how many teeth on the pinion, so what governs
> >this decision?
>
> I nearly always use a 1" pinion. This gives about 3" of travel per
> rev, and is large enough to have reasonable undercutting of the
> teeth in my preferred 16DP.
>
> What governs this decision is your speed and resolution
> requirements. Larger pinion equals faster speed, but a lower
> resolution, all else being equal.
>
> I use a 4:1 reduction from motor to pinion, so in the old days<G>

of

> halfstepping, I would get 240IPM at only 300 RPM at the stepper.
> This kept the drive cheap, and was plenty fast. The resolution was
> traded to get speed, as it was only about .002". (500SPI)
>
> Using modern drives and motors, you can easily exceed these

results.

>
> >I think is better if someone could really make clear this for me
> >and others, i guess other have the same doubts.
>
> Hope I've donethat!<G>
>
> Ballendo
>