Re: rack and pinion advice

Troy,

Several commercial cnc machines use size 34 motors (single or double
stack), a 4:1 ratio with about a 1 inch output wheel. Vytek, Shopbot,
Camtech, and Axyz are examples. Digital tool used 12:1.

14 degree racks are a better choice than 20 degree. Because they are
less sensitive to center distance variation, and the lower angle
means more force goes into moving the axis, rather than trying to
separate the rack and gear.

It's pretty easy (and worthwhile, IMO) to set up a spreadsheet which
will allow you to plug in the different specs and see all the results
at once. Things like ratio, resolution, oz.in., axis force, speed,
rpm of motor, pulses/second, drive type-means 200,4000,or XXXX
steps/rev are useful to have when designing a rack/pinion drive. From
these you can see the effect of various choices. Also will show you
how one thing balances(or works against) another...

Barring all this, you can use 4:1 with size 34 steppers, 14 degree
rack, and go about 240 IPM, with cheap drives. Resolution of .002
or .001. And have plenty of power for cutting 3/4 MDF all day long.

Hope this helps,

Ballendo






--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Troy" <superscooby3000@y...>
wrote:

> I am designing a 4'x8'gantry router with rack and pinion on the x
> and y. Does anyone have any advice on wether I should gear down
> nema 34 stepper motors with timing belts and pullys?
>
> I am thinking without gearing down I would get the fastest rapids
> and at cutting speed, and I think I would get the most torque
> because the motor wouldn't have to run as fast to move the gantry
> (I was under the impression that the faster you ran a stepper motor
> the less torque you get.) Am I correct in this thinking or am I
> missing something?
>
> I was planning on using pullys and timing belts anyways figuring it
> would be easier to upgrade to servos if I wanted to in the future.
> I just cannot decide if I need a 1:1 ratio or something else. If I
> did need a different ratio does anyone have advice on what that
> should be? I was figuring on using the smallest spur gear I could
> get, or do I need something with more teeth engaged.
>
>
> Another question: I was planning on using 14deg 20pitch gears and
> rack because that seems to be the cheapest route at Mcmaster, and

it

> seems mcmaster doesn't carry long lenghts of 20deg rack.
> Does 20deg make much of a difference on smoothness and is it worth
> the extra expense? (and does someone have a better source for

20deg

> rack?) How about gears and rack with helical teeth, I saw
> somewhere on the net but can no longer find the site, is that worth
> while?
>
> any other advice you rack and pinioners and give would be greatly
> appreciated.
>
> Troy