Re: CNC acetylene torch table drive

Marcin:

If I were to make a 4 x 8' oxyfuel machine which needs to run at 30
IPM (MAX !!), I'd make a single side drive for the 8' axis and use a
delta shaped frame requiring the main rail be maybe 3' longer on the
driven side. Thus you will need TWO steppers for your drives, and
you can use (2) Gecko Systems 10x microstepping amps.

Use rack and pinion drive for both axes. Rack can be gotten from
Standard Steel Specialties, www.stdsteel.com/gear.htm at a cost of
around $23 for each piece if 1/2 x 1/2 x 6 feet length in quantities
of (2) pcs. They stock 20 DP, 20 degree pressure angle rack. I would
ask if they could supply 24 DP, 20 degree rack as we used to buy
these from then in quantity.

Since the drive pinions need to have at least 18 teeth in order to
have no undercutting, these are the smallest pinions you should use
if you wanted direct drive. If you were to use 24DP, 18 tooth
pinions with 10X microstepping, you would have a resolution
of .0011781 inches per microstep.

As to the available force from this setup, I must respectfully
disagree with Les Newell regarding the torque/force available at low
speeds. Looking at the Pacific Scientific spec sheet for NEMA 34
frame single stack N31 motors with microstepping (.707 x torque)
gives about 315 ounce inches at this very slow speed. With an 18
tooth pinion, radius = .375 resulting in 840 Oz force or 52# force.

While I might practically opt for some sort of belt reduction
between the stepper and the pinion so as to use a larger diameter
drive pinion, the numbers above are still quite realistic.

To summarize,
1) Use rack and pinion drives
2) Drive the relatively narrow side (4 ft. wide machine) from one
side; design the main carriage and main rail to do this.
3) You can use direct drive with a resolution per step of .0012 per
step

I have pictures and data from my small test bed machine. You can
reach me at jccinc-at-owc-dot-net.

Regards,
Jack C.

> I am looking at Xylotex.com 425 in oz motors to drive the long

axis of

> a 4'x8' torch table. I'd like to use a 3/8" pitch by 1/2" wide belt
> that functions as a rack. The pinion is a pulley of 2" diameter.

I'd

> like to have one drive motor on each side of the long axis.
>
> Would I be able to drive the pulley directly with the motor, or do

I

> need some gearing on the motors? The carriage will be the is the
> lightest possible tube, just to carry an acetylene torch head.
>
> My concern is having adequate traction between the belt rack and

the

> pulley. Is gravity sufficient to hold onto an upward-facing belt

rack?

>
> I've considered ball screw assemblies, but it appears that I don't
> need such high accuracy, and simple belt drive will get me at least
> 1/30 inch accuracy on the torch table, and better if I use
> microstepping. Are the 3/8" pitch pulleys sufficiently accurate as

I

> indicate with the calculated 1/30 inch? I am trying to do the
> rack-and-pinion belt instead of ball screws for lower cost. The
> original design was screws- but I'm moving onto rack and pinion,

with

> 2 motors on the main axis to get the drive mechanism away from the
> working area.
>
> http://openfarmtech.org/index.php?title=Torch_Table