Re: [CAD_CAM_EDM_DRO] CNC acetylene torch table drive

Belt works well on plasma cutters. Screws are usually too fine as you
need speed more than resolution.

The holding torque figure for a stepper isn't actually very helpful. As
soon as you start moving the motor, the torque falls off. As a rough
rule of thumb you can only use about 1/4 of the motor's holding torque.
Your 425 oz-in motors will effectively produce only about 100 oz-in.
With a 2" diameter pulley you will have a maximum force of 100oz
(torque/radius). As you have two motors you will have a linear thrust of
about 200 oz or 12 lbs. At a guess you will probably lose getting on for
half of that in friction, giving you about 6 lbs useful force. For
accurate cutting you need good acceleration. I would think 0.5g would be
a minimum to aim for. Therefore your gantry can't weigh more than 12 lbs
total. That is pretty light. The good news is that you will have a very
high top speed, assuming you have enough acceleration to get there.

If you use a 1" diameter pulley the numbers look a lot better. The
thrust per motor now becomes 100/0.5 = 200oz. As you are using two, that
becomes 400 oz total, or 25 lbs. You still lose about 6 lbs in friction,
leaving 18lbs to accelerate your gantry. The gantry can now weigh 36
lbs. Assuming a maximum reliable speed of 1500rpm from the motor you
will have a top speed of 750IPM. That is pretty damn quick.

I wouldn't recommend using the belting flat . You will probably find
that the pulley engagement is not smooth and you will get slightly rough
motion. A far better method is to use two idlers to wrap the belt around
the pulley. Think of the Greek letter omega. I'll try for some ascii
art but it may not come out very well:
O
| |
__O O___

The idlers can be made out of a cheap skate bearings. Skate bearings are
8mm wide so two side by side will be wider than the belt. 3/8" pitch
seems quite coarse to me. On the next page in the McMaster Carr
catalogue they have some 1/2" wide XL belting. That would also allow you
to use a smaller pulley.

Les


marcin_ose wrote:

> Hello,
>
> 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
>
> Sincerely,
>
> Marcin
> http://openfarmtech.org/weblog/
> http://openfarmtech.org/index.php?title=Overview
>
>
>
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