diamond: Re: [DIY-CNC] Direct belt drive for 4'x8' torch table

Marcin, I have a few comments.

I very much recommend against the round rails.
Use linear guides, second hand if you cant afford the new ones.
The round rails will bend appreciably with the forces you get when using
a router on wood or an end mill on metal.
I know most hobbyists use them - but that does not mean its a good idea.

Mxl timing belts may not be strong enough.
Accuracy is not you problem, strength is.
Mxl is not able to transmit the power you need at the speeds you need.
Timing belts strength are dependant of speed - when cutting, your
effective rpm may be well under 10 rpm.
At that speed mxl and xl are simply not strong enough.

Use HTD pulleys and belts, 15 mm width, 5 or 8 mm pitch.
They are stronger, transmit more torque and would be a much better choice.
They are very accurate, more than you require.
Note that the belts are put on quite tightly for 8 mm htd. If they are
slack, they wont work nearly as well or accurately.

Use microstepping. Its really, really important.
Don´t even consider non-microstepping solutions.

The motors are good for this application.
Proper tensioning of the belts means that you overload the axels on the
Nema 23 motors -
they don´t fail immediately,but they may have a very short life.

Its more work, but a bearing and coupler will make it a much better
solution.

The design is classical, well proven, and should work very well.

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?
>
> 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, if I don't use microstepping.
> Are the 3/8" pitch pulleys sufficiently accurate for this? I am trying
> to do the rack-and-pinion belt instead of ball screws for lower cost:
>
> http://openfarmtech.org/index.php?title=Torch_Table
> <http://openfarmtech.org/index.php?title=Torch_Table>
>
> Sincerely,
>
> Marcin
> openfarmtech.org/weblog/
>
> __._,_