Re: Servo drives- exploding head - Router/plasma

> And if someone wishes to calculate a
> rack or belt drive and is not familiar
> with software like Motioneering we will
> be glad to help out as time permits.
>
> Les
>

Hi Les,

since Bridgeports seem to be the basis of discussion, maybe we can
create a router/plasma model that will be generic ?


As Erie offered, a pitch diameter of 1.25" is preferred over 1 inch.

I think the balance between a wide rack and low pitch # and a narrow
rack and high pitch number falls into what is commonly available.

McMaster Carr offers 20 pitch with 1/2 inch face rack. higher pitch
means a narrower rack. lower pitch mans a wider rack, but then the
available gears become larger.

A 20 pitch rack with a 20 deg pressure angle has 25 tooth gears
available in 1/2 bore size. This seems to be the place things cross
on the charts.

a 25 tooth gear has a 1.25" pitch diameter or 1.25 * PI = 3.92699
inches per gear revolution.

For timing belts, it appears the GT2 is preferred and a 28 tooth gear
is the smallest for a 1/2 shaft.

A 60 tooth driven pulley offers 2.14:1 ratio, but coupled with the
1.25 pitch dia gear, Using a 500 line, 2,000 quadrature encoder, that
equates to 0.00098960169 per step, or, 1,010 steps per inch. that
then has a cumulative error of 0.0233" over a 48 inch table. or
0.00048" error per inch.

28/70 pulleys - 2.5:1 and 30/60 is 2:1. Again, 28 is the smallest
tooth count and 72 is the highest available. A double gear would
offer different ranges, but as one moves away into other ranges, the
errors seem to get worse, especially the cumulative error.

My estimation of a light weight (plasma) gantry is about 300 pounds
and a router gantry is about 500 pounds. This would include all
wires, switches, routers, plasma cutter and wire, cable guides,
bearings and frame. I assume someone could make something much
lighter though, But I can't see how to get something much less than
100 pounds, if that low.

500 ipm seems to be the highest anyone uses for rapids.

I would be curious what would be the problem(s) and results using a
treadmill motor (115V, 5,500 RPM, 1-1/2HP ) in the 2.14:1 drive with
only the single reduction as well as the same thing with the max of
28/72 per set of gears, two sets for 6.12245:1 on the gear.
(0.593896759 inches table movement per motor revolution) that would
mean roughly a 840 RPM on the motor at 500 IPM on the table.

The open belt type drive would run a little different. Since 28 is
the lowest tooth count for a 1 /2" bore, that would be something
about 5.6" circumference. Or closer to 1:1 with a double gear reduction.

If you think this is pretty middle of the road, then I'll put it in
the files section as a reference point for everybody.

If there are things I missed or that would be changed to make it more
universal, then by all means add/change them.

Hopefully a generic layout will make it easier for everybody and also
you won't get bunches of similar requests.

Dave