Re: [CAD_CAM_EDM_DRO] Re: Servo drives- exploding head - Router/plasma

turbulatordude wrote:

>
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>>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
>>
>>
>>
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>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.
>
>

Preferred, but not if you are squeezing pennies, good quality xl belting
and pulleys work fine, as long as you purchase underbore, and have them
bored and reamed to finish size, as most mfrs. feel that several thou of
runout is acceptable. Of note is that many, many machines have been
built and running for years with xl or l series belts that are still
making money today.

>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 experience has been to avoid the largest pulleys, as they tend to be
pretty expen$ive. www.sdp-si.com has a drive calculator that is
indespensable for ratios, center to center distances, etc. The best
feature is the calc of how many teeth are in mesh, and warns if you go
below recommended no. of teeth. as far as the pulses per inch, I use
mach2/mach3, and it can use a decimal figure for your steps per inch. I
generally get in the ballpark with a dial indicator, then move 50" and
tweak till I'm consistently on. then I check at 100", I've said it
before, if you are careful, you can read a tape measure to 1/64"
(roughly .015") at 50", all errors are mulitplied by 50, so if you can
get within 1/64" at 50" (ignoring cyclical errors) then you are within
.0003" per inch. not bad without getting crazy. of course if you're
routing wood or mdf, a 10% change in humidity will cause it to grow more
than that. Routing aluminum, the heat of cutting will cause local
expansion that will swamp even 2 or 3 thou errors.

>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.
>
>
>

Depends on the structure used, I recently got a catalog from MK (similar
to 80/20, but more oddball bits) and I'm real impressed with some of the
extrusions, of note is a linear bearing system that is attached to their
extrusions and has several different types of carriages available,
either inside or outside the extrusion, using grooved wheels. stay tuned
for some empirical data, now that I have a full time job and have tons
more disposable income.....

>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.
>
>
>

or you can buy or make pulley stock and make your own, sdp is pretty
affordable ( I use alot of T-10 stock)

>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
>
>
>

Erie