Re: [CAD_CAM_EDM_DRO] Re: Servo drives- exploding head

Dave, a few things off the top of my head for you to consider....

1: Generally, a 1" pinion produces quite a bit of cogging and frictional
losses at the pitches typically used on a cnc machine, empirical data
(i.e. learned the hard way) has taught me 1.25" (30t on a 24 d.p. rack)
gives you the best tradeoff between fricitional loss and ratios. Pinion
circ. is nearly 4", and a reduction ration of circa 4 - 10:1 will give
you reasonable resolution for the task at hand, and great rapids. 480
ipm is 120 rpm at the pinion, so somewhere around 600 rpm for the drive
motor, this is one of those tradeoffs, if you're using steppers, run 4
or 5 to 1 reduction, for servos, somewhere closer to 10 or 15 to 1 is
ideal. (with low encoder counts on the servo, naturally)

2: On all but the smallest routers (36" or smaller), 240 ipm is
downright creeping, while I've been running around that speed lately, it
is only because I always have far too many non-machinery types around
the machine that have no clue what that yellow line on the shop floor
means, when I had it in my shop, and normally with no one else around,
500ipm rapids were the norm (and yes, I've had a control system fail,
and let the gantry, all 800 or so lbs freewheel into the shock absorbers
at 500 ipm, thought I was going to buy the farm that time, scared the
crap out of me) When cutting wood (and most plastics) even at 24krpm,
you need speeds of about 200-400 ipm to get proper chipload, of course
this depends on bit, # of flutes, dia, etc. I usually run abut 200 ipm
with a 1/4" straight single flute bit at about 30k, this gives me a
reasonable finish, and good chip loading to remove heat with the chips,
instead of catching the pywood on fire. (yes, I made that mistake early on.)


3. Best approach I've found is simply try something (preferably with
scrounged/surplus parts), you will learn more in a "lost" weekend
playing than all the time spent rummaging through the mailing list; that
much I can guarantee. Try some steppers, say about 1/2" to 1" per rev,
unless you're really a glutton for punishment, then try them at 10:1,
it's quite doable, and you'd be amazed at the acceleration potential.


erie

>"If you apply 10 Lbs on a 5" diameter crank, you should get 400 in-oz
>of torque."
>
>Using those as my reference,
>
>I can convert that from a 5 tpi at 400 oz-in to a 1 tpi at 2,000 oz-in
>and further to a rack and pinion (1 inch pitch or 3.14159 inches per
>rotation) to 6,283 oz-in on the spur gear. ( 1 inch / 3.14159=0.31.
> so, 2,000 /0.31 = 6,283 oz-in when using a 1 inch spur gear.)
>
>That tells me the force I need to be able to handle with bearings and
>mounts for the final gear part of the drive. (ie: the final drive
>gear much be beefy enough to handle the torque at that point)
>
>Working back from there, and assuming rapids of, say, 250 ipm on a
>router or plasma, we can figure that if one spur gear rotation is
>3.14159 inches, then a 1,000 RPM motor will be a tad too much. (not
>many people want 3,141 ipm rapids, or 52 inches PER SECOND is a tad
>fast for a 48 inch wide machine)
>
>A 10:1 reduction would spin the motor at 1,000 RPM, and the final gear
>at 100 RPM or 314 ipm. Good enough.
>
>The 10:1 reduction would be required to be in two stages as you cannot
>get that from timing belts in one set of gears. (actually 10:1 is
>hard to get in two stages.)
>
>But, although one has a theoretical 6,283 oz-in on the spur gear, with
>the 10:1, then the motor needs to be 628 oz-in.
>
>If, we used the 5:1 target ratio, then that would effectively double
>the 628 to 1,250 oz-in at the motor and still have the 6,283 at the gear.
>
>Now, the original thesis was for a person applying cutting forces on
>the Bridgeport, not transport speeds for a plasma or cutting speed for
>a router. I would assume that a metal router might use the same
>forces, so for a metal cutting router, the 1,250 oz-in might be a
>realistic top end. For plasma, it could certainly be less.
>
>Working in reverse, 3.14 inches per gear rotation and 250 IPM means a
>drive shaft speed of 79 RPM and a motor shaft speed of 400 RPM or so.
> That is in the range of stepper speeds.
>
>Now, the motor rotation is 400 RPM and the force is 1,250 OZ. So we
>do the 400 times 1,250 and divide that by 1,351 to get 370 watts.
>Further 1hp is 746 watts so 370/746= slightly less than 1/2 hp.
>
>OK, I took liberties in that the original number is for cutting
>forces. But a router table will also have cutting forces. Probably
>not as much as the Bridgeport, but much higher than the plasma.
>
>I didn't take in any factors for acceleration.
>
>(I should be looking for something to catch the grey matter soon)
>
>A servo is not great at 10% of it's full rated speed (treadmill at
>5,500 RPM) so running it an absolute max of 400 RPM leaves something
>to be desired.
>Even a specific duty servo motor at 1,700 RPM is missing that 80%
>sweet spot by major amounts.
>
>A simple treadmill motor is rated for 1-1 /2 hp at 5,500 rpm or some
>such, but a Gecko 320 likes less than 80 volts. A 48Volt transformer
>as the basis of a power supply offers 66 volts.
>
>That's 57% of the nameplate voltage, so one gets 57% of speed and HP.
>So, the treadmill motor at 66 volts offers 3,157 RPM and 0.86HP
>
>(grey matter starting to look for exits….)
>
>…something happens here with torque and hp and all that….
>
>
>(Quick. Look at wife, and smile…)
>
>change direction…
>
>A 370 watt stepper is also out of the sweet spot for steppers.
>
>OK, a 28 tooth pulley is the smallest listed for use on a 1 /2 inch
>shaft and a two stage gear reduction with a 32 drive and 72 driven on
>each section will yield a final drive ratio of 5.06:1 so that part
>can be achieved. OF course, there is NO simple set of pulleys that
>move to 0.001" or 1.0000 inches. Everything is off by some amount,
>and care needs to be taken so the cumulative error over the table is
>not too great.
>
>Then.. THEN,,, youse guyse toss in inertial matching and moment of
>inertia to add some excitement to help the veins stick out just that
>little bit more.
>
>And you think my feeble skull can keep in the throbbing grey matter ?
>
>I did put this into a spreadsheet to try to make it all easier to follow.
>
>Dave
>
>