New to me Shopbot PR (I think)

[Cantrma]

I just bought, last Saturday, a Shopbot PR (? strut rails, rack, pk296a1a-sg3.6). Somewhere in chain of owners the original controller and software was lost. Previous owner started converting to g540 and put 1/2 angle with v-rollers on outside of x-axis strut but never got it running. I've got it up and running, kind of, with Mach3. Issues:

(1) It rolls freely with the motors pulled back off the rack. I can give the gantry a gentle no-shoulder push and it freely rolls 8 feet. But I'd like to upgrade the x-axis motion (due to issue #2). There are Chinese 30mm supported round rail and bearings set, 3000mm long, for about $650 on eBay. Is there something better and cheaper to get better accuracy for the rails?

(2) If I run the X axis over about 150"/min it will skip, usually in the same place, usually resulting in racking and stalling. I can not find any resistance or bump in that location by observation or feel. Could it be in the rack itself? Runs fine up to about 150.

(3) When it loses steps (as in issue 2 above), I can't tell if the motor is losing steps or the gearhead is skipping. How can I test for a 'good enough' gearhead to make sure that's not my problem. These motors and gearheads are probably 15 or 20 years old with unknown usage.

(4) The motors are run with the yellow wires bundled so I assume it has to be series connected. What about running parallel on 1/2 windings? I've ordered a 48V power supply to replace the 28V on it. Can I run 48V on either series or parallel? Should I use 48V and should I use parallel? What should I limit current to in either case?

(5) If (no plans, just in case I can't fix the skips or if the gearheads are worn) I replace motors and keep the g540, what is a good choice in motors?

(6) Finally, thanks for your patience, I put a dial indicator on one of the X axes and zeroed it against the gantry. Zeroed the indicator and X-axis in Mach3. Ran a program g0 x 48, g0 x 0 (150"/min and accel of 30 in Mach3) and every four or five times I'd run it it would come back a thousandth or two too much and would accumulate over multiple slips. I assume this is acceleration related and is slipping the gearhead or losing steps. Is there a way to tell which based on how many thousandths it overruns? Anything I'm overlooking?

Thanks,
Mark Cantrell

p.s., is the user or build manual for this still online somewhere?

[Brady Watson]

(1) It rolls freely with the motors pulled back off the rack. I can give the gantry a gentle no-shoulder push and it freely rolls 8 feet. But I'd like to upgrade the x-axis motion (due to issue #2). There are Chinese 30mm supported round rail and bearings set, 3000mm long, for about $650 on eBay. Is there something better and cheaper to get better accuracy for the rails?

Those rails are fine, or you can convert it to angle iron rails, but you'd have to make them yourself. SB does set a PRT upgrade kit that are angle iron type supports with hardened BWC rails. Call them for pricing.


(2) If I run the X axis over about 150"/min it will skip, usually in the same place, usually resulting in racking and stalling. I can not find any resistance or bump in that location by observation or feel. Could it be in the rack itself? Runs fine up to about 150.

This is most likely the rack. Check it for square and eyeball the rails to make sure they are running straight & true. Those tools maxed out around 180 IPM with no load on them new. They run really well @ 120 IPM or less. Slow I know, but keep in mind that the structure of that tool was not designed to go very fast. It works well at it's designed speed.


(3) When it loses steps (as in issue 2 above), I can't tell if the motor is losing steps or the gearhead is skipping. How can I test for a 'good enough' gearhead to make sure that's not my problem. These motors and gearheads are probably 15 or 20 years old with unknown usage.

The noise you most likely hear is not mechanical at all - rather it is the motor stalling because it doesn't have enough voltage to go that fast (higher voltage power supply will help this) or your communication speed/pulse rate is not fast enough. Probably the power supply.


(4) The motors are run with the yellow wires bundled so I assume it has to be series connected. What about running parallel on 1/2 windings? I've ordered a 48V power supply to replace the 28V on it. Can I run 48V on either series or parallel? Should I use 48V and should I use parallel? What should I limit current to in either case?

The original control box probably used unipolar drivers with all 6 in use. You can run them bipolar parallel by just using the red, blue, green & black - omitting the white & yellow. Those are 1.5A motors, but set your current limit to 2A for a little bit more power.


(5) If (no plans, just in case I can't fix the skips or if the gearheads are worn) I replace motors and keep the g540, what is a good choice in motors?

Chances are the motors are fine. The SG3.6 designates straight gears; 3.6:1 reduction. You can get replacements for around $300 ea via Oriental. Otherwise, it gets complicated since you will need either a belt reduction setup with any stepper or another stepper with gearbox to get enough resolution & torque to make it worth while.


(6) Finally, thanks for your patience, I put a dial indicator on one of the X axes and zeroed it against the gantry. Zeroed the indicator and X-axis in Mach3. Ran a program g0 x 48, g0 x 0 (150"/min and accel of 30 in Mach3) and every four or five times I'd run it it would come back a thousandth or two too much and would accumulate over multiple slips. I assume this is acceleration related and is slipping the gearhead or losing steps. Is there a way to tell which based on how many thousandths it overruns? Anything I'm overlooking?

It is most likely losing steps. Set your program to run @ 60 IPM and repeat the test. A few thousanths repeatability is very good...unless you are making parts for NASA.

I don't have a link for a PR manual, but you may find this PRT Assembly Manual (http://www.shopbottools.com/ShopBotDocs/files/AssemblyBinder060828.pdf) helpful.

Also, see attached PDF for wiring your motors.

-B

[srwtlc]

You might also want to go through the motor tuning section of the Mach3 manual. That accel rate of 30 sounds a bit high and could also cause the motors to loose steps. It can also cause the machine to ramp into and out of corners more harsh and cause more 'jiggling' of a PR machine due to it's lighter weight. The trade off of going too low can cause excessive corner rounding in CV (constant velocity) mode though when using a higher feedrate.

[rb99]

The low power is a problem. When I got my G4 controller it added about 2.5 x the power and it showed.