Noisy PRS Alpha

I have a 2007 PRSalpha 120-60-8, 4hp spindle, version 3.6.44 with Artcam pro 2009 sp2a build 183 and windows 7. I believe the machine has less than 200 hours of usage.
The issue I have is when I move the X car from zero left corner with the key pad or running a file with an air cut, approximately the first 36-inches of travel is a little noisy, with chatter and vibration (hard to explain). Then in the center part of the machine the X car runs smooth all you hear are the motors (Model ASM98AA-T7.2) like it should be and then the noise starts back up again for approximately the last 18 inches at the opposite end.
What I discovered is when the X car is powered up while it is in the center of the table, its movement is quiet at each end and noisy in the center, if I power it up at either end, than it is noisy at each end and quiet in the center. I also noticed once that if it is powered up in the center and then moved to the 0 end and turn the power off, the end of the X car near 0 corner moved about .125’’ in the positive X direction, I am not sure if the other end moved or not – but this doesn’t always happen.
If you hang on to the yellow bar and put a very small pre load on the X car or when in jog speed you don’t notice the noise. I believe my move speed when using the key pad defaults to 2” per second – I changed it to 3” per second and the noise goes away but you can still feel a slight vibration.
When I put a dial indicator on either end of the X car just above the wheel bearing and with the bot powered up, I can move the car .005’’ in each X direction with little effort and it makes a clicking noise. I tried to tighten the pinion into the rack on both ends and it did not make any difference.
I then took the motors off to check the pinions (30 teeth, approx 1.602 O.D., .470’’ I.D. with key, top of the teeth have approximately .035’’ wide flat). With both motors removed from the car and powered up, when I grab the pinions and try to turn them, I can’t see it move but I hear a clicking noise inside both motors and they both act the same.
While the motors were removed, the car moved freely with no binding or noise very smooth. I checked the rail spacing in a few different places and it seems to be the same (77.812’’ outside to outside). I decided to readjust the back rail any way and then recheck the car movement, still very smooth.
The next step was to run both motors while they were still out and I noticed the number two motor was noisy while the motor was running in one direction, you can put your finger on the pinion while it was running and the noise would go away. Not sure if that is normal?
I then put the number two motor back in and the car ran smooth. Then I removed it and put the other motor back in and it ran smooth. When I repeated this step a day later, it didn’t seem to run as smoothly as it had the first time I did it.
I also checked the distance from the top of the rail and the bottom of the rack with calipers and I found up to approximately .012’’ variation along the 10-foot length but with calipers it’s hard to get a good measurement All bolts, screws, set screws are tight.
I checked the play in X car in several places while powered up and found that some areas are tight and some areas have a small amount of backlash (clicking sound). I moved one motor and pinion up in to the rack with a lot of pressure and then when moving the car it sounded like it was going down a gravel road, so that’s not a option. How I have been adjusting them is to push them up in to the rack with the power off until they stop and then I tighten them in that position.
I ran Gary Campbell’s Program (Tuning up the PRS Gantry). It was out of square by .085’’, I made corrections and now it is perfectly square. I checked all the wiring from the motors to the controllers, it all seems to be fine. The Bot is in a climate-controlled environment. Most of my work is 3D using a 1/8 ball nose.
I am not sure what to do next. Thanks for any assistance you may be able to offer. I will be away from my computer for a couple of days so I will reply to any responses when I return.
Thanks,
Gary

deleted because I saw you got an answer elsewhere :-)

That's not fair... What's the answer? Where is this other place with answers?

Gary, if I read this correctly it sounds like you have the same issue I have. At some point during moving from one place to another, it is possible for the drive motors to start creating a harmonic vibration. That is the noise I hear on my PRSAlpha. I video and sound taped it and sent to it Shopbot and they confirmed harmonic vibration being the culprit. I asked them for a fix and they said there was none. Just have to live with it. So I have...:(

Yes, the harmonics are annoying. Mine does the same thing. Aside from this, performance should not be affected and the numbers reported are in the norm for tolerances. Keep in mind that the gear rack is only good to like +/- .003-.005 per foot of rack anyway...so don't fret. These are not CNC milling machines that have to operate with ultra-low tolerances. The wood can expand/contract more than the backlash in these machines...

Don't go thinking this sort of thing only happens on a ShopBot...They ALL have issues like this in one form or another...or belt stretch, or debris getting jammed in the ball nut, or any number of other 'issues' that go along with owning one of these machines. If you wear hearing protection, you can't even hear it ;)

BTW, the original post is from 11-02-2012

-B

Hello everyone,
I am having a problem that sounds similar. I noticed a banging sound while cutting at a speed of 7 ips. When I first heard the sound I simply reduced the speed to 5 ips and the sound went away. Afterward I did some tests moving the router through air. I set the jog speed to its maximum of 30 ips and moved the router in both the X and Y directions with no problems. Next I set the move speed to its maximum of 12 ips and used the CR command to move through a 36 inch square. The initial direction of movement was along the Y axis. Immediately the machine made a very loud, rapid fire banging sound and I hit the E-stop. This seemed very odd since I had just seen the Y axis moving at 30 ips. I reduced the move speed to 5 ips and repeated the exercise with no issues. From here I increased the speed to 7 ips. This time I could hear the sound again when the router moved along Y, although it was much quieter. I heard nothing when the router moved along X. For a final test I set the move speed to 8 ips. This time I heard the sound as the router moved in both the Y and X axes. Does anyone have an idea of what might be causing this issue? Why would movements using the Move command behave differently that movements using Jog?

Thanks in advance,
Shay O'Brien

Your COM speed is too low to support those speeds. With SB3 closed, run SpeedTest.exe under the SB3/Diagnostic folder. See what your % is. If <70%, chances are you don't have a USB 2.0 hub plugged into your computer, and the Bot plugged into that. Any self-powered USB hub will increase your COM rate.

The motors full step when they jog & microstep when they move/cut, so COM demands are lower for jog moves.

-B

Thanks for the quick response Brady. I ran the Speed Test and recorded a Transmission Efficiency of 76.4 and Transmission Rate of 35.2 kHz. Also, I am using a powered USB hub. Does this efficiency rating still lead you to suspect that the problem is with my COM speed?

Thanks for explaining the full step vs. microstep. Now it makes sense.

Shay