Vertical chatter

[Shane]

I thought it best to create another thread then post in my other thread about needing a technician in Brisbane, Australia.

I've got a new PRS Shopbot 96x48 and I'm having some issues with vertical chatter. Having gone over everything to make sure it is all okay, I've found some issues but in fixing them they don't seem to have any effect with the vertical chatter.

Having completed some more testing today, I think I've narrowed down the issue or at least I can replicate the chatter and eliminate it by changing the method of cut.

In the attachments, one of them shows how the layout affects the quality of the cut. The chatter seems to be the worse when the machine is moving along the X axis. Y Axis chatter is very little.

If I place my hand against the top of the spindle and just feel the vibrations there is definitely a strange vibration when the machine is cutting along the X axis and it can even be felt by placing a hand on the spoilboard. It isn't a smooth movement.

Another issue that I'm not sure is related but I'll mention it anyway is when cutting a circle or curve where both X & Y axis are moving at the same time, the machine seems to struggle. Not struggle in cutting through the material as it was 9mm thick MDF, full pass depth cut using a 3/8th bit at only 40mm/sec. There was a noticeable change in the sound and it was like the machine was losing power. Kind of like there wasn't enough power to run all the motors at once, but after finishing the curve and going along Y, the sound changes back to normal. Sorry that I can't describe it better.

And I'd just like to point out that before my Shopbot arrived I had a new power cable hooked up to the electrical board with plenty of spare power needed to feed the machine power requirements. All new wires etc so it's not like I've got a faulty or dodgy wiring going to my machine.

The pictures show what effect it has when I rotate the word 90°. With most of the letters being long in only one axis and when the long parts run parallel to the X axis this is when the chatter is at its worse. Once rotated so the long parts run with Y Axis, all chatter is basically eliminated. No other changes where made to the file. 1/4 downcut spiral was used and it cut the 18mm MDF in two passes. First climb pass at 76mm/sec down to 8mm and the final conventional pass at 20mm/sec cutting the remaining 10mm.

I believe I've ruled out bits being an issue at it happens when using at least 3 different size bits and all at different speeds/rpms.

One thing that is different with my PRS was that the Y motor was Dead On Arrive and Shopbot sent out a new motor but they sent a PRT motor instead. They said it should work with the PRS and that I would just have to adjust the setting in the control box drivers. Would this have any effect?

Can the Y motor be swapped with the Z motor? I would swap it with one of the X motors but want to swap with Z to eliminate the problem just moving to X. And also I'd need to readjust the setting in the control box.

Oh and lastly I get a new Techniks collet nut but only noticed today that the inside lip is offset. This can't be right can it? I have taken this nut out of the machine so it is not affecting the above issues but just like some opinions about it before I send off an email to the company.

Sorry about the wall of text or for the lack of the right technical terms etc. But if anyone can help explain what might be causing my issues, as I'm just about to pull out my hair over it :(

Shane.

[myxpykalix]

I have a 2006 PRT Alpha and just recently i have noticed these same conditions described below:
Another issue that I'm not sure is related but I'll mention it anyway is when cutting a circle or curve where both X & Y axis are moving at the same time, the machine seems to struggle. Not struggle in cutting through the material as it was 9mm thick MDF, full pass depth cut using a 3/8th bit at only 40mm/sec. There was a noticeable change in the sound and it was like the machine was losing power. Kind of like there wasn't enough power to run all the motors at once, but after finishing the curve and going along Y, the sound changes back to normal. Sorry that I can't describe it better.

Although i did not notice a bog in my router and could not hear the steppers at the time due to the router I couldn't tell if the motors were bogging down or not. But it would get into the apex(?) of the curve where both axis' are moving and seemed to struggle to get around. I still haven't figured that one out yet.

One thing you might do is to see what the specs are for those motors and what the constant voltage delivered to them should be and hook some kind of meter to it while running that pattern and see if you are getting the proper juice to it under a load.

I think you need to locate where your vibration is coming from. Do you have a router you could replace your spindle with or another spindle? That might help eliminate issues with the mechanics of the table.

[Shane]

Thanks Jack for your reply. Yes that sounds like my issue. The stepper motors are making a different sound when doing the curve but the spindle sounds fine. I'll make contact again with Shopbot and see if they can send out a new PRS motor. This PRT motor looked used as the heatshrink tubing was already contracted around the connection.

And I've only got the one spindle. It's frustrating being on the other side of the world. I have to phone support at either midnight or just before 8am in the morning my time.

Shane.

[myxpykalix]

Today i had to make a small circle only 1.5" diameter and it went fine, it was only when i made the large circle (10") that i had that issue. Which leads me to say try laying out several circles incrementally larger 2,3,4,5,6,7,8" ect and cut on some scrap and see where it starts to have a problem. I don't know what, if anything, that might tell you, but you might learn something from that:confused:

[Simops]

Shane....as I see it if you bought a new Shopbot and it came faulty then Shopbot has a responsibility to replace the part with an identical item.....a new identical item.....no ifs or buts! Dont accept anything less than original new replacement part for something this new. I would get back to them pronto about this......a basic customer service complaint.

It's only when you have everything back to original ex-factory fit that you can check serviceability and operational capability as would have been done by Shopbot when they do the testing in the factory before despatch.

Cheers

[Brady Watson]

Shane,
The sound you describe is normal. It is mid-band resonance inherent of all stepper motors at certain speeds. It is certainly nothing to worry about.

In regards to your chatter issue, it will boil down to one specific area. Either your pinion is not fully engaged in the rack, it is worn (doubt it) or you have a defective pinion on one of your X motors - which is not impossible. I think I got a bad batch of pinions a while ago that made my tool vibrate like a fat man in corduroy. :D

Check that the pinion is tight on the shaft (remove the motor and look/feel), re-seat it in the rack and shake the gantry under power to make sure it is tight, then test again. If you still have the chatter, I'd look at replacement pinions. BEFORE you blame the pinions, swap the pinion on the Y with one on the X and see if there is chatter in the Y now.

Take NOTHING for granted.

-B

[Shane]

Success!! :)

I exchanged the Y stepper and moved it to Z and obviously changed the pinion to match. I did my two test cuts of "meh!" in both orientations and both now are coming out with no chatter. But now when the bot moves on Z axis you can feel a vibration throughout the machine. So I'd say the fault lies with either the pinion or the motor. My bets are on the motor as it is the wrong type for my machine.

I will phone Shopbot support and request they send a new PRS stepper motor. As Michael pointed out I shouldn't have to accept old parts in a brand new machine especially when the original part was DOA.

Shane.