ShopBot Circles cuts out of alignment

[CDarby67]

Hello,
I had some issues over the weekend while using a maker space ShopBot. I am cutting Polycarbonate 0.375' with a 1/4" single flute "O" up cut - Onsrud. (I have cut the same part with the same bit in, on a Laguana with no issues).

Speed 18,000 IPM 60, depth of pass 0.100" 4 passes. The center hole is not circular and the tabs cause a gouge in the periphery circle.

Suggestions?
Thanks,
Clint

[EricSchimel]

Looks like something is loose on that machine.

Try this:

Power it up and make sure the steppers are locked. Pull and push the gantry on the X axis and check for any play. Do the same for Y.

If it's a V roller machine (the larger format SBs) you can put your finger right on the V roller where it lands on the track and when you push and pull you can easily feel for any play.

For the Z axis flip a bit over and chuck it in the spindle/router so the smooth side is sticking out. Grab onto the bit and see if theres any wiggle to it.

If you feel any kind of play anywhere get out a dial indicator and start diagnosing further.

[CDarby67]

Eric,
I will try those recommendations. My thought was that the machine is WAY out of calibration. It is a public maker space. Additionally, the maker space takes commissions from the community and they recently made a few thousand face shields - as you can see the waster board was trashed. Ultimately, I can not do any calibrations - it is not my machine. I will however recommend they have it serviced. They had new waster boards on a cart waiting for install and surfacing.
Thanks,
Clint

[coryatjohn]

Your cuts are pretty aggressive. 0.1" per pass on polycarbonate takes a lot of power to do. I suggest trying a much lighter touch and cut half the depth per cut (0.05") at half the speed. It's the cutting force that really brings out the slop in a machine. The cut quality will improve too.

Another thing you can try is to leave an offset on the part and either sand it off or use a final pass (or another cut file) to trim the part to final size. The lighter the touch, the better the cut, especially with a worn out machine.

[EricSchimel]

Clint, what type of machine are we talking about? PRT, PRS, PRS Alhpa? Desktop? DesktopMax?

Cory, even on a PRS Standard, those speeds and feeds sound slow. Polycarbonate cuts like butter but can melt at slower speeds. If it were a PRS Standard I'd do 2.5 IPS, .25" depth per pass (with a .25" bit). If it were an Alpha .35 per pass at 4-5 IPS would do well (16-18k RPM)

Clint, if you were cutting too aggressively and you lost steps you'd see shifted circles, not oblong ones like you've got there. That machine, whatever version it is has something loose on it. You've got to figure out what part is loose. While it's up to the maker space to fix it, depending on what it is it might be very simple to do a field repair.

[CDarby67]

Cory - thanks. I used same bit/feed/speeds on the Laguna (Open Works Baltimore) and it cut perfectly.

Eric - PRS Alpha.
My first run of the same part I used the 0.25 cut per path - my initial thought was I was hogging too much with each pass. I run 18k per Onsrud recommendations.

This machine is used by the public with minimal experience and no vested interest in keeping the table running well. I sent an email to the management requesting a calibration and PM.

While I have your attention (greatly appreciated), How does ramping effect the simple parts I am cutting? I know it is easier on the bit - correct?

thanks
Clint