one corner of rectangles bad/ splayed out

[austinmmm]

Cant seem to cut perfect squares... its always the 4th corner near the end point of the cut - cut splays out or in.
motors are tightened and offset bearings are tight - no movement from xy car on the gantry.
overall i get slight jiggle on all axes but I think this is normal?
thought it was something to do with the de-acceleration tried cutting using several methods to avoid that with no luck..
happens on first pass, so this rules out slipping parts or tabs problems.

3365333654

[EricSchimel]

overall i get slight jiggle on all axes

I think that's your answer.

How much play are we talking about? Have you measured it with a dial indicator?

Also, what machine are you running?

[austinmmm]

running a prs standard. it appears after tightening the motors in place the gantry can move almost .01 according to the dial indicator..

[EricSchimel]

.01" is high. If you figure that your gantry has one motor at each end you've got a combined .02" of slop, and that's after you tightened things up. I'd check for worn pinions first as they're a wear item, then I'd look at your gearboxes. As those get older they get backlash in them as well.

[evan]

Don't forget that these pinion gears often have two set-screws in each threaded hole, one on top of the other.

[pro70z28]

If that cut was made with a 1/4" bit, that's a lot more movement than just a few .01".

[austinmmm]

the cut was made with a 3/8 bit

[EricSchimel]

If there is slop the machine will move no matter what bit is in there.

Dial out that slop if you can.

[srwtlc]

Along with that, something you can try in your design file, is to uncheck "Sharp external corners". This helps to keep a slight load on the tool as it makes the corner instead of a stop/start right angle path that unloads and loads the tool abruptly.

[bill.young]

Have you tried making changes to your toolpath to see if it helps? Do you do a ramp to minimize the amount of material it's cutting as it gets to the end of the pass? Or maybe try moving the start point away from the corner and along one of the sides?

Bill

[pro70z28]

If that's a 3/8" cut it makes my point even more valid. It looks like the cut has gone at least the width of the bit past the corner in that photo. That's .375". Where did that much play come from? Also sounds like it's just the "4th corner"? If there's play, why is it only showing up in one place and not every corner? At least the common axis corners? I don't have an answer either, so I'm asking?

[EricSchimel]

Scott, "sharp corners" mode just moves in one direction into the corner (so a command like MX 12) and then another move command to get out of the corner (so MY12) So essentially the machine will move into the corner, ramp down to a stop, and then ramp back up when it goes out of the corner.

With that option unchecked you get a super tiny arc at the corner. This causes the machine to ramp down less, and keep some load on the tool.

If there's some mechanical slop in the machine these two corner cutting methods might show different results. If there isn't, they should look the same.

Austin, if you can post either the VCarve file, or your SBP cutting file we can see if the problem is in the file for you.

[waynelocke]

Do you have “Lead In, Lead Out” checked? I think that that might give you that result in the one corner if that is the start point.

[chiloquinruss]

“Lead In, Lead Out” and that combined with "inside / outside" gives all kinds of differences. :D Russ

[EricSchimel]

Both good suggestions, but don't try to fix a hardware problem in software :)

Aluminum framed machines (SB and non SB) can flex if you push them hard. A common example of this is when you aggressively cut plywood you can get a part that's a bit out of square. If on your last pass you switch directions you can solve that problem. Lead and lead outs can help too. but again, if you've got a sloppy machine you've got to solve that first, and then dial in the cutting techniques.

I really think what we're seeing here is some worn pinions/gearboxes.

[waynelocke]

The splay is probably hardware slop but the size of the over-cuts and being only on one corner sure suggests like lead in and lead out.

[cnc_works]

If this were Mach 3 these exact symptoms would be linked to constant velocity settings. The only way I have been able to combat the issue is to slow down my cut speed...goes from one rounded corner to all four being identically straight. As an aside, I don't think Mach3 ever did cure this issue.

[bill.young]

Do your parts turn out correct other than the bumps on the corners? The correct size, corners square, etc? If so then trying some changes to optimize the toolpath like lead-ins or lead-outs or ramping into the cut seems like the sensible and easy first shot at fixing the problem.

Bill