Lots of issues losing accuracy during cutting- pics included

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Lots of issues losing accuracy during cutting- pics included

Postby wheelman » Wed Mar 18, 2015 11:12 pm

I have been building a slightly expanded buildlog2.x laser - approximately 24x32" build area with an 80w laser using the AWC608 from Lightobject. It's been up and cutting for a few weeks, but only mostly. Some prints are perfect, others just aren't cutting properly at all. I started noticing it when I would cut out a big object at say 12mm/s and pieces at the far end (longest laser distance) just didn't cut quite all the way through, likely due to beam divergence. I would hit start to rerun the cut, and it wouldn't cut the same path as before, and often left burn marks where I didn't want them. Even if I did a full system reset, this sometimes happens.

Additionally, on some cuts, the laser has a really hard time with losing accuracy during the print, especially if it needs to do a lot of traveling. Oddly enough, sometimes it seems to "self correct" on some errors later during the print. Below is a picture of a US map I cut, and the western half of the US is all messed up. Sometimes when the laser would try to retrace lines it would be off hugely, but other areas of the cut would be fine. I've also attached the settings from Lasercad. Would someone be kind enough to post a few screenshots of their settings from LaserCad just so I can compare mine?

Zoom in on the west coast of the US and on central Asia and you'll see where it looks like the cutter just decided to jump over a bit.
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I'm using NEMA17 steppers with microstepping and so far have loved most of the machine, but I can't quite seem to get a high quality cut. My cuts sometimes work pretty well, other times there's major inaccuracies. However, a lot of the time my raster engraving seems to work great.

Thoughts?
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Re: Lots of issues losing accuracy during cutting- pics incl

Postby r691175002 » Thu Mar 19, 2015 1:51 am

This is the hardest type of problem to debug because it can come from almost anywhere.

First off, the beam divergence is likely unrelated to losing position (although it could be a side effect of mechanical issues). I'd recommend taking another look at your beam alignment.

Losing position can come from several sources, from least to most likely:
Electrical issues between the controller and the stepper drivers:
- Stepper driver dir/step pulse length/frequency issues. This is unlikely to be a problem since the lightobject controller is quite popular.
- A laser tube generates a crapton of EMI and could be causing noise on the step/dir lines, generating false or missed steps.

General stepper motor failures:
- Intermittent wiring to the stepper motors causing missed steps.
- Mechanical issues/insufficient current causing stalling

General mechanical issues:
- Anything being loose.

I'd start by turning on the stepper motors (probably with tube unpowered) and trying to push the carriage around to see if anything moves. This will identify any obvious mechanical problems.

If nothing pops up there I'd take off the belts and put a piece of tape or otherwise mark the stepper motor shafts. You can then run a program and attempt to return to home. This will let you know if the stepper motors are losing position even when not under load.

If that is the case, try it with the laser tube on/off to check for EMI problems.

You can also run a few programs several times to determine if the issues are random or consistent, and if they are consistent, where they occur.

Pretty much test everything you can think of. If the problem has been getting worse over time I'd guess loose timing pulley.

Sometimes this kind of stuff can come from really weird places though, I had a stepper driver once that needed a direction change to precede the next pulse by a few microseconds or it would keep traveling in the old direction. Would slowly loose position every direction change.
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