A Shenhui accuracy problem

[Clark Pace]

Short answer is no. we've been having worse problems recently as the cuts aren't lining up. Tried changing the stepper, but didn't seem to help. And, actually, the other axis (Y) is worse apparently.

So, I've ordered up some closed loop steppers and drivers to try ($400 us approx) and will try and re-seat wiring and grounding at the same time. Maybe change out the controller box if those steps don't help, as I have a spare, but don't expect that is the issue.

Will update thread if I have a fix.

Have you checked your gantry alignment. Mine was of a bit, and it was causing issues. Another thing I'm seen is belts moving up and down on the gear.

[Greg Facer]

Have you checked your gantry alignment. Mine was of a bit, and it was causing issues. Another thing I'm seen is belts moving up and down on the gear.

Can you describe that a bit more (belt up and down on the gear).

As a continuation of the thread, I put in the hybrid closed loop steppers tonight, put on a new belt (well, old belt but since the one on the machine hadn't changed anything, and was a little melted, I went back to the old belt that came with the machine), and tweaked the X backlash slightly to .1mm.

Circles are coming out OK now, but cutting a line of 72mm wide rectangles still shows a fair bit of variation in length, although not as bad as before. I am about .5-.6mm from smallest to largest. But, the effect is somewhat random, or partly is. I cut two rows this time, They get larger and smaller together, but are about .1mm different from each other. So, I'm trying to nail down .4-.5mm of error, belt wandering might account for part of that. I'm assuming some error I will never eliminate and some error is probably beam related...different angles at different parts of the table.

I'll have to do more testing, but if I end up with .5-.6 error down from 1.5mm error previously, I suppose I'll be somewhat happy. Might still change out my controller too. I'd like to get my pieces within .3mm ideally.

Good thing was that the hybrid steppers was more or less a drop in fix, save for a little hacking metal holes to allow the encoder connector to pass through.

[Bill George]

Is this a new machine or is this something that just started? If from day one it sounds like a design/setup problem unless the gantry as someone said is out of square or loose.

[Greg Facer]

This has been from day 1....really, I don't use the machine for stuff that needs absolute accuracy anymore, use the CNC for that.

Sad to re-read my posts and realize that maybe current results are same as before, but I was "sure" that I had 1.5mm total variance, but it's been a few years too . Regardless, our "sister" laser (ie same machine bought at the same time, though different tube setup over time) is still much better, more consistent on same-position cuts (.17mm variance over 5 samples) and across the table cuts (.25mm variance over 5 cuts).

I think I might cut a series of circles next. Any electrical noise type issue (introduced before the closed loop at the stepper driver), or crappy controller, should show up in circles with a offset start stop line that varies. My square cuts would mask that somewhat. Anything that is just due to positional variance in the belt or gear performance would presumably still line up, which has mostly been my experience. Then I'll probably re-set all wires and change out the controller too, and re-align the beam. If that doesn't work, it is what it is.

I will check the squareness of the gantry and test bolts for looseness too. Hard to say a design problem when I have an identical machine that works acceptably. Still much worse than the machines spec's, but a spec of .002" repeatably on a belt drive should be ignored anyways!

Bill: right now (and from day one) I have the machine grounded directly to the building's ground rod, so directly grounded but not to a separately driven rod that I understand could cause a ground loop problem in certain cases. That should be good, yes?

[Bill George]

In the US if you have properly installed electrical service, grounded per NEC and your outlet is properly connected to either the bare or green grounding conductor that is all you need. Added a separate ground otherwise can cause ground loops. Building and earth grounds are not always at the same potential as the electrical service ground. But you said connected to the exact same ground rod as the electrical service so that should be ok.

I would check your machine for square and otherwise as you have planned.

[Rich Harman]

Regarding this accuracy problem... a few years ago I was cutting out 7" belt sprockets out of 1/4" ply for our FRC robot. I had to make dozens of them. Four would stack together to make a single pulley. I found that they had to be all rotated to a certain position for the teeth to line up perfectly. There was some inaccuracy in the way it was being cut.

So I sent the file to a friend that worked at very well equipped R&D place that had a 100W Universal laser. We went there after hours to get these parts cut. The parts all had the same problem. Obviously at this point we suspected it was an error in the file. It wasn't, we checked several times. The file was perfect.

The point here is that it may not be unique to Chinese lasers - others can have the same problem.