A Shenhui accuracy problem

[Dan Hintz]

The accuracy specified by western machines is across the entire working surface. If it wasn't, there would be a lot of cranky people out there.

[Rich Harman]

It is a few mm of an error, but seems consistent and unrelated to speed or direction of travel, so it must be stretch?

A few mm??? Holy cow that is a lot!

I have cut a lot of parts that are 30 inches long at all locations on the table. Symmetrical parts that share a cut line that interlock - box joints. When assembling if one piece gets flipped over it will still fit but will be a bit tight in places. If the error were were that big it would be impossible to assemble.

woodchuck.jpg

[john banks]

The back 100mm of the table has the cable chain hitting the cross member of the rear lid hinge, and the front 200mm have this error. So I guess we have a 1250 not a 1280. I must get it sorted, just haven't had the time and nothing has forced it to the top of the pile to sort. Embarrassing for a perfectionist, but it is my wife's machine and I only go in her workshop about once a month so I forget about the issues!

[Rodne Gold]

We cut lots of circles and squares and oddly shaped badge backs on both our machines , using the whole table , I just measured some of the stuff we do and there is no appreciable dimension change at all , no matter where the piece was cut on the tables. I would also imagine belt slop or stretch would show up as misregistration at the start / stop point of a cut?

[George M. Perzel]

I am currently cutting 3 inch ornaments out of 30x30 inch BB sheets. I just checked dimensions of the 4 corner pieces and see no difference in size but noticed small difference in cutting depth at max XY corner (front left).
Best Regards,
George
Laserarts

[Greg Facer]

Do these Chinese lasers have enough accuracy and reproducibility to do multiple passes? I cut thick balsa parts for model airplanes and often make 4 passes to cut through a hard section running through the sheet. My ULS 25ER with a Testra Accelerator controller does this easily. And the size of the part doesn't change no matter where it is cut on the table. is this not the case with Chinese systems? If so, what applications can they be used for?

Kay

Repeatability isn't the problem, so multiple passes should be fine. but when you buy a chinese machine the cost increase for more power is much less than a ULS would be, so you should really likely only use one pass.

Specifically to Rodne's question: Since the errors seem to be repeatable, there isn't a problem with the cuts lining up.

[Greg Facer]

Hi Guys,

I finally got to installing the new belt today. Not the improvements I was hoping for. I haven't tested as extensively as the old belt (yet) but I have about .6mm variance on a bunch of 100mm cuts across the width of the table. I guess that is better than before, but not exactly great.

I then figured out a test to check the roundness of the gear wheel. The movement per rotation of the final drive cog is 72mm, so I made some 72mm cuts spaced 72mm apart. If they all worked out the same, I had my problem (not round wheel). But, nothing changed, still about the same variation from piece to piece, so that wasn't it.

Next step I guess is to check the alignment of the beam, as that's about the only other thing that might cause the issue (but I would expect the changes to be progressive across the table, and they are not),

After that I guess it's change out the stepper and then on to the controller. Also on my list, if nothing else works, to make a roller for taking up the slack in the belt that mounts to the bottom of the gantry and goes up scorpion style. That would help with any belt droop issues.

Another idea, I guess I could change the dip switches on the stepper drivers to have less steps per revolution and that would point to electrical issues if that helped (revising the step length in the machine as well). 5000 steps per revolution seems a little overkill anyways, and maybe it's too many pulses for the system?

This is very frustrating as I really can't think of ANY problem that could have a non-progressive error (ie something like -.35, -.25, -.6, exact size (ie too big with the kerf), -.35, -.25, moving left to right on the table) and be as repeatable. Any other ideas? 50hz versus 60hz electrical?

[Dan Hintz]

I then figured out a test to check the roundness of the gear wheel. The movement per rotation of the final drive cog is 72mm, so I made some 72mm cuts spaced 72mm apart. If they all worked out the same, I had my problem (not round wheel).

If you are testing for eccentricity in the cog, you need to test at more positions that one full revolution... you need to test at least the 90 degree marks of a full rotation, so make a line every 18mm. If any one of those lines comes up as more/less than 18mm, then you have an eccentric cog.

[Greg Facer]

If you are testing for eccentricity in the cog, you need to test at more positions that one full revolution... you need to test at least the 90 degree marks of a full rotation, so make a line every 18mm. If any one of those lines comes up as more/less than 18mm, then you have an eccentric cog.

True, but I was more checking to see if hitting the same spot on the cog (in this case 1 rotation, but the spacing made the pieces one further full rotation apart) caused the problem to disappear. If yes, that would say the problem was the cog, and I could investigate further as you describe.

[Greg Facer]

An update:

I have done the beam alignment and I think it helped a little. The beam was consistent before, not quite as centered as would be ideal and is now more centered on the final mirror.

The cut pattern I am now using is 72mm cut, 72mm gap, and repeat along the X axis. I still have the same problem, low in some areas and one or possibly two pieces that are too large. As I have previously mentioned, the 72mm spacing is one rotation of the cogs the drive the belt, 3 rotations of the stepper motor. So, any errors to on eccentricity should be eliminated in this test pattern.

The weird part is that the error seems to be good (or slighly high), undersized, good, undersized, good undersized. I don't have the exact numbers with me right now or I'd post. The difference between pieces on the low or high "set" is only about .2mm (or plus/minus .1mm I suppose one might say). The difference between the high spots and low spots are in the range of about .4 best case to .6mm worst case.

I believe the issue would be compounded if the shape was spanning two low areas and only one high, esp. if compared to another part that spanned 2 high areas and one low.....assuming that the problem is tied (somehow) to the placement on the machine or the belt. That is to say, I think it's about as bad as I first determined, but I haven't directly compared that test pattern.

Obviously, what is weird in the above is that it has an larger/smaller/larger pattern at all. Progressive from one side of the table to the other, or progressive to a point in the center of the table? I could see both happening. But, the L/S pattern was unexpected since I had taken steps to remove any effects of the stepper cogs or the reduction cogs.

Anyways, if that gives anyone ideas, please share!

I haven't tried the stepper drive reduction of # of steps per revolution yet, ie changing the dip switches, but that's the next thing to try I guess.

I think I'll do some more tests as well, maybe moving the origin over by 36mm or similar and see what happens.....reading my earlier posts, I'm not sure it had that same pattern.

[Rich Harman]

What is your step length set to? 6.000?

[Greg Facer]

What is your step length set to? 6.000?

4.8, my calculations are different than yours, although I can't remember where the difference was. Shenhui had it set to 4.7932 or something similar. Close, but not quite the mathematically correct answer.

They said themselves via email that they cut, measure, and adjust the value (I think the software has a function for exactly that process as well.

[Greg Facer]

Another update (no success yet)

I have tried 10000 steps per rev, and 1000 steps per rev......figuring out how to set the "Y" axis value in the meantime (I had set both drivers the same but didn't realized there was a "Y" setting separately). Exactly the same results as the regular settings with either change.

My next step I guess will be to change the control board to the spare I had, as changing the stepper is more work :-), then the stepper / stepper driver in turn....that will be for another night.

Oh, I also ran the program after downloading to the machine as I thought I might as well rule out a windows "hiccup" of some sort from the USB connection, but the same results again. I haven't been measuring tonight, just placing the pieces on end and comparing one set to another.

And just for kicks, and because I've been avoiding it, I tested the Y axis. Same issue (not really the up and down part), but to a lesser degree than the X.

[jordan matthai]

Hey Greg,
Did you ever get the issue fixed? I've been having the same issues and can't seem to come up with a solution.

[Greg Facer]

Hey Greg,
Did you ever get the issue fixed? I've been having the same issues and can't seem to come up with a solution.

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.