A Shenhui accuracy problem

[Greg Facer]

Hello all,

It's been a while since I've dropped in since I'm not doing a lot of the laser work myself these days, mainly on the CNC router. My signature explains why that suits me just fine.

Anyways, we've noticed that some identically drawn parts vary about 1.5mm from point to point on the table. Doing a quick test now of 300mm rectangles shows the X low side (ie opposite the X motor) is out 1.5mm and the other two are about the same (actual is about 2mm too small and .5mm too small, no kerf compensation done). I've read Rich's posts about the timing belt stretch and even saw the new belt in person when I saw his machine prior to purchase....but I can't quite wrap my head around a size that is a) too small and b) only in one part of the machine as a stretching issue. It seems to be more of a crappy belt mold issue (assuming they use molds to make belts?)

Anyways, rather than changing out the belt and pully, I was thinking of maybe getting a better belt to try first. But, I can't seem to find one on-line long enough or continuous. (looking a little more before posting) I think I see why Rich did the change out he did.

So, two questions: Any other ideas that might be causing the problem? and any idea where to get a 3MM HTD Timing belt in the 2800mm plus length? Or suggestions on alternative setups (like Rich's) and suppliers for that?

Also, I tried the math Rich did for my machine's set up, and came up with a value of 4.8 um per step, based on stepper having a 24 cog gear, meshing with a 72 cog gear and then a 24 cog gear of 3mm pitch. Stepper driver at 5000 steps per revolution. So, 72mm travelled for 15000 steps = 4.8 um per step, the software has 4.79639. But that only changed the theoretical result on my 300mm rectangle by about .22mm. So, I don't particularly think it's the settings.

As always, I appreciate the help. If I can return the favour, let me know!

[Dan Hintz]

I don't know about the Shenhuis, but on western systems there is a GUI setting for such a problem... basically it gives you a percentage of increase/decrease to X/Y runs. Normally it's set to 1.000, but setting it to 1.001 would give you a 1% increase in length of any run, so a 10" box that always comes up short by 10 mils will have the error removed.

Anything like that?

[Greg Facer]

I don't know about the Shenhuis, but on western systems there is a GUI setting for such a problem... basically it gives you a percentage of increase/decrease to X/Y runs. Normally it's set to 1.000, but setting it to 1.001 would give you a 1% increase in length of any run, so a 10" box that always comes up short by 10 mils will have the error removed.

Anything like that?

There is, sort of, in the 1 step = x um value. Change that and it will be adjusted and that's what the factory must have done as the math puts the value at 4.8 and the current value is slightly off that. But, changing that doesn't solve the issue I have, which is specific to areas of the table (or areas across the length of the gantry anyways).

I have at least wrapped my head around the facts that a) belt stretch could make a part shorter rather than longer, and b) the belt stretch symptoms could in fact be worse if the bearing is further from the motor. I just wish I actually saw a problem that could account for the amount of error, 1.5mm is a lot.

[Dave Sheldrake]

Crab gantry Greg?

What are the corner angles like? 1.5mm is half pitch so if the belt jumped 1 tooth one side you would be 1.5mm out (roughly)

just a thought

cheers

Dave

[Greg Facer]

I haven't even really looked at the y axis with re two belts, this is just on the x across (or along) the gantry. So one tooth skip would equal 3mm and not too likely to repeat easily either.

[Allen Rawley]

Hello,

Not sure of the problem exactly but perhaps this might assist in debugging.

If it appears like a belt stretch, then it may be tested by reducing the acceleration (setup parameters) and velocity (job parameters). Different acceleration and speed should provide different results with a stretching or skipping issue.

Is the table square? In visiting many factories, including Shenhui, I see the workers eye-balling the part alignment then drilling holes. Measure the distance between the linear rails for the Y axis. For the x axis, it may be skewed. Techno has a pattern that they run to check for table alignment, it looks like a circle in a box and has cross-hairs in it. You could get that pattern, run it on the Shenhui, then have it diagnosed by Techno if you are unable to determine what is going on.

Sometimes the stepper motor can have a problem. Stick you hand on it while the X axis is moving. Do you feel vibration? (I have a bad motor on a new machine right now and it is making a noise). Will replace it before shipping in the next few weeks.

Regarding other issues, is this a test using your laser? That is a rather large table and the laser beam may not be aligned. Take a piece of acrylic and put it before the final beam bender and do a raw beam shot into it, at the four corners of the table. If the beam is moving around then that can contribute to the accuracy problems.

[Khalid Nazim]

I have exactly the same problem - a rectangular box changes its dimensions at different positions on the tabel. I have tried all adjustments in software and tightening belts etc to no avail. This is really frustrating as I cannot get predictable results from the machine.

I know that this does not help but you are not alone in having this problem.

Rodne/George/Rich - Have you observed this problem? Can you cut a rectangle at different locations on the table and see if all them measure the same.

I would be happy if all them have the exact same measurement even if the measurements are different than what the graphic has as I can then use compensation. I cannot compensate for different measurement at different positions on the table.

Regards
Khalid

[Rich Harman]

I have noticed the problem and have written about it before. I replaced the X axis belt and enjoyed a significant improvement - but the problem did not disappear. I now run my belts with low tension which seems to help. My theory is that tightening the belt stretches it unevenly. When you have a cheap belt to start with then the effect is more pronounced. I think the tendency to tighten the belts makes the situation worse and is irreversible. Also be sure that the X and Y axis are perpendicular, when they put the machines together they do not seem to care much about getting things lined up perfectly.

I have considered a couple of other solutions but the reality is that for the work I do, a small fraction of a millimeter variation does not make a difference. The first is to put a stepper motor on each end of the gantry driving the same belt - this should at least halve the errors, but I would expect the errors to be nearly eliminated. The second is to use a ball screw. There are some light duty ones that should work well for cutting but they would not do well for rastering at high speeds - too long of an unsupported shaft. The last is to use an encoder. Not on the motor but a linear encoder on the gantry. It would require replacing the stepper drivers with ones that support closed loop.

You can get open ended belts from polytechdesign.com - http://shop.polybelt.com/15-3M-Open-...B15-3M-MPS.htm

[Greg Facer]

I have noticed the problem and have written about it before. I replaced the X axis belt and enjoyed a significant improvement - but the problem did not disappear. I now run my belts with low tension which seems to help. My theory is that tightening the belt stretches it unevenly. When you have a cheap belt to start with then the effect is more pronounced. I think the tendency to tighten the belts makes the situation worse and is irreversible. Also be sure that the X and Y axis are perpendicular, when they put the machines together they do not seem to care much about getting things lined up perfectly.

I have considered a couple of other solutions but the reality is that for the work I do, a small fraction of a millimeter variation does not make a difference. The first is to put a stepper motor on each end of the gantry driving the same belt - this should at least halve the errors, but I would expect the errors to be nearly eliminated. The second is to use a ball screw. There are some light duty ones that should work well for cutting but they would not do well for rastering at high speeds - too long of an unsupported shaft. The last is to use an encoder. Not on the motor but a linear encoder on the gantry. It would require replacing the stepper drivers with ones that support closed loop.

You can get open ended belts from polytechdesign.com - http://shop.polybelt.com/15-3M-Open-...B15-3M-MPS.htm

Thanks for the link Rich, especially knowing you went with a different belt geometry!. I am not quite ready to pull the trigger, but just ran some thicker pieces that happened to be close to my earlier test, with the same results. I realized that my next step should be to dived the 300mm test area that had the error into 3 100mm areas and see if I can narrow the problem further.....and then hopefully find the defect on the belt or some other clue that gives me something concrete to fix.

Greg

[Greg Facer]

Yep, Rich is right, wonky belt.

Rather than 3 rectangles 300mm wide, I made 9 rectangles 100mm wide. Results vary across all the pieces, only about 3 were within .1 mm of what I am guessing was "right"....keeping in mind the kerf is no accounted for and the math works out that they all should be short about .08mm anyways. The X low (beam) side was consistently low on all three pieces, so that's why I noticed the issue there, but it wasn't a completely progressive issue from one side to the other.

So, I will try Rich's belt source and suggestions of low tension (how low are we talking about?) and correct the math issues and see what happens.

Thanks everyone, I will report back when I have installed the new stuff.
Greg

[Rich Harman]

If you purchased spare belts with your machine I would try that first - but tension them only enough to take the slack out.

[Rodne Gold]

I have not noticed anything untoward , albeit I dont use the Shenuis for anything requiring major precision.
I dont think the belts the chinese lasers use are great tho , nothing like kevlar reinforced stuff etc.
I havent replaced any and not sure at what tension they should be , we just flog the machine on a daily basis and it seems to work...

[Kay Bengtson]

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

[john banks]

Closer to the front of the machine, the more stretched things get in our y axis. It is noticeable on some jobs where parts cut in different areas of the table are overlaid. At the time I just changed the artwork, but did think it was related to Rich's previous descriptions of belt problems.

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

[Khalid Nazim]

So this issue of having small differences in dimensions of cut parts at different locations on the bed only visible in Chinese machines or this issue is there on Trotec/ULS/Epilog as well?

Would a rectangle cut at different locations on a large Trotec/ULS/Epilog bed be exactly the same size or would be out by 1 or 2mm? I am trying to understand if this accuracy issue is in all lasers or is it only with the chinese machines.

Regards
Khalid