Anyone else have pulleys go out-of-round?

[a542002]

hi there
I had the same probleme but I only converted a K40 chinese machine to the DSP system. this probleme did rive me crazy. I took out the whole mechanical system amny times and titen the belt many times till I finally drille 2" holes in the backsite to be aible to adjust the belt from the outside without taken the whole system out and afterwards re aligne the optic again.
the probleme with the inacruacy was that one of the pulleys had only one bearing in it what did make it wobbling under load so I did put a second bearing in
( bytheway out of a old computer PS fan) and now I dont have any more problemes
maybe this helps
greetings
walt

I'm not buying the backlash thing. First, I've never experienced this much of a distortion, and I've been running the machine for a few months now. It only started happening after the move. Additionally, if it was backlash, it wouldn't happen over as large a distance as it's happening. It would happen at the changes of direction, or the cardinal directions of the circle, as the carriage changed directions. This is happening at approximately 2 and 7 o'clock, far past where it should be affected by backlash.

Just to make sure, I went ahead and fired the machine up and tried to move the carriage in both the x and y directions. It was rock solid in both directions. I suspected it would be, as there isn't anything really flexible in regards to the belts, and the expected backlash should be a result of the belt teeth meshing with the gear teeth, which should be fractions of a mm at best.

[mikegrundvig]

I can only add that I've seen accuracy problems produce lobes in weird places on my little Chinese mill conversion. There are a variety of things that can go wrong but something I ran into (and why I don't like set-screws when I can use clamps instead) is where my lovejoy couplers worked slightly loose on the motor shaft. The set screw wasn't tight enough and so they "rocked" a bit and this looked like step-loss at times and backlash at others. To diagnose your problem, I suggest starting at the endpoint of the carriage and work backwards on every moving part - checking for tightness and wobble. Rhythmic noise is a dead giveaway too. Bearings and belts should be very quiet at the scale we are using. I'd also try cutting other shapes and sizes to see if it's always repeatable. It should be consistently visible in multiple things if it's a mechanical problem. Squares, circles, octagons, etc. Make them small and large to get a good feel for accuracy issues and if they always occur at the same spot in a curve or edge. For instance, a big French curve cut vector should also show the same issue as it will pass some of the same arcs but it's going to use different art and code. You could use your computer printer to print the image and then cut it in paper on the laser and hold them up together to see how close they are.

Also, if you have a plunge-style dial indicator you can very easily test for backlash using the same process we use for milling machines. Attach the indicator to the side of the table somewhere so the head can touch it. Jog the head of the laser cutter so it is pushing the plunger on the indicator a good bit - call it 1/2" or so. Either note where the needle is on the indicator very accurately or just re-position the face so it's on 0. Now have the computer move the head away from the indicator for something like 1/4" or so. The indicator should show EXACTLY 1/4" movement. If you see less, then you have backlash, some mechanical problem, or are losing steps. The amount of backlash will be how much less than 1/4" it moved. The one gotcha for this technique is that when you are first moving the head into the plunger, you can't reverse direction. Every time you reverse direction, you are taking up the slack. So you have to get it to it's final position for testing by moving it only in one direction once you get close. This is a little hard to describe but should be obvious once you think about it a bit. This technique is the "tried and true" way to measure backlash for all axis on a mill, I've used it many times to dial in backlash compensation for Mach3. Good luck!

-Mike

[naPS]

It looks like r691175002 was correct - it was *mostly* backlash! I'm shocked on this one!

The original pulley I had on there was out of round slightly, and replacing it made a pretty big difference in where the errors were occurring. But, even after replacing it, I was still getting some pretty decent wobble in my circles, and it was always flat spots on in the Y-direction, indicating problems with the X-axis not moving as much as it should.

I squared and re-squared. Checked and tightened everything. As a last resort, I said fuqit, and tightened the bejeezus out of the X-axis belt. I mean, it's so tight now I can almost pluck it like a guitar string. And guess what? No wobble whatsoever. My circles are perfect circles again, like they were before. I'm wondering if the belt that I have is past its useful life, and it's time to replace it, as I definitely didn't have to have it this tight before.

The other thing I did was flip which side of the carriage I was putting the belt through. Coming through on the top and folding it downwards allows the teeth to wrap-around the width of the carriage, and the connection there seems a lot more solid than it did before.

Thanks r691175002 - I'm glad I was wrong on this one!

[educa]

Can I ask what is the standard width and thickness of the belts you use on 2.x machines?

In my build I'll be using 2,54mm (1/10") thick Polyurethane steel enforced belt with a width of 10mm (about 3/8") on aluminium pulleys and thats about the only thing I think is strong enough to give good results in the end.

It all depends of course of the weight of your laser head, but in my opinion, this is certainly the most important mechanical part for correct operation of the machine.

[mikegrundvig]

I'm using 3/8" wide urethane belting with Kevlar-reinforcement. It's pretty cheap so if it wears out I'll just replace it. I got it from McMaster SKU #1840K101

Where did you get your metal pulleys? They are nice!

Oops, ignore that - I see I can get my same pulleys in metal from SDP-SI as well. Maybe I will do that as I'm not that happy with the nylon ones.

-Mike