Callibration of New Machine

That usually indicates one of your axes is slipping.

Screw or belt?

Its a belt Zootalaws.

I ran it extreamly slowly ( limited the feed to to about 10mm per min and limited the seek to 100mm per min) same result.

Another reason I think its not a slip because the "error" is exactly the same every time. A slip should show up as a step on one round.

I also made an Astrix with the Jog Buttons at full acceleration 50000mm/min and I get perfect repeatability with a 500mm Astrix still goes perfectly to the middle everytime. I thought this would prove no slippage? (BTW I am testing this by replacing the tool with a pencil attachment.)

With the current setting I have about .1mm accuracy (hope to improve it by adding micro stepping). Would this be a possibility? if my CAM is assuming microsteps then I will lose accuracy consistantly as the motors wont step? I am using CNC.js as my CAM program.

Wish I had a way to show you the images.

Thanks CMCGRATH5035. Here are my files:
CamGTextJpgs

The Cam.jpg shows the original file in CAM.js.

Then I have two examples of the affected prints.

There is an obvious X scaling issue. But you can see the italic type effect and the error between passes.

I checked again the tightness as you suggest. but it is solid and the pencil is wedged in very tight. I cant see the issue as a slip though. it is too repeated. consistantly.

How are you zeroing the machine? It might be that when it returns to the home position, it mechanically hits an end stop or obstruction before it hits a limit switch, even only by a tiny amount, which causes the stepper motors to lose a few steps in a predictable manner each time. Essentially, as an example, you could go 1000 steps right, 1000 steps left again, but after 998 steps you end up hitting something that causes you stop moving and lose those last two steps.That might only be a rough spot on the carriage mechanism if the torque is too low.

The end result would be each time you repeated the process, you'd move another two steps further out, giving something not dissimilar to what you're seeing.

I've had problems on a laser cutter that turned out to be something a little like that caused by some detritus on the rail which acted like a one way bearing, so the carriage would go over it in one direction without noticing and momentarily stall in a fairly consistent manner in the other.

Just a thought.

As an update to my post, I would suggest you create a gcode file by hand which is just a series of back and forth moves on one axis and accurately measure how far it goes in both directions over a number of repetitions at different speeds and distances. Check it's doing the same travel both ways, and very carefully observe your pulleys, stepper motor shafts, and any mechanical connections for unwanted movement. Do each axis in turn, then try diagonal movements too.

Hope this helps :)

Thanks PCA but I dont think thats it. I dont have end stops. I just set the machine home in the middle of the bed so it never goes near the end stops.

Also this error happens while printing. the depth is 3 mm and I set the steps to 1mm so the system passes three times to get to the depth. this is always offset.

Still puzzled:-/

If you don't have end stops, I suspect that may be your problem right there. Without some method to hard-zero it at some point, if it DOES drift for whatever reason, there's no way to correct for that. It would only take a small amount too little available torque for it to lose a step here and there and end up all over the place. If it rezeros ever time it homes, that's going to be minimized or eliminated.

Another thought strikes me as well; if you have a variable tightness on your belts, caused, for example, by one of the pulleys not quite being round, or the drive hole not being square to the center of the thing, and your torque is a little too low combined with the cutting forces, you can end up with missed steps from that. Each time it goes around you could lose a step or two, and it would add up to a considerable offset which would tend to cause similar results.

Whatever it turns out to be, I wouldn't be surprised to learn it was purely mechanical. Some of those sorts of problem can be very subtle.

Good luck with it in any case!

Cool,
I will try the repeating Gcode. I will write it myself. At least I know that is correct, eliminating an unknown. Good practical advice, thanks pca. Will let you know my results..
Matthew

I understand that you say nothing’s slipping, but the whole thing about CNC controllers is repeatability - if you are seeing cyclical differences, it’s nearly always something with a dodgy configuration, mechanical or electrical. If not mechanical, you are missing steps.

As you’ve confirmed that the drive system is solid, all that’s left is to check software configuration, electrical connections and interference.

If it was me...

Always start from a factory reset. Save your current settings, reset, then do no more than ensure your axis settings are right. Turn your stepper pots down to 8 o-clock.

Rather than using generated Gcode, just issue the commands you need, then you know nothing else is being introduced:

Tape a digital caliber to your y-axis
Pull the caliber wide open and nudge the carriage up to the caliber end and zero the caliper and axes.

G28.3 X0 Y0 // zero x & y

Issue a slow move command like G1 F500 Y50 then G1 F500 Y-50 and repeat a few times.

It should repeat with no variance

Your caliper should read 50mm - if not, your calibration is out.

Do the same for your X-axis.

I always use digital calipers for calibration - easy and accurate

thanks zootalaws. I like your process. I will try that. As an update, I did do a repeatability test with hand coded GCode. first I used a g0 based command

G0 x10.0 y10.0
g0 x10.0 y50.0
g0 x50.0 y50.0
g0 x50.0 y10.0
g0 x10.0 y10.0
g0 x0.0 y0.0

I copy and pasted this 30 times in the file to repeat it over and over with my pencil jig

The machine repeated the pattern perfectly with no slips.

Then I did the same test except changed all to g1 with f40

This was slow. BUT. I was seeing slips in my x axis (gantry). I did notice the torque was much reduced at this slow rate. so I am suspecting my stepper controller.

By the way the machine is clean. I removed all dust and grit from the drive system.

I will try your test,do it, to see If I can get any more clues

Getting close to the truth I think! Thanks guys.

No, I meant $defa=1

In my experience, when you get weird results, it’s a good idea to go back to defaults and try with basic settings applied (motor def, direction, etc.).

I just json it from a stock file.