Problem with job repeatability

LightBurn Hardware Compatibility
1

Hey guys, I have an Acmer P3 48W diode laser. I’m having a problem where after homing (with limit switches) and running a job straight from that position, the lasers moves to the top middle of the bed after completing the job (which is fine I like it). However, if I then try and repeat that job it’s ever so slightly misaligned.

The weird thing is - is that if I move the laser to any random position (via the move panel) after initially homing and then run a job, I can repeat it accurately and consistently from the end position.

2

Just for clarification, you home the machine then move the laser anywhere away from home then run the job. After the job finishes the laser goes to the top middle like before, but this time repaetability is correct?

Are you using absolute coordinates or user origin?

Does Lightburn report the positioning correctly in the move window? Does the end position read exactly the same no matter which way you run the job? If you home the laser before starting the repeat does it work correctly?

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3

Thanks for the reply Tim. Yes, you’re understanding correctly.

I am using absolute coordinates.

I’ll check to see what coordinates Lightburn is reporting, I’ll check that for you shortly.

I’m sure if I home before a repeat it is then missaligned. I’ll will triple check this though.

4

A quick test to verify the basics:

  1. Put some scrap material at the Home position
  2. Home the laser
  3. Fire a test pulse to mark the material
  4. Jog away from the Home position
  5. Repeat Steps 2-5 maybe five times without moving the material

If all the pulses go through the same hole, then the Home position is repeatable and the problem lies elsewhere.

If not, then there’s a (likely mechanical) problem with homing.

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5

When you run a job and it goes to a finish position, can you move the laser by actually pulling on it?

6

Yes, the dot is in the same position, as far as I can tell.

I’ve just engraved a circle, re homed, and ran it again and it was accurate. So annoyingly the issue seems to be temperamental - isn’t that always the way!

I sort of hate issues that seem to fix themselves like this.

7

Sort of feels like it has a little bit of travel on the Y axis, almost like it it’s inbetween steps - so if I wiggle it it wants to settle slightly north of where it is, or slightly south.

8

Cannot tell from your description, but it sounds like the controller is powering down the stepper motors. If this is true, cable tension could pill the gantry or trolley a bit.

$1=255 may keep the steppers powered up. Any other value is the shut-off delay on milliseconds.

9

I’ve just noticed, that if I’m framing something constantly, it can drift a little as it goes through the passes. Could this be symptom of the same thing that you’re saying?

10

This is an indication of play in your machine. To verify, scale this file proportionately to fit your laser bed and run it at high speed with optomizations turned off at just enough power to get a good mark on cardboard. Post a photo of the results so we can confer with you.
BacklashTest.lbrn2 (68.8 KB)

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11

Amazing thank you, how big should I scale the graphic in regards to my laser bed? Does it need to be big or does it not matter too much?

12

Big because you want it to reach max speeds.

13

IMG_0809
IMG_08091872×3658 623 KB

So the top square is ran straight after homing and then again from the end position (top middle)

The bottom square was ran from the end position and then again from the end position. As you can see it’s perfectly aligned.

Weird.

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14

The mismatched squares after homing tell you that the Home position is not repeatable, perhaps due to a cable snagging or another mechanical issue.

The overlaid squares from the End position tell you that the Home position mechanical problem doesn’t happen elsewhere.

You must find and fix the mechanical issue affecting the Home position.

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15

Running a job from home, re-homing and then running again is also accurate through! Which means that homing is repeatable.

Thank you for responding - I really appreciate your guys help.

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16

It would seem that setting $1 to 255 has resolved this issue. Is there any downside to doing this?

I’m assuming running an hour long job is just as stressful on the motors as keeping them locked for an hour, right? I read somewhere about heat concerns.

17

I would say more stressful than just a holding current. But I am not a Laser Engineer.

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18

Rant ahead!

I would like to know why any machine works better with this value other than 255.
It keeps raising its ugly head way too often. Something as fundamental as this should be the default setting.
There must be reason other than “we don’t want any body have repeatable positioning”

There should be a sticky on this

It must drive people mad with blaming it on mechanical problems.

Again it must be there for a good and important reason for it that I don’t know about
Rant concluded.

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19

I’ve been wondering about this myself the past few days. I don’t know why this wouldn’t just be the default even just as preventative measure. I don’t want the machine to be losing micro steps in between programs.

20

It is a decision made by the Chinese manufacturers. In a machine that is field assembled, we do not always assemble and cable it as designed. Where do we place the blame I am not sure.

The advantage is that when this becomes an issue, you know your machine better, and that is always a good thing. Just be grateful we know the solution.

The thinking behind that parameter is that it allows a cool down period, especially for larger stepper motors. Basically hold position until the machine quits shaking, then cut the power. Just be aware it is your responsibility to ensure your motors are not overheating. I will not state how much of a possibility that might be. :roll_eyes:

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