Closed shape multiple passes does not cut on same path

Hi all,

Please bear with me, I’ve only been doing this for about 3 months or so so I may be asking a very basic question.

I am carving out the shapes of Buffaloes and I’ve done this patterns hundreds of times. It takes 3 passes for a clean cut for these guys. The line up is 5 on the top row and 5 on the bottom row.

When I cut them, pass 1,2, and 3 are visible shifted on the x axis only on my stock and not cutting in the same path. If I press stop and rehome, and press start again, the first path will follow the previous path 1. To make things even more complicated, I completed about 30 of these things last night before I went to bed. First thing I did this morning was this and cannot complete a single cut.

Steps I took to troubleshoot:

  1. Realigned laser (I don’t believe this will help but wanted to eliminate it)
  2. Greased the x and y rails
  3. Cleaned the mirrors and lenses
  4. Tightened the belt on the gantry
  5. With the power off I moved both the gantry and the laser head by hand. I noticed some squeaking coming from near the stepper motors but I believe it’s the sound of the belt. There is no jerking motion or resistance.
  6. Ensured stock was secure with magnets
  7. Test pausing the job in the middle of passes. Stopped job and started job over, (no shift of the path). 8. Performed a 1 cut operation with 3 different program executions, working as expected.

Does anyone have any idea of what troubleshooting I can try next?

Thanks a lot!

The most common causes of this are:

  • Moving too fast
  • Accelerating too fast

The problem is that “too fast” can depend on what you’re cutting. If you are cutting items that are mostly long, continuous cuts, the motion of the laser will be quite stable, with brief spots in between where the motion jerks a little. If you are cutting designs where you are moving quickly through lots of very small cuts with rapid direction changes, you can cause resonance, eventually making the motors lose position.

Resonance is like feedback in a microphone - The speaker produces a sound, the microphone picks it up and sends it back to the speaker. If they’re “in sync”, it builds up and will eventually do damage. The laser can be the same - if you do lots of rapid vertical moves, the gantry will vibrate a little. If you happen to catch that vibration at just the right spot and move in the opposite direction of it, you end up reinforcing the vibration, adding more, and if it happens enough in a short period of time, it can cause the system to slip and lose position.

The best ways to combat this are go slower and/or reduce your acceleration, or add small delays to the start or end of each cut, giving the motion system time to settle a bit. Even adding 1/10th of a second (100ms) to the start of each cut will not add a great deal of time to the job, but will help prevent resonance buildup.

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This makes perfect sense to me. Thank you, Oz! I’ll try this as soon as I get home and update you but I guarantee you are correct. Thanks again! Brandon

I tried what you said… here are the outcomes:

  1. Decreased only speed by ~66%. I dropped the speed from 1.3 in/s to .5. - All cuts passed except a very large circle did not start and end exactly at the same spot
  2. Added only a delay of .2 seconds - No cuts passed but the skipping decreased by what I perceive to be 80-90%
  3. Decreased speed and added a delay. - All cuts passed with no visible difference between pass 1 and 3. The large circle ended at the same place.

One last question, do you believe I may have caused any permanent damage to parts in my machine? If so, what would I look for in the future?

For #2 - what do you mean by “no cuts passed”? If the skipping decreased by 80% to 90%, I would expect some of them to work.

I doubt you’ve caused any permanent damage - steppers will slip position under the right circumstances. I would try lowering the acceleration settings a little, and lowering the idle acceleration (used when moving between cuts). What are they set to now? (and how large is your gantry?)

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