Fusion 360 random depth changes

I just got my machine ready to cut things. I was never able to complete auto calibration, but on certain calibration files my machine was only 1 mm off, so im just going with it.

I cut out the shape of an animal that was about 12 inches by 8 inches with no issues. Then i tried up sizing that to a 22 inch cutout. The shape was supposed to be cut out in .15 max depth changes through a 3/4 inch birch ply. During the first pass, the bit kept moving down into the wood at slow increments untill it was over -.33 into the ply by the end of the first pass. Then the bit reset to -.15 and proceeded to do the same depth changes, i stopped the cut when it was going past -.45 in on the second pass. I was just watching the z coordinate drop every few seconds while it did the contour.

I have re-generated the g code for this multiple times with the same results the gcode size is 626kb. and is the grbl code.

Let me know what you think. I have watched the two great videos i found on this forum for fusion 360 and i hope i am just missing somthing stupid because it is such a nice program to work with.

What versions of firmware and GroundControl are you using? Was the z-axis motor turning to make the bit lower? (I ask because I’ve found that when my router isn’t clamped tight enough in the base, the bungee cords I rigged to maintain down pressure can pop it past the cog which the z-axis motor drives, sort of as you describe…) Are you using the default PID settings?

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I recommend using Fusion360’s simulator to ensure that the expected movements are what/where you want them before moving on to MaslowCNC and actually machining some material. Its a great point in the process to catch small (but significant) errors prior to cutting any materials, and there’s enough fiddly little bits in Fusion360, its great that it can simulate the milling process as a sanity check prior to actually sending it out to MaslowCNC.

good luck!

  1. newest versions of gc, and firmware. The z axis was turning, and ground control z axis was moving as well. 2) I took the same .svg and ran it through makerCAM, and it cut perfectly, only changing depths at the beginning and at the tabs. 3) the simulator on fusion ran perfectly as well, parallel cuts at .15 max.

@Arushur What post processor was used to export the G-Code. Can we take a look at the code changing depths? Zip it if it doesn’t want to upload. How did you upsize?

Bison25in.nc (625.9 KB)

Here is the gcode. The post processor was grbl in fusion360. I just took a look at the code and it seems right. I upsized by scaling the .svg when i imported it.

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Z looks ok in the code. Might be totally unrelated, but I’m just wondering how GC deals with the several lines without a “G” and some with only X or only Y value. The file runs on GC in my office with only a Mega attached and Z is flickering at start between -0.11 and -0.15 and moves its way up (out of the material) as the cut continues, so the opposite of what you have noticed. Again likely not related, as without a motor shield, I am perhaps totally confusing GC because it receives zero feedback.

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Not sure either, but I notice that the on-screen view doesn’t handle lines with only a Y properly - I noticed that on line 1081 the cursor jumps to X0 (love that Goto button!).

@Gero, are you stepping through the file? How are seeing the Z (or X? and Y?) values? :slight_smile: