Retract height too high?

[stiller7563K]

I'm experimenting with lithophanes. I used an online lithophane converter (itslitho.com) to make my STL file. I've imported that into Fusion 360 without issue. It's the correct size (apprx 75x75mm, 3mm thick) for my acrylic stock. I'm using parallel 3D milling, and my outputted gcode looks great in my sender (using bCNC). The only trouble is that when I run the program, my mill immediately retracts so high, it trips the Z safety switch every time.

I can't imagine what I did differently today, because yesterday's test runs worked fine. I learned then, I'd like to try with a tighter step-over. Today, no matter what I do, I can't run a single program because of the retract height. In Fusion 360, my heights are configured relative to my 3mm stock height, and I've set my retract height only 12mm above my stock, which is well within the limits of my machine. Even if I set it to only 5mm above my stock, this problem persists.

Any thoughts?

[daniel_lyall]

That is becouse it is going home first/ (machine zero) make sure you home the machine. G28, G30, and G53 are the go to home calls/machine zero

[stiller7563K]

daniel_lyall, thanks for the insight! That sounded about right, too, but
when I tried again-this time homing first-the same Z trigger happened, as
before.

I feel like there *must* be something wrong with my gcode, but given the
configuration of my heights in Fusion, I can't imagine what. In my Fusion
file, the total height of my body (the only body in the doc) is no more than
3mm. Is there place to limit the value of my default retract height?

[stiller7563K]

Here's a screenshot of my gcode visualized by the sender.  Right at the beginning, it makes this huge leap to the lithophane's lower right corner.  (There's another huge vertical line in the upper right corner.)  These two spots seem to be the culprit, and I don't mind revising the gcode manually if that will fix it, but I don't see where the Z axis is being sent especially high.

[johnswetz1982 · Accepted solution]

Right at the beginning like the previous poster said,

G28 G91 Z.0

See if your post processor has the option to use retract plane instead of G28, or G53

[stiller7563K]

johnswetz1982,

Thank you for the tip! I wasn't sure where to make this change at first, but finally located it in the Post Process dialog box for my project. In my case, I got there by getting into Manufacture view, right-clicking on my Parallel setup for this bit, and choosing Post Process. When that dialog box opened, I saw a Safe Retracts menu box on the right side, in the General section. My three choices were G28, G53, and Clearance Height. Choosing that last one fixed this for me. Its tooltip explains it as "'Clearance Height' retracts to the operation clearance height," which was the clearance height I had set in my file.  To me, that's the intuitive meaning of "clearance height," but I clearly had it set wrong with my previous configuration, which was indeed G28 and was the default.

Thanks again, to you and to daniel_lyall.