2D Contour Maximum Stepdown confusion

[billostratum]

I'm using a compression bit and need to step down to ensure the first cut is a down cut.

In the example shown (also uploaded) I have 20mm stock. I find that if I have maximum stepdown of 14mm, the two steps work as I expected. However, if I change the stepdown to 15 mm, I get "even" stepdowns instead of a 15mm step followed by a 5mm step.

Is there any explanation for why that is?

I know I can get the effect I want by working from the bottom with a 5mm finish stepdown, but it seems like I shouldn't have to do that calculation.

[programming2C78B]

you're right, that is the only difference. You can get around this by using a Finish Stepdown of 4mm. This will cut at -14 and then -18.

[matty.fuller]

I'm very curious about this behaviour too as it has stitched me up a few times on what I thought was a reliable template for a compression tool.

Looking at Compare & Edit in your file the only parameters that differ are the max roughing stepdown and max ramp stepdown. I played around with enabling ramping and entering different values, nothing I changed had any effect on the passes being generated evenly rather than taking the full depth of cut on the first pass.

[billostratum]

I've found that 14.4mm works, but 14.401 does not; 14.4 is exactly 80% of the desired full depth-of-cut.

It is almost like Fusion has an internal rule that says if your Maximum Roughing Stepdown is greater than 80% of the total, you didn't really mean that, and then automatically does Even stepdowns as a courtesy.

There must be a more logical explanation, with perhaps a workaround (other than converting to a Finishing Stepdown, as I pointed out in my original post).

Bill

[oleg.tikhomirov · Accepted solution]

Hi @billostratum,

You are almost right. Fusion tries to avoid small stepdown at the end so it might make two last z stepdowns equal. It does not make all other stepdowns even.

There was very similar question some time ago: Solved: 2D pocket stepdown failure - Autodesk Community - Fusion 360

[billostratum]

It seems more like a bug in that I can't use a compression bit without some unnecessary, and potentially error-prone arithmetic to calculate from the bottom instead of from the top. 

[billostratum]

After struggling with this when varying stock thicknesses for different models, I realized the easiest solution is to simply create two separate contour operations: 1) for the high part of the contour that does a one-pass cut at the proper depth for the compression bit, and 2) another to complete to full depth using multiple passes (starting at a top corresponding to the compression bit.