Trouble iwith concave models

[chemstock]

I have cut a number of hollow forms and concave models and am finding the edge of the model to be quite rough. It appears that the roughing pass is leaving some remnants that the fine pass is not finishing.
For Concave work, normally I set the boundary offset to zero and the machining allowance to zero, when I set the zero for the tool, I set it once at the table height then raise the tool 1.5mm and reset the tool to a new zero. This raises the rough model 1.5mm. above the model. Should be identical to the machining allowance to zero, When I use the machining allowance, I get a warning about the calculations, this is just my way of bypassing. I think the issue is not in the Z direction but in the XY direction ;the rough pass is cutting outside the model and the vectors. I am not sure.
On the attachments; The file ROUGH is a screen image of the rough model. The roughness I am speaking about is shown on the inside right hand of the model. File 20201126_xx is a photo of the model after the fine pass is made that shows the rough edge similar to the model. Using a 1/4" bull nose for the rough @40% stepover and an 1/8" tapered bit for the fine @ 10% stepover
When I do convex models or I cut off the edge, it is not a problem. Here I need the material outside the model. I thought of doing a profile pass to remove the roughness but I am not sure where the rough edge really is inside the model. Cannot cut too much material off as it would affect the finished product.

[adze_cnc]

You might want to re-read your post and clarify as I think you have swapped the terms convex and concave somewhere in there and it's quite confusing.

On the roughing pass why set the "Machining Allowance" to 0 but then mess around with the z-zero on the machine to 1.5mm higher than it should be? Why not just set the "Machining Allowance" to 1.5mm?

Every other time I've seen someone post that this is what they do I've never seen the point. Let the software do all the calculations is my motto. Less chance for error in the long run.

[scotttarnor]

Zero plane?

[Adrian]

I don't follow why you're messing about with z-zero and machining value allowances at all. Just do everything as normal.

Boundary offset should be at least the radius of the tool in my experience to allow the tool to properly cut the edge as the toolpath is calculated from the centre of the bit. Finally, with any concave model, use a zero plane to avoid artefact machining around the lip of the model.

[chemstock]

My Laguna has a touch off tool that I found to be unreliable so I started doing my own Z zero. After running some tests I found the rough cutting too deep even with an allowance set with the software.
Regardless I will use a zero plane to see if that helps. Use it alot on rotary, not used it in xyz.

[chemstock]

I added a zero plane and placed it at the top of the model tree. Found no difference when I ran the model, when I added a base height equal to the depth of my model, I found the remnants removed. Is this what you were talking about?

As an aside on setting the Z Zero; (should be a separate thread). I created a flat shape between two concentric circles and set the tool pass depth so the roughing pass would be complete in 1 pass. Ran a 0 machine allowance and a 2.54mm allowance and compared the generated toolpaths. The difference in Z was not the machining allowance. close but since this is a computer I would have expected exact.