I wanted to know what are reasonable stepover values for various endmill sizes in a scallop toolpath, so I created a quick test piece. I did get my stepover values (marked in red on the first photo), but I was surprised to see artifacts on the surface. I do not understand what causes them, so I thought I’d ask for advice.
There are two kinds of artifacts:
The hourglass shape seen when looking from the top (especially visible in A1 to A3 and B1).
“Veins” on the steeper part of the slope, which are strangely not visible on every slope.
I do not have Fusion 360 but it looks like you are using a constant step over value. Look for a setting that allows for you to step over based on scallop height. If Fusion 360 has this setting, set it to something like .0003".
@jwr The hourglass artifact you’re seeing is due to the consistent 90deg turns in the toolpath. I would try a Morphed Spiral toolpath instead. Scallop operations sometimes end up with odd paths.
From straight above:
@neilferreri Thanks for the morphed spiral suggestion, that might indeed improve the finish, and I’ll try that.
As for the “veins”, I noticed that they occur only on descending slopes, never on ascending, which makes me suspect they have something to do with the mechanics of my Nomad 3’s Z axis. I did lubricate the rails and screws recently, though.
My conclusion, after testing various combinations of scallop, morphed spiral, and parallel passes, is that a combination of scallop and parallel is the way to go. Here is an example of what this produces (look at the top of the keycap):
One thing to note. When doing scalloped ball mill cuts, I try to use the largest ball mill that will fit into the part details. This allows a bigger stepover for a given scallop height, or using a much smaller scallop height without increasing the cut time to something unreasonable. The larger tools are also more rigid & stable so result in better finishes.
Yes, I had the same thought — but I need to figure out how to split the toolpath into two (perhaps by height), as there is a narrow valley behind the keycap where a 2mm endmill doesn’t fit. For the rest of this project, a 2mm or even 3mm ball endmill would work just fine (and likely better, as @Tod1d said!).
Outlines in sketches to use as your Machining Boundary, then play with the ‘tool on boundary / inside boundary’ options.
1a) Go to “contact point boundary” mode
1b) Additional offset to put a little air gap in for safety
Avoid / Touch Surfaces and select the areas to avoid