Not much progress to show compared with my last post, but I did discover something interesting I wanted to share.
So many of the CNC cut parts are mirror images, which means to make them I just drew one side and then mirrored a duplicate in Inkscape. But I discovered that with all of the mirrored parts one side was slightly smaller than the other. On larger parts this wasn’t an issue, but on smaller parts it’s really obvious. It was quite perplexing!
After giving it some thought I realised what the problem is.
Styrene is not a great material for machining. It has a tendency to melt very easily so you have to have your cutter and feed speeds very correct for the depth you’re machining at. It turns out that feeding with the cutter rotation direction will take off more (or maybe less, I’m a bit confused now) material than feeding against the cutter rotation direction. When the parts are mirrored in Inkscape, it also inherently reverses the direction of the paths, so that the two mirrored parts will end up with slightly different sizes. Fortunately it’s easily fixed: simply use the reverse paths command on the mirrored paths.
The curved “intake” looking sections adjacent to the bridge area are where this problem was really evident. What’s more the multiple passes used to cut the shapes from the 6mm thick styrene (4x 1.5mm sheets laminated together) had left very prominent marks on the sides of the parts.
So I created new toolpaths with a new machining strategy: first the little undercut areas are milled out. Then 5 passes are made through the styrene to cut out the shape, with a 0.3mm offset (so that the part is 0.6mm bigger in each direction). The cutter can then be run around the actual part size shape at full depth, since only a 0.3mm shave is being taken off. This creates a perfect part with perfect, smooth edges!
Here’s what it looks like post machining:
Next up: engines!