Despite an angular resolution of 2°, the curves came out entirely smooth enough. The gritty scratchiness resulted in a pile of chaff covering the engraved area; perhaps some oil or lube or whatever would help.
Rescaling the pattern to fit a CD platter:
Polycarbonate seems to deform slightly, rather than scratch, leaving the final product with no chaff at all.
In this case, the doubled lines come from the reflection off the aluminized lower surface holding all the data.
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