Engraving the Tektronix Circuit Computer bottom deck on a scrap hard drive platter suggested I’m entirely too much of a sissy about downforce on the diamond drag bit:
That’s at Z=-5 mm for 350 g of downforce, with the spring preloaded with 100 g at a 50 g/mm rate. More or less, anyhow.
The GCMC code automagically scales everything by the ratio of the actual platter OD to the original Tek bottom deck. Using 93 mm for a hard drive platter (actual OD = 95 mm) sets the scaling to 0.197 = 93/197, which makes the scale legends just barely visible:
The thing looks lovely, though, with ticks engraved at 2400 mm/min and the text at 2000 mm/min. The problem turns out to be the time taken to run the Z axis down and up while engraving so many ticks and characters!
I cranked on another 2 mm = 100 g of preload:
The top graph shows the downforce in 0.1 mm increments, rising from 0.0 to 217 g in 0.3 mm, which illustrates what the Y intercept of the plot means in real life.
Engraving at Z=-3 mm will now produce 350 g of downforce and cut the Z axis travel time down by a bit less than half. I have no idea what the right force might be; more experiments are in order.