No proper mechanical engineer was around. Took about half an hour to cut a very clean hole. Used a 72mm downlighting holesaw, working from one side only.
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mm graph paper behind
42 teeth on the cutter so decided on keeping every 6th tooth and grinding back the rest. Black felt tip line marks one of the untouched teeth. Hole is quite round and very clean, no waveyness nicks etc and varying only between 71.9 and 72.2mm in diameter. Left image is the approach side and right image is the breakout side. The grey smudgey line on the left image is my felt tip mark to show where to put the plasticene bund, not anything due to the cutter. I carried on cutting until broken through 3/4 around and then hand snapped the remainder. The breakthrough side had a thin and narrow swarf ribbon perpendicular to the sheet which cut away easily with a stout knife, the only finishing I did before taking the pics, other than washing off the cutting oil. As a standard hand drill in a drill press, not geared down, I had to stab at it, ie spin up
/ drop+cut / release/ spin up ...
-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on
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