Also I would like to cut a round 4" diameter hole in the 1/8" aluminum
front
panel for the speaker grill. Is there a way to do this without a Greenlee
Pneumatic punch? I have a jig saw which I can cut square holes but round
holes are another matter. Is there a better way? I can drill round holes
up
to 7/8" with a unibit but I think 7/8" is as big as they get.
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Suggest you drill multiple holes ( as close to each other as possible) with
a 1/8 inch drill all around a circle with a diameter of
3 13/16 inch .
Then knock out the circular bit of aluminium and file with a half round file
to 4 inch diameter.
It is a bit of a job but will give a good result.
Alternatively you can cut out the 4 inch circular bit with a jig saw ,using
methylated spirit as 'lubricant'.
Finish with a fine file or emery paper.

Frank GM0CSZ / KN6WH