Archive for March 10th, 2010

Acrylic Sheet Thickness Variations

Milling plate thickness

Milling plate thickness

So I measured the thickness of the black acrylic sheet I’m using for the Totally Featureless clock and machined the rabbets to match. Went to assemble everything and the rabbets are too shallow!

Come to find out that the sheet varies in thickness from about 0.437 to 0.475 across the four pieces I’d cut and, of course, I’d measured the thinnest end of the thinnest piece. Makes no sense to me, as I’d expect the thickness to be pretty well controlled over a few feet of sheet, but that’s not how things went down.

The simplest solution was to mill a flat on the inside of the case to match the rabbet, so all four panel ends were the same thickness. The sketch below has the straight dope.

Acrylic sheet thickness fix

Acrylic sheet thickness fix

Milling with a 3/8-inch end mill at 2500 rpm, 10 ipm, in one pass with no cooling was OK.

I’ll insert some brass shimstock into the rabbets to make the outside edges wind up flush.


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