On Fri, 17 Dec 2010 10:23:31 -0600, Ignoramus30138
wrote:

On 2010-12-17, Pete C. wrote:

Ignoramus30138 wrote:

By the way, forgot to mention. I have three aluminum plates that are
approximately 12" wide, 30" long, 3/4" thick, and have handles on
them.

I wanted to drill a rectangular set of tapped holes in it, probably
1/4-20 holes on every inch of a 10x24" area, so it would be 240 holes.

I have a theory that then I could sell the plates for big bucks, as
layout/fixturing plates.

i

Remember that 1/2-13 hold down hardware is the norm for Bridgeport sized
machines, so 1/4-20 wouldn't be very popular.

Well, if so, I will make 1/2-13 holes. I will try to do it with a form
tap, since this is aluminum. I would space the holes a little wider
then, maybe one every 2 inches.


i

If a form tap raises a little metal around the hole, would you
counterbore very slightly afterward with an end mill? Just thinking
out loud. I've never used a form tap, but it works by displacing
metal, so I'd expect a slight ridge unless the hole is countersunk
first or cleaned up after.

Pete Keillor