Size of a Carved Region doesn't match from design to actual carve

[kelsky]

So I designed carving a box 1.5" x 1.5" and had it carve the region in that box .5" deep. When finished the actual carve ended up being 1 5/8" x 1 5/8" in size. I used a 3/16" bit for the pattern setting in the board definition. I hate that I have to second guess the end result of carving a region. I did not have any feather either. Why is this?

[bergerud]

The tip of the bit sweeps over the whole area of the square pattern. The sides at the bottom will be larger on each side by the radius of the ball. (If you use a tapered carving bit, the tops of the sides will be cut even wider by the taper.)

(Note that bit optimization will not help unless the pattern has sides.)

[Digitalwoodshop]

I am sure someone will chime in with the mathematical answer.

I took the dumb down approach...

I made a slot board in 2007.... and when I needed a quick fit I found the size on the physical board I needed and slapped in the numbers from the designer size. This all went out the window when I changed the internal computer in my CW and it had slightly different calibration data.

Plus it only worked for one machine.... the rest of the machines it is close enough with variations in the moisture content of the board swelling and shrinking...

AL

edit: O' Bit Taper... good catch... While I was typing...