Polycarbonate Shrinkage?

[dalesvp]

I have a question for the old timers. I drilled a 1/2" hole through 3/4" polycarbonate sheet. The resultant ID was .507 - larger than expected from a .500" drill bit - maybe some wabble - but a useable dimension. Then I drilled the same 3/4" sheet lengthways (about 3") with the same bit and the hole ID is .496. So what happened? Did the lengthways bore cause the plastic to shrink? Very puzzling....

[barts]

While drilling a deep hole, the plastic and drill will have warmed more... and could then deflect away from the bit?

info on polycarbonate...

- Bart

[dalesvp]

After contemplating this for a day or so I have come to the following conclusions none of which are really good but I can live with them. In the first hole through the 3/4" dimension the .507 from a .500 bit can be chalked up to 'bit wabble'. The second hole of .496 from a .500 bit appears to be the result of the material heating up (slower feed rate than the first) then expanding into the ID even though there is no detectable bulge or expansion on the outside of the material. Another thought is polycarbonate sheet must have a pronounced grain to it - expanding and contracting differently in different directions when slightly heated then cooled. It's an interesting material to work with - lots of personality to be dealt with.

[MotorradMike]

Did you drill a .500 hole without a pilot?
I'd probably use 3 or 4 sizes to get to .500 and that's just for the 3/4" depth.
I'm not sure what I'd do with the 3" deep hole.