Dimensional accuracy of endmills?

[Hubert S.]

When using an end mill as a reamer, your RPM is much lower than using it as an endmill. But your feed rate is quite high. Chip load per tooth is substantially more than normal machining operations

I tried it again and got a much better result. I used an ER40 collet holder and measured the run-out on the end mill before I cut the hole and could barely see the needle move. It seemed like I was getting less run-out than when I measured with the R8 collet. I ran the cutter at 600 rpm and was pecking and used WD40 to get rid of the chips. The hole is between 0.5 and 1.5 thou oversize. I don't have any gauge pins, so I cannot really measure very accurately. This is definitely close enough for my purposes, though.

Thanks again for your help, Nathan and Callum.