Cutting length (novice question)

[esolberg]

I think I know the answer to this really dumb question, but I'll ask anyway.

I'm boring some 3/16" holes 0.4" deep into aluminum (I'd drill but I want flat bottoms). I'm using a 1/8" center cutting end mill. The only 1/8 mill I had (past tense) had a 0.25" cutting length. My assumption was that as long as my max depth of cut was less than 0.25" it would be okay - basically as it got deeper into the hole the shaft would just rub a bit.

It made it through the first 2 holes okay, but made some unhappy noises down deep - I think during the profiling pass after pocketing out the holes. On the 3rd hole my bit went to a better place.

I take it that if I want to pocket out holes, that the cutting length of the bit has to be >= the pocket depth? I'm curious whats wrong about my assumption that the shaft would just rub a bit and on a small quick cut it should've been harmless...

Thanks!

[bleeth]

Theoretically this should have been OK. The key for what you were doing is taking very small steps as you went down and running slow enough. There is still a good chance though that the actual cutting diameter of the bit was less than .125 but the shaft was .125. That would screw it up! You may want to get a 3/16 bit and drill instead, or a 1/8 with a deeper cutting depth. You should be able to find one on a 1/4" shaft

[esolberg]

Thanks Dave. Yeah I posted on another thread the difficulty I had with getting dowel pins to fit in these holes - I had to cut them to a .004" bigger radius to get a fit. It sounds like my bit was off. It was a $15 YG1 bit I ordered online somewhere and had used only a little for cutting acrylic. Come to think of it though I did one horrible cut into a thick acrylic piece with it that mangled the acrylic. I must of messed up the bit then.

I've got an onsrud aluminum cutting bit coming with a 0.5" cutting length. This ought to keep me out of trouble (famous last words).

[jerry_stanek]

I would check the new bit with a micrometer to see if it is true to size