[monte55]

I have a project where I have to bore a hole in a hardened steel shaft to place a bolt to pin the shaft to a u-joint. I have to go through the outer part of the internally splined u-joint and through the male splined shaft. I am using
4 flute center cut 1/4" and 3/8" Atrax carbide sem-TIALN. The mills don't last
but maybe a couple holes if I'm lucky. They will start cutting and all of a sudden break. Each bore is less than one inch. Am I going about this wrong?
Suggestions.....................Nick

[Astonlee]

I am not surprise to learn that you are breaking your cutter, what you need to do, to maximise the number hole/bores per tool, is to circle interpolate hole with a TiAIN coated 2 flute Bull Nose Cutter.

[LeeWay]

I find cobalt drills work pretty well in hard steel.

[tobyaxis]

It is a little tough to say without being in front what your trying to do personally I would have tried a Carbide or Cobalt Drill first then remove the remaining material with the appropriate endmill. You can always drill faster than you can plunge with an endmill.

In your case your working with automotive parts which means they are heat treated and hardened. To what specs, I have no clue, but a smaller drill to remove the center for the endmill would seem a logical route rather than plunging with just an endmill.

Use a rich cutting oil as well, not coolant.

[monte55]

I am not surprise to learn that you are breaking your cutter, what you need to do, to maximise the number hole/bores per tool, is to circle interpolate hole with a TiAIN coated 2 flute Bull Nose Cutter.

Do you mean keep changing from one tool to the other as I progress?
Explain please

[monte55]

It was suggested to speed up my spindle. My max speed is 2580 rpm. With
cutting oil and light down feed, I plunged the 1/4" 4 flute center cut carbide mill through through the hardened hub on my small centrifugal clutch. I bored
6 holes and my end mill is still good. I was cutting at 300 rpm. Thanks for the help guys..................Nick