Drilling/milling hole in hardened steel - what's best?

This is a cam gear...hardened steel (don't know how hard or material).

I want to drill a 1/16" hole in the tooth land. The hole needs to go down .150", then it breaks through the other side which is what I want, i.e. create a through hole. I bought some 1/16" carbide end mills from McMaster Carr, and also a carbide drill bit.

The end mill worked....I could feel some variation in hardness as I went down. But, when it hits the last .010" or so, the end mill really doesn't like it (I assume due to the skin of the gear being harder) and pretty much breaks apart.

The twist drill bit didn't do any better, in fact it didn't even like starting the hole and I gave up on that before I broke it off.

So my question is...would a coated carbide end mill fare any better (I guess it wouldn't but...)? Is there a number of flutes that is best (I used a 4 flute but can get 2 or 3 flutes)?

I tried flooding with cutting oil, LP3, etc. and that didn't seem to hurt, or help. I also tried a slow/steady quill feed (by hand) and also tried locking the quill and slowly creeping up on the table (Z) but no big difference...the carbide end mill just doesn't seem 'tough' enough to cut the steel. The setup is plenty rigid, but at some point I can put enough force that the tool breaks...or if I go easier on feed, it never cuts.

If you could allow the hole to be larger, like maybe .09" I suspect you might be able to interpolate a path down with a quite shallow helix. Might suggest using a ball end mill, too, and peck to clear the chips.

What rpm? Sounds like you are on a knee mill. Run a straight flute carbide drill at max rpm and feed the table up steady using air to clear chips. I’ve drilled through HSS drill bits and taps like this. Hardened steel should be no problem

I'd go with a carbide spade. They are much less fragile than endmills. Maybe start the hole with the endmill to get a nice square start until it's got a flat at full width then switch to the spade. And peck down using the quill stop nut. Also want to know, what RPM were you running?

https://www.mscdirect.com/product/details/83245779

Thanks.

I was running fairly slow - around 2000RPM - because I (think) that's the rule for harder materials....slower. But I could be wrong.

I was using the Wells Index knee mill (manual) but could use the Tree CNC mill as it does have higher spindle RPM ability.

I'll try the spade, too. thanks.

+1 on the carbide spade drill, I've ground several of my own from used up tooling. work great for drilling out broken taps too!

GregSY said:

Thanks.

I was running fairly slow - around 2000RPM - because I (think) that's the rule for harder materials....slower. But I could be wrong.

I was using the Wells Index knee mill (manual) but could use the Tree CNC mill as it does have higher spindle RPM ability.

I'll try the spade, too. thanks.

Click to expand...

Slow is the rule, BUT you have a 1/16 diameter drill/end mill. If you're on a BP style mill, max out the rpm to 3k.
If you're on a cnc mill, run it at 6k-12k (100-200 sfpm) small pecks, with air blast.
HSS won't be you friend here because of the case.
Either a regular carbide drill or the spade style mentioned above, I've never used the spade style.

Yeah that's only 32 SFM - bad news. At that kind of speed you were probably getting B.U.E. - very bad news for carbide tooling. I'd want to hit at bare minimum 100 SFM in the hard part of the gear, faster in the soft part.