[QuinnSjoblom]

I need to punch a 4mm hole through a hardened 52100 sleeve. Sleeve is 12mm id, 17mm od. Haven't received it in the mail yet so not sure exactly how hard it is. I'll check with testing file when I get it, but its an inner race for needle bearing so I'm assuming somewhere between 55 and 60 hrc. I have solid carbide 4mm drills, but I've read that guys like to use masonry bits. Is there something special about masonry bits? Or is that just a cheap alternative when you don't have carbide?

[Jim Dawson]

More like 62-64 hrc. Masonry bits are much tougher than solid carbide twist drills. Won't drill as accurate hole, but have a good chance of getting through it without breaking. But give the twist drill a try and see how it works. A sinker EDM would be my first choice if you have one in your back pocket.

There is another way, copper tubing and some course valve grinding compound, or diamond dust. Will actually drill through hardened steel or glass.

[QuinnSjoblom]

More like 62-64 hrc. Masonry bits are much tougher than solid carbide twist drills. Won't drill as accurate hole, but have a good chance of getting through it without breaking. But give the twist drill a try and see how it works. A sinker EDM would be my first choice if you have one in your back pocket.

Ha! Yeah I'll just throw it on my edm, no problem.

Ill see what happens with the carbide. I'm not really sure exactly what type of hardening is done on these races, whether it's through hardened or just the surface. I'm guessing it would be pretty difficult to try a localised annealing. The hole is only a few mm away from where the bearing surface will be.

The other option is milling the hole. I can go from either side, so it's only going 2.5mm deep to get through the wall. Maybe try to helix in with a stub 2.5 or 3mm endmill. Not sure if that would be any easier

Hmm, copper tubing idea is interesting. I was wondering if i could somehow grind the hole. Sounds like that's basically what happens

[Jim Dawson]

Pretty sure those would be through hardened, pretty difficult to zone harden something with that cross section. I think any of the ideas that you propose would work fine.

Yes, the copper tubing trick is something I read about in Popular Mechanics over 50 years ago. And it is just like grinding, the media embeds in the copper and it becomes a grinding tool. I've only done it once about 50 years ago, a 1/4'' hole, 1/4'' through a rotary knife blade. I didn't have carbide tools back in the stone age.

[QuinnSjoblom]

Pretty sure those would be through hardened, pretty difficult to zone harden something with that cross section. I think any of the ideas that you propose would work fine.

Yes, the copper tubing trick is something I read about in Popular Mechanics over 50 years ago. And it is just like grinding, the media embeds in the copper and it becomes a grinding tool. I've only done it once about 50 years ago, a 1/4'' hole, 1/4'' through a rotary knife blade. I didn't have carbide tools back in the stone age.

No problem milling, pics attached. Had another similar sized race to test on. I used a 3 flute 2.5mm zrn coated maritool endmill. Totally wrong tool for this, but still worked completely fine. Didn't try drilling yet, but milling might be safer anyway. The race I need to drill has the hole only 2.5mm from the edge, so cracking it is also a concern. Checked the race I just milled and the 60hrc skated right over, BARELY made a mark with the 65.

[handlewanker]

There is a set of drills on the market (EBAY) called ARTU drills, comes in a set of 6 tipped drills from 2mm up to 8mm, and they will drill through HSS.....well according to the advert demo they do showing a HSS drill with multi holes through it, and various other materials with a hard nature.
Ian.

[Jim Dawson]

No problem milling, pics attached. Had another similar sized race to test on. I used a 3 flute 2.5mm zrn coated maritool endmill. Totally wrong tool for this, but still worked completely fine. Didn't try drilling yet, but milling might be safer anyway. The race I need to drill has the hole only 2.5mm from the edge, so cracking it is also a concern. Checked the race I just milled and the 60hrc skated right over, BARELY made a mark with the 65.

Looks like you got it.