[fettersp]

So I have a part made of 0-1. There is a through hole of .516 and and counter bore of .770. I have a HSS drill that's 33/64 but it seems that the o-1 is just destroying it? Is the o-1 just too hard or am I not drilling it coreectly with the mills speed? I'm not quite sure what to do about the .770 counter bore since I don't have a drill that size and don't think they make them that size. Currently I used a .5 carbide drill that uses carbide inserts and plan on EDMing it that last .016. I might be able to use a .75 carbide endmill to get the counterbore close to size then edm the rest of it out. All this would work but just seems very ineffient. Any ideas on what would be a better plan? I guess I could just make an electrode that does both too.

[Jim Dawson]

How fast are you turning the 33/64 drill bit? Somewhere in the 450 RPM range (~60FPM) would be about right for annealed O1, maybe a bit slower would be helpful, and don't baby it, if you let the drill rub it will work harden the work instantly. My preference is cobalt bits for working with tool steels. Sounds like you need a counterbore tool bit. If you don't have a set, you should get some. I have 2 sets, one set with onsize pilots and the other with clearance hole size pilots. Turn them at about 1/2 drill speed or less.

https://www.amazon.com/s?k=counterbo...ref=nb_sb_noss

[machinehop5]

So I have a part made of 0-1. There is a through hole of .516 and and counter bore of .770. I have a HSS drill that's 33/64 but it seems that the o-1 is just destroying it? Is the o-1 just too hard or am I not drilling it coreectly with the mills speed? I'm not quite sure what to do about the .770 counter bore since I don't have a drill that size and don't think they make them that size. Currently I used a .5 carbide drill that uses carbide inserts and plan on EDMing it that last .016. I might be able to use a .75 carbide endmill to get the counterbore close to size then edm the rest of it out. All this would work but just seems very ineffient. Any ideas on what would be a better plan? I guess I could just make an electrode that does both too.

SSpeed kills oil hardening ...work hardens when thin part is available.

[fettersp]

It turned out I was going way too fast. I turned my speed down to 135 and it was doing just fine. I had a squirt bottle of coolant spray as I was counterboring. The only issue was that the counter bore is a 3/4 but cut anywhere from .77 to .78. would tool pressure cause this ?

[Jim Dawson]

Were you using a piloted counterbore? If you use a 0.495 pilot in a 0.515 hole you are going to have a lot of slop. You would need a 0.510 or so pilot. What you really need to do is use a 13mm drill and a 19.5mm counterbore. Those are the correct sizes for what you are trying to do, for a nominal 12mm counterbored hole with standard clearance.