How To Counterbore?

[bladehunter]

From my limited experience.

• You would have to use a centre cutting endmill or slot drill.
• If you drill the hole first there is less material to remove, drills are cheap, end\slot mills not so.
• You can fine tune the counterbore depth, set your DRO to zero then easy to have equal depth counterbores.

As I said I have limited experience, and I could be wrong.

BTW I have found these useful:
https://www.carbidedepot.com/formulas-cb-metric.htm
https://www.carbidedepot.com/formulas-cb-inch.htm

 

[shorton]

OK, lets expand this. I use a 1/4" piloted CB to do a bunch of CB's "freehand" on my mill. Depth is set but the part "floats" on a table like a drill press. How does one determine the best RPM for the CB? In Alu I can usually find one that works, but if I try steel I'm prone to break the CB. The pilot hole is 1/8".

 

[Mark_f]

I counter bore at a slower speed than I drill usually. Just feed slowly at first and see how it cuts. If necessary, slow it down.

 

[Wreck™Wreck]

Not a lot of experience myself, but why couldn't you use an end mill and cut the counter bore first an then drill your thru-hole. Wouldn't that make sure you were centered as long you didn't move the work piece. I'm sure there's something I'm missing, but that's how we learn. Thanks, Mike

The less material that you have to remove using a center cutting end mill the better,(also a center cutting tool is not required if the center of the hole is absent) drill the hole first, chamfering the hole at the bottom of the spotface would be a second operation anyway.

 

[FOMOGO]

Thanks for the explanations, all makes sense. Mike

 

[Ulma Doctor]

I counter bore at a slower speed than I drill usually. Just feed slowly at first and see how it cuts. If necessary, slow it down.

Good Info Mark!
Run reamers and counterbores as slow as possible and generously lubricated (unless machining cast iron or brass ,but still turn it slow)

 

[18w]

OK, lets expand this. I use a 1/4" piloted CB to do a bunch of CB's "freehand" on my mill. Depth is set but the part "floats" on a table like a drill press. How does one determine the best RPM for the CB? In Alu I can usually find one that works, but if I try steel I'm prone to break the CB. The pilot hole is 1/8".

I think you will have better results with the part clamped, not floating. The pilot needs clearance so it doesn't try to seize in the bore also.. Counterbores and countersinks work best if everything is rigid. As everyone has said ,slow speed, moderate feed, and lots of cutting oil.

Darrell

 

[Mark_f]

I think you will have better results with the part clamped, not floating. The pilot needs clearance so it doesn't try to seize in the bore also.. Counterbores and countersinks work best if everything is rigid. As everyone has said ,slow speed, moderate feed, and lots of cutting oil.

Darrell

Darrell has it right,Definitely clamp the part in position if ,possible, especially if you are using an end mill or the part will scoot around when the end mill hits it, making an off center, crappy counter bore. ( learned from experience). A piloted counter bore won't do that as easily and does a better job.

 

[bfd]

2 things, the end of an endmill is not flat. there is a slight angle with the outside dia. hitting first. otherwise as you went across the part the inside would rub and overheat. also a bad finish would result. if you were using shcs. then you would point load the area next to the bolt shank. this would cause galling and loss of torque. maybe this would not matter. second thing is to make a flat bottom drill the size of the required counterbore. go slow with this method as the drill will tend to walk around, you can make more hole sizes as drill bits come in many more sizes then endmills. just grind the 118 degree point off and grind back clearance on the flat ends. bill

 

[4GSR]

Back in the days when we didn't have counterbores and used end mills to create the countersunk hole, we would seat the socket head cap screws at the required torque. A 1/2 SHCS would get seated using a long Allen wrench and about three to four hits from a engineers hammer. Break it out, check the seating pattern. Generally you get a good even pattern across the face of the cap screw. The back angle of the endmill is so small that it really matches the face angle of the SHCS, which has a slight back angle, too from the upsetting operation when it is made. You don't notice it on smaller ones as you do on larger ones. Also got to remember, the SHCS is much stronger/harder than the material that it is seating into. A yes, it will yield the material a bit to seat correctly. In over 40 years, I have never seen this come up in question as being an issue. Ken