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Countersinks

posted Dec 30, 2013, 10:43 PM by Sean M. Messenger   [ updated Dec 30, 2013, 10:50 PM ]

Background

The angle measure of countersinks and screws.[1]

One problem I seem to run into constantly is that of countersink chamfer angles. 82 degrees? 90 degrees? 100 degrees? Yes, there are different chamfer angles for countersinks and no, they are not all compatible. This is a quick rundown of the differences.

 Chamfer Angle End Use General
 82 degrees UNC/UNF screws  INCH
 90 degrees ISO Metric, Imperial screws  METRIC
 100 degrees Aviation  RIVETS

So if you are dealing with UNC/UNF screws such as 4-40 threads, the 82 degree countersink will be the matching size.

If you are using metric screws or just want a nice 90 degree chamfer, then go with that. 

If for whatever reason you are working on aviation projects and really want to pay top dollar for low profile, high strength screws, go for it. I wouldn't suggest getting a 100 degree countersink unless you really think you need it -- it won't be useful for much!

There are 3 other commonly marketed angles: 60 degrees, 110 degrees, and 120 degrees. I have never encountered these being used personally.

Personally, I have a 90 and 82 degree countersink for personal projects. I have only ever used the 90 degree countersink for making a chamfer (no screw application). I always have used UNC/UNF screws, so the 82 degree countersink is what my rocket screws and low temperature differential Stirling engine screws were done with.

Uses

A countersink can come in handy. You can use it to deburr holes (using a hand drill, drill press, CNC mill, lathe, or just by hand) or provide relief cuts for countersunk screws.  

Chatter

Drilling with countersinks often produces a lot of chatter. This can be hard to address, but the first thing you should do is ensure the part is held firmly and the countersink is secured properly. The shorter and more rigid the setup, the better. 

Lighter feed pressure is not better -- you want to keep constant pressure. As in other machining operations, it would actually be appropriate to increase feed rate (and hence pressure) and decrease rotation speed.

Speed

To help reduce chatter and provide a better finish, countersinks should be used at a slow speed. On drill presses, generally set the speed as low as you can go. On mills or lathes, around 80 RPM is appropriate.

References

[1] McMaster-Carr