Machinist or anyone who knows

[Dixieblade57]

I am putting together a KMG clone and am about to order a metal countersink from McMaster-Carr for counersinking the flat head machine screws that will be used in the project.
The screw heads measure approx. 1/2" in diameter. X 3/16" in depth. I think what I want to order is their Smooth Finish (Six Flute). Please refer to this link.

http://www.mcmaster.com/#3098a112/=3r7wx7


I am unsure from the chart which one to order and then it asks degree of angle and list from 60, 82, 90, 100, 110, 120. From what I could tell the back side of the screw head has a 30* angle. That is not one of the options!
Can someone point me in the right direction on this.
Which countersink?
What Degree?

 

[Macchina]

Diameter of countersink: If you want the screw head to be flush, you can use a 1/2" countersink. If you want the screw head to sit below flush, or you are countersinking a contoured surface, go one size bigger (5/8"). Six flute will give the best surface finish, don't ever get a single flute.

If the screw is a metric screw, it is 90 degrees. If it is a SAE screw, it will have an 82 degree head.

I'd recommend the 3098A116 countersink in the angle you need.
We order many of our supplies from McMaster at my shop and we absolutely love them. For our cutters and tooling we usually go through a local tooling company or J&L, but there shipping prices went up when they were bought by MSC.

 

[Dixieblade57]

Thanks for the info, and I love your avatar!

 

[J S Machine]

Diameter of countersink: If you want the screw head to be flush, you can use a 1/2" countersink. If you want the screw head to sit below flush, or you are countersinking a contoured surface, go one size bigger (5/8"). Six flute will give the best surface finish, don't ever get a single flute.

If the screw is a metric screw, it is 90 degrees. If it is a SAE screw, it will have an 82 degree head.

I agree. The three flute C'sinks work pretty good too. The one flute deals work well for chamfering holes, but that's about it. And once they dull, Forget about sharpening them..I am convinced it cannot be done

 

[drewfacehotm]

also you can always take a big enough drill bit and a steel protracter and grind what ever angle you need, had do to that on a saturday working on a test part and all the c'sinks we had were wore out pretty bad so i ground a drill bit to the 82* (set on the protractor 41*) worked out pretty good, they ended up using it for the whole job that monday.

 

[P.Brewster]

I sharpen single-flute counter-sinks all the time. You can't touch the conical portion; you grind inside the flute. You want a zero or a positive rake, so you need to grind a concave trough. You can do that on the side/corner of a a wheel. A cup wheel would be even better. After about 10 re-sharpens, you get too much clearance and the bit is done. Low RPMs (90 to 140) seem to work well with the re-sharpened countersinks.

Why countersink anyway? With countersunk holes and flat-head screws, you have zero room for error/adjustment. You're hole locations will have to be ±0.001 or so.

Why not use socket-head, hex-head etc screws and use standard thru holes, 0.030" oversized? That way you can adjust your fit. For example, you can adjust the play (clearance) between the receiver and the square shaft.