Counterbore tips needed

[EOsteam]

I'm about to use a counterbore for the first time. The material is 6061-T6 AL and the counterbore is for a #6 socket head cap screw in 6-32. The hole is already drilled to a #36 for eventual tapping but now I have to counterbore down into the piece. The piece will eventually be slotted to create a clamp which the #6 screw will then tighten and create the clamping force. Should I use a #26 drill for the clearance portion of the hole and then follow w/ the counterbore? Also what is a good RPM for the counterbore?

Thanks,
HJ

[GlennW]

Should I use a #26 drill for the clearance portion of the hole and then follow w/ the counterbore?

That would depend on the size of the pilot on the counterbore as not all are the same size.

Match the drill size to the pilot size and put a drop of lube on the pilot.

[Harold_V]

If your counterbore has replaceable pilots, be advised that they are exceedingly easy to break. All it takes is a tiny chip in the hole when you back off to clear chips from the conterbore operation. Be generous with an air hose if you go this route. Been there, done that, and have broken a lion's share of pilots.

An excellent way to create counterbores is to do them with a milling machine (not a drill press), using a short drill ground for flat bottoming. No issues with the pilot. I no longer use counterbores.

Note that a drill has tapered flutes. As it gets shorter, the web thickness increases. It has to be thinned, but that can be accomplished by hand with care, just as the flat bottom can be created by hand. It pays to use a wheel suited for grinding HSS.

H

[EOsteam]

Thanks fellas. The counterbore pilot tip was measured at .1460” so a #26 drill (.1470”) was used for the clearance portion of the hole. A drop of oil was used and the operation went smoothly. Thanks Harold for the tip on modified drill bits. That will be used when a counterbore shoulder is needed and a standard counterbore cutter is not available.

Thanks again,
HJ