View Full Version : How to drill this hole for cap screw

01-24-2005, 12:31 AM
What type of drill bit and where can i find them at.

01-24-2005, 09:40 AM
From your picture it seems you are wanting a counterbore drill, however the depth of the counterbore may be a problem and I am not aware of any off the shelf counterbore drills that will drill and counterbore to the depths you show.

Normally it is done in two steps, first the desired pilot hole is drilled then the counterbored.
If a precision holes is required then drilled under size .005-.010 or so, reamed and then counterbored.

In your case it may be simpler to just increase the bolt length and counter bore with a simple end mill of appropriate size which I have done, with what I can determine, perfect results.

01-24-2005, 09:51 AM
One way to do it without a counterbore is to pre-drill the holes, first, the smaller diameter, then open up the large dia. with a 3/8" drill, which will just clear the head of a standard 1/4" SHCS. Don't go full depth with the "counterbore" drill, leave a bit of material at the bottom for the next operation. Take that same sized drill but grind the tip to a flat profile. Providing a bit of rake to the two webs will help the cut, as will SLIGHTLY relieving the corners of the bit. There's not a lot of metal to remove at the bottom of the bore to produce the correct, flat profile.

A 3/8" end mill can be a handy substitute for a true 1/4" cap screw counterbore in many applications, remembering that an end mill does not produce a truly flat-bottomed bore... but this is not objectionable in most cases.

01-24-2005, 10:03 AM
I agree with Swede on the end mill. But if you have to have a perfectly flat bottom, then use a 2 flute straight 3/8 cutter . These will produce a flat bottom. I use them all the time finishing pockets for aerospace apps. They are not terribly expensive, but most tool places do not stock them. Mine does now :)