So... any words of advice on counterbores?
I see basically two types. Fixed pilot and interchangeable pilot.
The obvious advantage to interchangeable pilot is the ability to pilot
different size screw bores with different size head bores. They might also
be easier to sharpen with the pilot removed. Are there any disadvantages?
Are there any overwhelming advantages to the fixed pilot counterbores?
Yes -- they come with a pilot just right for a normal clearance
hole for an Allen head cap screw whose head is just right for the bore
part. For those, you just reach out and pick up the standard
counterbore. Nothing to worry about.
However, if you want something like a counterbore for a hex head
cap screw, or for a slotted screw, or whatever else -- you pick up a
changeable pilot bore to fit the head -- or to clear the hex socket to
fit the head, and either pick or make a pilot for the size of the screw
shank involved. Perhaps for model making, you want a hex size which
looks inch sized while modeling something which is mostly using metric
screws -- or vice versa.
Of course -- you *can* simply use a two-flute center cutting end
mill to counterbore before you drill for the screw's shank and not need
a piloted counterbore at all -- but if you are doing the counterboring
in a place where the milling machine can't reach -- the changeable pilot
counterbore might be the only thing which can reach and works.
For most purposes, I like a fixed pilot --- actually a set for
the common sized screws which I use. But the piloted ones are nice for
when you have something weird to do.
Note that there are also changeable pilot countersinks commonly
used in the ball bearing micrometer depth cages (called Microstop,
IIRC), for countersinking for rivets or screw to a constant depth to
keep the project looking nice.
O.K. *after* drilling the screw hole -- or if it is a
center-cutting 4-flute endmill. Note that some 4-flute endmills are not
center cutting. Look at the center if the end. If it has a center
hole, and a recess ending in a flat, you have to have a hole
pre-drilled, at least as large as the center recess.
The big advantage of a fixed pilot is that if the pilot starts to
seize in the hole it isn't ripped free of the counterbore. So they are
great in production situations. For infrequent use the interchangeable
pilot offers you the option of having different size holes and a pilot
that fits them.
When I've needed to counterbore for something other than a standard
cap screw head, the size wasn't necessarily the standard for a
different cap screw. More likely it was the root diameter of a thread.