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.
Enjoy, DoN.
I haven't had a problem countersinking with a 4-flute end mill, all I had in that size, after drilling the screw shank hole.
-jsw
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.
Enjoy, DoN.
Some of mine were center-cutting until I reground them. Now I have to zigzag them back and forth to plunge a blind slot.
--jsw
After some deliberation I decided to pick up a set of each. I considered also a metric set, but decided for the most part I could use the replaceable pilot set with custom turned pilots if necessary.
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. Eric
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.
-jsw
I had a chinese counterbore bust into several pieces when the interchangeable pilot seized. That damned thing was made out of velveeta. Replaced with a CL and made my own pilot.
Pete Keillor
Metric flathead fasteners have a different cone angle (90 degrees) as opposed to SAE (82 degrees). It might matter.
Counterbore. Not countersink.
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