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Counterbore questions?
- Thread starter Janger
- Start date
Dan Dubeau
Ultra Member
One thing to note about the clearance drill size, and those that think it's too big. A std size clearance drill/counterbore is not designed to locate. Dowels/taper/roll pins are for locating, and the capscrews are for fastening. If you need a screw to do both, you must accept that there will be reduced accuracy, and repeatability, and you should use a flathead capscrew/countersink. Std counterbores sizes, are great when you're laying out all the hole locations with scribe lines and punches, and drilling them in a drill press. Sometimes the extra clearance comes in handy....
Susquatch, your method is fine. There are many ways to skin the cat. Drilling through both parts with the tap drill, is a perfectly usable order of operation. I do things a few different ways depending on the task at hand. Sometimes I have a part with clearance/counterbore holes already in it, then I'll use transfer punches to locate for the tapped hole. Sometimes I have a part with the tapped hole in it, and I'll use transfer screws instead. Sometimes I have both parts clamped together and do as you do and drill through both with tap drill, then open up for clearance. Sometimes I use the readout, and just go to the numbers. Most times I use the CNC now and just do it all with magic.
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Having many different ways to accomplish a task with a few different tooling options is never a bad thing. There is no one perfect way of doing things for all jobs.
Susquatch, your method is fine. There are many ways to skin the cat. Drilling through both parts with the tap drill, is a perfectly usable order of operation. I do things a few different ways depending on the task at hand. Sometimes I have a part with clearance/counterbore holes already in it, then I'll use transfer punches to locate for the tapped hole. Sometimes I have a part with the tapped hole in it, and I'll use transfer screws instead. Sometimes I have both parts clamped together and do as you do and drill through both with tap drill, then open up for clearance. Sometimes I use the readout, and just go to the numbers. Most times I use the CNC now and just do it all with magic.
. Having many different ways to accomplish a task with a few different tooling options is never a bad thing. There is no one perfect way of doing things for all jobs.
I have used pilot point drill bits, they work fine for expanding a hole but they always leave the surface concave if you try using them to counterbore. I'm looking for some made with carbide teeth or stronger hss. I'm fairly confident that my new accusize counterbores will work well when cutting mild steel or standard stainless steel; but I have not had much success when cutting non hardened tool steels.
I ended up chopping the pilots off of my fixed counterbores. Just like @Susquatch I don't like the hole size you need in order for the pilot to work.
Last edited by a moderator:
ChazzC
Well-Known Member
Isn’t that an end mill?
I guess I think that depends on how much is removed.
In my opinion, if a stub remains, then it might still be called a counterbore, but in any case could easily be called a piloted end mill or perhaps a piloted counterbore. My preference would be for a pilot that barely clears the shank of a machine screw of the target size.