Counterbores
Weldon counterbores cut a flat-bottomed recess so screw heads sit flush or below the surface. Listed by screw size, including #6, #8, 5/16", 3/8", 7/16" and 1/2", each with a matching pilot.
Choosing a counterbore
A counterbore opens a flat-bottomed recess around a drilled hole so the head of a screw or bolt sits flush with the surface or below it. These are Weldon counterbores, listed by the screw they are made for and by pilot size, with shanks from 5/32" to 1/2".
The pilot on the nose rides in the hole you have already drilled, which keeps the recess centred. Pick the screw size first, then the pilot that matches your hole. One screw size comes with more than one pilot, such as 13/32", 3/8" and 25/64" for a 3/8" screw.
Drilling tips
Drill the through hole first, then counterbore at a slow speed with steady pressure. Use cutting oil on steel and back out now and then to clear chips. Clamp the work, because a counterbore can grab in thin material. Check the depth on scrap before you cut the real part. For flat-head screws you want a countersink instead.
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Common questions
What is the difference between a counterbore and a countersink?
A counterbore leaves a flat-bottomed step for a cap screw head or washer. A countersink cuts a cone for a flat-head screw. Choose by the head shape of your fastener.
Do I drill the hole first?
Yes. The pilot guides in an existing hole, so drill the through hole at the size you need, then counterbore. The pilot should fit that hole without slop so the recess stays centred.
What speed should I run a counterbore?
Slower than you would drill a hole of the same diameter, and the larger the cutter, the slower you go. Use steady pressure and cutting oil on steel, and stop to clear chips.
Buying for a crew?
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