Corby bolts come in sizes from 1/8" to 3/8" and in different lengths. Yu want a counterbore that matches the bolts you use. I highly recommend USA Knifemaker's bolts and carbide counterbore. Carbide will cut perfect seats for years ... buy once, cry once.
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Knife Making Supplies - Best prices, selection and service. Visit us Today!
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Here is one of my Corby tutorials:
All the suppliers sell Corby bolts and the drills for Corby bolts. Try USA Knifemaker, K&G, Jantz, Sheffield, Pop's, etc.
With some skill ,you can make a counterbore by carefully grinding down a drill bit that is the right size for the bolt body, but buying one will assure the alignment is good.
Corby Bolt installation:
NOTE - this procedure requires parallel tang surfaces, and scales with well mated flat inner surfaces. You also need your drill press properly aligned and trued.
When you drill the tang pre-HT, drill the tang holes about .10"oversize of the bolt shank size. You want a little room for error when inserting the bolts. This is normally a good policy for all tang holes, regardless of handle construction.
Once the tang is shaped and drilled (pre-HT), slightly relieve the inner areas on the tang ( the places that will be under the scales) by at least .010" to allow for a shallow epoxy reservoir in the final glue up. Leave a 1/8" circle around each bolt hole un-relieved to assure a firm seat in final assembly, and leave a 1/8" lip around the perimeter. A ball burr in a Dremel tool does this well. If you just bolt the flat scales on a flat tang with no reservoir, the glue may all squeeze out and create a glue starved joint.The slight relief also assures that the edges are seated flush, and thus cuts down on the small gaps that often show up after the glue is dry.
When the scales and tang are ready, clamp one scale in place on the tang and drill 1/8" pilot holes through it from the tang side, centering these holes in the tang holes. Drill pilot holes for all holes you will be needing, including thong holes and decorative pins/rivets. Remove the scale from the tang and set the blade aside. Completely finish the blade before final assembly.
Take the pair of scales, and tape or clamp them together. Drill the 1/8" pilot hole on the front bolt hole through both scales ( only this one hole for now). Re-drill with a bit the size of the bolt shaft. Separate the scales. Change the drill bit to the shoulder drill (counterbore) for the bolts you are using, and re-drill that hole with the shoulder drill until the shaft hole has about 1/8" left. Check the fit with a Corby bolt, and ream the hole a bit by carefully wiggling the scale against the bit if needed. A properly drilled shoulder hole will just allow the bolt to be inserted straight in with just a bit of snugness. The proper depth of the hole can be calculated by subtracting the tang thickness from the female bolt shaft length. Each shaft hole in the scales should be one half of this number plus .050" to allow the bolts to seat with just a little thread to go. This can be easily done by eye, too. Just remember that you can always drill the hole a little deeper, but adding wood back is a lot harder.
If all is good, screw in a Corby bolt and tighten just snug ( never torque down on a Corby). The bolt won't be tight on the scales, because the tang is not there, so tape or clamp the scales together to assure things don't move, and drill the other end 1/8" bolt pilot hole through both scales. Re-drill for the Corby bolt shaft hole, then drill the shoulder holes, and insert the second bolt to check the fit and alignment.
When both Corby bolts are seated, drill pilot holes for any other holes needed ( pins, thong holes, etc.).
At this point, I sand and polish the front edges of the bolted together scales. You won't be able to do this area once assembled.
Remove the bolts, and reassemble on the blade to test fit everything. If the holes need to be a tad deeper to allow the bolts to mate more, carefully deepen the shoulder holes. You need at least 1/16" of wood to allow the bolts enough wood to hold the scales firmly. Once the bolts are snug and all is good, you can sand the edges down to the tang. This will help with alignment and glue-up in final assembly.
Using a slow set epoxy ( I use T-88 ,and dye mine to match the wood color), put a little epoxy in the bolt holes in the scales and slide in the bolts, rotating them to assure the epoxy coats the entire inner hole surface. Using a tooth pick, clean out any epoxy that got in the threaded holes of the female bolt half. Now, put epoxy on both sides of the tang,. Set the female bolt scale in place, then set the male bolt scale in place on the other side. Mate the bolts and give each a gentle turn to make sure the bolts are not cross threaded. Once the bolts are started, tighten alternately until the epoxy starts squeezing out. Don't tighten more than snug. The bolts will be very strong, and only need to hold the scales in place securely, not clamp them down with force. Check that the scales are aligned to the tang as desired ( loosen and adjust if needed). Wipe off excess epoxy with alcohol, making sure to get it all off the front of the scales and ricasso, and set aside to cure for at least 24 hours. After the epoxy is set ( four to six hours for T-88) wipe the ricasso area with acetone to remove any squeeze-out or excess epoxy. You will thank yourself for this when doing the final polishing of the handle.
After the epoxy has fully cured, cut/grind off any excess bolts sticking out, re-drill any remaining holes to the size needed, and epoxy in any decorative pins, mosaic rivets, or thong tubes. Shape, sand , and finish the handle as desired.
Stacy
