Corby questions

G

gnappi

Joined
Mar 19, 2017
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3
Hi new member here and first post.

I retired recently and thought I'd take a shot at making knives. I have a couple of nice household blades that are in desperate need of new handles. So I found a couple of nice scales I want to buy but the vexing part I'm having is understanding corby rivets.

The ads just say something like:

5/32" x 7/8"

OK, I guess the 5/32" is the shaft diameter that goes through the blade, but what is the 7/8"

I have not seen a diagram like my crude one below that tells me what the max / min for A and B are or the dimension on C. What size bolt would I use for the blade in my diagram?

knife_xsection.jpg


Oh, Why would they use sizes like 5/32 when a decimal measurement like .156" measurable with a vernier.




Regards,

Gary
 
Last edited:
gnappi said:
Hi new member here and first post.

I retired recently and thought I'd take a shot at making knives. I have a couple of nice household blades that are in desperate need of new handles. So I found a couple of nice scales I want to buy but the vexing part I'm having is understanding corby rivets.

The ads just say something like:

5/32" x 7/8"

OK, I guess the 5/32" is the shaft diameter that goes through the blade, but what is the 7/8"

I have not seen a diagram like my crude one below that tells me what the max / min for A and B are or the dimension on C. What size bolt would I use for the blade in my diagram?

knife_xsection.jpg


Oh, Why would they use sizes like 5/32 when a decimal measurement like .156" measurable with a vernier.




Regards,

Gary
Click to expand...

Usually they would be giving you the largest O.D. x the max. length, but I guess it would depend on who you are buying them from.
 
Second question first - My very favorite rant:
In the USA and some other places it has taken a long time to go from fractional to decimal in measurements, and almost no gains have been made on the metric systems. Most industrial suppliers use decimal. Hobby catalogs use fractional or post both. Most hobby knifemakers still think in fractions. Since the hobby knifemaking catalogs are generally started by these guys, the catalogs are made with their measurements. It is slowly getting better.
Every technical person knows that decimal and metric are far better, but don't expect any housewife or factory worker to express his/her waist as970cm or 70cm. It also seems like speaking Russian to tell a woman she weighs 55Kg or a guy he has slimmed down to 80Kg. To 98% of Americans those numbers have no reference. I suspect it will be 50 to 100 years before we get fully integrated.
Meanwhile in Japan and India 6th grade students don't have any problem calculation precise metric volume in their heads, doing conversions from volume to weight of water, and complex calculations involving pressure, speed, distance, and volume, etc. are easily to do.
Rant over.




OK, a Corby has four measurements.
1) The shaft/shank diameter ( not always stated in the listings, but usually a standard for the head size.
2) The total length (B in your diagram)
3) The head diameter (C in the diagram)
4) Occasionally they also give the shank length total. (A in the diagram)



Here is how to choose and install them:

So, you start with the size of the rivet you want to see on the handle. Lets say you want it to show a 1/4" round rivet on the scales. They will call that a 1/4" Corby bolt. Then you look at the total handle thickness before you start shaping. On your drawing, it is .810". Pick a Corby that is just a tad longer in total length so the screw slots will be ground away in finishing.
Next see what size shaft that comes in. For a 1/4" Corby, the shaft will most likely be 7/32.

The total bolt length should be about .85-.90" or more for your suggested handle. If Corby bolts are way too long, the shaft can be ground down to about half its length if needed. On a really fat handle, make sure the counterbore hole is deep enough for the shank to go through one scale and the tang by at least .1" so the other scale will mate right. The screw does not need to bottom in the shank all the way, but it should go in about half way for the best fit.As long as the bolt fits, the head will be sawn/filed/ground down to the scales, so raely it that a problem. Because the bolts come in varying lengths, fitting is not an issue.

You need the proper counterbore ( AKA Step Drill) for the bolt you have. The best bet it to buy it from the same supplier as the bolts. They know it will fit exactly.
First, drill the guide hole ( 7/32") through both scales with the scales clamped together. I like to use a bit one size down, as the counterbore will make the shank hole fit perfect. I use a #3 bit for the guide hole. Just drill the front hole to start.
Next drill the seat with the counterbore.
I like to counterbore the rivet seat half the scale thickness. On your handle that would be about .185" deep.This gives plenty of support below the shoulder, an plenty of room for shaping the scales once assembled.
After boring both scales, dry assemble the Corby on the scales and tang. If they seat snug and the assembly is firm ... all is good. If not, make any adjustments to the shaft length needed.
Leave that bolt in place and snug ( but not too tight), clamp the scales, and drill the rear hole. Remove the front bolt and drill the seat for the back. Dry fit again to make sure all fits right. Now repeat for the center rivet if there is one.
Remove the scales and re-install the bolts. Hold/tape/clamp the scales together and shape the front of the scales where the ricasso will be. Sand to the final grit and buff. This area can't be finished once the scales are on the blade.
Assemble the handle on the completely finished knife blade ( except for sharpening) using a light layer of slow set epoxy ( I prefer 24 hour cure) on each scale. It is good to use a Q-tip and put a little epoxy down each bolt hole. Slide all the female bolts in one scale and make sure there is no resin inside the threaded hole. If so, remove it with a toothpick. Any amount of resin in the hole will prevent the screw part from going in because a liquid is incompressible. Once all the female parts are in place, lay the scale down on its back and set the tang over the shanks. Put the male screw parts in the other scale ( don't worry about resin getting on them) and place over the other half. Use your fingers or a screwdriver to gently start every screw. They should start smoothly and not bind. Ones that seem tight should be backed out and tried again to prevent cross threading. When all are started, evenly tighten them so they are all just finger tight. When they are there, give a little tweak more and wipe off all squeeze out resin from the blade with a rag wet with denatured alcohol. Pay special attention to the pre-finished front end. The rest will be ground down in shaping, so it is less of an issue. Let sit for a few minutes and snug the bolts a tad more if needed. You only need them just snug, as once the resin cures, they will be there forever. Over tightening will cause damage to the handle and possible warping of the scales as they dry. It will also squeeze out too much of the resin and make the seal between the handle and tang poor.
 
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