The following is a pictorial of my "hidden pins" experiment.
When you are laying out your metal bolsters plan for how many pins you want to put in and where. In the case of a rear bolster maybe you'd want two for added strength. In my graphics I will show one for simplicity.
Drill the holes with a 9/64 bit. If you have numbered bit use a No.29. Tap with an 8-32 tap. Use a little cutting fluid and de-burr after tapping.
Once your holes are drilled and tapped, you can go ahead and heat treat the blade as you desire.
This overall pin length is also dependent on Step 6 and how much material you remove from inside the hole.
My rule for this is SQUISH AN EIGHTH.
The ExperimentThis is what I used for my experiment. I didn't want to waste a bunch of good materials so I used reasonable substitutions
1/8" cold rolled steel
3/8" x 1/2" alloy bar, 2 pieces
Machine screw, 8-32 x 1" * Later found this was steel screw with brass coating. Caveat emptor!!
9/64" HSS bit - for tapping to #8
5/32" HSS bit
We don't want to drill any deeper than 3/16".
With the diamond ball moto-tool bit make the bottoms of the holes larger in diameter than the hole openings. This will create a taper.
My diamond ball moto-tool is a weird shape. I'd prefer to have a real Dremel such as 7103.
Now on to the tag and pin.
Drill tang with 9/64" bit, add a drop of cutting fluid and tap to 8-32.
You need to calculate the correct length for your application. Different tang and bolster thicknesses alter this length.
For 1/8" tang and 3/16" deep holes I used a 5/8" length of machine screw. For 1/8" tang and 3/16" deep holes + /8" squish = 5/8".
Basically you want to "squish an 8th" for an 8-32. So deeper holes or thicker tangs need to have longer pins.
Hole1 depth + Hole 2 depth + tang thickness + 1/8" = screw length.
If you really hog the holes in the bolster pieces, use 5/32" as your squish factor.
The screw appears to assimilate into the bolster. Nice!
I think this is going to be satisfactory way to do hidden pins. Two pins would certainly make a bolster very secure and add some good epoxy in the mix, this method of connection will last a lifetime. I will use and refine this process moving forward and ensure that mechanical fastening always works for the benefit of my customers.
Real World Application
Here I am using 8-32 stainless machine screws and 304 stainless bolster pieces. I drilled these to out with a 9/64" bit to a depth of 3/16" and the holes are hogged out with a Dremel. The pins are cut to 9/16"
The tang is drilled and tapped to 8-32 (do this machining before heat treatment) and the pins are threaded in 1/2 way.
Here I added some epoxy as a sealer. This will keep moisture, blood, food, whatever nasties want to collect in any microscopic cracks.
Pressing with the Hydraulic Pin Press. You can make something like the one I have for way under $100.