I saw a video a while back; the Wood Whisperer, I think. He was tapping some holes in wood, for what purpose I have forgotten. He claimed that the results were pretty strong, which surprised me. Machine threads seemed too shallow for wood.
As it happens, I have a set of taps; two actually. I didn't buy either of them. A retired electrician friend of mine gave me a set when I was dealing with yet another no-longer-standard (if it ever was) electrical box buried in my plaster wall.
The other was from my Dad's garage. My Dad's set is an artifact of an earlier age ('60s, probably), complete with a wooden case with recesses for each tap (and each die). The tap recesses have a hole at one end that's a little deeper than the rest of the recess. That's to allow you press that end of the tap down in order to tilt the other end up. Nice. It looks like my Dad might have used one size of tap: 3/8". The rest appear completely untouched.
Anyway, the all that "Whispering" made me curious. I recently reconfigured my dado jig to be width-adjustable. I used a pair of Rockler star knobs and some metal threaded inserts. It works fine, but what a pain it was to insert those ... inserts.
Yes, I know there's a tool, but I don't have it. I used a the widest slot screwdriver I have, but it still slipped out of the little grooves repeatedly, especially before the insert "bit" enough into the wood to keep itself upright.
On top of that, the knobs were a few bucks each. I got to thinking that - for jigs - I might try using machine bolts and tapped threads in wood. At least for jigs that would generally be set ONCE for a project.
I decided to do a test. I bought some 5/16" x 1.5" hex-head bolts. Even at the Home Depot "single unit" price they were only 20 cents each. I could make a lot of jigs for the price of a couple of plastic knobs.
I tried putting threads in Poplar, Oak and Birch Ply. I started out with the drill size that was listed on the inside lid of the lovely wooden case. That worked, and I couldn't pull the bolt out of the wood with a claw hammer, but the crests of the threads looked flat. I assume the recommendation was for tapping metal.
I reduced the hole size a 64th at a time until the threads looked sharp. That was at 13/64". I have to say, that bit, and the hole it produced, really looked too small for that tap, but it worked fine.
I wasn't particularly careful about tapping the holes. I used a drill to drive the tap, started off slow and progressed to a medium speed. It might have taken 15 seconds to tap each hole all the way through a 3/4" thickness.
The bolts fit well, with a small amount of wiggle. I decided to try one more test, which I only did with the Oak. I threaded the bolt into the hole, but left 1/4" of room at the bottom. I put the wood right over the leg of my workbench and hit the head of the bolt repeatedly with a hammer, hard. The bolt didn't move and the threads seemed undamaged.
I'm wondering if any of you ever find a use for tapped holes. If I were building a jig that I knew I'd use a lot, and would need repeated adjustments, I'd probably use the inserts. But this was quick, efficient and cheap. I think I may give it a try for some jigs I'll need to build soon.