Making a Tap to Thread a Hole
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11 March 2015 at 5:08 pm #125469
I’ve been experimenting with making threaded holes and eventually want to make screw boxes to make threaded rods. Thanks to Frank Joseph (@frankj) for some helpful tips on this. I bought a 3/4″ bolt and, using a triangular file with one side of the file in line with the center of the bolt, filed four grooves into the threads parallel to the axis of the bolt. This created cutting edges. To help the bolt get started in a pre-drilled hole, I put a gentle chamfer on the end of the bolt by filing so that the end had slightly smaller diameter. I also made a handle out of some scrap wood.
To try out the tap, I predrilled two holes in pine of 11/16″ and 5/8″ and screwed the tap into the holes. This worked well, though with the 11/16″ hole, the threads were not of full depth. The 5/8″ hole was the proper size. I later tried the 5/8″ hole in hard woods – cherry and maple, clamping the wood in a vise so that it would not split when running the tap through. These worked well, and I found that lubricating the tap made things go a lot easier. I’ve read that it might be good to oil the inside of the hole before tapping to make the wood less brittle and minimize tear-out.
Getting the tap to go in straight was also a challenge. Before tapping the last hole, I bored a 3/4″ hole in a board and clamped it just above the hole to be tapped and that helped me get the tap in much straighter.
Anybody have any tips on making the screw box? I’ve read Roy Underhill’s article on Roubo’s technique here: http://www.pbs.org/woodwrightsshop/project-guides/making-wooden-screws/ It looks challenging to get it right.
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You must be logged in to view attached files.11 March 2015 at 7:10 pm #125475@under2x, Thanks Joe. I looked at that episode yesterday. I’ve just ordered from McMaster-Carr some 1/4″ x 1/4″ O-1 steel to make a blade. I do have some old files I could use, but I’d have to anneal the steel and I’ve never tried annealing anything before.
I’ve also never tried hardening and tempering anything before, which I’ll have to do after shaping the cutter. That should be an interesting experience.
12 March 2015 at 3:20 am #125533Matt – Let me suggest the lazy man’s way to anneal. Next time you’re barbequing with a charcoal fire, build it a little bigger then you need. After you’re done cooking and the coals are still nice and hot, put in your old files, cover them up and close the lid. The next day the fire should be out and the files annealed. The slower they cool off the better.
12 March 2015 at 5:37 pm #125552@sidreilley Thanks Sid. But don’t you have to get the file to a cherry red glow before letting it cool slowly? Is is good enough to just let it heat in the fire without the red glow?
Also, if you close the lid on the fire, will that starve it of oxygen and make it go out and cool faster?
12 March 2015 at 6:28 pm #125553I can’t speak to getting the iron to cherry red, but I’d suggest having the BBQ closed to slow fire is probably a good idea. You want your steel to cool slowly, so having the fire run cooler for a longer time, is probably for the best. Another thing you can do is to heat it up and bury it in hot sand. I’m sure I read that somewhere anyway.
What I can say is that you probably shouldn’t use bolts too much for tapping threads in wood. Generally speaking wood screws want a much lower TPI than in metal. This serves the dual purpose of making them more durable, and allowing them to turn faster. Not sure how you’d make a custom tap from scratch though. I suppose you could use some old busted auger bits, with the spurs worn off.
12 March 2015 at 7:30 pm #125555You do need to get the steel to its critical temperature to fully anneal it. This is often described as cherry red, however that is a somewhat wooly description of the temperature as it depends on the amount of ambient light you’re in and your perception of colors. A charcoal fire the right size and shape should be capable of reaching critical heat (the Japanese swordsmiths used charcoal in their forges and furnaces) but you will have to watch the steel and try to get it to a dull red ( little work with a propane torch might help). It also depends on the size and composition of your files, obviously a big, thick one will require a good bit of heat and if it is made of complex or (gasp) stainless steel that will take a higher temperature. I shouldn’t worry about that however. IF you fall a bit short of heat and the file only turns, say, blue, it will be tempered. It will not be full soft but softer then it was (about like a saw plate) and will be easier to work with hand tools. Hot sand would work well if you want to get some and heat it up, but since you already have some hot ash at hand…..
Hope this helps, I’ve tried to keep this simple without going into a lot of theory.
13 March 2015 at 12:17 am #125560Thanks Sid and Moon. I do remember something else about the annealing – that when it gets to the proper temperature, it is no longer magnetic. I guess that’s how I’d check to see if it’s hot enough before cooling it slowly. I’ve already ordered some O-1 steel, which comes annealed, so I won’t have to worry about that part. But after shaping it, I will have to harden the steel and then temper it, which I think I can handle with a BBQ for the hardening and an oven for the tempering.
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