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By rxh
#1274701
Modern taps for wood have an angle of about 60 degrees. I have seen much larger angles on old wooden threads like this example from an olive press in Mallorca. I suppose that this one was made by drawing a helix and chiselling out the waste. I'd like to know how such large female threads were cut - a tap would require enormous effort to drive it.
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By AndyT
#1274703
rxh wrote:At a closer look you don't have a topslide so thread cutting is harder for you :)


No, I can't use that excuse. I did approximately split the angle when cutting the thread, so as to mainly work down one side, but this doesn't feature in the photos.
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By toolsntat
#1274715
Sorry for the wild goose chase Andy.
I've found it (p59) in the Colonial Williamsburg publication "Historic Trades" hiding behind a TATHS journal :oops: #-o

The Carpenters Tap
Roubo, L'Art du Menuisier III 1769-1775 translated by Robert Lyon.

Cheers Andy
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By Sheffield Tony
#1274775
I had a closer look at mine. I'm sure it is a poorly made replacement V cutter - it is too prominent for a start, sticking out a long way beyond the wooden threads. But the give away is the tap, which is a symmetrical 60 degree triangular thread profile. Interesting design on the tap - it is bored out up the middle, with cross-drillings to make the cutting edge, and somewhere for the shaving to go.
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By AndyT
#1276207
Quick update on this.

After playing about with some more experiments, using different scraps of wood and different sized preliminary holes, I found I was getting a very sloppy fit. This is the reason why:

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I'd made the tap 1/16" of an inch oversized. :oops:

So, it was back to the lathe, being pleased that I hadn't changed the way it was set up, or removed the end holes in the tap. A few passes with a file while the work was spinning removed enough material from the diameter, but that left teeth that were too shallow. I managed to line up a cutter, with the leadscrew set to work at the right tpi, and follow the original angle close enough. I really don't think I would have been able to do this with a powered lathe or a finer thread. As it was, I was able to pull the lathe round by hand, scraping away on one side of the thread, to deepen the teeth.

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A few passes and it felt like it would work, and it does.

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Sharp eyed readers will notice the addition of a cross-bar and retaining screw as well.

Now I could use the tap and box combo to make something, but I'll put that in the Projects, as it's wood, not metal.
By xy mosian
#1276262
Good save Andy.
xy
By woodfarmer
#1276399
A simpler way (lazy me) is to use over size round bar.
suppose you want a one inch tap. 25mm
use a 1 and a half inch round bar, mark the centre and scribe a 7/8 inch circle.
centre pop a few times around this scribed circle then drill holes. 4 equidistant is about right. Hole size not really critical 1/4 to 3/8 about right.
Now turn down the bar to 1 inch, you have your flutes and because the centres are inside the 1 inch diameter they will be a bit off set and will give a good cutting edge.
Cut your thread.. and you have a tap with not too much work.

suggest you make a wooden dummy one first to get the feel of what happens.

regards

hope this is of use to you
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By AndyT
#1276402
Thanks for the suggestion, but in this case I was also looking for something I could make with a bit of steel from the assortment of oddments I already have. Not always the easiest option and I didn't explain that in the original post.