I recently ordered a tap and die from Grizzly to cut 3/4 inch wooden threads. Great service — no instructions on the size hole for the tap. Called Grizzly — no clue. No big deal.
Anyone know? A 9/16 seems to be too tight. 19/32? A 5/8 is much too big.
Thanks. JET
Replies
Jet,
I find some metric specs at this European web site:
http://www.fine-tools.com/gewind.htm
Woody
Thanks a bunch. The chart indicates 16mm, which is 5/8. I'll try the 5/8 again.
1/8" less than the tap size unless you are using a very hard wood. Sometimes the threads crumble if the hole is tight. I have several including the 3/4" tap and die you have.I had to go to 15/16" on some maple pieces .This worked well, the smaller hole produced broken threads.Try 5/8" first on scrap.When using the die, the dowel should also be slightly under 3/4", even if you have to sand it.11/16" is about right.
mike
Most advice is NOT to sand something you'll be using an edge tool on, but the general advice is right. The dowel needs to be an easy fit in a 3/4 hole. Otherwise, the die will son bind and lock up.
I've used 5/8 hole size for 3/4 taps (6 tpi) in all sorts of woods. It sometimes helps to go a little larger in very hard woods. Although you can theoretically measure thread crown and root diameters, it isn't as useful as for metal. Wood threads don't cut as cleanly, and there is some 'springback' after the tap has passed. Trial and error will be quicker and more reliable. If 5/8 is too tight for your tap, go up a 32nd, and that ought to do it.
Wood threads need much more clearance than metal, because of seasonal movement. Once you've made a few, you get a feel for what will work all year round, and what will bind when the humidity rises. Since that will vary greatly depending on how severe the climate fluctuations are where you live, the only way to find out is experience.
Cheers,
Ian
Ian,you're right about sanding the dowel smaller. When I first typed my reply I said to turn the piece slightly undersize on the lathe. Then I realized he may not have a lathe and retyped the reply, using sanding smaller instead.
mike
Hi Mike - not a big deal, I just mentioned it, because sharpening and resetting those wretched cutters in a threadbox is likely to bring on a nervous breakdown. So if bits of grit DID get stuck in the surface, it would see off the edge all too soon.
In fact, I gave up on the traditional threadbox many years ago, and switched to a home-made jig for a router (published in FWW in the early 80's). With a carbide bit, I can thread anything from Balsa to the iron-like hardwoods of the Australian desert. Just try a bit of Gidgee in a threadbox to see what I mean!
Cheers, Ian
Wilkie, where you from eh?
I have a lovely piece of 3/4 thick Gidgee that has been sitting in the shed waiting for an excuse to use for about 6 years.
Any chance of passing on that jig design?
Dave
Hi Dave - Brisvegas - I see you're located at the source of all things.
Do you follow the U-beaut forum? I did a set of instructions and pics there a few months ago. If you can find that, it will save my lazy bones from having to re-do it. But if you prefer, and have a capacious mailbox, I can email you some written instructions and a few pikkies. The original article in FWW was about 1981 or 2 (i think!) and if you know your way around Taunton's archives, you would find it - it's very easy to follow.
Cheers from sunny Qld...
Ian
Ian,
Gidgee...wasn't she in all those beach party movies, back in the '60's...?
Cheers,
Ray
Yeah, Ray, but now she's grown up, we can't call her Gidg-ET any more............. ;-)
Using a dial caliper, measure the smallest diameter of the tap, at the bottom of the threads. The hole should be slightly larger than this since the threads shouldn't come to a point, the sharp edges would only crumble anyway. You may find that a metric bit will give the proper size if the jump between the inch sizes doesn't allow a good match.
John W.
It depends on the thread pitch and the length of engagement. Machinery's Handbook shows a tap drill of 0.642" - 0.653" for 3/4 x 10 tpi with less than 3/8" length of engagement at one end of the scale. At the other end, 3/4 x 28 tpi and greater than 1-1/8" length of engagement, they recommend 0.718" - 0.722".
The "rule" I learned from a machinist/instructor in college is the tap drill size is the diameter of the thread minus 1 devided by the number of threads per inch. So a 1"-8 thread will use a 1" minus 1/8" (0.875) tap drill. A 3/4" - 11 thread would use a 0.75 - 1/11 (0.659") tap drill. This works within a few thousanths for most threads - in metal. As mentioned above wood requires more clearance.
I just LOVED this post...
Last Sunday? I was watching old Roy Underhill show.. He was using/makin' a fixture for cutting threads in wood based on some OLD wooden threading tools...
I laughed so hard... He had about 7 stitches in his finger that looked like it was done with shoemakers thread.. You could see the knots in the stitches..
Damn.. I just love that guy!
By the way.. The cutter was made from a old file...
Seemed to work good! Inside and outside threads...
I purchased a 3/4" tape and die many years ago from Conover's.
They suggested an 11/16" hole. I found this to be just abit loose for my use so I took an 11/16 inch spade bit and filed the sides 1/64" on each side and this did the trick. I use that same bit for all my holes to be threaded.
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Garry
http://www.superwoodworks.com
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