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Thread: Taps for wood threads

  1. #1
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    Taps for wood threads

    I recently purchased a couple of taps for making wood threads. One is 2 1/4 diameter, the other 2 1/2. Good sizes for making bench vise screws right? The problem is that the thread angles are like 60 degrees. I know that after like 1860s or 1870s 60 degrees became a bit of a standard. Wood threads need to be 90 degrees for best results right? But if taps were made with 60 degree threads, I'm guessing enough people used them that it should be ok. What do you guys think? Aren't the new commercial taps you can buy, all kind of small, 60 degrees?
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  2. #2

    60 degree standard

    American Standard taps have been 60 for over 100 years. I don't know what angle has been standard for wood taps. Metal vise threads are often Acme--flat-topped, 29. I don't recall seeing 90 threads. I have used a standard 3/8-16 tpi tap cross grain in ipe with good results. Taps need to be sharp to cut wood without tearing. Jim Davis

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    I think the 60 degree geometry has been around for wooden screws for a long time. Certainly most of the commercially made 19th and early 20th century wood taps and thread boxes which I have seen have that geometry.

    I have a tap and box for 2" dia screws which is certainly early 19th century if not earlier, with handmade screws and nuts holding it together, and it has geometry which is a lot closer to 60 than to 90 degrees. It's too worn to use, though, so I can't vouch for its practicality. I do have a 1 1/2" box and tap of the sort which was made in the later 19th and early 20th centuries, with tap geometry like yours, and I have used it with success and the screws have held up so far. A lot of old workbench vises were made with this screw geometry and it seems to have been standard for a long time. Makers in the 18th century may have thought 90 degree geometry was better, and they may well have been right, but don't let that stop you!

    In any case these taps, if you can make thread boxes to match, will far outperform the modern cheap versions made in asia. Those tend to have very fine threads, making screws which look like large machine screws. The coarser geometry of the old style cutters makes screws which are stronger and also more forgiving of variable wood shrinkage, which is actually important. I cut some 1 1/2" screws from green beech, which worked beautifully, and after the screws shrank their fit is a bit sloppy but they still function perfectly because of the large bearing surfaces of the threads.

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    Thanks for the input John. how much slack should there be in the nuts and screws? Should the cutter for the box cut a tad extra? should the top of the threads be sanded a bit to make then not so sharp?

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    Blake, I wish I could give you more concrete guidelines, but I haven't done a lot of thread cutting, and I am sort of deliberately "not an engineer" about these things, preferring to work by feel rather than by numbers whenever possible. Do you have screw boxes to go with your taps, or do you need to make them? Because I used the large diameter end of the screw box as a guide when I turned the blanks for my screws, and of course that diameter was determined by a manufacturer with a lot more experience than I have. I did turn my blanks so that they were an easy fit to the box diameter. My assumption was that if they were too sloppy I could always make more. I was working with green wood from a tree I had salvaged from a neighbor, and so I wasn't worried about expense or wasted wood - just making potentially labor-intensive firewood. as it happens I was happy with my result first time out, but as I said above I really think the coarse thread size of these taps is intended to give you a lot of forgiveness about the fit.

    That said, you definitely don't want your threads to form a sharp point, since these will certainly break off. That goes for you internal threads as well, which means you might end up experimenting with the size of hole you bore for your "nut" as well.

  7. #7
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    Wood threads SHOULD BE 90,NOT 60. Just take a close look in the Diderot Encyclopedia where they show wooden bench screws. People have gotten lazy over the years and are ignoring the correct angle that wood threads should be. The 90 angle is more blunt at their tips,and makes the more delicate(than metal) threads less easy for bits of the tips to break off.

    When I made the giant cider press screw,it was correctly made with 90 threads. Look at the threads at the top of the screw. They show up the best.

    Trouble is,if tap and thread box makers are not making them the correct angle,unless you are a machinist,you are stuck with them. Well,it isn't the end of the World.
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    Quote Originally Posted by george wilson View Post
    Wood threads SHOULD BE 90,NOT 60. Just take a close look in the Diderot Encyclopedia where they show wooden bench screws. People have gotten lazy over the years and are ignoring the correct angle that wood threads should be. .
    Why is this George? Are 60 degree tap and dies easier for lazy people to make than 90's?

  9. #9
    I thought the OP was thinking of what is called "square" threads when he spoke of 90 threads. Perhaps not. The square thread was used in applications where wear of the faces was a concern. 60 threads are stronger, but can wear more rapidly in constant clamp/unclamp sequences. That's what the Acme thread was created for--more strength than a square thread and more wear resistance then a 60. A 90. as shown above, would wear faster than a 60. Most C clamps have acme threads. Lathe lead screws and milling machine feed screws are all Acme, unless home-made or lo-o-o-o-o-w budget items.

    Jim Davis

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    I found this link to a Woodwrights shop episode where Roy makes a tool for cutting the threads on a dowel. Interesting. He gets a 60 degree angle for the thread

  11. #11
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    60 is just a standard thread for metal working. over the years the wood thread's proper form has been forgotten,and 60 degree threads are now used. But,as I said,they are not the most ideal wood thread,that being the 90 degree type,which is harder for chunks to get broken off of.

    Roy needs to delve a bit more into the proper thread form for wooden threads.

    The square thread is a purely power transmission thread. It is the most ideal thread form (in metal) for power transmission. However,square threads are more difficult to cut,so a semi square thread,the 14 1/2 Acme thread was developed. Again,it is a short cut to making a really correct power transmission thread. a 90 degree square thread will not try to force half nuts on a metal lathe to open. The Acme,with its 14 1/2 sided threads will,to some degree,try to force half nuts apart. But,like most things these days,it is "good enough". I think they had decided the square thread was too much trouble to cut,and the cutters were too much trouble to grind correctly,even before WWI.

    We in the toolmaker's shop had to replace the 6" diameter wooden screw in the book binder';s standing press. Many years ago,when someone,probably in the maintenance dept.,made the thread,they used the metal working SQUARE THREAD(with 90 sides) on the wood screw. It gradually destroyed itself. Chunks of whole thread(all the way down to the root of the thread) kept breaking off. Finally,the press was unusable. This was an EXTREMELY poor choice for a wooden screw.

    Unfortunately,my picture of the 6"dia. standing press screw has vanished from my collection of pictures. But,here is a collection of taps for threading the holes that wooden screws go through. I have made them over the years when making authentic parts for antiques that were missing their wooden screws. Note that they all have the correct 90 thread angles. The double ended tap is from making a wooden cooper's caliper for a collector. I made the thread box also,but haven't found the mating picture of it. I think that when too many pictures are added,some others get pushed out of the attachment's allotted space.

    The longest tap is not 90,but I can't remember why I had to make it that way.

    Another thing that is nearly always wrong with modern wooden taps( But NOT the 2 taps the OP has shown. Those are as late as from the 1950's,and are coarse as they should be.)is that their threads are TOO FINE. Wood threads are MUCH COARSER than metal threads would be for a corresponding diameter. This is another way to make wood threads stronger against breaking bits off. That and the 90 degree form together make the strongest wooden thread. Note how coarse the little 3/8" dia. tap is. I don't recall the thread count on it. It was made to reproduce an 18th. C. embroidering frame. The larger 5/8" tap is 9 threads per inch.
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    Last edited by george wilson; 10-07-2016 at 10:00 PM.

  12. #12
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    I forgot to mention that use of a 60 tap is o.k.,if you make the screw 90. This is because the 60 tap makes threads that bears against the ROOT of the 90 thread instead of their crests. This makes the threads on the screw even harder to break pieces off of.

    So,go ahead and use the old 60 taps. But if you can,make the screw itself 90 threads.

    The taps I showed pictures of had to leave exact copies of 18th. C. threads and the holes they go into. As I have mentioned several times,my favorite customer is SUPER eagle eyed,so I have to make every effort to make things exactly like the originals. So,I made the taps 90,except for the long one,for which I have no recollection of its purpose. Might have been made back in the early 70's. I'll admit it is frustrating to not be able to recall what I did many years ago on some things I find in my tool box! Part of getting old!!
    Last edited by george wilson; 10-08-2016 at 12:34 PM.

  13. #13
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    I think the wider angle cut with the threadbox, and 60 degrees cut with the tap is actually what Roy Underhill was doing. He used a standard triangular file to make the cutter, so the angle cut into the metal is 60 degrees. However, he filed at a significant angle to the end of the cutter, so the angle cut into the wood is much wider. If you look at the threads he cut, they are closer to 90 than 60 degrees.

  14. #14
    Perhaps a bit of clarity would help.

    By 90 degrees, do people mean a 45 degree flank angle, which forms a 90 at both root and crest? Or are they talking the machine square thread?

    I imagine the 45 would be the strongest thread in wood. But then I am not a woodgineer.
    Last edited by Glen Canaday; 10-09-2016 at 10:29 AM.

  15. #15
    Quote Originally Posted by Glen Canaday View Post
    Perhaps a bit of clarity would help.

    By 90 degrees, do people mean a 45 degree flank angle, which forms a 90 at both root and crest? Or are they talking the machine square thread?

    I imagine the 45 would be the strongest thread in wood. But then I am not a woodgineer.
    They mean the former, a 45 flank angle. I'm sure that's what George means, anyway.

    I don't disagree with George about the 90 vs. 60 thing, but I think all the guys currently making wood screws for vises are using 60. However, they are using a very coarse thread that has a big flat on top. Those two things will make the thread pretty strong (in wood).
    "For me, chairs and chairmaking are a means to an end. My real goal is to spend my days in a quiet, dustless shop doing hand work on an object that is beautiful, useful and fun to make." --Peter Galbert

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