doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Tapered wood screws

For what it is worth, in Wooden Boat some years back they carried out holding tests for conventional wood screws and threaded machine screws in wood. The machine screw with a tapped thread had greater holding strength, in wood. Might be worth thinking about when deciding on new screws.

Makes sense to me. I cannot see why the parallel sided machine screw would NOT have greater holding capacity. The only way the tapered screw would have close to equal holding would be if the tapered screw were put into a tapered hole that allowed the threads somewhat near 100% grip, without tightening to the point that wedging (or splitting) occurred.

With a tapered hole and screw, when it is being pulled out by a force, once it moves any at all outward, the only thread having full engagement is the outermost one (largest diameter thread). However, with a parallel sided screw all threads are holding an equal amount when an outward force is applied, even if it moves.

Another way of saying it is, once a tapered screw is moved outwards of it's "seat", the only wood that has 100% holding capacity is that wood above the top thread. When a parallel sided screw is pulled outward, all the wood between all the threads has to shear for the screw to break free.

MY opinion only, SRH

I've often thought the tapered wood screw a terrible design based on a simple misunderstanding of how a wood screw works in pulling wooden or metal to wood things together. The tapered part, when drawn tight, adds little or no value to the pull. I get best results with a straight shaft screw with pilot hole the size of the shaft less threads in the "purchase" side, and a larger hole that the screw loosely slips in on the "pull" side.

Mike

I would think for the tapped hole with machine type screw in wood the thread would need to be relatively coarse. Screws for aluminum often use a coarser thread than those for use in steel. The coarser thread give a greater thickness to the thread, thus a higher shear strength. Necessary lengths of engagements for screws is based upon the strength of the crew body vs the shear strength of the threads. It is usually desirable to have this proportioned so that if one Over Tightens the screw the screw itself will be popped rather than the threads stripped. The neded length ratio is going to be longer for wood than even for aluminum. Wood screws should of course not be taken ou & back in any more often than is an absolute necessity. Even in aluminum when it is desirable for the screw to be often taken in or out steel thread inserts are quite often resorted to.

I believe you are all missing a factor when considering a tapered screw. Wood does not act as a solid as metal does. If a straight screw is used, only the threads have purchase and the wood can easily shear. A tapered screw will wedge its way into the wood and the wood will have a vise like clamping affect on the screw. This may double the holding ability of the screw. Longevity is dependent on the number of cycles in/out and the density of the wood itself.

Also, the first screws were filed out, by hand, and I suspect a tapered screw would be easier.
Mike

Bingo

This wedging has to be considered in light of whether or not a pilot hole is drilled for the screw, the size of the pilot hole, and whether or not it is a straight sided hole, or tapered like the screw shank, and the type wood being used. I have a set of tapered pilot hole wood bits. If sized properly for a piece of dense, dry wood, like gunstock walnut, not much wedging should take place, due to a danger of splitting. Same with heart yellow pine and some others. It is all about the application. There is a paper available to read that was published by the U. S. Bureau of Standards about 1926, I believe, which demonstrates the holding power of various wood screws in various woods. Interesting reading.

SRH

I really cannot say why wood screws are tapered, but I suspect they were originally used like a nail, no pilot hole, used in soft wood and screwed their way in.
Coarse machine screw threads, ie; 8 x 24 makes a good wood screws, Winchester used this screw thread for the guard screws on the M21, because that is generally cross-grain. But, machine screws don't hold well in end grain, ie; butt plate screws. If I need to make butt plate screws I cut the screws on a lathe and cut a coarse screw thread with a flat pointed threading bit so it cuts a wide root with a thin crest, as a wood screw has. These hold well in end grain. And a pilot hole is required.

In an earlier edition of Gunmaker, the publication of the American Custom Gunmaker's Guild, I did up a short article on these coarse screws with more specification.

Note that an 8-24NS would be "special" order. A #8 coarse is 8-32NC & fine is 8-36NF. When placing a tapered shank screw into a tapered hole it does not have to move far from full contact to establish good gripping powers. it would of course be less in dense wood such as Walnut than in a softer wood.
Dennis, is some firm regularly making these coarse thread screws & taps you mention at affordable prices? They are of course outside the realm of ordinarily procurable machine screws.