Spiral threads have been cut in wood for centuries. I discussed making thread by hand earlier. During the nineteenth century the tools had been perfected but still retained their original design. There are two components to threading; there is the screw and the nut. The screw has external threads and is referred to as the male element and the nut has internal threads and is referred to as the female. While these can be meticulously cut by hand, it is much easier to accomplish this by using some simple tools. There are two tools used to make the threads by hand and they are the tap and the die or die box or screw box. Wooden threads can also be cut on specialized lathes from a design first proposed by Leonardo da Vinci. This discussion will be about using the two hand tools to make the threaded screw and nut.
There is nothing quite like creating wooden threads. The process is a unique experience and the results can be rewarding. You are capable of making your own wooden screw clamps, veneer presses, vices and adjustable items such as music stools, embroidery frames and candle stands. Threading wood is something unique, a wooden nut and bolt is unusual, something that few others have. The projects, tools, furniture and fun things you can make with a set of these tools are endless. Wooden threads have always intrigued me; there is just something fascinating about them.
Wooden Thread Cutting Tools
There are two ways to go, the first is to buy a set of wooden thread cutting tools or you can make your own. The new wood threading tools are of good quality and generally follow traditional designs. If you choose to make your own you will need to have made a tap of the proper size that you are interested in making. This should have sharp edges to make clean cuts; a machinist or good blacksmith can fabricate a tap to your particular dimensions. These taps are similar to modern metal cutting taps except they are larger and have fewer teeth per inch.
Eight teeth per inch is about the minimum for a ½” tap and larger should have 6 teeth per inch and on very large screws for presses 4 TPI to withstand the pressure. To make a tap the threads can be cut in the metal for the tap and then the four sides flattened to produce 4 cutting edges at each thread. Regular taps have a tapered end to make it easy to start the tool into the wood. Bottoming taps are not tapered but will cut to the end or the bottom of a blind hole. Do not use a bottoming tap for initial threading as it can easily cut off centered internal threads.
There is another old design that has the threads machined on the outside and a hole drilled in the center of the end of the tap. On the end of the tap, the threads are machined down to form the taper and at the first thread a small hole is drilled at an angle into the large center hole. This forms a very sharp tooth that cuts and the chips go through the small hole and out the larger hole in the end of the tap. This type of tap does produce a very smooth cut, but the traditional 4-sided tap, if it is sharp and used properly will also produce clean cuts. The wooden handle should be strong and long enough to give leverage for the sometimes difficult process of cutting internal threads. The handle should be rounded and shaped to fit the hand as the hand is touching the handle a lot during tapping.
Once the tap is made it is possible to make the die box and all that is required is a V-shaped cutter that is secured in the box and cuts the external threads. Unlike the tap, which will cut with four cutters on each thread, the die has only one cutter that cuts all of the threads. A proper sized hole is drilled for the tap into a piece of hardwood such as beech or maple, see list below. This hole must be square to the body of the die box. The holes are then chamfered or countersunk to prevent the tap from splitting out the wood as it enters and exits the hole.
The tap should be lubricated with linseed oil to make the threading easier. The tap is then inserted carefully into the hole and started squarely to insure a straight threaded hole. It is very important that the tap is started perpendicular to the surface and square to the hole. If the tap binds up gently back a ½ turn then start again, if it becomes too difficult, remove the tap, lubricate and try again. Make sure the exit hole is countersunk to insure that the tap doesn’t break out any wood when it exits.
Now that you have the die box drilled and threaded, the next step is to cut a mortise for the V-shaped cutter that is on the front leading edge of the die box. The cutter is positioned right over the first complete thread peak at an angle of 30ºand the leading edge of the cutter should engage the wood at the widest part of the cutter first to score the wood being removed. The end of the cutter is ground at an angle of 15º with the top of the V leaning forward, with the bottom of the V trailing.
The cutter needs to be sharp and set to cut just slightly deeper than the threads that were cut by the tap. This insures that the newly cut external threads will not bind up in internal threads of the die box. The die box can also be equipped with a removable plate that will center round pieces as they are fed into the screw box. The plate needs to be thick enough to line up the piece to be threaded and removable so the external threads can be cut all the way up to the shoulder of a turned piece if necessary. The internal threads of the screw box should be well lubricated to make the cutting of the screw shaft easier.
Nut – The Nut is the part with the internal threads that are cut with the Tap. The nut is prepared by drilling the hole using the following starting holes sizes.
Starting holes:
½” threads use a 3/8” drill
¾” threads us a 5/8” drill
1” threads use a 7/8” drill
1 ¼” threads use a 1 1/8” drill
1 ½” threads use a 1 3/8” drill
1 ¾” threads use a 1 5/8” drill
2” threads use a 1 7/8” drill
2 ¼” threads use a 2 1/8” drill
2 ½” threads use a 2 3/8” drill
It is important that the hole is drilled square and perpendicular the flat surface of the nut. The wood for the nut should be a wood that is capable of taking the threads. While most hardwoods will hold the threads some are better than others. Beech, maple, hickory and oak can be used for nuts and will take threads, as can alder, elm and poplar. Some brittle woods such as cherry are difficult to thread, but it can be done. The thicker the piece of wood that is threaded, the stronger the threads will be.
On thin pieces for the nut, the threads can easily be cut at an angle, so make sure the hole is straight and the threading is done properly. When threading in an angled hole, the grain orientation is important as is beginning the cutting with the tap. The tap needs to go straight down the hole; if you get off at an improper angle the threads will be too deep on one side and too shallow on the other. While the internal threads on the nut are not as critical in terms of strength, the screw needs to be constructed of specific woods.
After the proper sized hole is drilled, the edges need to be chamfered or countersunk to prevent split out during the entry and exit of the tap. This must be done on both sides, as the tap will chip out the wood. The tap is lubricated with linseed oil or beeswax and it is inserted in the starting hole.
It is very important to make sure that the tap is perfectly square and lined up with the hole. The tap is twisted and forced into the wood, taking care to make sure that it is perpendicular to the surface of the nut. Enough downward pressure is exerted to engage the tap into the hole, after the cutting begins, the tap is self-feeding. If the tap binds in the hole, gently and carefully back it off a half a turn and start again. If it still binds, back the tap out of the hole, lubricate it again and carefully start the tap back into the cut threads.
Be careful when doing this so you don’t cross thread the screw and ruin your work. Every once in a while, back the tap out a half a turn and continue until cutting becomes more difficult, then repeat and go at it again. It is better to take your time and make sure that the work is done properly. Continue until the tap comes out the exit hole, clean out the shavings and back the tap out of the hole.
If you are threading a blind hole, your starting tap will hit bottom, then back out the tap, remove the dross and carefully place the bottoming tap into the threads and run it down until it cuts the internal threads on a blind hole. If the wood is fuzzy on the inside of the threads, I wet the piece with water and raise the grain. I allow it to dry completely and run the tap down the hole again to remove the raised grain and fuzz. Sometimes running the tap in from the opposite direction will remove the fuzz and clean up the internal threads.
Screw – The Screw is the part with external threads and is cut by the Screw Box or Die. The selection of the material for the screw is important as certain woods make excellent threads while others are more difficult. Softwoods are difficult without an extremely sharp cutter in the screw box. Hardwoods are preferable and woods such as beech, maple and hickory are the best for wooden bolts or screws, those with external threads. Strong, tough woods such as elm are better than brittle woods like cherry. Walnut also accepts threads as well. With care any wood can be threaded.
It is also important that the grain be as straight as possible. This is for strength and for a more uniform cut. The piece to be threaded should be turned to the size of the thread box. Therefore if the threads are for a 1 ½” screw then the dowel or piece should be turned to just under 1 ½”. All it takes is just a 32nd under to make the piece just the right size. It may take a slightly smaller say 16th under to get a proper fit, it may take some experimenting. But I guarantee if it is too large the threads will crumble as it is forced through the screw box. If it is too large it will not fit into the screw box and if it is too small it will not properly thread through the screw box. A slightly smaller screw works much better than one that is too tight. I like nice crisp threads, so I always turn the pieces just under the required size.
For some applications where you don’t necessarily need sharp peaks, such as heavy duty tools, the screw blank or dowel can be slightly undersized producing flat topped threads instead of sharp peaks. Small fine threads such as 8 threads per inch can be difficult and these fine external threads can easily break off. Denser woods work better for these fine threads. If the dowel is undersized, it is important to make sure that the screw box travels over the dowel in a uniform manner to insure proper threading. If flat-topped threads are required, I usually turn the dowel to the proper size, thread the piece, then re-chuck it in the lathe and turn off the peaks.
When you turn the screw or dowel on the lathe, just use your gouges and chisels, do not use sandpaper. The sandpaper can leave residue in the wood fibers that can dull the cutter in the screw box. Also you will want to chamfer the edge of the dowel or screw blank to make it easier to start the screw box.
I always dip the end of the screw blank or dowel in linseed oil to provide lubrication for the cutting process. I usually clamp the screw blank in a vice to hold it during the threading process. It is important to make sure that the screw box engages the screw blank or dowel perfectly square to insure accurate threading. I always look down the waste hole in the screw box where the chips come out to see how the cutter engages the threads. I try and exert enough pressure to engage the wood on flat grain first rather than the side grain. It just seems to start better if the cutter enters the wood on the flat grain.
Once the cutter has began to make threads they engage and pull the screw blank into the screw box, so the pressure can be reduced. After cutting begins, simply turning the screw box is sufficient. When the cutter has made the first part of the threads, they will engage the internal threads of the screw box and advance the screw into the tool creating a perfectly cut spiral thread. The first ½” or so is usually not perfect and I always allow for an extra half inch or so to cut off after the threads have been cut.
Once the cutting has started, the tool should ‘sing’ through the work. If the stuff is tight in the tool, the cutter may be set too shallow or the dowel is too large. Most screw boxes have a removable plate that guides the screw blank into the screw box. This is removed if the threads are to be cut up to a shoulder. The plate should be used to cut well into the screw blank and can be removed to thread just the last inch or so. A properly positioned and very sharp cutter will make the cutting much easier.
It is important that you keep your tools sharp and well maintained. The teeth on the tap need to be clean and sharp at their cutting edges. The new made tap and die sets require sharpening. The screw box has a V cutter that needs to be honed mirror shiny on the outside and perfectly flat on the inside. The V-shaped cutter in the screw box should be ground at the proper angle of 15º and should be very sharp and properly positioned. The bevel is ground on the outside of the cutter. Both edges must be honed for a good clean cut.
New set of ¾” taps and V-cutter, factory ground, NOT sharp. Note improper ground V-cutter, burr protruding from the bottom of the V.
New set Sharpened. Threads and slots have been dressed and V-cutter sharpened and honed.
The tap needs attention as well. The V-teeth need to be dressed to remove the grinding burr left during manufacture. Make sure to get both sides of the slot and both places where the slot and the V groove meet, especially on the leading (cutting) edge. Use a triangular file to dress the teeth. Use a thin flat mill file to dress the gullets so the cutting edge is sharp. Then go back and re-dress the V-grooves with the triangular file to remove the last of the burrs.
The sharper your tools the easier the work. I use a bit of sandpaper over the files to hone the edges to a mirror gloss. Your holes need to be clean, straight and countersunk and your turnings need to be of the proper size and chamfered to produce the desired results. Use linseed oil or beeswax to lubricate the parts being cut, it just makes the job easier. Make sure the tool engages the work squarely to produce quality work.
Clean out any shavings that can interfere with the cutting operation. Work slowly and carefully. This is not like cutting metal threads; it is a continuous operation, only backing out when the cutters jams or the cutting is complete. After you are finished using the tool make sure to clean off all excess linseed oil before it dries!
There is no end to the possibilities for using wooden threaded devices and the results are delightful. Wooden screws are capable of exerting incredible pressure when used for clamping applications and can provide for ease of assembly and disassembly for transportable furniture. There are endless applications and uses of wooden threads and they are fun to make.
Stephen
