thread cutting tools for wood

8/10/2019 Thread Cutting Tools for Wood

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Thread Cutting Tools for Wood1 of 7

Diego de Assis

To make threaded screws andnuts of wood a set of two toolsis needed:

the die, for cutting external(male) threads, and the tap,

for cutting internal (female)threads.

In Brazil manual tap and die sets are only available for metal or PVC tubing, whichare not appropriate for use with wood. I have experimented with several alternatives,but none was satisfactory. With wood, dies made for metal leave fragile threadsprone to breaking.

A common threaded nut can be made to work as a die, as shown on some videos

available on the Internet, but they are not as practical as they might seem.
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So I turned to old methods, and to the masters, including Roy Underhill, who, inaddition to his fantastic videos, describes how to make these tools in his book TheWoodwright's Workbook.

Another important source is Andr Roubos Le Menuisier bniste, which waspublished in 1774 and has a perfect description of these thread cutters, along with abeautiful engraved illustration. [Plate 311-trans.]
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And to understand the secrets of hardening and tempering metal parts I went to

Alexander Weygers. His bookThe Complete Modern Blacksmithshows how to makemetal both hard and strong, using simple and available techniques.
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Following these and other references I describe in this article how to make manualtap and screwbox set as an inexpensive alternative, using scraps and availablesupplies, and as efficient as those made centuries ago.

First tool: the tap

To cut the internal threads of the die, you first make the tap. I made mine of anordinary steel bolt 1 in diameter, with a pitch of 8 threads per inch. The cutting teethwere quickly shaped with the edge of an angle grinder.

The bolt is wrapped in masking tape and marked into quarters with a felt tippedmarker. For a better cutting shape, the channels along these lines are cut out with aslight angle, or hook toward the direction of the intended cut, as shown in thefollowing image of the end of the bolt.


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Next, to remove the burrs left by the grinder, the bolt is smoothed with emerycloth or fine sandpaper, directed counterclockwise, or opposite the direction it willcut, tapering the diameter somewhat toward the tip. This eases the edge leading intothe cutting channel, while sharpening the cutting tips, and helps avoid damaging thethreads as the tap is removed.


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I drilled a hole in the head of the bolt to receive another smaller bolt that serves asa handle for increased leverage in use. When drilling, its necessary to use plenty ofoil to draw heat away and reduce wear on the drill bit.


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The die: shaping the cutter

Inside the screwbox is a blade ground to a V shape, a small cutter which cuts themale threads, transforming round stock into a screw. This cutter should be of highcarbon steel, which can be hardened and tempered so that it will retain a sharp edge.

Low carbon steels cannot be adequately tempered, although they can be hardened bya surface application of carbon.

I used a piece of an old file. Files are made of good high carbon steel, and they canbe used to make various cutting tools, such as gouges and chisels. There are severaltypes of scrap steel with high carbon content that can be used, but how does onemeasure it?

A quick test is to grind the metal and observe the sparks it gives off. High carbonsteel gives off many sparks. Just touching it to the grinding wheel produces a small

explosion. Low carbon steel, in contrast,gives off few sparks. In the photos Icompare an ordinary bolt of low carbon steel (above) with a piece of an old file(below).


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Another test to heat the metal to the point at which it loses its magnetic attraction(cherry red), quench it immediately in oil or water, then try to file it. If after thishardening procedure the file skips, as if it were sliding on glass, it is high carbonsteel, which can be hardened and tempered.

To test for the desired temperature, simply touch it to a magnet while it is still

cherry red. When it does not stick it is hot enough to harden by quenching.


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In an improvised forge made of bricks and a propane torch I heated the metal tothe cherry red non-magnetic point, then let the piece cool in the air, withoutquenching. This anneals the metal, so that it can be easily worked.


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To shape the V in the cutter blank I clamp it in a vise and begin with a shallowhacksaw cut to guide the triangular file used to finish the job. Then the externalshape of the V is formed. Interestingly, the metal is very soft, making it impossibleto sharpen the edge.


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Heat treating hardening the steel

With the piece in the desired shape, I set up the torch and heated it again to thenon-magnetic cherry-red point, then quickly quenched in water.

In this experiment I used a water quench with no problem. Oil can also be used forquenching, which causes a softer thermic shock. The interior of the metal will beless brittle, and thus more durable.


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A water quench could lead to tiny cracks, if the structure of the metal cannot holdup to the thermic shock. In this condition the metal is hard and as brittle as glass,with a coating of slag.


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Tempering the metal

The next step is to polish the metal, so that the spectrum of colors is visible duringthe tempering process. After polishing I again apply heat to the piece, this timecarefully and paying attention to the color spectrum that appears as the metal heats

up.

The flame should be applied to the opposite end from the intended cutting edge,so that the spectrum travels toward that end. The reheating, or tempering, willleave the metal sufficiently hard to hold an edge. The metal is relaxed a little bythis process.

This is the most delicate step, because if the heat goes past the desired color, thewhole process has to be done over from the beginning.

The color spectrum gives an approximate indication of the temperature: thehighest temperature, nearest the heat source, is light blue, about 640F. Each color

of the spectrum corresponds to a specific hardness of the steel.

In the case of the cutter for this screwbox I let the area of the cutting edgeapproach peacock or almost purple, around 540F. This is the color recommendedfor delicate cutting tools, such as small gouges.

As soon as the edge reaches this shade I again quickly quench the piece in water.


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Now the cutter is ready for final sharpening.

Mastering the tempering of metal with these rudimentary techniques involvessome trial-and-error experience, and close observation. As Alexander Weigerscomments in his book The Complete Modern Blacksmith, . . . the advantage ofmaking ones own tools is knowing that the next one will be even better!.


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The Screwbox

Following the dimensions provided in Le Menuisier bniste, I simplified the designsomewhat, leaving out some details such as the turned handles and the decoration

on the opening for the workpiece.


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The bolt I used to make the tap has eight threads per inch. For metal bolts that isa coarse pitch (pitch measures the distance between the peaks of the adjacentthreads). For threaded parts that need to move a lot and are in constant use, such asthe screw of a bench vise, a very coarse pitch is preferable (fewer threads per inch).And with wooden screws another reason for a coarse pitch is that thicker threads willhold up better.


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guides the round stock at the beginning of the cut, and the main body. After theblock is cut to dimensions the guide plate is cut away.

The two parts are then put back together and secured with two screws on diagonalcorners (placing the screws in line with the grain could split the wood). Then the hole

in the lid is bored, with a chip drill, auger, or Forstner bit, marking the desired depthwith a bit of masking tape. I used a 1 chip drill.

The first cut should go through the lid, with the tip or lead screw going into themain body to mark the center. The bit is then changed to a smaller size, 7/8 in thisexample, and the smaller hole is bored through the main body. The difference in thediameter of the holes is exactly the difference between the two bits 1/8 in thiscase.

Once the hole has been bored the internal threads are cut in the main body.

A little vaseline in the hole makes this go more smoothly.


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With the internal threads cut, the next step is to locate the position of the cutterprecisely in the main body. The tap can be a help in this, by positioning the cuttingedge in the groove of its threads.

If the cutter is not seated correctly, it can be adjusted with the screws that hold it

in place. Andr Roubo shows three screws for this purpose, but I found that only twoscrews were needed. Roy Underhill suggests a better way - an L-shaped bolttightened by a nut on the outside of the main body.


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In addition to the slots in the main body for the cutter and the channel forshavings to escape, recesses are cut in the underside of the guide plate toaccommodate the heads of the screws that secure the cutter. With the cutter in placethe tool is tested and adjusted as necessary.


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Finally the optional curves are cut on the ends of the box with a bandsaw, and thescrewbox is smoothed and edges relieved. With the tools ready, lets put them to use.

Attaching threaded screws to blocks

Of the several species of wood I tried out, the one that stood up to the cutter thebest was Itaba preta (Mezilaurus itauba). I think its cousins in the Lauraceae family,such as imbuias, louros, and canelas, would also be good for this purpose.


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A good way to use threaded screws is to make them separate pieces, which canthen be attached to just about any type of wood. Such threaded attachments have anunthreaded shaft that can be glued into a tight-fitting hole in any other piece.

To make these pieces, after cutting threads on a length of dowel it is returned tothe lathe to cut the pegs. Several separate pieces can be cut from a short piece of


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threaded dowel.

This technique can be used for furniture, toys, and utensils that can be assembledand broken down simply and securely. If you want to make your ownscrewbox,download the PDFadaptation of Roubos design, and go to it!

Diego de AssisFebruary 2011

Translated from Portuguese by Tom Holloway.

CAUTION: Many workshop procedures come with risk of injury. They should not be done without the

appropriate protective equipment, including safety glasses, dust masks, ear protection, and other safetymeasures. Only qualified people should use cutting tools; doing otherwise could lead to serious accidents.

Related Info:

A Jig For Grinding Chisels/Gougesby James D. Thompson

Using a Dressing Tool on a Grinding Wheelby James D. Thompson

Making a Scratch Stockby James D. Thompson

Sharpening a Tapby James D. Thompson

1913 -Steel: Its Selection, Annealing, Hardening and Temperingby E. R. Markham
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