Wood Thread

8/19/2019 Wood Thread

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Shop-made Thread Cutting Tools for Wood

To make threaded screws and nuts of wood a set of two tools is needed: thedie, for cutting external (male) threads, and the tap, for cutting internal

(female) threads.

In Brazil manual tap and die sets are only aaila!le for metal or "#$ tu!ing, whichare not appropriate for use with wood. I hae experimented with seeral alternaties,!ut none was satisfactory. %ith wood, dies made for metal leae fragile threads prone

to !reaking.

& common threaded nut can !e made to work as a die, as shown on some ideosaaila!le on the Internet, !ut they are not as practical as they might seem.

'o I turned to old methods, and to the masters, including oy nderhill, who, in


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addition to his fantastic ideos, descri!es how to make these tools in his !ook The%oodwright*s %ork!ook .

&nother important source is &ndr+ ou!os -e enuisier /!+niste, which waspu!lished in 0112 and has a perfect description of these thread cutters, along with a!eautiful engraed illustration. 3"late 4005trans.6

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&nd to understand the secrets of hardening and tempering metal parts I went to&lexander %eygers. 7is !ook The $omplete odern Blacksmith shows how to

make metal !oth hard and strong, using simple and aaila!le techni8ues.

9ollowing these and other references I descri!e in this article how to make manualtap and screw!ox set as an inexpensie alternatie, using scraps and aaila!lesupplies, and as efficient as those made centuries ago.

9irst tool: the tap

To cut the internal threads of the die, you first make the tap. I made mine of anordinary steel !olt 0 in diameter, with a pitch of ; threads per inch. The cutting teethwere 8uickly shaped with the edge of an angle grinder.

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The !olt is wrapped in masking tape and marked into 8uarters with a felt tippedmarker. 9or a !etter 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 !olt.


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I drilled a hole in the head of the !olt to receie another smaller !olt that seres asa handle for increased leerage in use. %hen drilling, its necessary to use plenty ofoil to draw heat away and reduce wear on the drill !it.


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The die: shaping the cutter

Inside the screw!ox is a !lade ground to a =# shape, a small cutter which cuts themale threads, transforming round stock into a screw. This cutter should !e of high

car!on steel, which can !e hardened and tempered so that it will retain a sharp edge.-ow car!on steels cannot !e ade8uately tempered, although they can !e hardened !ya surface application of car!on.

I used a piece of an old file. 9iles are made of good high car!on steel, and they can!e used to make arious cutting tools, such as gouges and chisels. There are seeraltypes of scrap steel with high car!on content that can !e used, !ut how does onemeasure it>

& 8uick test is to grind the metal and o!sere the sparks it gies off. 7igh car!onsteel gies off many sparks. ?ust touching it to the grinding wheel produces a small

=explosion. -ow car!on steel, in contrast, gies off few sparks. In the photos I

compare an ordinary !olt of low car!on steel (a!oe) with a piece of an old file(!elow).


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&nother test to heat the metal to the point at which it loses its magnetic attraction(cherry red), 8uench 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 car!on steel,which can !e hardened and tempered.

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

cherry red. %hen it does not stick it is hot enough to harden !y 8uenching.


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In an improised forge made of !ricks and a propane torch I heated the metal to thecherry red non5magnetic point, then let the piece cool in the air, without 8uenching.This anneals the metal, so that it can !e easily worked.


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To shape the =# in the cutter !lank I clamp it in a ise and !egin with a shallowhacksaw cut to guide the triangular file used to finish the @o!. Then the external shapeof the =# is formed. Interestingly, the metal is ery soft, making it impossi!le tosharpen the edge.


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Heat treating – hardening the steel

%ith the piece in the desired shape, I set up the torch and heated it again to thenon5magnetic cherry5red point, then 8uickly 8uenched in water.

In this experiment I used a water 8uench with no pro!lem. Ail can also !e used for8uenching, which causes a =softer thermic shock. The interior of the metal will !eless !rittle, and thus more dura!le.

& water 8uench 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 !rittle 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 isi!le duringthe tempering process. &fter polishing I again apply heat to the piece, this timecarefully and paying attention to the color spectrum that appears as the metal heatsup.

The flame should !e applied to the opposite end from the intended cutting edge, sothat the spectrum =traels toward that end. The reheating, or tempering, will leaethe metal sufficiently hard to hold an edge. The metal is =relaxed a little !y thisprocess.

This is the most delicate step, !ecause if the heat goes past the desired color, thewhole process has to !e done oer from the !eginning.


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The color spectrum gies an approximate indication of the temperature: thehighest temperature, nearest the heat source, is light !lue, a!out 2CD9. Each color of the spectrum corresponds to a specific hardness of the steel.

In the case of the cutter for this screw!ox I let the area of the cutting edgeapproach =peacock or almost purple, around F2CD9. This is the color recommendedfor delicate cutting tools, such as small gouges.

&s soon as the edge reaches this shade I again 8uickly 8uench the piece in water.


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astering the tempering of metal with these rudimentary techni8ues inolessome trial5and5error experience, and close o!seration. &s &lexander %eigerscomments in his !ook The $omplete odern Blacksmith, =. . . the adantage ofmaking ones own tools is knowing that the next one will !e een !etterG.


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The 'crew!ox9ollowing the dimensions proided in -e enuisier /!+niste, I simplified the designsomewhat, leaing out some details such as the turned handles and the decoration on

the opening for the workpiece.


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The !olt I used to make the tap has eight threads per inch. 9or metal !olts that is acoarse pitch (pitch measures the distance !etween the peaks of the ad@acentthreads). 9or threaded parts that need to moe a lot and are in constant use, such asthe screw of a !ench ise, a ery coarse pitch is prefera!le (fewer threads per inch).&nd with wooden screws another reason for a coarse pitch is that thicker threads willhold up !etter.

7ard woods should !e used for these parts, as with softer woods the internal threadscould wear away. %oods such as ?ato!H, Tauari, "ero!a5de5campos, Ip, and oxinhoare excellent for making the screw!ox. I used ?ato!H (7ymenaea cour!aril), which is


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hard and ery ersatile.

The screw!ox has two parts: a lid or guide plate, which secures the cutter and guidesthe round stock at the !eginning of the cut, and the main !ody. &fter the !lock is cutto dimensions the guide plate is cut away.The two parts are then put !ack together and secured with two screws on diagonalcorners (placing the screws in line with the grain could split the wood). Then the holein the lid is !ored, with a chip drill, auger, or 9orstner !it, marking the desired depthwith a !it of masking tape. I used a 0 chip drill.The first cut should go through the lid, with the tip or lead screw going into the main!ody to mark the center. The !it is then changed to a smaller size, 1J; in this

example, and the smaller hole is !ored through the main !ody. The difference in thediameter of the holes is exactly the difference !etween the two !its K 0J; in thiscase.


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Ance the hole has !een !ored the internal threads are cut in the main !ody.

& little aseline in the hole makes this go more smoothly.


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%ith the internal threads cut, the next step is to locate the position of the cutterprecisely in the main !ody. The tap can !e a help in this, !y positioning the cuttingedge in the grooe of its threads.

If the cutter is not seated correctly, it can !e ad@usted with the screws that hold it in

place. &ndr+ ou!o shows three screws for this purpose, !ut I found that only twoscrews were needed. oy nderhill suggests a !etter way 5 an =-5shaped !olttightened !y a nut on the outside of the main !ody.


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In addition to the slots in the main !ody for the cutter and the channel for shaingsto escape, recesses are cut in the underside of the guide plate to accommodate theheads of the screws that secure the cutter. %ith the cutter in place the tool is testedand ad@usted as necessary.


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9inally the optional cures are cut on the ends of the !ox with a !andsaw, and thescrew!ox is smoothed and edges relieed. %ith the tools ready, lets put them to use.

Attaching threaded screws to blocks

Af the seeral species of wood I tried out, the one that stood up to the cutter the

!est was ItaL!a preta (ezilaurus itau!a). I think its cousins in the -auraceae family,such as im!uias, louros, and canelas, would also !e good for this purpose.


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& good way to use threaded screws is to make them separate pieces, which canthen !e attached to @ust a!out any type of wood. 'uch threaded attachments hae anunthreaded shaft that can !e glued into a tight5fitting hole in any other piece.

To make these pieces, after cutting threads on a length of dowel it is returned to thelathe to cut the pegs. 'eeral separate pieces can !e cut from a short piece ofthreaded dowel.


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This techni8ue can !e used for furniture, toys, and utensils that can !e assem!ledand !roken down simply and securely. If you want to make your own screw!ox,download the PD adaptation of ou!os design, and go to itG
http://www.wkfinetools.com/contrib/dAssis/art/TapDie/cossinete2.pdfhttp://www.wkfinetools.com/contrib/dAssis/art/TapDie/cossinete2.pdf

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