Plan B. The Carpenter’s Tap
While Roubo offers advice on the tools for making smaller screws, the following translated excerpt from Bergeron’s 1792 Manuel du Tourneur (chapter 1, section 3, plate 7) addresses how to make big screws for bench vises and presses using the carpenter’s tap — taraud de charpentier. Bergeron was the pen name of Louis-Georges-Isaac Salivet, a prominent lawyer and official in the French Ministry of Justice. When he wasn’t considering the fate of the miscreants of Paris, he was treadling away at his lathe.
The tap used to make large nuts is called the “Carpenter’s Tap,” figures 3 and 11, plate 7. It is composed of two principal pieces, the cylinder, figure 4, cut with a spiral, and the nut, figures 1 and 2. In figure 11 the nut is replaced with the press A, B. We will begin by describing the manner of making these different parts.
Turn a cylinder of sufficient length and width perfectly equal from one end to the other. While the cylinder is still on the lathe, draw a series of pencil lines around the circumference, spaced at the pitch spacing of the threads. Draw lines down the cylinder along its length dividing it into six or eight equal parts.
Points placed on these lines will delineate the path of the thread. Divide each of these spaces along its lengthwise lines in as many parts as you have made divisions down the length of the cylinder. To avoid confusion,mark the points that indicate a full turn of each thread.
Starting at the bottom right end of the cylinder, at the end of one of the lines drawn along its length, use a pencil and a flexible ruler to draw a line up to the angle at the left that forms the first lengthwise subdivision, from there to the second, and so on. When you have made a complete circumference, you will have arrived at the second main division — one complete turn of the screw. Continue right to the end. This will give a very exact screw, which ends about seven inches from the end of the cylinder. On the last three inches on this smooth part, make a square head so you can fit the tap with a lever.
Make a saw from a steel blade, from 1/2 to 3/4 inch wide and five to six inches long. Tooth it very fine and mount it in a wooden back, figure 5, so that no more than a quarter inch sticks out. Fix it there with three or four pins such as those you see in a, b, c, d. This handle is just a bar of wood, nine to ten inches long, sawn along its thickness with a rounded handle. With this saw, follow the line marked on the cylinder. The back will serve as a stop to keep the blade from penetrating more than necessary. You can see in figure 4 the course of this screw on the cylinder.
Pierce at A, following the diameter of the cylinder, a flat hole capable of containing a cutter shaped like a barley-grain, figure 6, that fits it exactly and is held by a wooden wedge, figure 7. This cutter is ground to an angle of 60 degrees.
For extra strength, you can give the cutter the form shown in figure 8, seen in profile, where you have a reduction in the shank in order not to weaken the cylinder with a large hole. The shoulder you see below the bevel gives you a seat to tap the cutter back in. Instead of leaving the top surface smooth, scrape a hollow there, always protecting the two bevels that form the angle of the cutter. This ensures that the wood is cut instead of being scraped out and crushed as it would be with a plain scraper. By exposing very little iron at a time, with several repetitions, you will make smooth internal threads.
The false nut is made in several ways. We will detail the most common. Take a strong piece of wood, such as cormier (Pyrus sorbus, servicetree) or walnut. Make it four inches square for a tap of two inches, five for three, and so on, following the size of the tap. Make the thickness four times the height of the steps of the screw. After you have dressed it with the jointing plane on each face, mount it in the lathe with the universal chuck. Draw two concentric circles in the middle, figure 1. The interior circle’s diameter is equal to that of the cylinder that carries the cutter and the exterior about an inch away from that. Cut a hole the exact diameter of the inner circle. Making this hole exactly perpendicular is of the greatest importance. Next, cut away all the wood, A, that is outside the outer circle. Continue thus right up until the projecting part B has at least the height of one step of the screw drawn on the cylinder. Figure 1 shows an end view. Figure 2 shows the cross section, where A is the cylindrical hole, B is the projection, and D is the thickness that you leave raised.
Draw on the exterior of the cylindrical part — of which a, a is the diameter — a thread equally spaced with those on the tap, using the same process you used to draw the screw on the tap. Now, cut the cylindricalpart B, exactly following this line in a circular inclined plane, called a “snail,” which is made clear in figure 1 and again in figures 2 and 3. Take a piece of sheet iron equal in thickness to the kerf of the saw used to make the screw, and give it the form seen in figure 1. The interior circle in the iron must be 1/4 inch less in diameter than the opening of the hole. The exterior circle of the iron must exactly equal the diameter of the cylindrical part. After having cut out this ring, whether on the lathe or with the file, cut it along its diameter from a to b. Pierce seven holes on this circle for screws from 3/4 to one inch long. Countersink these holes and fix the circle on the inclined plane, A, B. You see by the dotted line, figure 1, that the iron circle exceeds the inner circumference of the hole. It is this excess part that catches the screw traced on the tap.
We now turn to the manner of making a nut with this tap.
Begin by making a hole the size of the cylinder in the piece that you want to thread. Push the cylinder through it with the cutter sunk in its mortise — as you see in B, figure 3, where A is the crossbeam of a press that you want to thread. Fit the false nut B into the thread of the screw and push it flush on the crossbeam. Place it squarely and fix it there with four nails a, a, a, the fourth not being in view here. To make them easier to withdraw, grease them with a little tallow.
Advance the cylinder by turning it to the right, and when the cutter has nearly entered the nut, knock the end opposite the point with the face of a hammer and a piece of flat iron until the point sticks out ever so little from the surface of the cylinder. Then, putting a lever on the head of the cylinder, turn it from left to right, feeling the cutter passing through the wood. When you reach the other side of the piece of wood (which you will easily notice by the end of resistance) take care to empty the shavings from the groove C that connects with the hollow F that you made beneath the plate B. This hollow has a somewhat greater diameter than that of the screw. Turn the cylinder backward until the cutter exits from part A. Drive the cutter a little more, and turn it again through the nut, repeating until it is the proper size for the screw.
After having repeatedly run it through, set a bit deeper each time, judge by the length of the protruding cutter if the thread of the nut is deep enough. Withdraw the cylinder entirely and see if the screw, which must have been made first, enters easily enough. If it is wanting a little, put the cylinder back in the false nut and drive the cutter deeper, taking only a very little wood at a time so that the nut will be clean and smooth. It is good that the screw be more tight than slack in its nut, seeing that the wood, no matter how dry it is, always contracts a bit. Rub the screw with a bit of soap and not grease.
Now to make the screw. If the screw does not exceed three inches in diameter, make it with the ordinary screw box. If larger, use the following methods. Turn a big cylinder, figure 10, with a stout head A and a spindle B. If the screw must meet with great force, as that of a heavy press, furnish the head with two iron bands D, D, so that the lever passed through the mortises E, E will not make it split. Pierce a hole in the center of the head suitable to receive an iron bolt C on which the head and the collar are turned. The body should be square with a square hole to receive a key, d, which keeps it tight in place.
Divide the cylinder in eight, or better yet, in twelve parts, with lengthwise lines. The more the division is multiplied, the more accurately you can lay out the thread. Determine how many turns the screw must make over a certain distance, carefully matching the nut, and mark these divisions down the cylinder. Subdivide each of these divisions into as many parts as you have put lengthwise lines on the circumference, and with a chalk or graphite, trace the thread in the manner that we have detailed above.
Now, trace a second helix in the middle of each turn of the first helix, which will seem to double the screw. If you fear making a mistake, make one line in black and one line in red. Now use a saw in a handle like that in figure 5, or indeed an ordinary saw, but the first is more sure. Set the blade to project all the depth that the thread of the screw must have. Saw a line to its full depth exactly following the black or red line, as you have determined.
When this saw cut is made along the length of the screw as you see in a, a, a, a, a, take a mallet with a well sharpened firmer chisel, and cut down, little by little, all the wood, beginning about 1/8 inch from the line of the saw cut, following a slope, seen in b, b, b, b. When you have thus rough-hewn the thread on one side, do as much on the other; next, with a chisel that cuts very keenly, finish giving the thread the form it should have, cutting from the line to the bottom of the saw cut, taking care that you don’t leave chisel marks. This will give the screw the form you see in c, c, c. To finish it off, pass a half-round rasp over all its surfaces to remove flats of the chisel that always remain. This operation is done with the cylinder lying on a bench, where it is held by its weight if it is heavy, and by a holdfast if not. It is not necessary to follow a thread to the end, endlessly turning the cylinder. You can give a cut of the firmer chisel to all the threads, on the same point of the circumference, as, for example, here at the summits a, a, a, a, a, a, a, a, etc. Then, turning the cylinder a little, you can hew the next summits that present themselves.
Ordinarily, for the screws of screw jacks, wine presses or large machinery, you take some very sound wild stock, some service tree or unhewn walnut, that is to say, the round log — never the quarter — or, finally, some good elm. Carpenters make all their cylinders with the besaiguë with much art. But they never attain the perfection of those made on the lathe.
We will not give here the description of a wine press. These works of carpentry are not our subject of discussion. We will content ourselves to say that the part in which the screw passes in a strong press is called the sommier and is made as seen in A, figure 3. It is assembled in the two uprights by means of the fork at either end.
Tapping by the preceding method, one is obliged to fix the false nut on the part with four nails or screws, leaving unsightly holes. It is also inconvenient to back out the cylinder from its guide. An extremely clever means to overcome both disadvantages was communicated to us by the distinguished artist, Hulot. Two strong crosspieces of wood A, B, figure 11, notched in the middle, are assembled by means of two wooden screws C, C, which enter square to part B, and whose other end, partly tapped, enters holes bored in part A. These are retained by means of the two nuts L, L, which one sees better in figure 14, which is part A, figure 11, seen endwise.
Toward the edge of the notch are small, pointed eye bolts B, B. These points stick into the part D that one wants to tap. They are used to prevent the tap from shifting during the operation. If you fear that the marks of the points will spoil the part, you can back them off.
The top of part A carries a false nut, like that in figure 1, that sits on or is set into the piece. If the nut is not yet to the final size, replace part E, F, figure 11, with the cylinder fitted in the guide, after having advanced the The Carpenter’s Tap 157 cutter a little bit. The cylinder is similar to the previous one, except much longer. Part B is pierced with a tapped hole much larger than the cylinder, as one sees at c, c. Turn a piece of wood, figure 12, which is also tapped, to enter the threads c, c, with a rim F, of a larger diameter, which shoulders against part B. In the center of this disc is a hole A, figure 12, just fitting the cylinder G, figure 11. The edges F, F are grooved, so that one can more easily screw and unscrew this part according to need. It is represented in cross section in figure 15.
When one wants to test if the screw goes well in its nut, part F is unscrewed. The hole it exposes is much larger than the cylinder, and when the cutter is withdrawn, there is room to enter the screw, figure 16, to test it.
Figure 14 is, as we said, part A, figure 11, seen from the side. One sees the two nuts L, L, and the false nut M.
Instead of a lever, one can use a wooden crank I, K, figure 11, to get a continuous and more uniform motion. This works only for smaller nuts. It is always necessary to use a lever for heavier parts because of the turning resistance.
Figure 16 is a screw from a carpenter’s bench vise or a sawing press. This is the size of the screw for the nut that we are making — part D is the leg of a carpenter bench. The head of the screw is fitted with a broad iron hoop, inset well into the head and resting against a shoulder at a, a. Turn a good flat shoulder at b, b. Fix the hoop on the head screw with four long wood screws, as one sees at d.
Bore the head through its diameter to receive the lever, which tightens the jaw of the vise against the bench. To make the front jaw return when one screws backward, make a small groove c, c on the screw neck in which one or two keys ride, which pass through the thickness of the front vise jaw. By this means, the screw can turn easily, but when one turns backward, it takes the vise jaw back with it.
"The Woodwright's Guide: Working Wood with Wedge & Edge" By Roy Underhill
© 2012 The University of North Carolina Press