For the purpose of properly measuring nut-screws during the screw-cutting, two or three sorts of gauges are employed. These consist of sheet gauges, wire gauges, the author's valin, and the screwed gauges, such as hobs, taps, and plugs.

The simplest mode of measuring a nut-screw consists in using the flat sheet gauge, to which the hole was bored. If this method of measuring is adopted, the hole to be screwed is bored so that the gauge fits tight therein, a minute quantity of metal being left for the screw-tool to remove while finally adjusting the smallest diameter of the screw to the finished size. In order to make a number of nut-screws to one exact size, by means of a sheet gauge, a two-teeth screw-tool must be used which has been properly cut with a hob, to make the two teeth to the exact required shape and length ; consequently, if the screw-cutting is continued with such a tool until the sheet gauge will exactly fit the smallest diameter of the screw, the largest diameter of it must be that which is required, if the right tool is used, because its teeth are of a known and prescribed length, and therefore extend into the metal a known distance. A sheet gauge in use for measuring a nut is shown in Fig. 1056.

A wire gauge for screw-cutting is a piece of wire pointed at each end, and should be of steel, to avoid the risk of wearing the points, and thereby making the gauge too short while in use. The length from one extreme to the ether is not analogous to the extreme diameter of a sheet gauge, because a wire gauge is intended to measure the largest diameter of the screw, and not the smallest. The length of the wire is a trifle greater than the greatest diameter of the intended screw, and this trifle is more or less according to the relation between the diameter of the hole and the step of the thread to be cut. The points of the gauge are smoothly filed and thinned to exactly fit the bottom of the nut's thread-groove; consequently the points should not be circular, but oblong, in order that about an eighth of an inch of surface may exist for contact with the bottom of the groove during measurement. A piece of wire thus shaped can be gently screwed into the nut as soon as the groove is deep enough, the mode of entering the gauge being similar to the entering-in of a bolt-end. A gauge of this class is shown by Fig. 1057, and a similar one, but with a wire handle, is indicated in Fig. 1058.

To adjust the wire to a proper length, it can be fitted into a nut which is already screwed, if such a nut is known to be of the desired size, and can therefore be considered as a standard; in which case it is not requisite to know the exact length, because the length is known to be correct as soon as the gauge will properly screw into the standard. But if a number of nuts are to be screwed to a specified diameter, and no standard nut exists to be referred to, the length between the extremities must be adjusted with regard to a measure and to the step of the thread. For example, suppose a quantity of bolts are to be made, and that their screwed ends are to be five inches in diameter, furnished with threads of four steps to an inch, the nuts for the bolts should, in a regular way, be first bored and screwed previous to screwing the ends of the bolts. To be sure that all the screws of the nuts will exactly fit the five-inch bolts, the longest diameter of the screws must be at least five inches; and consequently an accurate mode of measurement must be adopted to ascertain when the nuts are sufficiently enlarged. In nearly every such case it will be found necessary to make the thread-groove deep enough to prevent the bottom touching the summits of the bolt-thread, so that for a five-inch bolt the longest diameter of the nut-screw must be more than five inches.

The adjustment of a wire gauge for screwing five-inch nuts should be effected by means of a diagram similar to Fig. 1066. This is marked upon a flat piece of sheet iron, or piece of smooth board, and consists of a couple of parallel lines as far apart as the diameter of the bolt, and a few lines marked across, which denote the step of the thread and the extreme length between the points of the gauge to be made. The Figure is one-fifth of the actual size that would be required for five-inch nuts, and the one inch between the two lines S and S therefore denotes the bolt's or nut's diameter. The line L is scribed exactly square to the bolt's length, and is a sort of base from which the length of the step is marked, which, in this case, is a quarter of an inch. The step is therefore the space between the parallel lines L and T, and when these are scribed a rectangle is delineated. The angle of the thread with the screw's length is to be next shown by scribing the diagonal D from one corner of the rectangle to another. The length of this line is the exact length to which the wire is to be adjusted for producing nuts whose screws are to be five inches diameter. The amount of length which the gauge is to have in addition to the specified diameter of the screw, is shown by scribing the arc, A, with a compass-point at the centre, C. The extra length thus denoted is, for a five-inch gauge, only about a fiftieth of an inch. The wire must therefore be at least a fiftieth of an inch longer than five inches. But because the gauge is to suit nuts for bolts five inches in diameter, and because the summits of the bolt's threads should not touch the bottoms of the grooves in the nuts, the gauge should be a thirtieth or a twentieth of an inch longer.

In order to plainly show the relation between the length of a gauge and the diameter of a screw, a full-size diagram for a screw one inch in diameter is given. This is denoted by Fig. 1068, and has a step of half an inch, which would be much too great for a bolt only an inch thick; but the diagram serves to plainly show the difference alluded to. By this Figure it will be seen that the extreme length of gauge would be about an inch and an eighth, for the production of a screw an inch in diameter.

Fig. 1069 is a full-size sketch for an ordinary Whitworth screw of an inch and a quarter in diameter. The step of this is a seventh of an inch, and the diagonal shown is the length for a gauge suitable for screwing a nut or any other article with an inch and a quarter screw, supposing it could not be screwed with a tap. Fig. 1077 is a full-size sketch for four-inches nuts, which are to have screws with the length of step shown, the length of the diagonal showing the exact length of gauge, as in the other Figures.

The author's valin, or springy divider for measuring nut-screws, is denoted by Fig. 1067. This is a very efficient tool, the one implement being capable of measuring several different sizes, when the nuts in progress are only about three, four, or five inches. For larger sizes the straight wire gauges are preferable, because the large divider which would be required for such sizes would be liable to bend in use after being adjusted, and would therefore mislead the operator.

If the feet of the valin are bent to subtend the angle subtended by the feet shown in the Figure, the instrument can be used for a variety of sizes without causing the thicker parts of the feet to come into contact with the thread while the tool is in the hole. While adjusting the points to the distance required for a nut of a specified size, they are applied to the extremities of the diagonal shown in the diagram for the purpose; consequently, the distance between the two points is analogous to that between the points of a wire guage. The valin can be also adjusted to fit the screw of a standard nut, when such a nut is to be employed.

Screwed plugs, hobs, taps, and similar gauges, for screwing nuts to stated diameters, are only available for such as are not more than two or three inches in diameter, because of the comparative great amount of time required for the making of screwed gauges; but it may be mentioned, that they, of all classes, are the best, being the least likely to mislead.

In order to facilitate the screw-cutting of nuts, a bevel should be formed at the entrance or mouth of the hole at which the screw-tool enters. This bevel is about as deep as the step of the thread to be formed, and the diameter is equal to the largest diameter of the screw. In Fig. 1074 a bevel of this class is indicated by the oblique dotted lines at the right-hand end of the nut, the two diameters of the same bevel being shown by B, in Fig. 1076.

Screw-cutting a deep hole in a nut, or in any other object, is also facilitated by the author's channelled stock, denoted by Fig. 1079. Such a stock would be highly useful for screwing the further end of a chambered nut similar to Fig. 1078. The peculiarity of the implement consists in its having a shallow groove or gutter formed along its upper surface, as seen in the Figure. The gutter has a very smooth bottom, and is broad, but not deep, in order that it may contain enough oil without weakening the end of the tool. That extremity of the gutter at the extremity of the tool is the deepest part, to allow the oil to flow freely along and fall upon the metal of the nut or other article to be screwed. There are also two other outlets for the oil situate near the cutter-slot, one at each edge of the stock, and directly over the slot; these outlets cause a portion of the oil to flow from them to the tool and to the nut, instead of allowing the entire quantity to flow to the extremity.

By means of a stock furnished with a channel of this sort, oil can be supplied at any moment during the screw-cutting, without directly pouring it into the hole of the nut. The operator can pour oil into the outer end of the channel, and it will flow along to the cutter-point, although this may be at the middle or further end of the hole out of sight.