In order to accurately bore a nut so that the hole shall have the exact diameter required, the operator should make or be provided with a sheet gauge similar to Fig. 1055, especially if a number of nuts are to be bored to one diameter. The gauge is a piece of steel, if it is intended to remain a permanent tool for a large number; but an iron one is available for most purposes. One gauge of this class can be made to suit two sizes, if the two sizes differ greatly from each other, in which case the operator is not liable to use the larger dimension of the gauge instead of the smaller.

The two opposite narrow surfaces termed edges, of a sheet gauge for boring should not be exactly parallel to each other, nor quite flat, as appears by the Figure, but each end or point should be slightly tapered, the taper part extending to about a quarter or a sixth of the gauge's length. The intermediate part between the two taper ends, is to be parallel, and of the exact diameter required for the hole to be bored. It is also proper to curve the edges, which is done with a smooth file, the curve produced being about the same as that of the hole required. By bestowing the necessary attention to the gauge while being made, it will accurately measure a hole, will retain its original size a greater length of time, and will readily enter a hole by reason of its taper form. By the gauge entering only a short distance into a hole, the operator is also able to know how much more is to be removed.

The diameter to which the hole is to be bored before commencing the screw-cutting, is the shortest diameter of the intended screwed hole, usually termed the diameter at the bottom of the thread. This distance is therefore the diameter of the gauge to be used ; and while a gauge is being adjusted to a proper size, it can be either fitted to a screwed hole or nut which is known to be correct, or it can be adjusted to some stated diameter to suit the purpose. When one of the usual Whitworth threads is to be used, the proper diameter for the gauge may be known by referring to Table 6, pages 178 and 179. Suppose now that a nut intended for a bolt three inches in diameter is to be made, and that a screw-tap three inches in diameter can be used for forming the thread, the diameter of the hole should be two and five-eighths inches, as appears by the last fine of the Table. Therefore, three-eighths of an inch of the hole's diameter is the amount occupied by the thread, and this is the difference between the shortest diameter and longest diameter of the screwed hole to be made. This difference is the same, whether the nut is a three inch one, or a ten inch one, if the thread referred to is formed therein, and is exactly analogous to the difference in a three-inch hob referred to in Table 5, column the sixth, because the thread of a three-inch hob is the same in shape and size as that of a three-inch tap.

But the sort of screw-thread referred to in these Tables is too large for a bolt or rod only three inches in diameter, being too broad and too deep, the pitch being, as indicated in column eighth of Table 5, 31/2 per inch. This size of thread is large enough for a bolt seven inches in diameter, as indicated in Table 7. Consequently, if a nut is now to be screwed for a seven-inch bolt, the diameter of the hole when bored ready for screwing must be 6f inches, allowing that the usual Whitworth-shape thread is to be adopted, such as that used for three-inch nuts. It may therefore be inferred from the foregoing remarks that each sheet gauge requires to be named in order to indicate two things, which are, the extreme or largest diameter of the screws for which the gauge is made, and the step and shape of the screws.