Hollow or interior screws are commonly cut by means of a steel tap, which is simply a screw of which great part of the worm is removed by filing flat faces along its whole length, the angles left ,ley this operation forming a series of obtuse cutters. The head of the tap is squared, to fit into the middle of a long handle, by which it can be turned with considerable purchase. The taps fur cutting screws in wood are commonly fluted on the sides, to make them cut the inure readily, and to afford more room for the escape of the cuttings. The tap invented by Mr. Jones answers the purpose of a tapering tap for commencing the screw, and a cylindrical one for completing it : the lower part being tapered, while the upper is left cylindrical. The tap is shown at e, more adapted for cutting than either a or to, which are the more usual forms. In large taps of this kind a steel cutter may be inserted, as shown in the section d, at the commencement of the cylindrical portion of the tap; the cutter being made to project a little, so that the tap follows it. without difficulty. An ingenious kind of tap for cutting a square-threaded screw consists of a hollow screw of steel, having a hole drilled obliquely from the front end of the thread to the hole in the centre of the tap. The edges of this oblique hole, being made sharp, cut their way through the wood when the to is turned round ; while the hole itself forms a channel by which the cuttings escape into the cavity in the centre. For cutting internal screws in wood, where great accuracy is not required, the tap invented by Mr.
Siebe, represented in the following cut, is useful. Though the wooden part is cylindrical, the steel plate tapers towards the end that enters the hole, in order that, by the first teeth projecting but little, the Instrument may cut gradually. A groove is cut on each side of the tap, where the plate is inserted, to afford room for the escape of the cuttings ; and the upper end of the cylinder is made flat or square for the purpose of fitting the lever by which it is turned.
The large iron screws used in vices, presses, waggon-jacks, &c., are formed by means of dies, turned with immense power by very long levers; the thread being made without cutting, by indenting and squeezing up the metal. In the ordinary method of cutting screws in a lathe, the size of the worm, or the distance between the threads, is regulated by a pattern screw, and cannot be varied from it. An ingenious machine is used in the Woolwich dock-yard for cutting a great variety of different screws from one pattern.
In making screws for mathematical or astronomical instruments, where the greatest accuracy is essentially necessary, it is especially desirable to avoid the risk of error arising from irregularities in the pattern screw, or in any part of the machiuery used. The late Mr. Holtzapffel was highly skilled in this art ; but Mr. Whitworth has recently carried it to a degree of refinement never before attained.