External and internal screws of all diameters, may be commenced and completed on the traversing mandrel so as to fit each other; or, which is sometimes convenient, so soon as the screw thread has been once transferred from the guide to the work, the traverse of the mandrel may then be exchanged for continuous revolution, and the diameter of the required external or internal screw may be reduced or enlarged, with the flat tool and screw tool, used in the manner already described.

The one constant position of the tool, whether a single point or a line of points, figs. 519. 520, the parallelism of its edge to the mandrel, and the certainty of direction, are perfectly secured when the tools are used in the slide rest. But the adjustment of the tool for distance with respect to the advancing traverse of the work, that the external tool may not be struck by the shoulder, nor the end of the internal tool, by the surface at the bottom of the internal screw, while it is very easily made with the slide rest, is yet more necessary than with the hand tools. With the latter, the contact usually results in damage to the thread alone; in the slide rest, as the tool can neither yield nor be instantly withdrawn, it also is liable to injury; the collision besides damaging the thread, sometimes breaking the internal tool also. The tool is clamped in the slide rest with its face exactly at the height of center of the mandrel, the main slide parallel, or at right angles to the latter, for external or for internal screws respectively. The lateral adjustment of the external screw tool to the shoulder of the work, is made by the slide parallel with the mandrel, and the depth of cut is given by the upper or transverse slide; for the internal screw this is reversed, the tool is placed in position by the transverse, and advanced in cutting by the main slide.

In chasing the external or internal screw with the traversing mandrel when the tool is used in the slide rest, the tool is first placed so that its cutting edge is just out of contact with the work; then as the mandrel traverses, the tool is gradually advanced to the work by the screw of the appropriate slide, moved round by its micrometer head or winch handle, through a small space at a time. So soon as the cutting edge is found to arrive in contact with the work, the advance of the tool is only continued intermittently, and always at the moment when the work has receded, previously to the commencement of every forward traverse of the mandrel. Every advance of the tool when cutting is but small, the amount also depending upon the hardness of the material of the screw being cut; and the tool is not retired from the work until the thread is completed.

Softwood screws are cut in the lathe upon the traversing mandrel, and most conveniently, when the tool is applied by hand. The softwood hand screw tools, figs. 516. 517. 518, have a single point formed as an angular groove, the sides very similar to the edges of two chisels meeting in a point, but sharpened almost entirely upon their inner sides forming the groove, the same tool serving for either fine or coarse threads. The shaft of the external tool, fig. 516, is held upon the rest at a small horizontal angle, and slightly tilted towards the left, that the two sides of the cutting edge may agree with the inclination of the thread; occasionally, as in fig. 377, Vol. II. the angular groove does not lie in the line of the shaft, the tool then requires to he held at an increased angle. The cutting edge of the internal tool, fig. 517, is at right angles to its rectangular shaft, which is held slightly underhand, and parallel with the axis of the mandrel; sometimes as in fig. 518, the shaft is cranked as with other softwood tools to increase its surface bearing on the rest. All these tools require holding with considerable firmness to avoid any lateral displacement, to which they are rather prone, both from being single points, and from the avidity with which they cut; the shafts of the external tools are held inclined vertically, fig. 335, to present the cutting edges at the appropriate angle for softwood, which is that of the gouge or chisel upon the cylinder. When keen, the tools cut very readily and the finer threads are often completed at a single traverse. The tops of the threads are usually slightly truncated with the chisel, or a sufficient portion of the original cylinder may be permitted to remain when chasing the screw.

Fig. 516.

Similar Screws Cut With The Traversing Mandrel Con 400317

Softwood screws, especially those exceeding their diameter in length, are also very generally cut with the screw box, figs. 554 - 557. Vol. II.; a tool combining both guide and cutter. Every diameter of screw requires a separate screw box, and these, which all cut a comparatively coarse screw, a necessity in softwood, range in size for screws of from one eighth, to three or four inches in diameter; the smaller screw boxes serving for many of the screws required in the following examples of softwood turning.

The piece for the external screw is turned cylindrical, rounded or pointed at the end, the shoulder true and square; it is then simply twisted into the hole in the screw box, which both guides the work and carries the cutter. The smaller screw boxes are used held in the left hand, the work being twisted in by the right, a moderate pressure in the line of its shaft being employed at the commencement of the cut. Long screws to about one inch diameter, such as those for tambour frames, are commenced in this manner, they are then replaced between the lathe centers, and the left hand is placed on the lathe pulley, to twist the work round and into the screw box, which is held by the right. Larger works, are placed upright in the vice, and the screw box twisted round by both hands. The thread is completely finished at one operation, and it may be arrested at any distance along the shaft of the screw, or it may be carried close up to a shoulder.

The internal screw is cut by a taper tap, the work having been previously surfaced, and a hole turned completely through it, or of sufficient depth, to allow the action of the tap. The smaller taps held in a hand-vice, are twisted into the hole with moderate pressure, care being observed to keep them perpendicular or in the line of the hole; the larger are used with a tap wrench. The wood tap with separate inserted cutter, fig. 552. Vol. II., requires less force than the large metal taps, and is a much superior tool for tapping large internal screws. The hollow metal tap, the principle of which is indicated by fig. 553. Vol. II., is the most efficient, but it is rarely made, unless it be required for the production of a large number of one size of internal softwood screws.