One of the oldest, most simple, and general apparatus for cutting short screws in the lathe, by means of a mechanical guidance, is the screw-mandrel or traversing-mandrel, which appears to have been known, almost as soon as the iron mandrel itself was introduced.
* The twisted moulds for upholsterers' fringes, are frequently screwed by hand; a thin gouge, or a carpenters' fluted bit of the width of the groove, is ground very obliquely from the lower Bide bo as to leave two long edges or fangs projecting, and the tool is sharpened from within. An oblique notch is made by hand at the end of the mould as a commencement, and the tool wedging into the groove is guided along the rest at the same angle as the notch, whilst the lathe revolves slowly, and completes the twist at one cut. To make the second groove parallel with the first the finger is placed beside the gouge, and within the first twist; and so on with the others. The process is very pleasing from its rapidity and simplicity, and is also sufficiently accurate for the end proposed.
Fig. 592 is copied from an old French mandrel mounted in a wooden frame, and with tin collars cast in two parts; the upper halves of the collars arc removed to show the cylindrical necks of the mandrel, upon the shaft of which are cut several short screws. In ordinary turning, the retaining key k, which is shown detached in the view k, prevents the mandrel from traversing, as its angular and circular ridge enters the groove in the mandrel; but although not represented, each thread on the mandrel is provided with a similar key, except that their circular arcs are screw-form instead of angular. In screw cutting, k is depressed to leave the mandrel at liberty; the mandrel is advanced slightly forward, and one of the screw-keys is elevated by its wedge until it becomes engaged with its corresponding guide-screw, and now as the mandrel revolves, it also advances or retires in the exact path of the screw selected.
The modern screw-mandrel lathe has a cast-iron frame, and hardened steel collars which are not divided; the guide screws are fitted as rings to the extreme end of the hardened steel mandrel, and they work in a plate of brass, which has six scollops, or semicircular screws upon its edge. When this mandrel is used for plain turning, its traverse is prevented by a cap which extends over the portion of the mandrel protruding through the collars.*
* For further details of the construction of the old screw -mandrel lathes, the reader is referred to Moxon, Plumier, Leupold, etc.; and to pages 30 to 42 of the the screw-mandrelof requires but a very small change of apparatus, and whatever may be the diameter of the work, it ensures perfect copies of the guide screws, the half dozen varieties of which, will be found to present abundant choice as to coarseness, in respect to the ordinary purposes of turning.
In cutting screws with either the old or modern screw-mandrel, the work is chucked, and the tool is applied, exactly in the manner of turning a plain object; but the mandrel requires an alternating motion backwards and forwards, somewhat short of the length of the guide screw, this is effected by giving a swinging motion or partial revolution to the foot wheel. The tool should retain its place with great steadiness, and it is therefore often fixed in the sliding rest, by which also it is then advanced to the axis of the work with the progress of the external screw, or by which it is also removed from the center in cutting an internal screw.
To cut a screw exceeding the length of traverse of the mandrel, the screw tool is first applied at the end of the work, and when as much has been cut as the traverse will admit, the tool is shifted the space of a few threads to the left, and a further portion is cut; and this change of the tool is repeated until the screw-attains the full length required. When the tool is applied by hand, it readily assumes its true position in the threads, when it is fixed in the slide rest its adjustment requires much care.
In screwing an object which is too long to be attached to the mandrel by the chuck alone, its opposite extremity is sometimes supported by the front center or popit head; but the center point must then be pressed up by a spring, that it may yield to the advance of the mandrel: this method will only serve for very slight works, as the pressure of the screw-tool is apt to thrust the work out of the center. It is a much stronger and more usual plan, to make the extremity or some more convenient part of the work cylindrical, and to support that part within a stationary cylindrical bearing, or collar plate, which retains the position of the work notwithstanding its helical motion, and supplies the needful resistance against the tool.* fourth volume. And also to pages 90 to 92 of the same, for the figures and explanation of the modern screw-mandrel lathe, with cylindrical collars of hardened steel; the durability of which has been occasionally brought into question by those who, it must be presumed, have not personally tried them. See remarks, page 52, of Vol. IV.
* In cutting the screws upon the ends of glass smelling-bottles, and similar works incapable of being cut with steel tools, the bottle is mounted on a traversing mandrel, which is moved slowly by hand, and the cutting tool is a metal disk revolving rapidly on fixed centers, and having an angular edge fed with emery and water; in some rare cases a diamond is used as the cutting tool.
The amateur who experiences diffliculty in cutting screws flying, or with the common mandrel and hand-tool unassistedly, will find the screw mandril an apparatus by far the most generally convenient for those works, in wood, ivory, and metal turning, to which the screw box, and the taps and dies are inapplicable.