5. A large and a small master-tap m, fig. 613, were cut on the traversing mandrel with a fixed tool, the threads were about an inch long, and situated in the middle of a shaft eight or ten inches long; the small master-tap was of the same diameter as the finished screw, the large master-tap measured at the bottom of the thread the same as the blank cylinder to be screwed, as in figs. 572 and 576, page 600. The master-taps m, were used in cutting up the rectangular dies required in the apparatus shown in fig. 613, and now to be described.

6. On the parallel bed of a lathe, were fitted two standards or collar-heads h h', intended to receive the pivots of the screw to be cut, on the extremity of which was placed a winch handle, or sometimes an intermediate socket was interposed between the screw and the winch, to carry the latter to the end of the bed.

* Mr. A. Ross considers that the friction of Mr. Allan's apparatus is apt to retard the traverse of the screw, and therefore to cut the bottom of the thread too wide or rounding. In bis practice be uses the large and small dies for a short period at the commencement and conclusion of the process, but he cuts out the principal bulk of the material, by a fixed tool inserted within a radial mortise in a semicircular copper die; the copper is indented more and more with the progress of the work, and serves as an efficient guide, whilst the cutting is accomplished with considerably leas friction, and in a superior manner, by the cutter or turning-tool.

The bed had also an accurate slide-plate s s', running freely upon it, the slide-plate had two tails which passed betide the head h, and at the other end, a projection through which was made a transverse rectangular mortise for the dies, the one cud of the mortise is shown by the removal of the front die d, and the back die d' is seen in its proper situation; one extremity of each die was cut from the large master tap to, and the other from the small. The clamp or shackle c c', was used to close the two dies upon the screw simultaneously; it is shown out of its true posi-tion in order that the dies and mortise may be seen, but when in use the shackle would be shifted to the right, so as to embrace the dies d d'. The plain extremity c rested against the back die, whilst the screw c bore against the front die, through the inter-vention of the washer loosely attached to the clamp to save the teeth from injury; the pressure screw c had a graduated head and an index, to denote how much the dies were closed.

Fig. 613.

Maudslay s Methods Of Originating Screws Part 3 200117

7. A cylinder about two feet long, prepared for the screw, was placed between the heads h h', and the large dies, whose inner edges were of the same diameter as the cylinder, were closed upon it moderately tight, and the screw was turned round with the winch, to trace a thread from end to end; this was repeated a few times, the dies being slightly closed between each trip.

A screw-tool was next fixed on the slide s s', in a chamfer slide t t' with appropriate adjusting screws, so as to follow the dies and remove a shaving, much the same as in turning; the dies having armed at one end of the screw, the same screw tool, or a second tool, was placed on the opposite side of the slide-plate, so as to cut during the return movement. With the progress of the screw, the screw-tool was applied at a variety of distances from the pair of dies, as well as on opposite sides of the screw, so that the metal was cut out by the tool, and the dies were used almost alone to guide the traverse. Of course the dies were closed between each trip, and when the screw was about half cut up, the small dies were substituted for the large ones used at the commencement of the process.

9. The screw thus made, which was intended for a slide-rest, was found to be very uniform in its thread, and it was used for some time for the ordinary purposes of turning. When however it was required to be used for cutting other screws, it was found objectionable that its rate was nearly nine, whereas it was required to have eight threads per inch; it was then used in cutting a new guide-screw by means of a pair of change wheels of 50 and 56 teeth, which upon calculation were found to effect the conversion with sufficient precision.

10. From 9, the screw of 24 inches in length, one of 8 feet in length was obtained; the thread was cut one-third its depth, with the wheels, successive portions being operated upon, and the tool being carefully adjusted to the termination of the part previously cut. The general truth of the entire length was given by a repetition of the tedious mode of correction represented in the figure, with the dies and tool applied upon a bearer rather exceeding the full length of the screw.*

Although the processes 7 and 8 will produce a most uniform screw, Mr. Clement attaches little importance to the use of the dies and guide-frame alone, when several screws are wanted strictly of the same length. Of some few thus made, as nearly as possible under equal circumstances, two screws were found very nearly to agree, and a third was above a tenth of an inch longer in ten inches. This difference he thinks to have arisen in marking out the threads, from a little variation in the friction of the slide, or a difference in the first penetration of the dies.

The friction of the slide, when sufficient to cause any retardation, he considers to produce a constant and accumulative effect; first as it were, reducing the screw of l5 threads per inch, say to the fineness of 15 1/4; then acting upon that of 15 1/4 reducing it to 15 1/4, and so on; and that to such an extent, as occasionally to place the screw entirely beyond the correctional process. This cannot be the case when the thread is first marked out with the change wheels, instead of the dies.

* Mr. Clement also made a very superior steel screw of about five feet in length and three inches diameter, precisely by the method 10, before he bad completed the screw lathe he now commonly uses: and Mr. Whitworth followed precisely the same method in obtaining his standard screw, of about the length of 24 feet and half-inch pitch; except that a clasp nut was used instead of the dies. It was produced from a short screw cut by Mr. Clements; the correctional process occupied two months, and was carried with a most strict regard to avoid the unequal expansion of the screw and apparatus employed upon it.