The Spiral Apparatus Part 2 400140

Fig. 167.

The Spiral Apparatus Part 2 400141

Fig. 168.

The Spiral Apparatus Part 2 400142

Fig. 169.

The Spiral Apparatus Part 2 400143

170.

The Spiral Apparatus Part 2 400144

171.

The Spiral Apparatus Part 2 400145

The socket fig. 171, by means of a similar arrangement, fig. 170, carries a wheel with either a large or small central hole, upon the slide rest screw. The cylindrical portion c, is received within a circular, recess, countersunk in the end of the slide rest, around the extremity of the screw; this portion of the socket is hollowed and accurately fits the plain extremity of the screw, upon which it is retained by a transverse pin passing through them both. The retaining pin is slightly taper, and in length, does not exceed the external diameter of the socket, not to impede the revolution of the latter within the circular recess; a vertical hole bored through the slide rest gives access to the conical pin, which is driven out by a small round punch, when it is desired to separate the socket from the screw. The wheels are carried by the socket instead of directly upon the end of the screw, which in the latter case would have to project, in order that the removal of the socket may leave the end of the slide rest free of any such projection; frequently very necessary in turning, other than screw cutting, to enable the end of the slide rest to be approached close up to the work.

The mandrel carrying the work, and the slide rest screw traversing the tool, revolve at relative speeds, governed by the diameter or number of teeth, and the arrangement of the wheels connecting them. The rate or pitch of the resulting screw upon the work, being either a copy or a multiple of the slide rest or guide screw, according to the wheels employed. The simplest connection, would be by two wheels, one upon the mandrel and one upon the screw. When these two wheels are equal in the number of their teeth, the mandrel and the screw revolve turn for turn, and the screw produced is an exact copy as to coarseness of the screw of the slide rest, which in fig. 146, and in the slide rest for ornamental turning, is ten threads to the inch.

When the two wheels are unequal in diameter, the pitch of the guide screw is multiplied or divided upon the work, as the wheels may be in the proportion of one to two, one to three, one to four, or otherwise. Thus, taking the first proportion of one to two, when the wheel on the mandrel is twice as large as that on the screw, the latter, will make two revolutions to one of the mandrel, and the screw produced upon the work, will contain between every one of its threads twice the space or interval of that of the guide screw, and have five instead of ten threads to the inch. In the opposite case, if the wheel upon the mandrel be half the diameter of that upon the screw, the mandrel will make two revolutions for one of the latter, and the screw resulting will be twice as fine as the guide or copy, and be twenty threads to the inch; and so on for pairs of wheels of other proportions.

The supposed two wheels however are not used alone in practice, for in all probability they would not place the slide rest at a convenient distance from the work; and, they turn in opposite directions. The wheel on the mandrel necessarily turns with the work towards the tool, that on the screw therefore turns in the opposite direction; and if as is usual, the slide rest screw has a right handed thread, the tool travels from left to right, cutting a left handed thread, that is, sloping the reverse way to that of the slide rest and screws in general. When three or any odd number of wheels are in gear, the first and last turn in the same direction, with two or any even number, in opposite directions; a third wheel therefore, is interposed between that on the mandrel and that on the screw, to change the direction and produce a right handed thread.

The third wheel employed, is mounted upon a separate arbor or axis, it acts simply as a carrier of motion from the one wheel to the other, and in no way interferes with the results arising from then relative proportions. The third wheel may also be of any diameter or number of teeth, its position may be varied along the straight mortise fig. 165, and its axis may also be raised or lowered by altering the height of the radial arm; so that it may gear with the two wheels, and at the same time accommodate the varying distances, at which it may be convenient to place the slide rest from the axis of the lathe, as required by various diameters of work. In cutting left handed threads, sometimes required, the wheel on the mandrel and that on the screw are also seldom used alone, but, two intermediate wheels are introduced, to give the slide rest freedom of adjustment for distance. Any number of intermediate wheels, so long as all the wheels of the train are in one plane, or in other words, drive each other from separate axes, in no respect influence the thread produced by the relative proportions of the wheel on the mandrel and that on the screw.

A considerable variety of threads, both of whole and fractional values, may be obtained from a moderate number of change wheels, more especially if a fair proportion of them be multiples of 6 and 8; the first of the following tables, gives the results obtained from a set of 15 change wheels, with a screw of ten threads to the inch, with one wheel upon the mandrel and one upon the screw. A wheel of any convenient size attached to an extra arbor, being employed for the third or intermediate wheel. If required for any special purpose also, there is little difficulty in adding a change wheel of a suitable number of teeth for producing a required pitch. Thus, a wheel of 53 teeth has been introduced in the table, in order to enable the apparatus to cut a thread of the same pitch as the screws of the mandrels of Holtzapffel & Co.'s 5 inch center lathes; the approximate value of which is stated on page 673, Vol. II., as 9.45. The table shows that 53 on the mandrel and 50 on the screw give a pitch of 9.433 as the result of the combination, which, although not theoretically correct, is a sufficient approximation for practical purposes.