The arrangement for the change wheels of a screw-cutting lathe given in fig. 603, resembles the mode frequently adopted.

The guide-screw extends through the middle of the bed, and projects at the end; there is a clasp nut, so that when required, the slide-rest may be detached from the screw and moved independently of the same. The train of wheels is placed at the left extremity of the lathe; there is a radial arm which circulates around the end of the main screw, the arm has one or two straight mortises, in which are fixed the axes of the intermediate wheels, and there are two circular mortises, by which the arm may be secured to the lathe bed, in any required position, by its two binding screws.

Fig. 603.

On Cutting Screws In Lathes With Traversing Tools  200111

On comparing the relative facilities for cutting screws, cither with the slide-rest furnished with a train of wheels, or with the traversing or screw-cutting lathe, the advantage will be found greatly in favour of the latter; for instance:

With the slide-rest arrangement, fig. 599, the work must be always fixed in a chuck to which the first of the change wheels can be also attached; the wheels frequently prevent the most favourable position of the slides from being adopted; and in cutting hollow screws the change wheels entirely prevent the tool carriage of the slide-rest from being placed opposite to the center, and therefore awkward tools, bent to the rectangular form, must be then used. The slide-rest also requires frequent attention to its parallelism with the axis of the lathe, or the screws cut will be conical instead of cylindrical.

With the traversing lathe, from the wheels being at the back of the mandrel, no interference can possibly arise from them, and consequently the work may be chucked indiscriminately on any of the chucks of the lathe; every position may be given to the slide carrying the tool, and therefore the most favourable, or that nearest to the work, may be always selected, and the tools need not be crooked. As the tool carriage traverses at once on the bearers of the lathe, the adjustment for parallelism is always true, and the length of traverse is greatly extended.

The system of screw-cutting just explained is very general and practical: for instance, one long and perfect guide-screw (which we will call the guide), containing 2, 4, 6, 8, 10, or any precise number of threads per inch having been obtained, it becomes very easy to make from it subsequent screws (or copies), which shall be respectively coarser and finer in any determined degree. The principal is, that whilst the copy makes one revolution, the guide must make so much of one revolution, or so many, as shall traverse the tool the space required between each thread of the copy; and this is accomplished by selecting change wheels in the proportions of these quantities of motion, or, in other words, in the proportion required to exist between the guide-screw and the copy.

In explanation, we will suppose the guide to have 6 threads per inch, and that copies of 18, 14, 12 1/2, 8, 3, 2, 1, threads per inch, are required: the two wheels must be respectively in the proportions of the fractions 6/18, 6/14, 6/12 1/2, 6/8, 6/3, 6/2, 6/1, the guide being constantly the numerator. The numerator also represents the wheel on the mandrel, and the denominator that on the guide screw; any multiples of these fractions may be selected for the change wheels to be employed.

For example, any multiples of 6/18, ad 12/36, 18/54, 24/72, etc, will produce a screw of 18 threads per inch, the first and finest of the group; and any multiples of -6/1, as 90/15, 120/20, etc, will produce a screw of 1 thread per inch, which is the last and coarsest of those given.

Screws 2, 4, or 6 times as fine, will result from interposing a second pair of wheels, respectively multiples of 1/2, 1/4, 1/6, and placed upon one axis.

For instance, the pair of wheels 24/72, used for producing a screw of 18 threads per inch, would, by the combination A, produce a copy three times as fine, or a screw of 54 threads per inch.*

Combination A.

M

Interm.

S

24

-

60

20

-

72

Combination B.

M

Interm.

S

120

-

24

72

-

20

Combination C.

M

Interm.

S

27

-

53

39

-

107

And the wheels 120/20, used for the screw of one thread per inch, would, by the combination B, produce a copy three times as coarse, or of three inches rise. Whatsoever the value of the intermediate wheels, whether multiples of 3/4, 7/8, 9/4, etc., they produce screws, respectively of 3/4, 7/8, 9/4, the pitches of those screws, which would be otherwise obtained by the two exterior wheels alone; and in this manner a great variety of screws, extending over a wide range of pitch, may be obtained from a limited number of wheels.

For instance, the apparatus Holtzapffel & Co. have recently added to the slide rest, after the manner of figs. 599 and 601, has a series of about fifteen wheels, of from 15 to 144 teeth, employed with a screw of 10 threads per inch; several hundred varieties of screws may be produced by this apparatus, the finest of which has 320 threads per inch, the coarsest measures 7 1/5 inches in each coil or rise; and the screws may be made right or left handed, double, triple, quadruple, or of any number of threads. The finest combinations are only useful fur self-acting turning, those of medium coarseness serve for all the ordinary purposes of screws; whilst the very coarse pitches are much employed in ornamental works of the character of the Elizabethan twist: and in cutting these coarse screws, the motion is given to the slide-rest screw, and by it communicated to the mandrel.

• Fig. 601, represents the wheels referred to in combination A, and fig. 602, those in combination B.

The value of any combination of wheels may be calculated as vulgar fractions, by multiplying together all the driving wheels as numerators, and all the driven wheels as denominators, adding also the fractional value, or pitch, of the guide-screw; thus in the first example A: