Figs. 86, 87, 88.
One great difficulty in cutting the screw-threads to the top of a box, or the inside of its cover, arises from the necessity for stopping short, and removing the tool instantly as soon as it touches the shoulder, or the top of the cover. The latter should be made rather deeper than is necessary, so that there may be a turn or two of screw to spare. This will give more room for the play and removal of the inside chasing tool.
The ordinary form of the latter is as shown in [Fig. 87], the part under the plane upper surface (a) being either slightly hollowed or flat, generally the former, from having been cut by a revolving cylindrical hub.
Now, although this form may be suitable for outside screw tools, which have to work on cylindrical pieces, it does not appear equally suitable for inside tools, which are to act on concave work. The writer of this article has experimented upon many patterns of chasing tool, and has found it perfectly easy to chase an inside thread with an ordinary grooved tap, which seldom makes a false cut, or crosses the threads. From this the idea naturally arose of a convex edged tool for inside chasing, and a concave one for outside work, as [Fig. 89].
Figs. 89, 90.
Practically, however, the convex edge, [Fig. 90], will answer satisfactorily for an outside cylinder. In order to obtain an efficient cutting edge from this form, the rounding must be very slight. The out or inside tool is used with a rolling movement on the rest as it advances. If for hard wood, a notch cut across in the line e, f, [Fig. 90], with a saw-file, will, by making a partially cutting edge on the convex part, cause the tool to enter more readily at starting. Tools like the above must be necessarily the work of the amateur himself. The regular makers have a great objection to make any tool or machine out of the ordinary routine. Hence the same patterns are constantly reproduced year after year, until some one connected with the manufacture invents an improved form, or some one else of mechanical genius, and possessed of means, registers a new design. Amateurs are apt to cavil at this system, and in some cases it no doubt interferes with, and checks improvement in tools and machines, but the evil is almost a matter of necessity. Tools are made not singly, one of each pattern, but so many score or hundreds of one form are forged out, and handed over to the grinding and finishing department, and it would sadly interfere with the system and order of the manufacturer to make a single tool or two for individual purchasers of different pattern to those ordinarily used. If a design is sent in by a retail dealer who can order a hundred or so at his own risk, the above objection is obviated, and the new pattern of tool or machine is at once introduced. If, however, any new design by an amateur, being submitted to such men as Holtzapffel, Buck, Fenn, or Whitworth, appears to them good and saleable, they will not only not object to introduce it, but may possibly give a premium to the inventor. We have thought it necessary to make these remarks to obviate the possible disappointment of amateurs in this respect. It is but natural to suppose that some ingenious device must have long since arisen to obviate the difficulty of thus cutting screws by hand. Every turner finds the difficulty, and few perhaps have failed to try some plan or other to counteract it. There are two methods whereby this can be done: one by causing the tool to traverse at a given rate according to the proposed pitch of the screw, the other by giving similar movement to the mandrel, while the tool remains still. For such work as screwing the lids of boxes, the traversing mandrel is commonly used, but for cutting long screws in metal, the tool is fixed in a slide rest, and the latter is made to traverse, if necessary, the whole length of the lathe bed, by means of a guide screw driven by suitable gearing put in motion by the mandrel itself, the speed being adjusted by a series of cog wheels which can be interchanged to cause various rates of motion. The latter method belongs to machine lathes, and will be treated of hereafter in this series.
The author has great pleasure in here introducing a device, not exactly for cutting, but for starting the threads of a single, or double screw, or, indeed, a quadruple one. It is the invention of a gentleman whose nom de plume is East Norfolk Amateur, and was by him kindly communicated to the English Mechanic, of June 21, 1867. The description is here given in his own words:—
"I think the plan to be described will produce to a certainty any required number of screws and turns to the inch. The screws are entirely cut with a common comb tool, but started by a revolving cutter set to the required angle, and applied firmly to the work, on the T-rest. I call it 'the universal screw guide tool,' contrived and made by myself, and I believe will prove as useful to others as it has to me: the [drawing] will almost explain the tool. The cutter, A, is 9/16ths of an inch in diameter, turned to a cutting edge, and finely tempered. The stem, B, in which it revolves, is round, and fits into the shoe, C, having a graduated collar, D, in front of C, to set the cutter to the required pitch or angle, the set screw, E, makes it fast; having turned a piece of rod, of brass, iron, or steel, a little above the size necessary, and supposing a quadruple screw is to be cut having ten turns to the inch, there would, of course, be forty threads when complete; if one of these four can be truly traced, the comb tool will easily follow by inserting the outside tooth, either right or left hand, as found convenient, in the line traced, when the other three will soon appear with perfect accuracy, provided the first one exactly corresponded to five points of the comb, which is easily accomplished after a few trials, and if not successful at first,[4] can be removed by a dead flat file several times, without reducing the rod too much. When found to exactly fit the five points, the cutter may be applied with more force to leave a good chase for the comb. The T-rest requires a smooth surface for the shoe, C, to slide freely on, and to be set parallel with the work, and the tool held at a right angle as it proceeds along the rest, or the lines formed would be of unequal distance. After a little experience it will be found to work with beautiful accuracy, and for those who have not screw-guide mandrels, and are not practised hands at flying common screws, it will be found a great assistance, as it sets to anything. I described only the quadruple, but the same rule applies to all quick screws; for a double the chase must correspond to three points, and so on for any number, that is, one more point of the comb than the number of screws to be cut, and for a common one the chase must fit the comb altogether."