ENGLISH MECHANIC AND WORLD OF SCIENCE: No. 881, FEB. 10, 1882. (p. 544-5)
LATHES AND LATHE-MAKING
[19712.]–My attention has been called to Dr. Edmunds's rejoinder to a previous correspondent, appearing in your last issue.
Dr. Edmunds is well known as a physician of mark; he is also an enthusiast, as evidenced by the tyle of his letter, and, while I agree in much that he says, and would consider any of his utterances with respect, I venture to think he shows that he's not yet exhaustively acquainted with the vast subject of screw-threads, and that consequently he sometimes soars into the clouds.
I do not propose to enter into a discussion upon screw-threads, upon which prolific question so much has been already written and published, neither have I the time–my every spare moment being devoted to Vol. V., which, Dr. Edmunds, I know, will be glad to hear, has now made considerable progress, but I venture to trouble you with this letter to convey a word or two that I hope may afford the information your correspondent has been unable to obtain.
For about the last fifty years, Holtzapffel and Co. have made the threads of all their measuring and leading screws, and numberless others, aliquot parts of the inch; they have also made continuously the corresponding screw-tools from 4 to 100 threads to the inch, as part of their ordinary stock. Dr. Edmunds is correct in his views that our firm has never made any single thing an enigma, neither, on the other hand, do they propound conundrums, and I willingly answer his questions.
All the taps and dies designated and marked from D to U, are of J. J. Holtzapffel's twelve selected screw-threads. Table, page 673, Vol. II., "Turning and Mechanical Manipulation." The two largest diameters, D and E, are both of No. 4 thread; the next largest, F, is No. 5; H and G are No. 6; I, J, and K are No. 8; M is No. 9; L. N, and P are No. 10; O and Q are No. 11; and the four smallest, R, S, T, and U, are all No. 12 thread. The letter is, therefore, simply a convenient workshop mark, and one that soon becomes familiar to the mind as representing both the thread and the size of the tap. That there may be something to be said in favour of these particular taps and dies–beyond their "charming" working, so gracefully alluded to by your correspondent–albeit they are not aliquot parts of the inch, lies in the circumstance that they are largely employed for those numerous cases for which Whitworth's threads of small diameters prove too coarse, as shown by an unabated demand for them from the Government, eminent engineers, and others.
Dr. Edmunds makes merry at some length on an insignificant detail, i.e., the numbers 1 to 6 marked on the screw-guides of traversing mandrels, and having quoted a foot-note from my fourth volume, asks questions, the replies to which I conceive are answered by that note. Further explanation is simple. Most ornamental turners find the six screw-guides, and the six pairs of screw-tools that match them, ample for their wants, and, therefore, desire to possess no more. The numbers 1 to 6 are only engraved as a matter of convenience to select, say, the coarsest or the finest at the first glance as the guides may lie in their case or drawer, and I have never heard that they have given inconvenience to intellects even far less acute than that of Dr. Edmunds.
To my mind one part of his joke falls rather flat–viz., "that the work may run at one pitch, and the screw-tool cut another, and so necessarily spoil the work." No one will deny the impossibility of cutting a screw when the screw-guide and the screw-tool are of two different threads; but most people take care that they are of one and the same before they begin, by the very obvious process of matching them; that is, applying the teeth of the tool to the threads of the guide about to be used, and observing that the one drop into the other. I might almost call this an intuitive proceeding, and it is certainly nearly invariable with guides or taps by practical men, and I hope I shall not be indecorous in reminding Dr. Edmunds that in the course of a recent conversation with him on the subject, he admitted that he "had never thought of that." May I now be permitted to thank you by anticipation for inserting these replies to your correspondent's queries.
John J. Holtzapffel
64, Charing-cross, Feb. 6.