Before standardized screw threads became ubiquitous, manufacturers used screws of their own peculiar series. It was widely believed, and cynically, that they did this to ensure that any repair work needed would would come back to their own shop.
The problem is well-described in Charles Holtzapffel's magisterial six-volume work on ornamental turning. Holtzapffel and Co. was the most famous maker of ornamental turning lathes in the 19th century, perhaps ever.
Between the years 1794-1800, the author's father made a few varieties of taps, dies, hobs, and screw tools, after the modes explained at pages 635 and 636; these varieties of pitch were ultimately extended to twelve kinds, of each of which was formed a deep and shallow hob, or screw tool-cutter. These, when measured many years afterwards, were found nearly to possess in each inch of their length, the threads and decimal parts that are expressed in the following table.
|Threads in 1 inch||6.58||8.25||9.45||13.09||16.5||19.89||22.12||25.71||23.88||30.10||39.53||55.11|
The angle of the deep threads is about 50 degrees; of the shallow 60 degrees.
This irregularity of pitch would not have occurred had the screw-lathe with change-wheels been then in use; but such was not the case. For a long series of years I. I. Holtzapffel, (in conjunction with his partner, I. G. Deyerlein, from 1804 to 1827,) made, as occasion required, a large or a small screw, a course or fine, a shallow or deep thread, and so forth. By which accumulative mode, their series of working taps and dies, together with screw tools, gages, chucks, carriers, and a variety of subordinate apparatus, became extended to not less than one hundred varieties of all kinds.
About one-third of these sizes have been constantly used, up to the present time [ed-1856], both by H. & Co., and by other persons to whom copies of these screw tackles have been supplied, and consequently many thousands of screws of these kinds have been made: this implies the continual necessity for repairs and alterations in old works, which can only be accomplished by retaining the original sizes.
Since the period at which H. & Co. made their screw lathe, they have employed the aliquot threads for all screws above half an inch; indeed, most of these have also been cut in the screw lathe. To have introduced the same method in the small binding screws which are not made in the screw lathe, but with the die-stocks and chasing tools, would have doubled the number of their working-screw tackle, and the attendant apparatus; with the risk of confusion from the increased number, but without commensurate advantage as regards the purposes to which they are applied.
Doubtless the same reasons have operated in numerous other factories, as the long existence of good useful tools has often lessened, if not annulled, the advantage to be derived from a change which refers more immediately to engineering works.
Turning and Mechanical Manipulation…,vol II.
London: Holtzappfel and Co., 1856.
Pages 673 and 674
Later customers found it impossible to cut pitches (threads per inch) they required with the change wheels supplied with the screw-lathe. Naturally enough, this situation infuriated the owners of the lathes. Frustration found expression in an article by James Edmunds. If you are fortunate enough to own a Holtzappfel lathe, you may find the following table helpful.
|Taps and Dies|
|1||—||6.58||A||1 in. = 1.0000|
|2||—||8.25||B||7/8 in. = 0.8750|
|9/8 in. = 1.1250|
|15/16 in. = 0.9375|
|13/16 in. = 0.8125|
|3||1||9.45||C||¾ in. = 0.7500|
|¾ in. = 0.7500|
|4||2||13.09||DD||5/8 in. = 0.6250|
|4||2||13.09||E||½ in. = 0.5000|
|8||5||25.71||K||¼ in. = 0.2500|
|10||6||36.10||N||1/5 in. = 0.2000|
|12||—||55.11||U||1/10 in. = 0.1000|
For examples of the mind-boggling work Holtzapffel's lathes were capable of, visit holtzapffel.org
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Last revised: 27 June 2011.