VIII.—The Royal Microscopical Society's Standard Screw-Thread for Nose-Piece and Object-Glasses of Microscopes.
Being the Report of a Sub-Committee of the Council,
drawn up by Conrad Beck, F.R.M.S., Secretary to the Sub-Committee.
(Read 17th June, 1896.)
The so-called Standard Screw-Thread of the Royal Microscopical Society has been but an imperfect standard, and has not ensured that interchangeability which it originally promised. It has been our duty to investigate the causes of this state of affairs, and to formulate a plan by which such an inconvenience should be remedied in the future.
Without going too closely into the entire history of the subject, we propose to briefly explain the reasons why the original standard was not efficient for practical purposes, and then to state the plan which the Council of the Royal Microscopical Society has now adopted for the future.
The specification of the original standard screw was as follows:—
Form of Thread.—Whitworth thread, i.e. a V-shaped thread, sides of thread inclined at an angle of 55° to each other, one-sixth of the V depth of the thread being rounded off at the top of the thread, and one-sixth of the thread being rounded off at the bottom of the thread.
Pitch of Screw, 36 to the inch.
Length of Thread on Object-Glass, 0.125 in.
Plain Fitting above Thread of Object-Glass 0.15 in. long, to be about the size of the bottom of male thread.
Length of Thread of Nose-Piece not less than 0.125 in.
Diameter of the Object-Glass Screw at the bottom of the screw, 0.7626 in.
Diameter of the Nose-Piece Screw at the bottom of the thread, 0.8 in.
When the exact form of the Whitworth screw-thread is calculated it will be found that this allows a difference between the male and female screw of 0.0018 in., which is in itself quite sufficient margin of looseness to make an easy fit.
The Society had two plug and ring gauges, one 0.8 in. and the other 0.7626 in., made by Whitworth as standards for the use of the Society, and it has been shown that if an adjustable tap and die (as recommended by the late Mr. Richard Beck in a paper printed in the 'Transactions of the Microscopical Society,' 1859, p. 92) be made which could be accurately adjusted to these standard sizes so that the tap exactly fitted the 0.8 in. ring size, and the die exactly fitted the 0.7626 in. plug, the exact standard screw as originally suggested could be adhered to. These adjustable taps and dies were not used for cutting the thread, but for passing over each thread after it had been cut to approximately the right size. That this method will work satisfactorily, is evidenced by the fact that in the late Mr. Richard Beck's firm the method has been in successful operation ever since.
The use, however, of such a system involved the necessity of every maker being provided with adjustable tap and die, and also the two pairs of plug and ring Whitworth sizes, together with a means of accurately sharpening the adjustable tap and die. And it was found in practice that Microscope makers were not universally prepared to go to such an outlay for a matter which at that time did not appear to be of such importance as has since proved to be the case.
Therefore the Society issued solid taps, and finding that, as is well known to be the case, a solid tap could not be made to an exactly accurate size owing to the alteration of the steel during the process of hardening and tempering, they had them made somewhat larger than the standard 0.8 in. gauge. An additional reason for their being larger was to allow for the slight wearing of the tap after prolonged use.
Here, however, there was no record of the amount larger which the taps were made, and although the first set appear to have been carefully manufactured, those which were from time to time obtained were less and less like the original, and in this manner a discrepancy arose which the arrangements now adopted by the Council are intended to correct for the future.
Beyond the fact that the Council specify that the diameter of the plain fitting of the object-glass should be as near as possible to, but not exceeding 0.759 in., and that the length of this fitting has been reduced to 0.1 in., the original specification of the standard screw is only altered as to the exact diameters of the screw itself.
The original specification of these diameters allowed only 0.0018 for clearance between the male and female screw.
If absolutely exact sizing taps and dies could be made which should not wear, the original diameters might have been adhered to, but as has been previously pointed out, adjustable dies in connection with gauges, &c., are requisite for this.
The Council has been able to obtain taps and dies which are guaranteed not to vary more than 1/1000 of an inch larger or smaller than the nominal size. And they are therefore having manufactured a series of taps of the nominal diameter on the top of the screwthread of 0.8015 in. which will not vary more than from 0.8005 in. to 0.8025 in. To ensure this the Council has ordered a Whitworth plug and ring, size 0.803 in. in diameter, and no tap will be allowed to be stamped with the Society's stamp unless it will pass easily through this 0.803 in. ring, and unless it is of such a size that it will not enter the 0.8 in. standard gauge already in the Society's possession.
They are also having made a series of dies of the nominal inside diameter on the top of the thread of 0.7611 in., which will not vary more than from 0.7601 to 0.7621. To test this the Council has ordered a Whitworth plug and ring, size 0.7596 in. diameter, and no die will be allowed to be stamped with the Society's stamp unless it will pass easily over the 0.7596 in. plug and will not pass over the 0.7626 in. plug.
These taps and dies will be for sale almost immediately, at cost price, 2l. 15s. for each pair of tap and dies, and it is earnestly requested that every maker of Microscopes will possess himself of a pair of these sizing gauges.
The Council believe that at such time as these sizing taps and dies have come into universal use the standard screw-thread will have been put upon a permanent basis, and complete interchangeability of all object-glasses will have been established.
SPECIFICATION OF THE ROYAL MICROSCOPICAL SOCIETY STANDARD SCREW.
Thread.— Whitworth screw, i.e. a V-shaped thread, sides of thread inclined at an angle of 55° to each other, one-sixth of the V depth being rounded off at the top and the bottom of the thread.
Pitch.— 36 to the inch.
Length of Thread on Object-Glass 0.125 in.
Plain Fitting above Thread of Object-Glass 0.1 in. long, not to exceed 0.759 in. in diameter.
Diameter (C) of Thread on Object-Glass at top of thread not to exceed 0.7982 in., or to be less than 0.7952 in.
Diameter (D) of Thread on Object-Glass at bottom of thread not to exceed 0.7626 in., or to be less than 0.7596 in.
Length of Screw of Nose-Piece to be not less than 0.125 in.
Diameter of Screw of Nose-Piece (A) at top of thread not to exceed 0.7674 in., or be less than 0.7644 in.
Diameter of Screw of Nose-Piece (B) at bottom of thread not to exceed 0.803 in., or be less than 0.8 in.
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Last revised: 1 Jan 2010.