~ ia 
ON THE SCREW GAUGE. 529 
II. Tue PRESENT, 
The Committee were formed ‘to consider means by which better prac- 
tical effect can be given to the introduction of the Screw Gauge proposed 
by the Association in 1884.’ They have held many meetings. They have 
added Colonel Watkin, C.B., R.A., Mr. E. Rigg, and Mr. W. A. Price to 
their number. They have received great assistance from the Pratt and 
Whitney Company of Hartford (Connecticut, United States of America), 
who supplied each member of the Committee with a copy of their book on 
‘Standards of Length and their Practical Application.’ They were unfor- 
tunately deprived of the services of Mr. Hewitt, who was seized with a very 
severe illness after the first meeting, but they received from him his paper 
‘On the Manufacture of Standard Screws for Machine-made Watches,’ read 
before the Institution of Mechanical Engineers in October 1894 ; a paper 
which has been of great service. Mr. Griffith, representing the Council, 
attended regularly, and took advantage of his presence in the United 
States of America to visit the Pratt and Whitney works. 
Evidence was taken by the Committee from large users of the screws. 
Mr. Willmot, of the Post Office Factory, stated that the Post Office had 
used some tens of millions of screws made to the British Association gauge, 
and he had never received a single complaint. 
Various apparatus for measuring screws and different methods of testing 
their accuracy were carefully considered and discussed. 
The Committee came to the conclusion that it was necessary to con- 
sider the subject from the three points of view of the Standards Office, 
the Works Manager, and the Workman. 
1. The Standards Office. 
This must include, not only the custody of recognised and authen- 
ticated standards, but also a scientific mode of measuring the dimensions 
of commercial gauges and screws themselves, and of comparing their 
accuracy with the authorised standards. The peculiarity of the British 
Association gauge is this, that material standards are not impera- 
tivelynecessary. Thetable of dimensions given at page 527, together with 
the formula, enables any draughtsman to reproduce the form and pitch to 
any desired scale on paper. Colonel Watkin has shown to the Committee 
how to throw side by side, for purposes of very accurate comparison, a 
photographic image of — 
(a) The screw to be examined. 
(6) The standard with which it is to be compared. 
(c) Aseale which may be divided to ,,},,5th of an inch, the images of 
these three objects being so close to one another that a comparison to 
a very high degree of accuracy can be made. The Appendix to this Re- 
port contains a description of Colonel Watkin’s method. 
Mr. Price submitted to the Committee a microscopical method of 
measuring screws. The screw to be measured is attached to the stage of 
the microscope, the traversing slide of which is provided with a vernier 
and scale, while a vertical cross-hair in the eye-piece forms the index of 
the instrument. When the microscope has been adjusted for clear focus 
the screw is traversed across the field until the cross-hair intersects the 
thread of the screw at the desired point. The traversing screw of the 
slide is then turned until the corresponding point of the next thread is 
intersected by the cross-hair, and the reading of the vernier on the scale 
gives the measurement of the pitch with great accuracy. Mr. Buckney 
1896. MM 
