ON THE SMALL SCKEW GAUGE. 



441 



threat force would be required either to squeeze the metal into sharp 

 re-entrant angles, or to make it flow past sharp corners. On this point 

 Mr. C. J. Hewitt writes to the Committee respecting the proposed altera- 

 tion of the British Association thread : ' A die of this operating character 

 for screws Hatted top and bottom soon loses its contour, and needs con- 

 stant replacement ; and in addition my experience leads me to believe 

 that it requires more force than a rounded thread ; therefore it sets up 

 more torsional strain of the metal, a factor of great moment where such 



Tadle of Dimexsioxs. 



AA=-] U1 iiitcli. 

 BB=-2,S8 „ 

 /■ =-lS2 „ 



small diameters are being produced, breakage in the dies being a con- 

 stant source of trouble even at the best.' In the same letter Mr. Hewitt 

 explicitly approves the proposals of the Committee for the larger 

 threads, both as regards the flat top and the provision of clearance. 

 iMr. Hewitt's experience at the Prescot watch factory is so large, and 

 his knowledge of the manufacture of watchmakers' screws is so intimate, 

 that the other members of the Committee have no hesitation in accepting 

 his suggestion to divide the present series into two sections. The large 

 section, consisting of what may be called instrument-makers' screws, from 

 No. to No. 11, includes screws from G mm. to 1'5 mm., or in English 

 measure from J inch to -059 inch. The small section, from No. 12 down- 

 wards, consisting of watchmakers' screws, includes screws below 1'5 mm., 



