442 REPORT— 1900. 



or in English measure below -059 inch. The Committee propose to 

 modify the form of thread of the screw of the large section only. 



The above considerations lead the Committee to propose to replace 

 the present form of thread of screws from No. to No. 1 1 inclusive by 

 the form shown in tig. 2. 



Here the line A''~A A . . . represents the outline of the nut, B' B B . . . 

 of the screw ; and rf e/ . . . is the outline of the present British Association 

 thread. It will be observed that the flat part of the side, or the working 

 surface, is increased by nearly 60 per cent. Accurately formed screws 

 of this pattern for special purposes can be cut on the lathe with much 

 greater ease than those with a rounded thread. The screw is cut with 

 a single-point tool from a cylinder, and in the larger sizes the nut can be 

 cut with a single-point tool from a cylindrical hole. The difficulty of 

 forming chasers of a complicated form is entirely avoided. These observa- 

 tions apply equally to the construction of taps and plates, and of gauge- 

 pieces. Given that the pitch of the screw and the angle of the thread 

 are accurate, and the sides straiglit, the ht of the screw in a correct gauge 

 is determined by the length of the diametral line terminated by the in- 

 clined sides of the thread, and this dimension is called the effective diameter 

 of the screv. If this dimension is the same in the screw and the nut, they 

 will fit without shake independently of the exact values of the external 

 and internal diameters, or of the exact form of the ends ; and the lengths 

 of the effective diameters of screws and nuts are definite numerical 

 measures of their fits one with another. The point which it is important 

 should be right is the straight between A' and B in both nut and screw. 

 The nut must not pass A', nor the screw pass B ; but so long as the nut 

 is cut as far as B or farther the shape of the excess does not matter. The 

 same thing holds with the screw at A', but here excessive clearance is 

 objectionable because it weakens the core of the screw. 



In constructing plug-gauges for testing nuts the straight sides of the 

 thread can be corrected after hardening by grinding with a lap, and 

 this process corrects at once the irregularities of pitch Mid angle, and is 

 continued till the effective diameter is reduced to the desired value. The 

 top of the thread, being cylindrical, presents no difficulty. The form of 

 the bottom of the thread is immaterial, since the clear hole in the nut is 

 most conveniently tested with a cylinder plug- gauge. In the specimens 

 submitted to us last year by the Pratt and Whitney Company this 

 cylinder was constructed in one piece with the screwed plug. 



In ring or nut gauges for testing screws a slit is cut through the 

 tapped hole, and closed with a screw. The hole can be polished by a 

 corresponding screwed piece, but could only be con-ected by grinding by 

 the use of very refined appliances. After polishing the slit is closed to 

 fit a prepared screwed plug, and the clear hole brought to its correct value 

 with a lap. The process is not so satisfactory as with a plug-gauge, but 

 the pieces which the ring is designed to test can be satisfactorily measured 

 in other ways, so that the gauge is of less importance. 



The effective diameter of a screw is readily measured in a micrometer 

 gauge between a conical point and a V notch, both having an angle of 

 47^°. An instrument of this kind constructed for 60° is figured in tool- 

 makers' catalogues. 



Ordinary taps for nuts or for the working holes in larger pieces will 

 be different in form from the screws, and diSerent from the taps employed 

 to make dies or screw-plates. The ordinary dies or plates in a workshop 



