_Answer_. The rule given by Mr. Sellers for proportioning the thread is as follows: “DIVIDE THE PITCH, OR, WHAT IS THE SAME THING, THE SIDE OF THE THREAD, INTO EIGHT EQUAL PARTS; TAKE OFF ONE PART FROM THE TOP AND FILL IN ONE PART IN THE BOTTOM OF THE THREAD: THEN THE FLAT TOP AND BOTTOM WILL EQUAL ONE-EIGHTH OF THE PITCH, THE WEARING SURFACE WILL BE THREE-QUARTERS OF THE PITCH AND THE DIAMETER OF SCREW AT BOTTOM OF THE THREAD WILL BE EXPRESSED BY THE FORMULA: _diameter_ - 1.299 ------------------------ _no. threads per inch._” In order to make the form and proportions of this standard thread as plain as possible, we have had an enlarged diagram, fig. 202, engraved, so as to represent the different parts clearly. _x_ represents the pitch, _d_ the diameter of the screw in inches, _v_ the number of threads to the inch, _n_ the diameter at bottom of thread, _m_ the width of back part of thread at the top and bottom, and _s_ the length of the side of thread. From Mr. Sellers’ rule the following formulæ can be deduced: 1.299 _n_ = _d_ - ----- _v_ _x_ _m_ = --- 8 3_x_ _s_ = ---- 4 From this rule any thread can be constructed, it being only necessary to know the pitch or number of threads to the inch. This, with all the dimensions of the standard threads for bolts from ¹⁄₄ to 2 inch diameter, is given in the table on page 348. For practical use in the shop a gauge like that shown in fig. 203 will be found most convenient for grinding the tools to the proper form for making the standard screws. With this gauge the screw-cutting tool can first be ground to the proper angle by fitting it to the deepest notch, and the requisite quantity should then be taken off the point by fitting it to the notch representing the form of thread for the sized bolt or number of threads to the inch which it is intended to cut. Wherever this standard for threads is used, if any pretense at all is made to accuracy of workmanship, careful attention should be given to the form and proportion of the threads as well as to the number to the inch. In buying taps and dies the purchaser should see that they conform in every respect to the standard, and in making specifications for new work similar care should be exercised to secure the true standard, form and proportion of screws. In many shops the workmen who have the care of those tools are entirely ignorant of the peculiarities of the Sellers system, and have only the vague idea that so long as they get the proper number of threads to the inch they are doing all that is necessary to secure uniformity. Unless, therefore, some care is exercised to insure accuracy of workmanship in this department, the adoption of a “standard” for screws will not insure the advantages which would result from uniformity of screws and threads. [Illustration: Fig. 203. Full size.]