CHAPTER IV.

=SCREW-THREADS.=

=Screw Threads=, the various forms of 85 The pitch of 85 Self-locking 85 The Whitworth 86 The United States standard 86 The Common V 86 The requirements of 86 Tools for cutting 87 Variation of pitch from hardening 87 The wear of thread-cutting tools 88 Methods of producing 88 Alteration of shape of, from the wear of the tools they are cut by 89 =Screw Thread Cutting Tools.= The wear of the tap and the die 89 Improved form of chaser to equalize the wear 90 Form of, to eliminate the effects of the wear in altering the fit 90 Originating standard angles for 91 Standard micrometer gauge for the United States standard screw thread 91 Standard plug and collar gauges for 91 Producing gauges for 92 Table of United States standard for bolts and nuts 93 Table of standard for the V-thread 93 United States standard for gas and steam pipes 93 Taper for standard pipe threads 95 Tables of the pitches and diameters at root of thread, of the Whitworth thread 95 Table of Whitworth's screw threads for gas, water, and hydraulic piping 96 Whitworth's standard gauges for watch and instrument makers 96 Screw-cutting hand tools 96 =Thread-Cutting Tools.= American and English forms of stocks and dies 97 Adjustable or jamb dies 98 The friction of jamb dies 98 The sizes of hobs that should be used on jamb dies 99 Cutting right or left-hand thread with either single, double, or treble threads with the same dies 99 Hobs for hobbing or threading dies 100 Various forms of stocks with dies adjustable to take up the wear 101 Dies for gas and steam pipes 101 =Thread-Cutting Tool Taps.= The general forms of taps 102 Reducing the friction of 102 Giving clearance to 102 The friction of taper 103 Improved forms of 103 Professor J. E. Sweet's form of tap 104 Adjustable standard 104 The various shapes of flutes employed on taps 105 The number of flutes a tap should have 105 Demonstration that a tap should have four cutting edges rather than three 106 The position of the square or driving end, with relation to the cutting edges 106 Taper taps for blacksmiths 106 Collapsing taps for use in tapping machines 107 Collapsing tap for use in a screw machine 107 The alteration of pitch that occurs in hardening 108 Gauging the pitch after the hardening 108 Correcting the errors of pitch caused by the hardening 109 For lead 109 Elliptical in cross section 109 For very straight holes 109 Tap wrenches solid and adjustable 110 =Thread-Cutting.= Tapping 110 Appliances for tapping standard work 111