Table for Measuring Whitworth Standard Threads by the Three-wire Method Measuring Flat or Radius at Point of Thread Tools. To measure the width of the flat of a U. S. thread tool or the radius of a Whitworth thread tool, a micrometer arranged as shown in Fig. 4 may be used. The angle ABC of block D must, of course, be made to fit over the angle of the tool, and a different block must be used for the U. S. and the Whitworth thread tools, on account of the different angles of the threads. By this micrometer, the distance EF, Fig. 5, can be measured from which the exact width of flat or the correct radius can be determined. Tables are given herewith showing what the distance EF should be for thread tools for different numbers of threads per inch, for correct flats or radii. Table for Determining Flat or Radius at Point of Thread Tools Table for Determining Radius at Point of British Association Standard Reference Thread Gages. The table of "Reference Screw Thread Gages" shows a type of gage used for reference purposes only. At each end of the gage a knurled collar is provided, larger in diameter than the thread. These collars prevent nuts from being screwed onto the threads and also prevent damage to the thread in handling and storage. The blank diameter at K gives the inside diameter of the corresponding ring thread gage. Up to 2 inches diameter, the following limits for thread gages are commercially obtainable: Pitch, +0.0007 to 0.0007 inch per inch of length; diameter of plug gage, 0.000 inch to 0.0003 inch; inside diameter (smallest) of ring gage, 0.000 to + 0.0003 inch. The material used for reference thread gages is commonly a good quality of mild steel, the blanks being annealed before and after rough turning. After the second annealing, they are again turned all over, rough threaded, and the collars knurled. They are then carbonized to a depth of from 2 to 6 inch, allowed to cool in the carbonizing box, and then reheated to about 1500 degrees F. and quenched in water at about 75 degrees F. They are then drawn to a bluish purple. TAPS AND THREADING DIES Hand Taps. - These taps are usually made in sets of three, termed taper, plug and bottoming taps. The point of the taper tap is turned down to the diameter at the bottom of the thread for a length of about three or four threads, and then about six threads are chamfered or tapered until the full diameter of the tap is reached. On the plug tap, three threads are chamfered at the point. On the bottoming tap only about one thread is chamfered. The diameter of the straight portion of the thread of all the taps is the same in the regular type of hand tap. Taps are made in sets, however, where only the bottoming tap is of the full diameter, while the other taps gradually decrease in diameter, so as to distribute the work between the three taps. A simple rule used by one tap manufacturer for proportioning U. S. standard thread taps when made in this manner, is to make the diameter of the first tap in a set equal to the diameter of the finishing tap less the depth of the thread, and to make the diameter of the second tap equal to the diameter |