As there is no standard of diameter for the finer pitches, the columns for diameter and caliper reading are left blank. The column on the right gives the number to be subtracted from the diameter to obtain the caliper reading.

or explanation of screw thread micrometer caliper, refer to

As there is no standard of diameter for the finer pitches, the columns for diameter and caliper reading are left blank. The column on the right gives the number to be subtracted from the diameter to obtain the caliper reading.

For explanation of screw thread micrometer caliper, refer to page 26.

THE Brown & Sharpe thread micrometer is fitted with pointed spindle and "V" anvil as in Fig. 10, to measure the actual thread on the cut surface. Enough of the point is removed and the bottom of the "V" is carried low enough so that the anvil and spindle clear the top and bottom of the thread and rest directly on the sides of the thread.

As it measures one-half of the depth of the thread from the top, on each side, the diameter of the thread as indicated by the caliper, or the pitch diameter, is the full size of the thread less the depth of one thread.

This depth may be found as follows;

As the U. S. thread is flatted of its own depth on top, it follows that the pitch diameter of the thread is increased on each side, equaling of the whole depth and instead of the constant .866 we use the constant .6495, which is of .866.

When the point and anvil are in contact the o represents a line drawn through the plane A B, Fig. 10,

and if the caliper opened, say to .500, it represents the distance of the two planes .500" apart. The preceding tables are used in connection with the micrometer.

MEASURING EXTERNAL SCREW-THREAD DIAMETERS WITH MICROMETERS AND WIRES

It is frequently necessary, especially in making a tap or threadplug gage, to measure the thread diameter on the thread angle in addition to measuring on top of the thread and at the bottom of the thread groove, and unless calipers made expressly for such work are at hand, the measurement on the thread angle is not made with any degree of accuracy or is omitted entirely. The accompanying sketches, Figs. 11, 12 and 13, formulas, and tables, are worked out for convenience in screw-thread inspection, so that by using ordinary micrometer calipers and wire of the diameter called for in the table the standard threads can be compared with the figures given.

Threads of Special Diameter

For threads of special diameter the values of x, x1 or x2 can be readily computed from the formula corresponding to the method of measuring to be used. The method shown in Fig. 11 at x is liable to lead to an error unless care be taken that the diameter on top of the thread is correct, and also that the flatted surface on the top of the threads is concentric with the rest of the thread. The concentricity of the flatted surface can be tested by measuring, as at x, Fig. II, at several points on a plane through the axis and at right angles to it. The wire used must be round and of uniform diameter.

a

B
D2

=

=

=

=

=

=

=

=

N

FIG. II. Measuring U. S. Standard Threads

outside diameter of thread.

root diameter measured in thread groove.
number of threads per inch of length.

depth of thread.

distance from center to center of adjacent threads.
width of flat on U. S. Standard thread.

diameter of wire used.

distance from apex of thread angle at root, to center of wire. diameter of cylinder touched by apexes of thread angles. diameter from top of threads on one side of tap or bolt, to top of wire laid in thread groove on opposite side.

=

(D2 + 2 B)2 +

2 + a.