Tc 9-524 fundamentals of machine tools

7-52

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m = (D +3 W)- 1.5155
n
TC 9-524
M= PD+CPD=M-C
Where m = micrometer measurement over wires,
D = diameter of the thread,
n = number of threads per inch,
W = diameter of wire used
Example: Determine m (measurement over wires) for 1/2
inch, 12-pitch UNC thread. We would proceed to solve as
follows:
where W = 0.04811 inch
D = 0.500 inch
n=12
Then m = (0.500+ 0.14433) - 15155
12
m = (0.500 + 0.14433) -0.1263
m = 0.51803 inch (micrometer measurement)
When measuring a Unified National Fine thread, the same
method and formula are used. Too much pressure should not
be applied when measuring over wires.
Metric threads can also be checked by using the three-wire
method by using different numerical values in the formula.
M = measurement over the wires
PD = pitch diameter
C = N constant (This is found in Table 7-11 in
Appendix A)
The “best” wire size can be found by converting from inch
to metric, or by using Table 7-11 in Appendix A.
An optical comparator must be used to check the threads if
the tolerance desired is less than 0.001 inch (0.02 mm). This
type of thread measurement is normally used in industrial
shops doing production work.
CUTTING INTERNAL THREADS
Internal threads are cut into nuts and castings in the same
general manner as external threads. If a hand tap is not
available to cut the internal threads, they must be machined
on the lathe.
An internal threading operation will usually follow a boring
and drilling operation, thus the machine operator must know
drilling and boring procedures before attempting to cut
internal threads. The same holder used for boring can be used
to hold the tool bit for cutting internal threads. Lathe speed is
Three-wire threads of metric dimensions must have a 60°
the same as the speed for external thread cutting.
angle for this method.