The wire size you should use to measure the pitch
you have this wire size available. Now make the
diameter depends on the number of threads per inch.
calculation. The data collected so far arr:
You will obtain the most accurate results when you
Thread = 3/4-10 UNC 2A
use the best wire size. The best size is not always
available, but you will get satisfactory results if you
Pitch diameter (PD) = 0.6832 in.
use wire diameters within a given range. Use a wire
Pitch (P) = 0.100 in.
size as close as possible to the best wire size. To
determine the wire sizes, use these formulas:
Wire size (W) = 0.0577 in.
Best wire size = 0.57735 inch × pitch
The standard formula for the measurement over the
wires was M = PD (0.86603 × P) + (3 × W).
Smallest wire size = 0.56 inch × pitch
Enter the collected data in the correct positions of the
Largest wire size = 0.90 inch × pitch
M = 0.6832 in. (0.86603 in × 0.100 in.)
For example, the diameter of the best wire for
measuring a thread that has 10 threads per inch is
+ (3 × 0.0577 in.)
0.0577 inch, but you could use any size between
0.056 inch and 0.090 inch.
M = 0.6832 in. 0.086603 in. +0.1731 in.
M = 0.769697 in.
NOTE: The wires should be fairly hard and
uniform in diameter. All three wires must be the same
The measurement over the wires should be
size. You can use the shanks of drill bits as substitutes
0.769697 in. or when rounded to four decimal places,
for the wires.
As mentioned in the beginning of the section on
classes of threads, the major diameter is a factor also
Use the following formulas to determine what the
considered in each different class of fit. The basic or
measurement over the wires should be for a given
nominal major diameter is seldom the size actually
machined on the outside diameter of the part to be
threaded. The actual size is smaller than the basic
Measurement = pitch diameter - (0.86603
size. In the case of the 3/4 - 10 UNC - 2A thread, the
× pitch) + (3 × wire diameter)
basic size is 0.750 in.; however, the size that the
M = PD - (0.86603 × xP)
outside diameter should be machined to is between
0.7482 and 0.7353 in.
+ (3 × W)
CUTTING SCREW THREADS ON THE
Use the actual size of the wires in the formula, not
the calculated size.
Example: What should the measurement be over
Screw threads are cut on the lathe by connecting
the wires for a 3/4-10 UNC-2A thread? First,
the headstock spindle of the lathe with the lead screw
determine the required pitch diameter for a class 2A
through a series of gears to get a positive carriage
3/4-10 UNC thread. You can find this information in
feed. The lead screw is driven at the required speed in
charts in several handbooks for machinists. The
relation to the headstock spindle speed. You can
limits of the pitch diameter for this particular thread
arrange the gearing between the headstock spindle
size and class are between 0.6832 and 0.6773 inch.
and lead screw so that you can cut any desired pitch.
Use the maximum size (0.6832 inch) for this example.
For example, if the lead screw has 8 threads per inch
Next, calculate the pitch for 10 threads per inch. The
and you arrange the gears so the headstock spindle
formula, "one divided by the number of threads per
revolves four times while the lead screw revolves
once, the thread you cut will be four times as fine as
inch", will give you pitch
For 10 TPI, the pitch
the thread on the lead screw, or 32 threads per inch.
is 0.100 inch. As previously stated, the best wire size
With the quick-change gear box, you can quickly and
for measuring 10 TPI is 0.0577 inch, so assume that
easily make the proper gearing arrangement by