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

formula:

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.

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.

0.7697 in.

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

pitch diameter.

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)

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

6-54

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