Chapter 4. MECHANICAL EQUIPMENT

TM 750-245-4

CHAPTER 4

MECHANICAL EQUIPMENT

4-5. Unknown Tap Drill Diameters

4-1. Introduction

The diameters of tap drills can be found by the formula

This discussion of mechanical equipment is necessarily

D= T-0.75x2d in which D = drill diameter, T = diameter

limited. The basic information and theory required by

of tap or thread, and d = depth of thread. The depth of

the inspector is presented; however, the breadth of the

thread for various numbers of threads per inch and

subject does not permit a detailed listing of all attaching

thread forms, are given in various tables in appropriate

hardware and attendant inspection procedures.

mechanical handbooks. The diameter obtained by the

Mechanical equipment is designed to perform a function

above formula allows for a thread having 75 percent of

at the least cost and with a minimum of required

the standard depth which is sufficient for general work.

maintenance during the life cycle of the item. The best

The formula applies to the American (National) thread

reliability and operation must be built into the item.

form. The diameter of the tap drill should not be smaller

Missile equipment must have as near 100 percent

than is necessary to give the required strength of thread,

reliability as possible. Therefore, the best inspection

as every decrease of even 0.001 inch in diameter of the

obtainable is required.

tap drill materially increases the power required for

tapping and the percentage of broken taps.

4-2. Attaching Hardware

Attaching hardware for the equipment is subjected to:

4-6. Simplified Rule for Tap Drill Diameter

a. Sever stresses

If a table of tap drill sizes is not at hand, the following

b. Shock

rule may be used: Rule-To find the tap drill diameter in

c. Corrosion

inches, subtract from the outside diameter of the tap an

d. Contraction and expansion of metals

amount equal to one divided by the number of threads

e. Warpage

per inch. In the practical application of this rule, the

nearest commercial drill size is always used.

f. Temperature changes

g. Atmospheric conditions

4-7. Thread Gage Tolerances

h. Shelf life

Gage tolerances for the four classes of American

Standard screw thread fits are designated as W, X, and

4-3. Tapping and Thread Cutting - General

Y tolerances (table 4-1 and 4-2). These tolerances are

Tapping troubles are often caused by using tap drills

applicable to the National Coarse (NC) and National

that are too small in diameter.

For ordinary

Fine (NF) Series, and to comparable diameters and

manufacturing, not more than 75 or 80 percent of the

pitches. The recommended uses for W, X, and Y gage

standard thread depth is necessary, and for some

tolerances are as follows:

classes of work, not more than 50 percent is required.

a. Working Gages. For classes 1 and 2 fits, use Y

Tap drill sizes, especially for machine screws, should be

tolerances; for class 3, use X tolerances; for class 4, use

varied according to the material to be tapped and the

W tolerances.

depth of the tapped hole.

b. Inspection Gages. Recommended uses are the

same as given under working gages.

4-4. Types of Holes (Tapping)

c. Setting Gages. These thread-plug gages are

Soft material, such as copper, soft iron, drawn

used in adjusting thread-ring gages, thread-snap gages,

aluminum, etc., should have a larger hole for the tap

or other thread comparators. For classes 1 and 2 fits,

than hard crystalline materials such as cast metals.

use X tolerances; for class 3 fit, use W or X tolerances;

When tapping soft materials, if the hole is too small, the

for class 4, use W tolerances.

threads will be torn off to some extent, thus actually

decreasing the effective thread depth as compared to

what it would be if the tap drill had been of larger

4-8. "GO" Gages for Screw Threads (Fig. 4-1)

diameter, but if the hole is drilled rather large, -when

A "Go" gage should check simultaneously as many

tapping tenacious materials, the metal at the top of the

elements as possible, whereas a "Not Go" gage usually

thread is drawn somewhat, thereby increasing the

checks one element only. A "Go" gage

depths of the threads. This is more likely to occur after

the keen edge of the tap has been slightly dulled by use.

4-1