Table 1. American Standard Screw Threads.

Use And Care Of Handtools And Measuring Tools
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a tendency to flow towards the root of the minor diameter of the tap while

being tapped. The minor diameter of tapped holes in such material will be

smaller after tapping. Take this into consideration when selecting the tap

drill of sufficient size.

Use the smallest thread depth possible on

materials that are very tough or of high hardness. This is accomplished by

using the largest drill possible, which nevertheless will be smaller than

the one used for softer materials, under similar circumstances.

Larger tap drills giving 50 percent

thread depth may be used when holes are deeper than 1 1/2 times the tap

diameter. Punched holes often cause binding and tap breakage, especially in

thin sheet metal, which creates an "oilcan" effect.

When the hole is

punched, the metal is flared out. As the tap is reversed after threading,

it draws in this flare and binds. Consider this when selecting the punch

size. The larger the punched hole, the less tendency for the tap to bind.

Holes formed in castings and forgings should be checked for 75 percent

depth, and reamed to size if necessary before tapping.

(2) Tapping a Hole.

(a) After the proper size and type hole (figure 140 on

the

following page) is drilled and cleaned, secure the tap in a tap wrench.

(b) Apply cutting oil to the tap and to the hole. Place the point

of the tap in the hole and rotate the tap (figure 141 on the following page)

clockwise for right-hand threads and counterclockwise for left-hand threads.

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