THREADING.

a. Purpose of taps and dies. Taps and dies are used to cut threads in metal, plastics, or hard rubber. The taps are used for cutting internal threads and the dies are used to cut external threads.

b. Types of taps. There are several types of taps issued by the supply system. However, the most common are the taper, plug, bottoming, and pipe taps (fig 29). Special purpose taps (fig 30) such as mud or washout taps, boiler taps, and staybolt taps are also discussed in this paragraph.

(1) Taper hand tap. The taper or starting hand tap (fig 29) has a chamfer length of 8 to 10 threads. These taps are used when starting the tapping operation and tapping coarse threads in through holes especially in harder metals.

(2) Plug hand taps. Plug hand taps (fig 29) are designed for use after the taper tap and in through holes when tapping softer metals or fine-pitch threads. They have a chamfered length of 3 to 5 threads. These taps. are the most popular.

(3) Bottoming hand taps. Bottoming hand taps (fig 29) are used for threading the bottom of a blind hole. They have a very short chamfer length of only 1 to 1-1/2 threads for this purpose. This tap is always used after the plug tap has already been used and for hard materials. Both the taper hand and plug hand taps should precede the use of the bottoming hand tap.

(4) Pipe taps. A taper pipe tap (fig 29) is used for pipe fittings and on other places where extremely tight fits are necessary. The tap diameter, from end to end of threaded portion, increases at the rate of 3/4 inch per foot. All

threads on this tap cut as compared to the straight taps where only the nonchamfered portion does the cutting.

Figure 29. Types of common taps.

Figure 30. Special purpose taps.

(5) Boiler taps. Straight boiler taps (fig 30) range in diameter from 1/2 to 1-1/2 inches, The chamfered portion of the thread simplifies starting. Taper boiler taps have no chamfered portion and the threaded portion is tapered 3/4 inch per foot. The overall and shank diameters are similar to those of straight boiler taps.

(6) Staybolt taps. Staybolt taps (fig 30) are used principally in boiler locomotive and railroad shops for tapping the staybolt holes in the outer and inner plates or shells of boilers.

(7) Mud or washout taps. Mud or washout taps (fig 30) have 6 flutes with a 1-1/4-inch taper per foot and 12 threads per inch. They are used to cut special American National or V-form threads when tapping mud plug drain holes.

(8) Fusible plug tap. The fusible plug tap (not illustrated) is made of carbon steel and has a 1-1/4-inch taper per foot. It has a thread length of 4-3/8 inches, is 1-inch square on the shank, has 4 flutes, and is 7 inches long.

(9) Pipe tap and drill. A pipe tap and drill (not-illustrated) is a

combination twist drill and tap. The tap portion follows the twist drill tip. The tool does drilling and tapping in one operation. They are made of carbon steel with a tapered square shank and sized to drill and tap holes 1/4--18, 3/8--18, and 1/2--14.

c. Types of dies. Dies are made in several different shapes and are of the solid or adjustable type.

(1) Solid dies.

(a) Square pipe dies. The square pipe die (fig 31) will cut American Standard Pipe Thread only. It is issued in sizes that will cut threads on pipe from 1/8--27 to 3-8.

(b) Rethreading dies. A rethreading die (fig 31) is used principally for dressing over bruised or rusty threads on screws and bolts. However, it may be used for cutting occasional new threads. It is available in a variety of sizes for rethreading American Standard Coarse and Fine threads. These dies are usually hexagon in shape and can be turned with a socket, box, open-end, or any wrench that will fit. Rethreading dies are available in sets of 6, 10, 14, and 28 assorted sizes in a case.

(2) Adjustable dies.

(a) Round split adjustable dies. Round split adjustable dies (fig 32) are called "Button" dies and can be used in either hand diestocks or machine

holders. The adjustment in the screw adjusting type is made by a fine-pitch screw which forces the sides of the die apart or allows them to spring together. The adjustment in the open adjusting types is made by means of three screws in the holder--one for expanding and two for compressing the dies. Round split adjustable dies are available in a variety of sizes to cut American Standard Coarse and Fine threads, special form threads, and for the standard sizes used in Britain and other European countries. For hand threading, these dies are held in diestocks (fig 33). One type has three pointed screws that will hold round dies of any construction, although specifically made for open adjusting-type dies.

Figure 32,. Types of adjustable dies.

(b) Two-piece collet dies. A two-piece collet die (fig 32) is used with a collet (fig 33). The collet consists of a cap and guide. The die halves are placed in the cap slot and are held in place by the guide which screws into the underside of the cap. The die is adjusted by means of setscrews at either end of the internal slot. This type of adjustable die is issued in various sizes to cover the cutting range of American Standard Coarse and Fine and special form threads. Diestocks (fig 33) to hold collets are issued in three sizes.

Figure 33. Diestocks, die collet, and tap wrenches.

(c) Two-piece rectangular pipe dies. Two-piece rectangular pipe dies (fig 32) are available to cut American Standard Pipe Threads. They are held in ordinary or ratchet-type die-stock. The jaws of the dies are adjusted by means of setscrews. An adjustable guide serves to keep the pipe in alinement with respect to the dies. The smooth jaws of the guide are adjusted by means of a cam plate; a thumbscrew locks the jaws firmly in the desired position (fig 34). Non-ratchet- type diestock and plain guides are also available for pipe threading (fig 35).

Figure 35. Threading sets.

d. Use of taps.

(1) Selecting a tap drill. The size of a tap is the outside diameter of its threads; therefore, theoretically the hole drilled for tapping could be smaller than the tap by twice the depth of the thread if a full thread is cut. The shape of the thread partly determines the amount to be subtracted from the tap diameter. Do not use a tap drill that is too small in diameter, as this will cause tapping troubles. A drilled hole should be of sufficient diameter to produce a thread depth of approximately 75 percent. This 75 percent is only an average. Actually, the percentage may vary from 50 to 53 percent for small or deep holes to a maximum of 83 percent on any size. A definite thread depth for all sizes, under all

conditions, is not practical. You must determine which is most suitable by analyzing the following conditions: diameter of tapped hole, nature of material being tapped, depth of tapped hole, and the pitch.

THE THREADING SET WITH RECTANGULAR