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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. Extractors are used to remove broken taps or screws that have been broken off inside of the hole or fastener. In this unit, you will learn about different types of taps, dies, and extractors and their uses. You will also learn how to select the right tap, die, or extractor for the job, use various types, and provide the proper care and keep them in good working condition.
When you have completed this unit, you will be able to do the following:
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Figure 1 � Parts of a tap.
Taps (Figure 1) are made of hardened steel and have the following parts: a square end, a round shank, a body (threaded) section, and a chamfer. The square end is used to turn the tap with either a straight or T-handled tap wrench. The shank is a smooth, rounded section that is immediately behind the threaded section. The body (threaded) section contains four flutes which have threads cut into their upper edges. They have a hollow section near the center to permit metal shavings to fall away from the cutting edges. The chamfer is the non-threaded end of the tap that allows the tap to be positioned squarely in the metal to be threaded without engaging the threads of the tap.
The taper (starting) hand tap (Figure 2) has a chamfer (nonthreaded) length equal to 7 to 10 threads. The taper lead distributes the cutting force over a large area. The taper hand tap is used to start tapping operations.
Figure 2 � Taper (starting) hand tap.
Plug/pipe taps (Figure 3) are used for pipefitting and places where extremely tight fits are necessary. The tap diameter, from end to end of the threaded portion, increases at the rate of 3 /4 inch per foot. All the threads on this tap do the cutting, as compared to the taper taps, on which only the non-chamfered portion does the cutting.
Figure 3 � Plug/pipe hand tap.
Bottoming hand taps (Figure 4) 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. Use of both the taper and plug taps should precede the use of the bottoming hand tap. This tap is also used when tapping hard materials.
Figure 4 � Bottoming hand tap.
Staybolt taps (Figure 5) are used in boiler, locomotive, and railroad shops for tapping holes in the outer and inner plates or shells of boilers. The staybolt tap has two separate threaded areas. The first is for cutting threads and the second is for guiding the tap into another piece of metal for threading by the cutting threads. The spindle-type staybolt has an adjustable spindle, which changes the distance between the cutting threads and the guide threads.
Figure 5 � Staybolt tap
The mud or washout tap (Figure 6) has six flutes, tapers 1 1 /4 inches per foot, and has 12 threads per inch. It is used for cutting Unified or National form threads in mud plug drain holes.
Figure 6 � Mud hand tap.
A rethreading die (Figure 7) is used principally for dressing over-bruised or rusty threads on screws or bolts. It is available in a variety of sizes for rethreading National Coarse (NC) and National Fine (NF) threads. Square pipe and rethreading dies are usually hexagonal 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.
Figure 7 � Rethreading die.
Figure 8 � Two-piece collet die.
Two-piece collet dies (Figure 8) are used with a collet cap and collet guide (Figure 9). 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 setscrews at both ends of the interval slot. This type of adjustable die is issued in various sizes to cover the cutting ranges of NC, NF, and special-form threads. Diestocks to hold the dies come in three different sizes.
Figure 9 � Cap and collet guide.
Round split adjustable dies (Figure 10), also called button dies, 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 type is made by three screws in the holder, one for expanding and two for compressing the dies. A die holder or handle is needed for proper operation of round split adjustable dies.
Figure 10 � Round split adjustable die.
Thread cutter sets (Figure 11) are available in many different combinations of taps and dies, together with diestocks, tap wrenches, guides, and necessary screwdrivers and wrenches to loosen and tighten adjusting screws and bolts.
Figure 11 � Thread cutter set.
The following steps describe how to use a hand tap properly:
| Warning Wear eye protection when working where flying particles may cause eye injury. |
 Figure 12 � Position the tap. |
 Figure 13 � Check the tap for squareness. |
The following steps describe how to use a die and diestock properly:
| Note The work to be threaded must be clean and free of burrs. |
Figure 14 � Assemble the die and diestock.
| Caution After assembling the die to the diestock, ensure the setscrew is tight. The die can fall out of diestock and become damaged. |
Figure 15 � Rotate the diestock.
Use the following guidelines when working with taps:
Use the following guidelines when working with dies:
Screw extractors (Figure 16) are used to remove broken screws without damaging the surrounding material or the threaded hole.
Figure 16 � Screw extractor.
Screw extractors are straight, with spiraling flutes at one end. These extractors are available in sizes to remove broken screws having 1/4- to 1/2-inch outside diameter (OD). Spiral tapered extractors are sized to remove screws and bolts from 3/16- to 2 1/8-inch OD.
The spiral-tapered type requires a twist drill for drilling a pilot hole.
Tap extractors (Figure 17) are used to remove broken taps. Tap extractors are sized to remove taps from 3/16- to 2 1/8-inch OD. The tap extractor is usually of the flute type and requires the use of a wrench to turn the tap. It is used for removing taps with no external area. The tap extractor has fingers that enter the flutes of the tap.
Figure 17 � Tap extractor.
Most extractor sets include twist drills and a drill guide.
The following steps describe how to use a spiral tapered screw extractor properly:
| Warning Wear eye protection when working where flying particles may cause eye injury. |
 Figure 18 � Drill a hole in the broken screw. |
 Figure 19 � Turn the extractor counterclockwise. |
Use the following guidelines when working with extractors:
1. The body of a tap contains how many flutes?
2. What section of a tap allows the tap to be positioned squarely in the metal to be threaded?
3. To start tapping operations, what tap should be used?
4. When extremely tight fits are necessary, what tap should be used?
5. For tapping holes in the outer and inner plates or shells of boilers, what tap should be used?
6. A rethreading die is usually hexagonal in shape and can be turned by what type of tool?
7. What type of die is used with a collet cap and collet guide?
8. When using a die and diestock, what distance should the diestock move in the forward direction?
9. What type of tool is used to remove a broken screw without damaging the surrounding material or threaded hole?
10.Screw extractors are straight with what type of flutes at one end?
11.Screw extractors are available to remove broken screws having what size outside diameter?
12.Tap extractors can remove taps from what minimum size outside diameter, in inches?
Answers to Exercises
Copyright � David L.
Heiserman
All Rights Reserved