TAPS, DIES, AND EXTRACTORS

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:

Contents
Major sections of this unit

  1. Taps
  2. Dies
  3. Extractors

Review Questions


TAPS

Types and Uses

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.

Taper (Starting) Hand 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 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 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 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

Mud Hand Tap (Washout 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.

 

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DIES

Types and Uses

Rethreading Die

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.

Two-Piece Collet 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 Die

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 Set

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.

Using a Hand Tap

The following steps describe how to use a hand tap properly:

Warning

Wear eye protection when working where flying particles may cause eye injury.

  1. Clamp a steel plate securely in a vise. Drill and ream a hole of the desired size.
  2. Select a tap and secure it in a tap wrench.
  3. Apply cutting oil to the tap and the hole.
  4. Place the point of the tap in the hole (Figure 12); rotate a right-handed tap clockwise for right-hand threads or a left-handed tap counterclockwise for left-hand threads.

Figure 12 Position the tap.

Figure 13 Check the tap for squareness.

  1. Remove the tap wrench and use a square to check the tap for squareness (Figure 13). Check at least two different positions on the tap.
  2. Replace the tap wrench and continue the tapping operation. It is not necessary to apply pressure, as the threads will be pulled through at all times.
  3. Remove the tap by turning in the opposite direction.
  4. Wipe the excess oil and metal shavings from the metal plate.
  5. Check the newly-cut threads with a screw pitch gage before inserting a screw or stud.

Using a Die and Diestock

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.

  1. Secure the work firmly in a vise.
  2. Assemble the die and diestock (Figure 14).

Figure 14 Assemble the die and diestock.

  1. Tighten the setscrew.
  2. Loosen the two thumbscrews to adjust diestock.
Caution

After assembling the die to the diestock, ensure the setscrew is tight. The die can fall out of diestock and become damaged.

  1. Apply cutting oil to the die and to the work.
  2. Position the diestock over the work.
  3. Rotate the diestock clockwise for right-hand threads (Figure 15) or rotate a left-handed tap counterclockwise for left-hand threads, slowly but firmly, until the die takes hold.

Figure 15 Rotate the diestock.

  1. After several threads have been cut, use a square to check squareness.
  2. Turn the diestock one turn forward and one-quarter turn backward. Repeat this procedure until the desired thread length has been cut.
  3. Carefully back the diestock off the threads by turning in the opposite direction.
  4. Clean the threads with a clean rag.
  5. Check the threads with a screw pitch gage before using.
  6. Disassemble the die and diestock by loosening the setscrew.
  7. Wipe clean with a rag.

Care of Taps

Use the following guidelines when working with taps:

Care of Dies

Use the following guidelines when working with dies:

 

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EXTRACTORS

Types and Uses

 Screw Extractor

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 Extractor

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.

Using a Spiral Tapered Screw Extractor

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.

  1. Drill a hole in the broken screw (Figure 18). Use a drill size guide if available. If one is not available, drill the hole slightly smaller than the diameter of the extractor (when drilling larger screws it may be necessary to drill a small pilot hole first, then a larger hole).

Figure 18 Drill a hole in the broken screw.

Figure 19 Turn the extractor counterclockwise.

  1. Insert the extractor in the drilled hole.
  2. Remove the broken screw by turning the extractor counterclockwise (Figure 19). The extractor may be turned using a tap wrench or open end wrench.

Care of Extractors

Use the following guidelines when working with extractors:

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Review Questions

1. The body of a tap contains how many flutes?

A. 4
B. 6
C. 8
D. 10

2. What section of a tap allows the tap to be positioned squarely in the metal to be threaded?

A. Body
B. Chamfer
C. Round shank
D. Square end

3. To start tapping operations, what tap should be used?

A. Bottom
B. Plug
C. Staybolt
D. Taper

4. When extremely tight fits are necessary, what tap should be used?

A. Bottom
B. Staybolt
C. Plug
D. Taper

5. For tapping holes in the outer and inner plates or shells of boilers, what tap should be used?

A. Bottom
B. Plug
C. Staybolt
D. Taper

6. A rethreading die is usually hexagonal in shape and can be turned by what type of tool?

A. Hammer
B. Pliers
C. Screwdriver
D. Socket

7. What type of die is used with a collet cap and collet guide?

A. Rethreading
B. Round adjustable
C. Staybolt
D. Two-piece

8. When using a die and diestock, what distance should the diestock move in the forward direction?

A. One-quarter turn
B. One turn
C. Two turns
D. Three turns

9. What type of tool is used to remove a broken screw without damaging the surrounding material or threaded hole?

A. Screw die
B. Screw extractor
C. Tap and die
D. Tap extractor

10.Screw extractors are straight with what type of flutes at one end?

A. Crisscross
B. Perpendicular
C. Spiraling
D. Straight

11.Screw extractors are available to remove broken screws having what size outside diameter?

A. 1 centimeter
B. 1 /2 inch
C. 3 /4 inch
D. 2 centimeters

12.Tap extractors can remove taps from what minimum size outside diameter, in inches?

A. 3 /16
B. 5 /16
C. 3 /8
D. 3 /4

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Answers to Exercises

1. A
2. B
3. D
4. C
5. C
6. D
7. D
8. B
9. B
10. C
11. B
12. A

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Copyright David L. Heiserman
All Rights Reserved