Taps and Dies

Taps and dies are tools used to cut threads for nuts and bolts. One of the earliest forms of tap was illustrated by Leonardo da Vinci toward the end of the fifteenth century. It was a set of three short square sectioned bars with the thread cut deeper on each face, mounted like a star so that each tap could be turned using the other two as handles. These were used for tapping the screw box, an early form of die, which cut the corresponding screw thread on a turned cylinder, usually made of hardwood in those days.

SCREW THREAD TYPES

When screw threads were cut with smith-made taps and dies there was no standardization, as each workshop used its own pattern. Even now there are many different screw thread types, far more than necessary, some differing from each other by only small amounts. Those most likely to be encountered are as follows:

Metric Coarse

This thread is made in diameters from 1 to 300mm and is recommended for all general work.

Metric Fine

There are various fine threads for special purposes such as in machinery where vibration would loosen the fixing.

B.A. (British Association)

Used on the small screws extensively used in electrical equipment, available in 16 sizes, Nos. 0-15 (0.236 to 0.031in.)

B.S.W. (British Standard Whitworth)

Made from 1/4in. diameter upward, rising in 1/16in. steps. Now obsolete, this was the first standard thread in Britain, but was found to be too coarse for some applications. The steepness of the thread gives too little clamping force in the presence of vibration. It is now used for soft or weak materials such as aluminum or cast iron or for cheaply made nuts and bolts where accuracy is not important.

B.S.F. (British Standard Fine)

This was introduced to overcome the lack of clamping force of B.S.W. and is made in the same sizes.

UNF Unified Fine, UNC (Unified Coarse)

The American equivalent of Whitworth and B.S.F. Made from 1/4in. diameter, rising in 1/16in. steps.

Taps

SIZE: See above

MATERIAL: Steel

ACCESSORIES: Tap wrench

USE: To cut internal screw threads as for a nut

Tap wrenches

A variety of tap wrenches are available; most are adjustable with two handles. You can use an ordinary wrench on a tap but it is not advisable when starting a new thread because a firm grip and a symmetrical handle are important.

Matching a thread to a bolt

If a bolt hole has been stripped of its thread, it is necessary to match the tap to a larger bolt. If you know the thread type and size, look for the appropriately marked tap. If you do not, measure the bolt with calipers to ascertain the right diameter.

Alternatively, use a screw pitch gauge. Compare the thread of the bolt with that of the tap by fitting them together: they should marry exactly. If you are in any doubt, take the bolt to be matched by a tool supplier.

Starting a threaded hole

To make a thread mark the position of the hole and centre punch it. If the work is important or is likely to be subjected to high stress, use the correct tapping drill for the tap and the material being worked.

Alternatively, select a drill slightly larger than the minor diameter of the thread. If the drill is the same size or smaller than this the tapping will be stiff and the tap may break.

Fit the taper tap into the wrench and position it in the hole. Make sure you align the tap with the hole as it will be impossible to correct mis-alignment once cutting has begun.

For accuracy, use a lathe or drill press to start the thread.

Do not use the power unless you have an automatic tapping device but turn the machine by hand to avoid breaking the tap. In each case provide pressure into the work while turning. Once the thread is started it can be continued by hand.

Marrying thread and bolt

Press the thread of the tap to the bolt to see that they fit flush together.

Keeping the tap straight

Use a try square to check for true in two directions at right angles.

Completing the thread

Use tallow or oil lubricants for steel and turpentine for copper or aluminum. Thread brass and cast iron dry.

Turn the tap back 1/4 turn to break the swarf then proceed with a 1/2 turn back and so on.

Threading deep holes

Cut part of the thread using the three taps in sequence then start again with the taper tap and repeat the process. This will reduce the strain on the taps and lessen the chance of breakage.

Any stopped holes should be drilled deeper than the required depth of thread because it is impossible to cut right to the bottom. Remove taps frequently to clear them of swarf. Remove swarf from the hole by shaking the work or, where this is not possible, use compressed air (wear safety glasses) or a greased rod. Take care when approaching the bottom of a hole; if you go too far, the tap can easily break.

Dies

SIZE: Various, see above.

MATERIAL: Steel

ACCESSORIES: Stock or handle

USE: To cut external screw-threads as for a belt

ROUND SPLIT DIE

The most common type of die is the split ring or round split die. This has a central threaded hole with a slight chamfer at one end.

Three or four holes grouped around the central one form cutting edges. The die is split to allow for adjusting by means of screws in the stock. To open it, the locking screws are backed off and the adjusting screw screwed in; the locking screws are then tightened to secure it.

The die can be closed by backing off the adjusting screw until the natural spring of the die closes the gap. To close the die further, tighten the locking screws. This adjustment provides a good fit between nut and bolt and also allows metal to be removed in easy stages.

SPLIT DIES

The other common type of die is the split die, which consists of two rectangular jaws assembled in the stock and operated by means of a single adjusting screw. This type of die allows more adjustment than the round split die.

Threading a rod

First cut a 5° to 7° chamfer on the end of the rod for about 1-1/2 diameters. Open the die (and tighten the locking screws in the case of a split ring die) and place it on the end of the rod which must be firmly held. Press down and turn while keeping the die square to the rod. Once the die has begun to bite, no further pressure is required. Proceed a half turn forward and a quarter turn backward to break the swarf and lubricate as for tapping.

Preparing for another cut

Having completed the required length of thread, wind the die back to the beginning and tighten for another cut. Check the fit of the nut after each cut.

Using a drill press

Rest the die stock on the bed of the machine, chamfer uppermost, and hold firmly. Insert the rod in the chuck which should be hand turned. You can also use a lathe, fitting the work in the chuck and the die stock on a face plate.

Pipe Die

SIZE: 1/4 to 1-1/4in.

MATERIAL: Steel

ACCESSORIES: Diestock, guide bushing

USE: To cut a screw thread on pipework

The pipe die can be a one piece cutter made to fit a particular pipe size, or adjustable if constructed from separate jaws, You need a guide to align the die with the pipe: this can be either part of the stock itself or a separate bushing which fits into it along with the die. The cutting sequence is the same as that for normal thread cutting, with the die being lubricated every 2 to 3 turns. Some die-stocks have ratchets so the tool can be used in confined spaces.

Die Nut

CITHER NAME: Rethreading die

SIZE: Various, see above

MATERIAL: Steel

USE: To recut damaged or rusty machine screw threads

A die nut can be used to clean fup an existing thread. It is not adjustable and must be matched to the screw thread. It can be driven with any convenient wrench.

Screw Box

SIZE: To thread from 2 to 3in. diameter

MATERIAL: Box: wood; Cutter: metal

USE: To cut threads in wood

The screw box is made in two parts and is held together by screws. The cutter is mounted between the two. The first section of the box is smooth bored to act as a guide when starting the cut. With the wooden dowel held in a vise, the box is engaged on the end and turned clockwise until it begins to cut. Internal threads are cut with a tap like those used for metal.

While now rare, wooden threads were once extensively used in hand tools such as clamps and rabbet planes. Today, wooden screws are used in marking gauges.