Taps and dies

Taps and dies are metalworking tools for the creation (cutting) of screw threads in metal parts.

Tap

The tap cuts a thread on the inside surface of a hole, creating a female surface which functions like a nut. The three taps in the image are of different sizes and types.

The top tap has full size thread to the end and is called a bottoming tap; it taps to the bottom of the hole.
The middle tap is an intermediate tap where the thread tapers before the bottom.
The lower tap is a taper tap where the thread significantly tapers towards the end. This taper allows the tap to ease into the freshly drilled hole in a gradual cutting action, relieving the cutting pressure on the first few teeth of the tap.

Before using the tap, a hole is created (usually drilled) of minor diameter according to the tap size. This is the equivalent of the blank size (major diameter), less thread depth. The order of usage when hand tapping is to use the taper tap first, the intermediate next (if the material is hard and it is felt that the tap is still working too hard), and finally the bottoming tap is used to create thread for the full depth of the shank or hole.

Practical use and safety of the tap is in tap wrench.

Die

The die cuts a thread on a preformed cylindrical rod, which creates a male threaded piece which functions like a bolt. The dies shown are

top left: an older split die, with top adjusting screw
bottom left: a one piece die with top adjusting screw
center: a one piece die with side adjusting screw (barely visible on the full image)
right: two dies without adjusting screws

To use, a cylindrical blank, which is usually slightly less than the required diameter, is machined with a taper (chamfer) at the threaded end. This chamfer allows the die to ease onto the blank before it cuts a sufficient thread to pull itself along.

The adjusting screws allow the die to be compressed or expanded to accommodate slight variations in size, due to material, manufacture, or die sharpness. The two rightmost dies shown in the image have no adjusting screws, however the die holder can be used to exert pressure and close the cutting size down if required.

Each tool is used independently, but are usually sold in paired sets of both types, one die and three taps. Some sets however may provide a lesser number of taps. The common sets shown are designed for hand operation, but different types such as helical or spiral may be used in production tools such as CNC machining tools.

Tap Drill Bit Size Table (source: http://www.boltdepot.com)

Imperial Tap & drill bit size table

Metric Tap & drill bit size table

Tap	Fractional Drill Bit	Number Drill Bit	Letter Drill Bit
0-80	3/64	-	-
1-64	-	53	-
2-56	-	50	-
3-48	-	47	-
4-40	3/32	43	-
5-40	-	38	-
6-32	7/64	35	-
8-32	-	29	-
10-24	5/32	25	-
10-32	5/32	21	-
12-24	11/64	16	-
1/4-20	13/64	7	-
1/4-28	7/32	3	-
5/16-18	17/64	-	F
5/16-24	-	-	I
3/8-16	5/16	-	-
3/8-24	21/64	-	Q
7/16-14	23/64	-	U
7/16-20	25/64	-	-
1/2-13	27/64	-	-
1/2-20	29/64	-	-
9/16-12	31/64	-	-
9/16-18	33/64	-	-
5/8-11	17/32	-	-
5/8-18	37/64	-	-
3/4-10	21/32	-	-
3/4-16	11/16	-	-
Drill sizes are for 75% depth of thread.

Tap	Metric Drill	Imperial Drill
3 mm x 0.6	2.5 mm	-
4 mm x 0.7	3.4 mm	-
5 mm x 0.8	4.3 mm	-
6 mm x 1.0	5.2 mm	-
7 mm x 1.0	6.1 mm	15/64
8 mm x 1.25	6.9 mm	17/64
8 mm x 1.0	7.1 mm	-
10 mm x 1.5	8.7 mm	-
10 mm x 1.25	8.9 mm	11/32
10 mm x 1.0	9.1 mm	-
12 mm x 1.75	10.5 mm	-
12 mm x 1.5	10.7 mm	27/64
14 mm x 2.0	12.2 mm	-
14 mm x 1.5	12.7 mm	-
16 mm x 2.0	14.2 mm	35/64
16 mm x 1.5	14.7 mm	-
Drill sizes are for 75% depth of thread.

Pipe

Threaded pipe is often used in plumbing and pneumatic applications. Because pipe joints must form a seal, the threaded portion is slightly conical rather than cylindrical. As a result, threaded pipe requires specialized taps and dies.

Rolled threads

Not all threads are made by cutting. In large production runs or where great strength is required, the threads may be rolled, both for the male and female portions.

In the case of the tap there are no cutting edges but instead the tap is lobed. The tap is forced into the hole and the material is deformed by the lobes into the required thread form. The male portion (bolt) is fed between rollers that have the full thread form ground into their outer diameter. The action of feeding the rollers into the work piece deforms the material into the required shape.

Rolled threads have the advantage of increased strength (the material flows into shape, similar to forging) along with reduced material cost as the bar or rod used is actually smaller than the finished size due to the material squeezing into shape.

Other uses

An ordinary tap and die can also be used for repairing threads in stripped holes or bolts. Die nuts are dies made for cleaning up old threads, they have no split for resizing and are made from a hexagonal bar so that a wrench or shifter spanner can be used to apply them (rather than the purpose built die wrench).

Drilling and threading | Screws

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