How do I work out the tapping drill size?

In metric threads, subtract the pitch from the thread diameter. For M8x1.25: 8 − 1.25 = 6.75, so a 6.8 mm drill.

Why do my taps keep breaking?

Usually the tapping hole is too small, the chip is not being broken, or there is no cutting oil. Backing off a quarter turn every half turn breaks the chip and prevents most breakages.

Why does the order of a three-tap set matter?

The taper tap starts the thread, the second deepens it and the bottoming tap cuts the full profile. Skipping a step spoils the thread, and in blind holes the bottoming tap is what carries it to the base.

What is the difference between a tap and a die?

A tap cuts an internal thread in a hole (a nut thread); a die cuts an external thread on a rod (a bolt thread). Both are defined by the same size and pitch system.

How do I tell metric from imperial threads?

Metric sizes start with M and quote pitch in millimetres (M8x1.25). Imperial quotes diameter as a fraction and pitch as threads per inch (1/4"-20). The two do not interchange.

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Taps & Dies

A tap cuts an internal thread and a die an external one; success in both depends on getting size and pitch right and drilling the tapping hole to the correct diameter. This page covers the metric system, the tapping drill calculation and the order of a hand tap set. The catalogue holds over 480 sizes.

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Tapping drill diameter

The hole drilled before tapping is smaller than the thread by one pitch: M8x1.25 needs about 6.75 mm (8 − 1.25). That rule of thumb works directly in metric threads and prevents both failure modes.

Too large a hole leaves too little thread depth and a weak joint; too small overloads the tap and snaps it. Extracting a broken tap usually costs the workpiece.

Thread	Pitch	Tapping drill
M4	0.70 mm	3.3 mm
M5	0.80 mm	4.2 mm
M6	1.00 mm	5.0 mm
M8	1.25 mm	6.8 mm
M10	1.50 mm	8.5 mm
M12	1.75 mm	10.2 mm

Hand tap sets and their order

Hand taps usually come in threes and are used in order: the taper tap starts the thread, the second deepens it, and the plug (bottoming) tap cuts the full profile. In blind holes the third tap matters most, because it carries the thread to the bottom.

Machine taps are single tools for a bench drill or magnetic drill, designed for continuous feed rather than the back-and-forth of hand tapping.

Back off a quarter turn every half turn to break the chip.
Use cutting oil — tapping dry ruins the tap.
Start square to the axis; a tilted start ruins the thread.
In blind holes, do not skip the bottoming tap.

Frequently Asked Questions

How do I work out the tapping drill size?
In metric threads, subtract the pitch from the thread diameter. For M8x1.25: 8 − 1.25 = 6.75, so a 6.8 mm drill.
Why do my taps keep breaking?
Usually the tapping hole is too small, the chip is not being broken, or there is no cutting oil. Backing off a quarter turn every half turn breaks the chip and prevents most breakages.
Why does the order of a three-tap set matter?
The taper tap starts the thread, the second deepens it and the bottoming tap cuts the full profile. Skipping a step spoils the thread, and in blind holes the bottoming tap is what carries it to the base.
What is the difference between a tap and a die?
A tap cuts an internal thread in a hole (a nut thread); a die cuts an external thread on a rod (a bolt thread). Both are defined by the same size and pitch system.
How do I tell metric from imperial threads?
Metric sizes start with M and quote pitch in millimetres (M8x1.25). Imperial quotes diameter as a fraction and pitch as threads per inch (1/4"-20). The two do not interchange.