Taps: thread, material & coating

The standard thread in Europe, 60° flank angle. Named by diameter and pitch in millimetres: M8 x 1.25. Coarse pitch is the default for general use.

Same diameter as metric, but a finer pitch (M8 x 1 instead of 1.25). It holds better under vibration, works in thin walls and allows finer adjustment.

The American coarse thread (Unified Coarse), 60° flank angle. Named by diameter in inches and threads per inch: 1/4" x 20.

The American fine thread (Unified Fine). More threads per inch than UNC, so it holds better under vibration and adjusts more precisely.

British Whitworth, with a 55° flank angle instead of 60°. An older standard found on British machinery and old work. It is not interchangeable with UNC.

A parallel pipe thread (55°) for hydraulic and pneumatic connections. It does not seal on its own: sealing is done with a washer or PTFE tape.

The American tapered pipe thread (60°). It seals on the thread itself as it tightens, which is why it needs a tapered tap and the correct depth.

A trapezoidal thread for transmitting motion, not for clamping. You will find it on leadscrews, vices and feed screws, where smooth movement under load is the point.

Machine tap. Straight flutes, the most common form. The chip is pushed forward, which is why it suits through holes and shallow blind ones. Simple, economical and tough.

Machine tap. Helical flutes that pull the chip backwards, out of the hole. The answer for blind holes, where the chip has nowhere else to go and would otherwise pack.

Hand tap. Turned with a wrench rather than a machine, and sold as a set: the taps in the set have different chamfers on the nose and go in sequence: from the most tapered, which starts easily, to the flattest, which reaches the bottom of a blind hole. Slower, but it gives you the control and needs no machine.

Machine tap. It does not cut; it forms. It presses the material into the shape of the thread, so it produces no chip at all. For ductile materials (aluminium, copper, mild steel). It gives a stronger thread, but needs a larger pilot hole than a cutting tap.

Plain and alloy steels: the largest and most forgiving group. They give a long, continuous chip, so chip control is what matters.

Stainless steels. They work-harden locally as you cut them, weld to the edge, and do not carry heat away. They want cobalt, a steady feed with no dwelling, and plenty of coolant.

Cast irons. They give a short, crumbling chip, but the material is abrasive and eats the cutting edge. Here you need abrasion resistance, not heat resistance.

Non-ferrous: aluminium, brass, copper. Soft and fast, but they throw a bulky chip that sticks. They want few flutes, large flute valleys and high revs.

Superalloys and titanium (Inconel, Ti). Very low thermal conductivity: the heat does not leave with the chip, it stays in the edge. Low speeds, steady feed, lots of coolant.

Hardened materials, typically above 45 HRC. They demand carbide and a thermally stable coating; plain HSS simply dulls immediately.

High speed steel with 5% cobalt for increased resistance to high temperatures. Suitable for stainless (INOX) and hard steels.

The classic high speed steel with high hardness and resistance to fracture, ideal for straightforward work.

Powder metallurgy steel with a perfectly uniform structure and excellent wear resistance. A top-tier solution offering longer tool life than conventional high speed steels.

Tungsten carbide with cobalt, extremely hard and rigid. For very high cutting speeds in CNC and maximum tool life, but sensitive to vibration and impact.