Thread Tapping: Cut Taps vs Form Taps and Thread Classes

Most threaded holes in machined parts are made by tapping — running a tap into a drilled hole to cut or form the internal thread. It sounds simple, but the choices behind it (cut tap or form tap, the right drill size, blind or through hole, the thread class) decide whether the thread is strong, gauges correctly and doesn’t snap a tap in your part. This guide covers how tapping works, the difference between cut and form taps, and the design rules that make threaded holes reliable.

How Tapping Works

A threaded hole starts as a plain drilled hole at a specific tap-drill diameter, then a tap creates the thread. Get the tap-drill size wrong and you either strip a too-large hole or break the tap in a too-small one, so the drill size is matched to the thread and the desired thread engagement. The two ways a tap makes the thread — cutting it or forming it — are fundamentally different.

Cut Taps vs Form Taps

A cut tap (the traditional kind) has flutes that remove material to carve the thread, producing chips. It works on virtually any machinable material, including brittle ones like cast iron, and is the general-purpose choice. The catch is chips — in a blind hole they must clear, or they pack and break the tap.

A form tap (roll/forming tap) has no cutting edges; it cold-forms the thread by displacing material, making no chips at all. Because the metal is pushed rather than cut, the resulting thread is stronger (the grain flows around it) and there are no chips to clear — ideal for blind holes. The trade-off: form taps only work on ductile materials (aluminium, mild steel, copper, many stainless grades) and need a slightly larger tap-drill size. On the right material, form tapping gives a stronger, cleaner, chip-free thread.

Thread Classes and Fit

Internal threads have a class of fit that controls how tight they mate with the bolt — in inch threads, 2B is the general-purpose standard and 3B is a tighter, more precise fit; metric threads use tolerance classes like 6H. Call out a tighter class only where the application needs it, since it costs more to hold. Matching the internal class to the external fastener is part of getting the joint right — see our fits & tolerances guide.

Designing Threaded Holes That Tap Cleanly

• Don’t over-specify thread depth. Thread engagement of about 1.5× the diameter is plenty in steel (more in aluminium) — deeper just costs time and risks tap breakage.
• Allow clearance at the bottom of blind holes. Drill deeper than the thread so the tap (and chips) have room; a tap can’t thread to the very bottom.
• Add a chamfer at the hole mouth so the fastener starts straight and the first thread isn’t damaged.
• Choose form tapping for ductile, high-strength or blind-hole threads; cut tapping for brittle materials or where forming isn’t suitable.
• For soft or repeatedly-assembled materials, consider a threaded insert instead of a tapped hole for far greater strength and reusability.

The Bottom Line

A reliable threaded hole is more than “drill and tap.” Match the tap-drill size to the thread, pick a cut tap for general and brittle materials or a form tap for stronger, chip-free threads in ductile metals, and specify only the thread depth and class the joint actually needs. Add a starting chamfer, leave room at the bottom of blind holes, and reach for a threaded insert when the base material is soft. Get those right and threads gauge correctly, hold their rated load and don’t leave a broken tap buried in your part.

MechPart Pro taps and thread-mills threaded holes to the right class and depth — cut or form tapping to suit your material — and installs threaded inserts where strength demands. Share your drawing and our engineers will confirm thread specs, hole depths and the best method as part of our free design-for-manufacturability review. Explore the alloys we thread in our materials guide.