Cut Taps & Form Taps

Taps are the cutting tools that make internal threads, the fluted screws that cut or form the thread inside a drilled hole, and they come in two fundamentally different kinds. The tapping operation — the machine feeding the tap into the hole, one pitch per revolution, to cut the thread — is this wiki’s entry on the process; this entry is about the tool itself, the tap, and the choice that shapes every tapping job: cut taps and form taps. A cut tap makes the thread by cutting, its edges shearing the thread form out of the wall of the hole and producing chips. A form tap makes the same thread by displacement, its lobed body pressing and rolling the metal up into the thread form, producing no chips at all. The two tools make threads that look alike and behave very differently, and choosing between them follows the material, the hole and the machine. This entry sets out how each tap works and when each is the right tool.

The cut tap

The cut tap is the classic tap, the fluted, threaded tool that cuts an internal thread in the way that most people picture: the tap is a screw of the thread to be made, with flutes — grooves cut along its length — forming cutting edges and carrying chips away, and its end chamfered so that a few threads lead into the cut. As the tap turns and feeds into the hole, the leading chamfered threads cut the thread form progressively, each tooth taking its share, until the full thread is cut. The cut tap is named by its chamfer: the long, gradual taper of a taper tap leads into the thread gently; the shorter chamfer of a plug tap, the common general-purpose tap, suits most jobs; and the bottoming tap, with only a thread or two of chamfer, is the tap that cuts the thread to the very bottom of a blind hole, following the plug tap after it has done the main cutting. The cut tap is made in high speed steel or carbide, coated for the material, its geometry ground to cut cleanly and to break and clear the chip.

Cut-tap chips and their flutes

The cut tap’s whole working is bound up with its chips, because a tap cutting a thread in a blind hole is cutting in a confined space and the chips must go somewhere. The flutes of the tap decide the direction. A tap with straight flutes, the ordinary tap, pushes its chips ahead of it, so it is the tap of the through hole, where the chips fall out the far side; in a blind hole the straight-fluted tap packs its chips at the bottom and breaks. The spiral-point tap — the gun tap — has its flutes angled so that the chips are pushed forward, ahead of the tap, which makes it the fast, reliable tap of the through hole, its chips pushed out the bottom as it cuts. The spiral-flute tap reverses the idea: its flutes spiral so that the chips are pulled up and out of the hole, which makes it the tap of the blind hole, lifting its chips clear as it cuts down. Choosing the cut tap is thus choosing its chip control as much as its thread: the spiral point for the through hole, the spiral flute for the blind, the straight flute for the general and the simple.

The form tap

The form tap — the forming tap, the roll tap — makes its thread without cutting at all. It has no flutes and no cutting edges; instead it is a lobed tool, its body the shape of the thread with three, four or more lobes round its circumference, and it works by pressing. As the form tap turns and feeds into the hole, its lobes squeeze the wall of the hole, and the metal, being ductile, is displaced rather than cut: it flows up into the spaces between the lobes and is rolled into the thread form, the tap’s lobes burnishing the thread as they pass. Because the metal is moved and not removed, the form tap makes no chips, and the thread it leaves is different in kind from a cut thread: its grains are pushed and work-hardened into the form, making a thread that is often stronger and smoother than a cut thread, with a burnished surface that resists galling. The price is that forming needs metal that will flow, and a hole prepared differently, as the next sections describe.

Forming needs its hole and its material

The form tap makes different demands on the hole and the material, and the difference is the heart of choosing it. Because the form tap displaces metal rather than removing it, the metal that becomes the thread must come from the wall of the hole — so the tap drill for a form tap is larger than for a cut tap of the same thread, the hole sized so that the displaced metal fills the thread form without overstressing the tap. The material must be ductile enough to flow: the form tap is at home in aluminium, the soft alloys, copper, brass and the low-carbon and stainless steels that flow well, and it is the favoured tap of the aluminium and the soft-metal shop; it cannot form threads in the hard, brittle or cast materials that would crack or resist the flow, where the cut tap is the only answer. And forming takes more torque than cutting — the tap is squeezing metal, not shearing it — so the machine and the holder must drive it firmly. Within its bounds the form tap repays the care: no chips to clear, a stronger and smoother thread, and a tap that lasts far longer because nothing cuts it.

Choosing cut or form

The choice between the cut tap and the form tap follows the job, and the tapping process entry sets out the operation around it. The cut tap is the generalist: it cuts any material the tap is made for, its chamfer and its flute type are chosen for the hole — the spiral point for the through hole, the spiral flute for the blind — and it needs only a conventional tap drill and the chip control the hole allows. The form tap is the specialist of the ductile material and the blind hole: it makes no chips, so a blind hole is no problem, and its rolled thread is stronger and smoother, at the price of the larger hole, the higher torque and the limit on material. In practice a shop keeps both on the rack, reaching for the form tap when the material flows and the thread must be its best, for the cut tap when the material is hard or brittle or the hole and the machine are ordinary. Both are made to the same thread standards, both are run at the feeds and speeds their material calls for, and both do their work in the hole that the drill made first.