Tapping

Tapping is the machining process that cuts an internal thread — the female thread inside a hole — using a tap, a fluted tool whose cutting edges carry the thread form. It is how a machining centre puts a thread in a drilled hole, and it looks deceptively simple: the tap spins in and the thread appears. What makes tapping unlike every other machining process is one hard constraint — the tap must advance into the work by exactly one pitch for every revolution it makes. The thread is cut by that advance, no more and no less, so the tap’s feed is not a free choice but is locked to its rotation. Everything else about tapping exists to make that one synchronised motion happen without breaking the tap.

How a tap cuts

A tap is a screw of hard tool steel or carbide whose flutes are ground so their edges carry the thread profile. As it turns and advances, each flute’s edge slices the thread form into the wall of the hole, a little deeper with each pass, until the full thread depth is cut. The tap’s chamfer — the tapered leading portion that does the cutting — sets how the tap enters and how deep it can reach. A long-chamfer taper tap eases in and starts cleanly, a medium plug tap is the general-purpose choice, and a short-chamfer bottoming tap can thread to within a thread or two of the bottom of a blind hole. Because the chamfer cannot reach the hole’s end, a blind hole must be drilled deeper than the thread, or a bottoming tap must finish the last threads after a plug tap. In a through hole the tap simply runs through and out the far side.

Cut taps and form taps

There are two ways to make the thread, and they behave very differently. A cutting tap removes material: its flutes slice chips from the hole wall, so it works in almost any material, brittle or ductile, and is the conventional, forgiving choice. A form tap, also called a roll or fluteless tap, makes no chips: it has no flutes and squeezes the metal of the hole wall up into the thread shape, cold-forming it between the tool’s lobes. Because the thread is pressed rather than cut, its flanks are work-hardened and burnished — stronger and smoother than a cut thread — and with nothing to evacuate, the tap lasts far longer and can run faster. But forming has hard limits: the material must flow rather than crack, so form taps suit aluminium, brass and low-alloy steel and are useless on cast iron or hardened steel, and forming takes more torque and a larger hole because the displaced metal must come from somewhere.

The hole decides everything

A tap does not create its hole; it threads one that drilling has already made, and the tap drill size is the single most influential decision in the process. The drilled hole must be smaller than the thread’s major diameter so there is material to form the thread, but not so small that the tap has to cut a full 100 percent thread. Engagement of around 65 to 75 percent is the working optimum: nearly full thread strength at moderate torque. An over-large hole leaves a thread that strips under load; an under-size hole makes the tap cut an almost-solid thread, torque climbs, and the tap overheats, galls and snaps. The familiar rules of thumb — for a metric thread, drill roughly the major diameter minus the pitch — exist to land in that band, and charts and calculators carry the same numbers. Form taps, which displace rather than cut, need a larger hole again than cutting taps for the same thread.

Blind holes: chips are the whole game

In a through hole, the chips a cutting tap makes have an easy exit, so the choice of tap is simple. In a blind hole they do not, and chip direction becomes the deciding factor in tap selection. A spiral-point tap, sometimes called a gun tap, has flutes angled so chips are pushed forward, ahead of the tap — perfect for a through hole, where they fall out the bottom, and disastrous in a blind hole, where they pack against its end until the tap jams and shears. A spiral-flute tap angles the other way and lifts chips up and out as it cuts, which is why it is the blind-hole tap. The deeper the blind hole, the harder evacuation becomes, and at some depth the answer is to stop cutting chips altogether: a form tap’s chip-free action is the cleanest possible solution to a blind hole in a ductile material. Even then, the drilling stage must plan ahead — the hole is drilled deeper than the thread depth required, to give the tap’s chamfer and the chips somewhere to go.

Rigid tapping and why taps break

On a CNC machine the pitch-locked advance is done by rigid tapping: the control drives the spindle and the Z-axis feed together, electronically synchronised so the tap’s feed in millimetres per revolution exactly equals the thread’s pitch, and reverses the spindle to back the tap out along the same path. Older machines used floating or tension-compression holders whose spring travel absorbed the mismatch; rigid tapping removed the need for them and is now the normal method. Because the feed is fixed by the pitch, the only speed decision left is spindle speed, chosen conservatively — tapping runs slower than drilling the same material, since a tap turning too fast heats, welds chips into its flutes and tears the thread, while one far too slow tears the flanks instead of shearing them. Taps break for a handful of avoidable reasons: an undersize hole that overloads the flutes, chips packed in a blind hole, misalignment at entry, a crooked start that cross-threads, or speed too high for the material. A broken tap in a finished part is a costly extraction job, which is why good tapping practice is mostly caution — correct hole size, correct tap, clean entry, and a speed that suits the material.

Tapping in the job flow

Tapping is the last step in the holemaking chain that begins with drilling: the hole is drilled, its mouth is chamfered or deburred to give the tap a clean, true start and to remove the burr the tap would otherwise roll over, and the tap follows. On a machining centre the whole sequence runs in one program — drill, chamfer and tap, each loaded in turn — which is what makes tapped holes so cheap in production. Tapping suits the smaller, standard threads that dominate assemblies; very large threads, threads in awkward positions, and threads in work-hardening materials are more often made by thread milling or single-point cutting, and the lathe makes its own internal threads by single-pointing rather than tapping — each a process this wiki covers in its own right. But wherever a drilled hole is to take a bolt, a screw or a stud, tapping is the process that puts the thread there, its speeds set under the same feeds and speeds discipline as every cut in CNC machining, most often on the spindle of a milling machine.