Threading Tools for Tapping, Thread Milling and Turning

An internal thread is cut inside a hole and an external thread on the outside of a stud or shaft, and the tool families divide along the same line: taps, thread mills and internal laydown inserts cut internal threads, while dies, external laydown inserts and thread turning with an ER insert cut external ones. Both sides are measured by the same tolerance classes — 6H and 6g metric, 2B and 2A inch — and a mating pair is only a fit when the internal and external classes are called out together.

Tap when the hole is accessible, the machine reverses reliably and speed per hole matters. Thread mill when the diameter is large, the material is hard, the part thin-walled or the workpiece does not rotate — and always where a broken tap would scrap the job. A thread mill also fixes a pitch problem: if the pre-hole is slightly oversize, the interpolation can still cut a correct thread, while a tap would simply cut oversize.

Through hole: a spiral point or gun tap pushes the chip ahead of the tap and can be run fast in a machine. Blind hole: a spiral flute tap lifts the chip back out of the hole, or a form tap makes no chip at all. Straight flute taps handle both when the material breaks its chips short — the mistake to avoid is a gun tap in a blind hole, where the chip has nowhere to go.

A callout carries a size, a pitch and a class: M10 x 1.5 - 6H is a metric internal thread with a 1.5 mm pitch in class 6H; 1/4-20 UNC - 2B is an inch internal thread, 20 threads per inch, class 2B. Beyond the common 60° forms sit the specials: buttress is asymmetric, with a near-vertical load face and a slanted back, used where force goes one way; Acme is a 29° trapezoid for lead screws and motion; NPT and BSP are pipe threads, tapered and parallel respectively. Tell us the callout and we match tap, mill or insert to it.

A sharp, coated tap — HSS-E or powder metal with TiCN or AlTiN — a spiral flute form for a blind hole, a generous lead, and a real tapping fluid rather than general coolant. Stainless work-hardens ahead of a dull edge and grabs at the first sign of rubbing, so run slower than you would in mild steel, feed continuously rather than pecking, and replace the tap at the first squeal instead of pushing it through another hundred holes.

For a cutting tap, subtract the pitch from the major diameter: M10 x 1.5 wants an 8.5 mm hole, a 1/4-20 UNC wants 5.4 mm (0.201 in), an M6 x 1.0 wants 5.0 mm. Form taps and some materials need a different pre-hole, so check the tap drill size chart on this site for the exact value and quote the drill with the tap — that way the hole and the thread arrive matched.

Threading tools are sourced from vetted tooling partners and inspected before dispatch. Thread form, tolerance class, coating and shank are confirmed against the quotation, dimensional reports and material certificates are supplied on request, and each shipment is packed for transit. If a batch does not perform as confirmed, send the batch reference and your measurements and we will put it right.

The thread callout exactly as drawn (size, pitch and class), the workpiece material and hardness, whether the hole is through or blind, the machine and tap holder, and the quantity. For anything outside the common forms — buttress, Acme, tapered pipe, multi-start — send the drawing or thread specification so the profile, lead and pitch can be quoted properly rather than approximated.

Quotations are normally issued within hours during business hours — email and WhatsApp reach us directly. Orders ship worldwide from Guangzhou, China with tracking, and payment terms are stated on the quotation. Trial quantities and mixed-item orders — taps, drills and a tap drill size chart set on one PO — are both welcome.