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Tap Chamfers — Taper, Plug & Bottoming

The chamfer on a tap is the tapered lead at the tip that does most of the cutting. Taper, plug and bottoming taps differ only in chamfer length, which decides how easily the tap starts and how close to the bottom of a blind hole it can thread.

Ref CT-0016ASME B94.9ISO 529

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

A tap does not cut full threads at its first tooth. The first few threads are ground down at an angle, called the chamfer. These chamfered threads share the cutting work. A long chamfer spreads the load over many teeth and starts easily, but leaves several partial threads at the bottom of the hole. A short chamfer threads close to the bottom but puts more load on each tooth.

How to identify

Count the chamfered threads at the tip, from the first partial tooth to the first full-height thread. The chamfer type is often marked on the shank or the package. Hand tap sets usually come as a taper, plug and bottoming tap of the same size.

Chamfer types

Chamfer	Chamfered threads (about)	Best use
Taper	7 to 10	Starting threads by hand, hard material, through holes
Plug	3 to 5	Most through holes and blind holes with room at the bottom; the general-purpose choice
Semi-bottoming	2 to 3	Blind holes with a little room; machine tapping
Bottoming	1 to 2	Finishing threads close to the bottom of a blind hole

Key points

In a blind hole, drill deeper than the needed full thread so the chamfer and chips have room. The extra depth is about the chamfer length plus chip room.
By hand, start with a taper or plug tap and finish with a bottoming tap. A bottoming tap does not start well by itself.
On a machine, plug chamfer taps are common. Semi-bottoming and bottoming taps are used when the drawing calls for full thread near the bottom.
Spiral point (gun) taps push chips ahead of the tap and suit through holes. Spiral flute taps lift chips out and suit blind holes. Chamfer and flute type are separate choices.
Short chamfers load each tooth harder. Use good lubricant and reduce speed in tough materials.
Use the right tap drill. See the tap drill chart.

Common mistakes

Starting a hole with a bottoming tap and getting a crooked thread.
Running a plug tap into the bottom of a blind hole and breaking it.
Drilling a blind hole only as deep as the needed thread length.
Thinking the chamfer name tells the flute type.

Governing standards: ASME B94.9 (taps, ground and cut threads, inch and metric); ISO 529 (short machine and hand taps).

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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