Regrind or Replace a Tap: Wear Location, Chamfer Taper and Rake Regrinding

Most shops throw a dull tap away and draw a new one. Yet when the wear sits in the chamfer, grinding the front taper shallower brings the tap back — one textbook case took an M12 tap from about 30 nuts to about 150. This guide covers how to read the wear location, how to regrind the taper and the rake face, whether thread size drifts afterwards, and when to simply replace.

1. Read the wear location first

The general rule is that a worn tap is scrapped, not reground — on small sizes the labour and risk of resharpening cost more than a new tool. There is one exception: larger taps, where tooling spend is significant and repeated regrinding works well.

The first step is not measuring how much wear there is, but seeing where it is. Three locations behind the same "it will not cut" complaint call for three different decisions.

For the formal part names and their functions see Tap Structure Guide; for how far wear may go before a tap is pulled, see Carbide Tap Guide.

2. Regrinding the front taper: an M12 stainless case

The front taper of a first tap is defined as (D－d)/L, where D is the major diameter, d the small-end diameter of the chamfer and L the chamfer length. The standard value is about 1/5, giving a chip thickness of roughly 0.27mm.

The case is an M12 nut in 1Cr18Ni9Ti stainless: one tap produced only about 30 pieces, and the failure sat at the chamfer tip rather than along the whole flute. Because only the tip was gone, reducing the taper was enough to make those edges sharp again.

A shallower taper puts more teeth into the cut, so each tooth takes less and the load case improves. Three practical gains follow: it runs lighter than a new tap, it no longer needs reverse backing out, and it can tap holes below roughly 10mm diameter. The textbook case reports about four times the life.

The table holds measured values from one documented case, not a general specification. The major diameter D is not listed; measure it on the tap itself. Verify your own regrind allowance against the actual size and thread tolerance.

3. Regrinding the rake face without touching the form

The second typical failure is a badly dulled rake face on the chamfer while the thread form itself survives. Here the target is the face that carries the rake angle: grind it back to a sharp rake and the thread geometry is unaffected, which is why this is far safer than reworking the form.

Does the tap shrink each time? It can be calculated. The relieved back angle of a tap is generally 4°~6°. Taking 4°, removing 0.5mm from the rake face reduces the radius by:

x = tan4° × 0.5mm ≈ 0.035mm, so the cutting diameter drops by roughly 0.07mm — still inside the thread tolerance band.

In other words one 0.5mm pass has a controlled effect on the finished thread, which is what makes repeated regrinding of larger taps defensible. Wheel diameter is chosen to suit the tap size: pick a wheel that reaches into the flute without fouling the neighbouring teeth. For rake angle itself, see Tap Cutting Geometry.

4. When not to regrind at all

If the problem is repeated chipping or breakage rather than plain wear, that is a different line of countermeasures — see Tap Chipping and Breakage. If what you want to change is chip flow and torque rather than wear, see Special Tap Edge Geometries.

This article is part of Tool Regrinding: The Complete Guide — Decide Whether to Grind, Then Look Up How, With What, and How to Check; that guide shows how the whole topic fits together.

Published: 2026-08-17｜Last updated: 2026-08-17