How to Choose Type A, B and R Center Drills

A center drill produces a center hole that provides a locating datum for turning. It can also be used for spot drilling and chamfering. Types A, B and R differ mainly in front-end geometry and machining purpose.

Structural Differences among Type A, Type B and Type R Center Drills

The drawings and product profiles show different front-end geometries for positioning, hole protection and machining stability.

Type A Center Drill structural diagram — Type A Center Drill

Type A Center Drill product photo — Type A Center Drill

Type B Center Drill structural diagram — Type B Center Drill

Type B Center Drill product photo — Type B Center Drill

Type R Center Drill structural diagram — Type R Center Drill

Type R Center Drill product photo — Type R Center Drill

Type A Center Drill uses a basic 60° cone without an extra protective cone and is the most common choice for routine positioning.

Type B Center Drill adds a 120° protective cone around the machined 60° cone and suits precision shafts where hole-mouth integrity matters.

Type R Center Drill uses a radius transition to improve locating stability and reduce eccentricity and vibration, especially in hard materials.

Shape Differences among the Three Center-Hole Types

Type A forms a basic 60° center hole; Type B adds a protective cone outside the 60° working cone; Type R uses a radius transition to improve locating contact.

When to Switch from Type A to Type B

Type A is simple and economical, but impact, a rough end face or burrs may damage the center hole. The Type B protective cone improves hole-mouth quality and subsequent stability.

60° working cone and 120° protective cone of a Type B center hole

When to Choose Type R

An excessive center angle may give insufficient contact, while a small angle may cause interference. Misalignment also creates errors. Type R provides more stable contact than Type A.

Center-hole angle greater than center angle — Type A — Center-hole angle greater than center angle

Center-hole angle greater than center angle — Type R — Center-hole angle greater than center angle

Center-hole angle smaller than center angle — Type A — Center-hole angle smaller than center angle

Center-hole angle smaller than center angle — Type R — Center-hole angle smaller than center angle

Center-hole and center axes misaligned — Type A — Center-hole and center axes misaligned

Center-hole and center axes misaligned — Type R — Center-hole and center axes misaligned

Center-Drill Type Selection Guide

Based on structure and use, the following is a practical starting point:

Application	Recommended center drill
General-purpose work	Type A center drill
Rough machining or poor end-face condition	Type B center drill
High-precision machining or demanding locating stability	Type R center drill

Author: Simmons Wu, a mechanical engineer.

Please credit the source when reposting.

View center drill dimensions

Structural Differences among Type A, Type B and Type R Center Drills

Shape Differences among the Three Center-Hole Types

When to Switch from Type A to Type B

When to Choose Type R

Center-Drill Type Selection Guide

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