Center drills

An ideal 60° included seat between a 2.00 mm pilot and a 4.00 mm mouth is 1.732 mm deep from the pilot-to-cone junction. A 4.20 mm mouth gives 1.905 mm. Check the tool-tip reference, actual seat profile and center engagement separately.

Purpose and terminology

A center drill combines a small pilot and tapered seat to support work on centers. Match the required seat to the actual tool. OSG, “Center Drill,” published September 5, 2025; updated December 10, 2025

ISCAR explicitly separates making a center hole from spotting a hole for a subsequent drill. Its MULTI-MASTER MM ECS carbide heads make 60° center holes; the documented forms include A, without a protective chamfer, and B, with a conical protective chamfer. The guide associates its metric and inch offerings with DIN 332 and ANSI B94.11M respectively. Those statements concern the listed heads and hole forms, not universal interchangeability of center-drill sizes. ISCAR, Machining Titanium, printed p. 50.

Geometry and catalog dimensions

The pilot's drill-point angle and the larger center-seat angle describe different surfaces. For example, Gühring series 613 has a 118° drill point and a 60° seat angle. Its sheet identifies DIN 333 form A tooling for DIN 332 part 1 form A holes, without a protective countersink. It lists HSS construction and distinguishes pilot diameter d1, body diameter d2, and overall length l1. A listed 2.00 mm pilot has a 5.00 mm body and 40.00 mm overall length. Body diameter is not automatically the required seat-mouth diameter. The sheet also states that sizes at or below 0.8 mm are not double ended; double-ended construction is therefore not inherent in the tool name. Gühring series 613, printed p. 37, undated product sheet.

These distinctions separate four quantities: pilot size, seat angle, seat-mouth diameter and tool body size. Two tools with the same nominal pilot diameter can still produce different hole forms. The cited manufacturer descriptions do not reproduce the standards' complete profiles, tolerances or acceptance rules.

Center holes versus drill-starting spots

A conical support seat and a locating spot have different geometric purposes. Gühring series 567 is a 120° NC spot drill, explicitly limited to spotting; it is not described as a 60° center-hole tool. Gühring series 567, printed p. 71, undated product sheet.

For carbide drills, Harvey recommends a spot angle slightly larger than the following drill's point angle, and accepts equal angles. A narrower spot can make the outer cutting edges contact first and impose damaging shock loading. This is guidance for the subsequent drill's engagement, not a reason to change a drawing's center-seat angle. Harvey Performance, “Spot Drilling: The First Step to Precision Drilling,” August 25, 2022.

Gühring's technical guidance makes the applicability more specific: 120° spotting for subsequent 118° HSS/HSCO drills, and 142° spotting for subsequent 135–140° carbide drills. It separately describes 90° spotting for HSS/HSCO drills with a relatively large chisel edge. Thus, a blanket rule about all drill materials would miss a documented exception. It also limits its NC spot drills to the point geometry's depth because their short-flute, unrelieved-body design is intended for spotting. Gühring, Centering and Pilot Drilling Recommendations, printed pp. 449–450, undated extract.

Worked example: straight conical seat depth

Use an ideal straight 60° included cone from the 2.00 mm pilot to the 4.00 mm mouth. Exclude edge radii, protective chamfers and tolerances.

h = (D - d) / [2 × tan(α / 2)]
  = (4.00 - 2.00) / [2 × tan(30°)]
  = 1.732 mm

Here α is the included seat angle; h starts at the pilot-to-cone junction. It is not depth from the drill tip or total tool travel. With the same pilot and angle, increasing the mouth to 4.20 mm gives h = 1.905 mm, an additional 0.173 mm of ideal cone penetration. This follows directly from the radial growth of a cone. It does not model curved profiles or additional chamfers.

Applicability and coverage

The examples establish terminology and geometric relationships. They do not establish allowable center load, workpiece accuracy, lubrication, spindle speed, feed, tool life or suitability for a particular work material. Catalog geometry and a depth calculation alone cannot establish a qualified support arrangement. Source dates above are displayed dates where available; the undated catalog extracts have no verified issue date, and the ISCAR file path is not treated as proof of an edition date.

Related reference

Cutting tools and tooling references