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ASME B94.11M-1993 scan — Twist Drills (Straight and Taper Shank, Combined Drills and Countersinks)
🛠️ ASME B94.11M-1993 is the authoritative American National Standard for twist drills, maintained by Technical Committee No. 7 on Twist Drills of the ASME B94 Standards Committee and issued as a revision of ANSI B94.11-1979. The Standard covers nomenclature, definitions, sizes, and tolerances of high-speed steel straight shank and taper shank twist drills, core drills, and combined drills and countersinks (plain and bell types), in both inch and metric size systems. Its lineage reaches back to the 1940 U.S. Standard B5-12 drill nomenclature and major dimension standard, with revisions issued in 1950, 1958, 1967, and 1979. The 1993 edition incorporated suggestions of the United States Cutting Tool Institute, corrected numerous numerical and editorial errors, and reorganized the document for easier use, making it the de facto dimensional benchmark for drill interchangeability in North America.
💡 A “standard” twist drill fits drills sleeves, chucks, and sharpening machines from any manufacturer without rework because B94.11M unifies key parameters — diameter, shank, overall length, and point geometry. That standard-part interchangeability is the core value of this Standard.
1. Scope and General Structure 📌
Chapter 1 (Scope) states that the Standard covers nomenclature, definitions, sizes, and tolerances of high-speed steel straight and taper shank drills, and combined drills and countersinks (plain and bell types), in both inch and metric sizes. The body is organized in two main parts: ordinary two-flute twist drills, then three- and four-flute core drills, followed by combined drills and countersinks. Section 2.1 defines the standard terminology and classification of drill features; Section 3 defines core drill features; and a comprehensive set of figures and tables supplies the dimensional series and tolerances.
⚠️ Note that B94.11M covers solid high-speed steel drills. Carbide drills, carbide-tipped drills, center drills, step drills, and other special drills fall outside this Standard and are governed by other B94 standards or manufacturer specifications.
2. Nomenclature and Key Definitions 🔬
Section 2.1 of the Standard precisely defines every feature of a twist drill — the common language for manufacturing, inspection, and drawing annotation:
- Axis: the imaginary straight line forming the longitudinal center line of the drill.
- Back taper: a slight decrease in diameter from point to back of the body, which prevents rubbing against the hole wall.
- Land: the peripheral portion of the body between adjacent flutes; its cylindrical portion left unrelieved is the margin, while the cut-away portion is the body diameter clearance.
- Chisel edge: the edge at the end of the web that connects the cutting lips; the chisel edge angle is the angle between the chisel edge and the cutting lip viewed from the end.
- Flutes: helical or straight grooves forming the cutting lips, permitting chip removal and delivering cutting fluid; flute length runs from the outer corners of the lips to the extreme back end of the flutes.
- Helix angle: the angle made by the leading edge of the land with a plane containing the drill axis.
- Lips: the cutting edges — on a two-flute drill they extend from the chisel edge to the periphery; on a three- or four-flute core drill they extend from the bottom of the chamfer to the periphery.
- Lip relief angle: the axial relief angle at the outer corner of the lip, measured across the margin portion of the land.
- Drill diameter: the diameter over the margins of the drill measured at the point.
- Overall length: from the extreme end of the shank to the outer corners of the cutting lips, excluding the conical shank end and the conical cutting point.
- Point and point angle: the cutting end of the drill resembling a cone but relieved behind the lips; the point angle is included between the lips projected on a plane parallel to the axis.
- Shank and tang: the portion by which the drill is held and driven; the tang is the flattened end of a taper shank that fits into the driving slot.
🚨 The most common inspection error is confusing “drill diameter” with “margin diameter”: the drill diameter must be measured at the point across the margins, not at the back of the body, otherwise the back taper yields an undersized reading and good parts may be rejected.
3. Dimensional Series and Tolerances 📏
The core data are provided by ten dimension tables. Straight shank twist drills are grouped into three series by application: Jobbers Length, Taper Length, and Screw Machine Length; each series is split at 1/2 in. (12.7 mm) diameter into two sub-ranges with dedicated tolerances. Table 4 gives the general dimensions of tangs for straight shank drills; Table 5 covers taper shank twist drills; Table 6 defines the specifications for split point drill grinding; Tables 7 and 8 give three- and four-flute straight and taper shank core drills; Table 9 gives combined drills and countersinks (plain and bell types) with tolerances; and Table 10 gives element tolerances for high-speed steel general-purpose twist drills.
| Series | Diameter Range | Typical Use | Tolerance Features |
|---|---|---|---|
| Jobbers Length (straight shank) | Through and over 1/2 in. (12.7 mm) | General drilling, moderate depth | Dedicated tolerances on diameter, length, flute length |
| Taper Length (straight shank) | Mainly over 1/2 in. (12.7 mm) | Deep holes, difficult chip evacuation | Extended flutes for chip space |
| Screw Machine Length (straight shank) | Through and over 1 in. (25.4 mm) | Automatic lathes, machining centers | Short overall length for rigidity |
| Taper Shank | Graded by machine taper (B5.10) | Direct mounting in drill spindles | Unified taper and tang dimensions |
| Core Drills | Three-/four-flute series | Enlarging cored or drilled holes | More flutes for guidance |
An important change of the 1993 edition is the continuation of dual dimensioning: since the 1979 edition, metric sizes were added to the jobbers, screw machine, taper length, and taper shank series to reflect then-current U.S. drilling practice, while metric sizes were not included for core drills or combined drills and countersinks. The 1993 revision corrected several numerical and editorial errors and made the tables easier to use.
💡 When ordering, remember that metric values in the Standard are rounded equivalents, not exact conversions, of the inch values. State explicitly which dimensioning system applies in your purchase order — mixing systems constitutes nonconformance.
4. Engineering Practice Essentials 🛠️
- 🔍 Point style selection: the Standard covers the conventional point and the split point. The split point reduces axial thrust and improves centering for CNC machining and difficult materials; point geometry (point angle, lip relief) should be controlled per Table 6.
- 🔧 Shank fit: the fit of straight shanks in chucks and taper shanks in spindle sockets directly affects runout and hole position. Taper shanks are made to ANSI B5.10 (Machine Tapers); tangs follow Table 4 — never hand-file them.
- 📊 Tool life management: standardized dimensions let a shop agree on wear limits (e.g., allowed reduction in land width) and set uniform resharpening intervals.
- 📋 Inspection: verify against Tables 1–10, focusing on drill diameter, overall length, flute length, and land width; use standard gages and keep first-article inspection records for batch purchases.
⚠️ Core drills and twist drills are not interchangeable: a twist drill cuts a hole from solid with lips running from the chisel edge; a core drill enlarges an existing hole with lips running from the bottom of the chamfer. Using a twist drill to enlarge a hole causes wander and runout, while a core drill cannot cut the center of a solid hole.
5. Frequently Asked Questions ❓
❓ Q: What changed between B94.11-1979 and B94.11M-1993?
A: The 1993 edition adopted suggestions from the United States Cutting Tool Institute, corrected many numerical and editorial errors, and was reorganized for easier use. It continues the metric/inch dual series introduced in 1979.
A: The 1993 edition adopted suggestions from the United States Cutting Tool Institute, corrected many numerical and editorial errors, and was reorganized for easier use. It continues the metric/inch dual series introduced in 1979.
❓ Q: When should I choose straight shank versus taper shank drills?
A: Small drills (typically up to 1/2 in.) are usually straight shank in a chuck for quick change; larger drills transmit higher torque and tend to slip, so taper shanks mounted directly in the spindle, with the tang driving and retaining, are preferred.
A: Small drills (typically up to 1/2 in.) are usually straight shank in a chuck for quick change; larger drills transmit higher torque and tend to slip, so taper shanks mounted directly in the spindle, with the tang driving and retaining, are preferred.
❓ Q: What are combined drills and countersinks used for?
A: They produce a hole plus a counterbore or countersink in one operation — e.g., rivet holes or countersunk screw holes. The Standard covers plain and bell types, reducing operation count and improving hole-position consistency.
A: They produce a hole plus a counterbore or countersink in one operation — e.g., rivet holes or countersunk screw holes. The Standard covers plain and bell types, reducing operation count and improving hole-position consistency.
❓ Q: Does the Standard specify drill material?
A: It covers dimensional requirements for high-speed steel drills without fixing a specific HSS grade; material selection is the manufacturer’s responsibility, though Table 10 constrains element composition for general-purpose HSS drills, so request material certificates on acceptance.
A: It covers dimensional requirements for high-speed steel drills without fixing a specific HSS grade; material selection is the manufacturer’s responsibility, though Table 10 constrains element composition for general-purpose HSS drills, so request material certificates on acceptance.
