Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
ASME B94.20-1977 — Specifications for Carbide Blanks for Twist Drills, Reamers, End Mills, and Random Rod
💎 ASME B94.20-1977 is the foundational American National Standard for carbide tool blanks. It covers dimensional specifications and designations for carbide blanks for twist drills, reamers, end mills, and random rod. The document was developed jointly by the Cemented Carbide Producers Association, the Solid Carbide Tool Institute, and the Metal Cutting Tool Institute, approved as an American National Standard on July 19, 1977, and traces its lineage to B85.1-1963. It establishes a common “blank language” between blank suppliers and tool manufacturers — the starting point for quality control throughout the powder-metallurgy, sintering, and grinding chain.
💡 A blank is an unground carbide solid. The Standard defines only blank dimensions and designations; it does not prescribe carbide grades (e.g., C-2/K20 or P30 types), which the toolmaker selects according to the workpiece and cutting application.
1. Scope and Definitions 📌
This Standard covers dimensional specifications and designations for carbide blanks for twist drills, reamers, end mills, and random rod. Two definitions frame the document:
- Carbide: a sintered carbide of refractory metal suitable for use as a cutting tool material.
- Blank: an unground carbide solid.
In the 1976 revision, the Metal Cutting Tool Institute recommended substantial changes to male and female centers in blanks for reamers, drills, and end mills, and proposed new standards for circuit board drill blanks, blanks for drills and countersinks, and blanks for fluted countersinks. All of these changes are incorporated in this edition.
2. Blank Designation System 🏷️
The Standard establishes a letter-plus-number identification system. Different blank families use different prefixes:
| Blank Family | Designation | Size Range | Finished Tool |
|---|---|---|---|
| Flat-type twist drill blanks | TD, sizes 1–5 | Drill diameters 1/16 in – 1/4 in | Carbide-tipped twist drills |
| Fluted-type twist drill blanks | TD, sizes 6–25 | Drill diameters 9/32 in – 1-1/2 in | Carbide-tipped twist drills |
| Radius-type reamer blanks | RT, sizes 0–12 | 3/64 × 1/2 in – 1/8 × 1 in | Carbide-tipped reamers |
| Rectangular reamer blanks | RT family (Table 4) | Per Table 4 | Carbide-tipped reamers |
| Solid carbide end mill blanks | Tables 5/10/11 | Per tables | Solid carbide end mills |
| Random rod blanks | Table 8 | Various dia. × length | General-purpose rod stock |
⚠️ A designation such as TD-1 is a range code: each number covers a band of finished drill diameters (for example, TD-1 covers drills over 1/16 in through 1/8 in). It is not a single exact diameter — specify the actual dimension and tolerance in addition to the code when purchasing.
3. Principal Blank Families 🔧
The body of the Standard presents complete dimensional data in fifteen main tables (Tables 1–15 and 15A), including:
- Flat- and fluted-type twist drill blanks (Tables 1, 2) for carbide-tipped twist drills.
- Radius- and rectangular-type reamer blanks (Tables 3, 4) for carbide-tipped reamers.
- Solid carbide end mill, reamer, twist drill, and fluted end mill blanks (Tables 5, 6, 7, 9, 10, 11).
- Random-length rod blanks (Table 8) for general downstream processing.
- Circuit board drill blanks (Tables 12, 14 — common-shank and straight-shank), drill and countersink blanks (Table 13), and countersink blanks for 60° and fluted 82°/90° included angles (Tables 15, 15A).
A supplement provides conversion tables so the inch dimensions can be converted readily to metric equivalents.
💡 The 1976 revision added dedicated blanks for circuit board drills in response to the electronics industry’s rapid growth in PCB micro-hole machining — fine-diameter PCB drills demand exceptionally high concentricity and uniform grain structure in the blank.
4. Tolerances and Quality Control 📏
The Standard specifies tolerances on key blank dimensions (diameter, length, center form, etc.) so that sintered blanks, after grinding, can meet finished-tool accuracy requirements. Because a blank is an unground solid, its tolerances must be coordinated with the subsequent grinding allowance:
- Blank diameters carry sufficient grinding allowance for finished cutting-edge diameters to reach final precision.
- Center form (male/female centers) and accuracy directly affect grinding setup and concentricity — a focus of the 1976 revision.
- Blanks must be free of defects that impair cutting performance (porosity, cracks, decarburized layers); inspection methods can be agreed between supplier and purchaser.
🚨 Cracks and under-sintering in carbide blanks are common root causes of premature tool failure. For production quantities, require density, hardness, or metallographic data from the supplier, and consider nondestructive testing on critical lots.
5. Engineering Practice Essentials 🛠️
- 🔍 Selection workflow: choose the blank family by finished tool type (TD/RT, etc.) → pick the size number from the finished diameter range → verify center form and grinding allowance → agree grade and tolerance with the supplier.
- 🔄 Grinding: machine carbide blanks with diamond wheels; flute and edge grinding should be done on dedicated tool grinders with controlled feeds to avoid burning and micro-cracks.
- 🧪 Incoming inspection: sample-check dimensions, surface quality, and density on receipt to confirm conformance.
- 📦 Storage: keep blanks segregated by grade and size to avoid mix-ups; record lot numbers for full traceability.
6. Frequently Asked Questions ❓
❓ Q: What do the TD and RT prefixes mean?
A: TD designates twist drill blanks and RT designates radius-type reamer blanks; end mill, rod, and other families use their own table-based designations within the same system.
A: TD designates twist drill blanks and RT designates radius-type reamer blanks; end mill, rod, and other families use their own table-based designations within the same system.
❓ Q: Why is a blank number a range rather than a single size?
A: Because a sintered blank is ground into the finished tool, one blank number can cover a band of finished diameters. This reduces the number of stock-keeping units and simplifies inventory while still leaving adequate grinding stock.
A: Because a sintered blank is ground into the finished tool, one blank number can cover a band of finished diameters. This reduces the number of stock-keeping units and simplifies inventory while still leaving adequate grinding stock.
❓ Q: Does the Standard specify carbide grades?
A: No. B94.20 covers blank dimensions and designations only; grade selection (C-series or ISO-series classifications) is left to the toolmaker based on workpiece and cutting conditions.
A: No. B94.20 covers blank dimensions and designations only; grade selection (C-series or ISO-series classifications) is left to the toolmaker based on workpiece and cutting conditions.
🔍 Q: Why are circuit board drill blanks listed separately?
A: PCB drills are small in diameter, have high length-to-diameter ratios, and run at very high speeds, demanding far better concentricity, grain uniformity, and stiffness than ordinary twist drills — hence the dedicated common-shank and straight-shank blank tables added in 1976.
A: PCB drills are small in diameter, have high length-to-diameter ratios, and run at very high speeds, demanding far better concentricity, grain uniformity, and stiffness than ordinary twist drills — hence the dedicated common-shank and straight-shank blank tables added in 1976.
