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ASME B94.26-1969 — Indexable (Throw-Away) Insert Holders
🛠️ USAS B94.26-1969 (ASME B94.26) is the American National Standard for holders of indexable (throw-away) inserts. It covers dimensional specifications, styles, and designations of holders for indexable inserts. Prepared by Technical Committee 1 (Carbide and Oxide Tools) of USA Standards Committee B94 and approved by the USA Standards Institute on September 10, 1969, as a partial revision of B80.1-1959, it establishes an eight-position identification system in which insert shape, lead angle, rake, hand, shank size, insert size, and position accuracy are all encoded in the holder designation — the fundamental reference for selecting and purchasing clamp-on carbide tooling for CNC lathes and turning centers.
💡 A holder is a tool holder with the cutting insert mechanically held in place. Unlike brazed tools, indexable inserts are clamped into the holder pocket, so changing an insert means releasing one clamp — no regrinding and no re-brazing, dramatically shortening tool change time.
1. Scope and Definitions 📌
This Standard covers dimensional specifications, styles, and designations of holders for indexable (throw-away) inserts. Key definitions include:
- Insert: a carbide solid which is mechanically held for use as a cutting tool.
- Indexable (Throw-Away) Insert: an insert with a number of cutting surfaces, all of which are used until dull, and the insert is then discarded.
- Holder: a tool holder with the cutting insert mechanically held in place.
- Position Accuracy: the location of the cutting edge from the locating surfaces of the holder.
⚠️ Position accuracy is a key quality indicator of the holder. The position accuracy class affects how consistently the cutting edge sits relative to the machine coordinates; for batch production choose a holder class that meets the process accuracy requirement.
2. The Eight-Position Identification System 🏷️
Section 3.2 defines the holder identification system of 8 positions. Each position defines a characteristic of the holder in the following order:
| Position | Characteristic | Code Example |
|---|---|---|
| 1st | Clamping method | P = lock pin (insert with center hole), C = clamp (normally top clamping), etc. |
| 2nd | Insert shape | C = 80° diamond, D = 55° diamond, V = 35° diamond, T = triangle, S = square, R = round |
| 3rd | Holder style | A = straight shank, 0° side cutting edge angle; B = 15°; D = 45°; F = offset, 0° end angle; etc. |
| 4th | Rake | N = negative rake, P = positive rake |
| 5th | Hand | L = left hand, R = right hand |
| 6th | Shank cross section | e.g., 1 × 1-1/4 in |
| 7th | Insert inscribed circle | e.g., 1/2 in IC, 1/4 in IC |
| 8th | Position accuracy | U, N, and other accuracy classes |
💡 Reading the examples: CCLNL-86-4U = mechanically clamped holder, 80° diamond insert, negative rake, left hand, 1 × 1-1/4 in shank cross section, 1/2 in IC insert, class U position accuracy; PCLNR-86-4U = lock-pin holder, 80° diamond, negative rake, right hand, 1 × 1-1/4 in shank, 1/2 in IC insert, class N position accuracy.
3. Holder Types and Tables 📏
The Standard presents complete dimensional data in roughly forty tables:
- Clamp-type holders for negative-rake inserts (Tables 1–19): straight- and offset-shank holders with round (style G), triangular (styles A/B/D/E/F/G), and square (styles B/S/D/K/A) pockets.
- Pin-type holders for negative-rake inserts (Tables 20–40): holders for center-holed inserts in round, triangular, square, 80° diamond, and 55° diamond shapes.
🚨 Clamping type must match the insert hole geometry: pin-type (P) holders require center-holed inserts; C-clamp holders suit hole-less positive-rake inserts (e.g., SPG). Mismatching clamping styles causes insecure seating, insert shift, and even insert ejection — a serious safety hazard.
4. Position Accuracy and Holder Quality 📏
Position accuracy, as defined in the Standard, is the location of the cutting edge relative to the locating surfaces of the holder. Contributing factors include:
- Machining accuracy of the pocket (flatness and angles of the seating faces).
- Repeatability of the clamping structure in relocating the insert.
- The insert class’s own dimensional tolerance (see B94.25).
💡 The holder and the insert together determine the actual edge position. For high-precision work, combine a high-class insert (G-class) with a position-accurate holder and verify with a trial cut before production.
5. Engineering Practice Essentials 🛠️
- 🔍 Selection workflow: pick insert shape by operation (OD turning/face/ profiling) → choose clamping method (P/C) → select holder style and lead angle → choose left/right hand → match shank cross-section and insert IC to the machine turret.
- 🔧 Mounting and tool setting: clamp the holder firmly in the turret/pocket; set the tool to the program using the cutting edge as the datum and verify the edge location.
- 🔄 Insert indexing: release the clamp, rotate or replace the insert, re-clamp, and make sure the insert seats fully against the pocket bottom and side walls.
- 🧪 Cleaning: periodically clean chips and debris from the pocket to prevent insert lift and position drift.
6. Frequently Asked Questions ❓
❓ Q: How do I read the designation CCLNL-86-4U?
A: Decompose it through the eight positions: 1st C = clamping method, 2nd C = insert shape (80° diamond), 3rd L = holder style, 4th N = negative rake, 5th L = left hand, 6th 86 = 1 × 1-1/4 in shank, 7th 4 = 1/2 in IC, 8th U = position accuracy class.
A: Decompose it through the eight positions: 1st C = clamping method, 2nd C = insert shape (80° diamond), 3rd L = holder style, 4th N = negative rake, 5th L = left hand, 6th 86 = 1 × 1-1/4 in shank, 7th 4 = 1/2 in IC, 8th U = position accuracy class.
❓ Q: When should I choose a pin-type (P) or clamp-type (C) holder?
A: P-type suits center-holed negative-rake inserts (CNMG, TNMG) with reliable seating for heavy cuts; C-type top clamping suits hole-less positive-rake inserts (SPG, TPG) with simple construction and good edge visibility.
A: P-type suits center-holed negative-rake inserts (CNMG, TNMG) with reliable seating for heavy cuts; C-type top clamping suits hole-less positive-rake inserts (SPG, TPG) with simple construction and good edge visibility.
❓ Q: What is the difference between left-hand and right-hand holders?
A: Right-hand holders carry the cutting edge to the right, suiting most right-spindle machines for OD turning; left-hand holders serve back turning, facing, or specific turret layouts — choose by machine and feed direction.
A: Right-hand holders carry the cutting edge to the right, suiting most right-spindle machines for OD turning; left-hand holders serve back turning, facing, or specific turret layouts — choose by machine and feed direction.
🔍 Q: How do B94.26, B94.25, and B94.4 relate?
A: B94.25 defines the insert body, B94.4 the complete identification system, and B94.26 the holders that clamp the inserts; together they form the American indexable tooling system and must be selected as a set.
A: B94.25 defines the insert body, B94.4 the complete identification system, and B94.26 the holders that clamp the inserts; together they form the American indexable tooling system and must be selected as a set.
