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ASME B94.3-1965 (1972) — Straight Cut-Off Tools for Lathes and Automatic Screw Machines
🔪 ASA B94.3-1965, sponsored by ASME and reaffirmed in 1972 (hence the archival designation ASME B94.3-1965 (1972), original UDC 621.937/.94), is the U.S. national standard for straight cut-off blades for lathes and automatic (bar) screw machines. The standard defines the size designation, dimensions, and structural types of straight cut-off blades — also called parting or grooving blades — mounted in tool posts of lathes and automatic bar machines. Its purpose is to make blades from different manufacturers interchangeable and to give tool designers, manufacturers, and users a single, unambiguous dimensioning and identification system. It is one of the earliest foundation standards of the B94 series (cutting tools, tool holders, drive accessories, and bushings), published by ASME as sponsoring body.
💡 Straight cut-off blades are used to part a finished part from the bar, and also for grooving and face-grooving. The edge geometry — side clearance, rake, chamfers, and land width — directly determines the flatness of the parted face, burr size, and tool life, making this blade one of the most common tools on automatic screw machines, CNC lathes, and sliding-headstock machines.
1. Subject and Scope of the Standard 📌
The subject of the Standard is the standardization of straight cut-off blades for lathes and screw machines:
- Equipment covered: conventional lathes and automatic screw machines, including automatic bar machines and sliding-headstock machines.
- Product covered: the straight cut-off blade — a bar-type cutting blade of rectangular section with a ground parting edge at the front, clamped in a dedicated tool holder.
- Designation system: a numeric code identifying blade width, height, and length, and distinguishing the structural “Types” of cutting edge.
- Size range: widths from 0.040 in. to about 0.125 in. (larger sizes also listed), with height expressed as a “nominal height.”
The Standard states that the cross-sectional shape and dimensional limits of blades conform to each manufacturer’s factory standards, that heights are nominal values, and that some sizes are flagged as “preferred sizes” recommended for future design and development, while the remaining listed sizes are standard off-the-shelf sizes.
⚠️ Because cross-sectional shapes and limits follow individual manufacturers’ factory standards, mixing blades from different vendors in the same holder requires checking that the section dimensions match the holder slot — otherwise the blade may be clamped at an angle or left loose, causing deflection or breakout during parting.
2. Size Designation System 🔧
The Standard uses a numeric code to identify blade sizes. Examples extracted from Table 1 of the Standard illustrate the pattern (width in inches, height nominal in inches):
| Designation | Width (in.) | Nominal Height (in.) | Remarks |
|---|---|---|---|
| 030818 | 0.040 | 4-3/4 | Standard size |
| 040818 | 0.062 | 4-1/4 | Standard size |
| 050818 | 0.078 | 4-1/4 | Standard size |
| 080817 | 0.125 | about 4-1/4 | Standard size |
| 080818 | 0.125 | about 4-1/4 | Type T height corrected to 0.477 (not 0.499) |
| 081117 / 081120 | 0.125 | about 5-1/4 to 5-1/2 | Standard size |
| 101117 / 101120 | 0.156 (about 5/32) | about 5-1/4 to 5-1/2 | Standard size |
| 121120 / 121121 | 0.188 (about 3/16) | about 5-1/2 | Standard size |
| 081324 / 101324 / 141324 / 161324 | 0.125 to 0.250 | about 6-1/2 | Standard size |
The code is prefixed with a letter indicating the cut-off type (such as “C” or the appropriate cut-off-tool type letter) and must be accompanied by the blade or cutting-head material. Some Type T designs carry a specific nominal height of 0.477 in. rather than 0.499 in. and must be selected per the corrected value in the standard’s table.
💡 The code is essentially a “dimension encoding”: the leading digits encode width (08 → 0.125 in., 10 → 0.156 in., 12 → 0.188 in.), the middle digits the height bracket, and the trailing digits the length/type. Always select from the standard’s tables rather than decoding by guesswork.
3. Cutting Edge and Cutting Geometry 📏
The Standard defines the edge geometry of the different blade types, balancing clearance against edge strength:
| Geometry Element | Typical Value / Requirement | Function |
|---|---|---|
| Top rake | about 12° (Type L illustrated) | Reduces cutting force and improves chip flow |
| Side clearance | 8° (Types L and B) | Prevents the blade sides from rubbing the groove walls |
| End relief | 2° (optional design) | Reduces burr and friction at the end of the cut |
| Land width | 0.005 in. max (height ≤ 0.68 in.); 0.010 in. max (taller blades) | Strengthens the edge, prevents chipping |
| Side clearance (parallel) | 0.004–0.005 in. (optional design for widths under 3/32 in.) | Reduces side friction on narrow blades |
| Radius transition | about 8.0R (optional design) | Reduces stress concentration at the edge |
The Standard emphasizes two rules: blades narrower than 3/32 in. (about 0.094 in.) must not use angled clearance — they should use the optional parallel side-clearance design to preserve edge strength; and the land must not exceed 0.005 in. on blades no taller than 0.68 in., or 0.010 in. on taller blades.
🚨 The land width is the critical factor for edge strength: too wide a land makes cutting forces rise sharply, roughens the parted face, and increases vibration; too narrow a land invites chipping. Blades narrower than 3/32 in. must never have enlarged clearance angles — that severely weakens the narrow edge and can cause the blade to fracture and fly out during parting.
4. Materials and Grinding 🛠️
The Standard requires the designation to state the blade or cutting-head material. Common engineering choices include:
- High-speed steel (HSS): tough, easy to resharpen, suitable for general steel, aluminum, and interrupted cutting — the primary parting-blade material.
- Carbide: high hardness and wear resistance for cast iron, hardened steel, and high-volume production, but impact-sensitive and requiring rigid machine support.
- Powder-metal HSS (PM-HSS): combines toughness and wear resistance for difficult materials and demanding jobs.
- Brazed composite heads: a steel body brazed with a carbide head, balancing holder rigidity with edge wear resistance.
Grinding should reproduce the standard geometry on a dedicated tool grinder — rake, side clearance, and land — keeping left and right sides symmetrical; asymmetric clearance makes the parted end slope.
💡 High-precision parting demands strict left-right symmetry: rake and side clearances on both flanks must be ground identically, otherwise the parted end tilts or leaves a center “pip.” Verify symmetry and land width with a tool-setting gauge after grinding.
5. Engineering Practice Essentials 📋
- 🔍 Selection flow: determine blade width from part diameter and bar size (typically 1/8 to 1/4 of the diameter) → pick the nearest standard width → select section height to match the holder slot → choose the edge type (L/B/T) and material.
- 🔧 Clamping: keep overhang as short as possible (generally no more than 1.2–1.5 times the blade height) and ensure the holder grips firmly to prevent bending during the cut.
- 📊 Cooling and lubrication: parting concentrates heat, so coolant supply must be generous; where narrow slots make chip evacuation difficult, choose edge geometries with chip-breaker grooves.
- 🔨 Maintenance: inspect edges for chipping and land wear and regrind or replace at thresholds; manage spare blades of the same size by the standard designation.
6. Frequently Asked Questions ❓
❓ Q: How is the cut-off blade width determined?
A: Generally 1/8 to 1/4 of the part diameter, or from the minimum stock-removal requirement of the finished part, then rounded to the nearest standard width (0.040–0.250 in. series). Too wide wastes material and raises cutting force; too narrow risks breakage.
A: Generally 1/8 to 1/4 of the part diameter, or from the minimum stock-removal requirement of the finished part, then rounded to the nearest standard width (0.040–0.250 in. series). Too wide wastes material and raises cutting force; too narrow risks breakage.
❓ Q: What is the difference between Type L, Type B, and Type T?
A: The letters denote different edge/section designs — for example, Type T has a specific nominal height correction (0.477 or 0.675 in.), while Types L and B differ in clearance and rake geometry. Select by matching the section geometry in the standard’s figures — never by code alone.
A: The letters denote different edge/section designs — for example, Type T has a specific nominal height correction (0.477 or 0.675 in.), while Types L and B differ in clearance and rake geometry. Select by matching the section geometry in the standard’s figures — never by code alone.
❓ Q: Why must narrow blades (under 3/32 in.) not use angled clearance?
A: Angled clearance sharply reduces the edge section of a narrow blade, dropping its strength and inviting breakage during parting. Narrow blades should use the optional parallel side-clearance design to preserve full edge width strength.
A: Angled clearance sharply reduces the edge section of a narrow blade, dropping its strength and inviting breakage during parting. Narrow blades should use the optional parallel side-clearance design to preserve full edge width strength.
🔍 Q: How does this standard relate to the rest of the B94 series?
A: B94.3 is the foundation standard for parting tools within the B94 series (cutting tools, holders, drive accessories, bushings). Together with lathe tools (e.g., B94.5 single-point brazed tools) and form-tool blanks (B94.32), it forms the standardized turning-tool system for lathes and screw machines.
A: B94.3 is the foundation standard for parting tools within the B94 series (cutting tools, holders, drive accessories, bushings). Together with lathe tools (e.g., B94.5 single-point brazed tools) and form-tool blanks (B94.32), it forms the standardized turning-tool system for lathes and screw machines.
