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ASME B94.28.1M-1984 — Die Buttons — Basic Straight Relief, Press Fit (Metric)
⭕ ANSI/ASME B94.28.1M-1984 is the metric (SI) version of the die button standard for precision stamping dies, titled “Die Buttons — Basic Straight Relief, Press Fit (Metric)”. Sponsored and published by the American Society of Mechanical Engineers (ASME), it is structurally identical to the inch version, ANSI B94.28-1970, but expresses every dimension, tolerance, and size in millimetres. It defines metric round, oblong, square, and rectangular hole buttons in both headless and head types, with a maximum pierced hole size on the order of 25.4 mm (1 in.), giving metric-based toolrooms a direct selection and stocking reference without inch-to-millimetre conversion.
💡 The engineering value of the metric edition: metric-based stamping shops can select and stock buttons directly in SI units while keeping exactly the same structural logic and fit concepts as the inch version — dual coverage of one design in two unit systems.
1. Scope and General Framework 📌
This Standard covers metric press-fit, straight-relief die buttons with the same categories as the inch edition:
- Hole shapes: Round, Oblong, Square, and Rectangular.
- Structural types: Headless Type and Head Type.
- Unit system: all dimensions stated in millimetres (SI).
- Relief form: straight relief — a straight, enlarged slug bore for easy slug passage.
As in B94.28-1970, headless buttons are retained entirely by press fit in the die block or retainer (optionally in counterbored holes of thick blocks), while head-type buttons add a shoulder for axial retention and increased bearing area. The Standard again controls component tolerances (body diameter “D”) rather than prescribing the mating fit, leaving the actual press-fit amount to the die designer for the specific application.
⚠️ The metric and inch editions are not exact conversion equivalents: metric sizes are rounded to their own SI series. State one unit system clearly in drawings and purchase orders, and never mix dimension values from the two editions in a single assembly.
2. Terminology and Structural Elements 🔧
The metric edition uses the same terminology and symbols as the inch edition:
| Term | Symbol | Metric Definition / Engineering Role |
|---|---|---|
| Hole Diameter | P | Diameter of the working (land) hole in mm; across flats for square holes; “P” larger and “W” smaller for other shapes |
| Land Length | B | Length of the precisely sized working hole, including grind allowance |
| Body Diameter | D | Cylindrical portion fitting the die block or retainer; transmits stamping forces |
| Head Diameter | H | Larger head diameter for load distribution and shoulder retention |
| Head Thickness | T | Thickness of the head |
| Overall Length | L | Total length, selected for the die block thickness |
| Lead Diameter | — | Reduced end of a headless body for primary entry and piloting |
| Fillet | — | Blend from body to head to avoid stress concentration |
| Diagonal | G | Diagonal of a rectangular hole; limiting circumscribed diameter GMAX |
| Relief Hole Diameter | R | Diameter of the straight relief bore, larger than the working hole |
💡 International unification of terms and symbols is one of the most practical benefits of the metric edition: across borders and companies, using the same P, B, D, H, T, L, G, R symbols eliminates drawing ambiguity and rework.
3. Metric Size System 📏
The Standard defines eight types combining structure and hole shape, mirroring the inch type numbers:
| Type | Structure | Hole Shape | Metric Application Notes |
|---|---|---|---|
| 22.00M | Headless | Round | Most common; fully press-fit in thick die blocks |
| 22.01M | Headless | Oblong | Long slots, louver and heat-sink shapes |
| 22.02M | Headless | Square | Square openings with square punches |
| 22.03M | Headless | Rectangular | Narrow long slots; limited by GMAX |
| 22.10M | Head | Round | Shoulder-located; light press or slip fit |
| 22.11M | Head | Oblong | Head-type long-slot piercing |
| 22.12M | Head | Square | Head-type square-hole piercing |
| 22.13M | Head | Rectangular | Head-type rectangular piercing |
Size tables provide metric body diameter, land length, relief bore, lead diameter, and overall length for each size. For rectangular and oblong holes, the larger dimension P must keep the circumscribed circle within GMAX to preserve wall strength.
⚠️ For metric fits, prefer ISO limits-and-fits (e.g., H7/r6, H7/s6 for interference) to define the retainer-bore/body relationship. Directly copying inch interference amounts without converting fit classes causes over- or under-interference.
4. Tolerance Design and Fits 🛠️
The metric edition continues the core principle of B94.28: specify the body tolerance, let the die designer choose the fit. In practice:
- Press fit: headless buttons default to a firm interference fit (e.g., H7/r6) for centering and load capacity.
- Shoulder + light press / transition fit: head-type buttons may use H7/h6 or H7/js6 for easier replacement.
- Lead diameter: pilots the button into the retainer bore square and protects the land.
Metric tolerance values are taken from the standard size tables; the bore and land are finalized by internal grinding or honing, with the piercing clearance typically 3%–8% of stock thickness by material and thickness.
🚨 Align the button axis with the retainer bore when pressing. Use a pilot or dedicated arbor-press tooling — never strike the land end. After installation verify land size, roundness, and concentricity with the punch using internal micrometers or air gages.
5. Engineering Practice Essentials 📋
- 🔍 Selection workflow: choose hole shape and nominal size → select headless or head type → pick overall length L from block thickness → define the ISO fit class and verify interference.
- 🔧 Metric clearance: button bore = punch diameter + total clearance (3%–8% of stock thickness), finalized by grinding.
- 📏 Slug passage: confirm relief bore R accommodates the slug with allowance for free drop.
- 🔨 Maintenance: stock buttons by metric type number; replace worn buttons as whole units rather than reworking the relief geometry in place.
6. Frequently Asked Questions ❓
❓ Q: How do B94.28.1M-1984 and B94.28-1970 relate?
A: They are identical in content and structure, differing only in unit system: B94.28-1970 is inch, B94.28.1M-1984 is millimetre. Choose one based on the design and procurement unit system; never mix.
A: They are identical in content and structure, differing only in unit system: B94.28-1970 is inch, B94.28.1M-1984 is millimetre. Choose one based on the design and procurement unit system; never mix.
❓ Q: Can metric buttons be used with inch punches?
A: Not recommended. Metric and inch sizes are rounded to their own series and are not exact equivalents; mixing loses clearance control. Use matching metric punches and retainers.
A: Not recommended. Metric and inch sizes are rounded to their own series and are not exact equivalents; mixing loses clearance control. Use matching metric punches and retainers.
❓ Q: How is the metric interference fit chosen?
A: Use ISO limits-and-fits: retainer bore H7 with button body r6/s6 gives firm interference; h6/js6 suits light press. Also account for block material, bore size, and button length.
A: Use ISO limits-and-fits: retainer bore H7 with button body r6/s6 gives firm interference; h6/js6 suits light press. Also account for block material, bore size, and button length.
🔍 Q: Why must the rectangular hole’s large dimension P also be limited here?
A: For the same reason as in the inch edition — to preserve wall thickness and strength, the circumscribed diameter must stay within GMAX; otherwise the wall distorts under press fit and may crack in service.
A: For the same reason as in the inch edition — to preserve wall thickness and strength, the circumscribed diameter must stay within GMAX; otherwise the wall distorts under press fit and may crack in service.
