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ASME B94.28-1970 — Die Buttons — Basic Straight Relief, Press Fit
⭕ ANSI/ASME B94.28-1970 (“Die Buttons — Basic Straight Relief, Press Fit”) is the American National Standard governing press-fit die buttons for precision stamping dies (also called permanent dies). Developed by Technical Committee No. 16 (Punches and Dies) of American National Standards Committee B94 and published by the American Society of Mechanical Engineers (ASME) in collaboration with the National Association of Punch Manufacturers (NAPM), it specifies dimensions, tolerances, nomenclature, sizes, and hole shapes for headless and head-type die buttons with a maximum pierced hole size of 1 in. (25.4 mm). Together with the standard punches (B94.14, B94.18), punch guide bushings (B94.23), and retainers of the B94 series, it forms an interchangeable, standardized punch-and-die system widely used by diemakers for long-run production of individual metal parts in many kinds of presses.
💡 A die button is the female cutting element of a stamping die: the punch pierces the sheet into the button’s precisely sized hole, and the land of the hole together with the punch diameter establishes the desired clearance, controlling edge quality and tool life. Standardization makes buttons from different manufacturers interchangeable, simplifying stocking and maintenance.
1. Scope and General Framework 📌
The Standard covers press-fit, straight-relief die buttons in four basic hole shapes and two structural types:
- Hole shapes: Round, Oblong, Square, and Rectangular.
- Structural types: Headless Type and Head Type.
- Size limit: maximum pierced hole size of 1 in. (25.4 mm).
- Relief form: straight relief — a straight, larger-diameter slug hole that lets the slug pass through easily.
Headless buttons are intended primarily for retention in the die block or retainer entirely by press fit, and for installations in thick die blocks, optionally in counterbored holes. Head-type buttons are retained by press fit and/or by the shoulder action of the head, and suit applications requiring increased bearing area. The Standard deliberately states only a tolerance on the body (“D”) diameter rather than prescribing ever-larger press-fit allowances with nominal size, because the fit inside a retainer hole made by others should vary with the application; the standard controls component tolerances, not the mating fit itself.
⚠️ The scope is limited to pierced holes up to 1 in. diameter. For larger bores or non-standard shapes, B94.28 does not apply; forcing oversized buttons into these specifications risks inadequate strength and slug passage.
2. Terminology and Structural Elements 🔧
Section 2 establishes a complete, industry-common terminology:
| Term | Symbol | Definition / Engineering Role |
|---|---|---|
| Hole Diameter | P | Diameter of the working (land) hole; for square holes, distance across flats; for other shapes, “P” is the larger and “W” the smaller dimension |
| 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 diameter of the head for load distribution and shoulder retention |
| Head Thickness | T | Thickness (length) of the head |
| Overall Length | L | Total length of the button, selected for the die block thickness |
| Lead Diameter | — | Reduced end of a headless body for primary entry and piloting during assembly |
| Fillet | — | Blend between body and head diameters 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 |
💡 Straight relief is the defining feature of B94.28, in contrast to the taper relief of B94.27 and the variable press fit of B94.30. The straight, cylindrical slug bore has no step, allowing slugs to pass through essentially undeformed when their shape is stable.
3. Button Types and Size System 📏
The Standard defines eight types combining structure and hole shape, identified by NAPM type numbers:
| Type No. | Structure | Hole Shape | Typical Application |
|---|---|---|---|
| 22.00 | Headless | Round | Most common; fully press-fit in thick die blocks |
| 22.01 | Headless | Oblong | Long slots, heat-sink and louver shapes |
| 22.02 | Headless | Square | Square openings with square punches |
| 22.03 | Headless | Rectangular | Narrow long slots; observe limiting diameter |
| 22.10 | Head | Round | Shoulder-located; light or slip fit |
| 22.11 | Head | Oblong | Head-type long-slot piercing |
| 22.12 | Head | Square | Head-type square-hole piercing |
| 22.13 | Head | Rectangular | Head-type rectangular piercing |
Systematic size tables provide body diameter D, land length B, relief hole diameter R, lead diameter, and overall length L for every size. For rectangular and oblong holes, the larger dimension P must be calculated so the circumscribed circle stays within the limiting diameter GMAX, preserving wall strength.
⚠️ For a given smaller dimension W, the larger dimension P must keep the circumscribed diameter within GMAX. Ignoring this constraint thins the wall, causing distortion under press fit or bursting during piercing.
4. Press Fit and Tolerance Design 🛠️
A distinctive feature of B94.28 is that it abandons the older practice of ever-larger press-fit allowances on headless buttons with increasing nominal body size. Instead it states a uniform tolerance on the body “D” diameter, identical to that used for head-type buttons and head-type punches (reference USAS B94.14-1968). The rationale: a standard can control the tolerances of components, but it cannot dictate the actual fit in a retainer hole that is made by others for specific applications.
- Press fit: for high centering accuracy and load capacity where disassembly is infrequent; the default for headless buttons.
- Shoulder + light press / slip fit: for head-type buttons, the head shoulder carries axial load and eases replacement.
- Lead diameter: eases initial entry and pilots the button square into the retainer hole, protecting the cutting land.
🚨 Press the button in with its axis aligned to the retainer hole. Use a pilot or dedicated arbor press tooling; never hammer the land end. After installation, verify concentricity with the punch — misalignment produces one-sided clearance, uneven wear, and burr problems.
5. Engineering Practice Essentials 📋
- 🔍 Selection workflow: choose hole shape (round/square/oblong/rectangular) and size → decide headless vs. head type → select overall length L from die block thickness → verify body diameter D against the retainer bore and define the fit (press, light press, or slip).
- 🔧 Clearance: button bore = punch diameter + total clearance; clearance is typically 3%–8% of stock thickness depending on material, finalized by internal grind or honing.
- 📏 Slug passage: confirm the straight relief bore accommodates the slug shape with allowance for free drop; blocked slugs cause second shearing and die damage.
- 🔨 Maintenance: replace worn buttons with same-type standards; periodically check press-fit looseness, head shoulder wear, and land cracks.
6. Frequently Asked Questions ❓
❓ Q: B94.28 (straight relief) vs. B94.27 (taper relief) — which to choose?
A: B94.28 uses a straight, cylindrical slug bore — simple, slugs pass through directly, ideal when slug shape is stable. B94.27 uses a tapered relief that self-guides slugs and resists jamming — better for thin stock and irregular slug shapes. The landing designs differ; do not mix them.
A: B94.28 uses a straight, cylindrical slug bore — simple, slugs pass through directly, ideal when slug shape is stable. B94.27 uses a tapered relief that self-guides slugs and resists jamming — better for thin stock and irregular slug shapes. The landing designs differ; do not mix them.
❓ Q: When to use headless versus head-type buttons?
A: Headless buttons are fully press-fit and suit thick die blocks and locations changed infrequently. Head-type buttons add a shoulder for axial load capacity and retention, permitting light press or slip fits and easier replacement.
A: Headless buttons are fully press-fit and suit thick die blocks and locations changed infrequently. Head-type buttons add a shoulder for axial load capacity and retention, permitting light press or slip fits and easier replacement.
❓ Q: Why must the large dimension P of a rectangular hole be limited?
A: To keep sufficient wall thickness and strength, the circumscribed diameter of a rectangular hole must not exceed GMAX. Otherwise the wall is too thin — it distorts during press fit and may crack in service. Fix W first, then compute the largest allowable P.
A: To keep sufficient wall thickness and strength, the circumscribed diameter of a rectangular hole must not exceed GMAX. Otherwise the wall is too thin — it distorts during press fit and may crack in service. Fix W first, then compute the largest allowable P.
🔍 Q: How does this Standard relate to NAPM and to B94.14 punches?
A: NAPM (National Association of Punch Manufacturers) was the major contributor; B94.28 shares its tolerance system with USAS B94.14-1968 head-type punches so punch-button pairs stay coordinated. Request NAPM type numbers and conformance statements when purchasing.
A: NAPM (National Association of Punch Manufacturers) was the major contributor; B94.28 shares its tolerance system with USAS B94.14-1968 head-type punches so punch-button pairs stay coordinated. Request NAPM type numbers and conformance statements when purchasing.
