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ASME B94.31-1981 — Die Buttons — Variable Press Fit, Metric
⭕ ANSI/ASME B94.31-1981 is the metric companion to the U.S. National Standard for die buttons retained by a variable press fit, formally titled “Die Buttons — Variable Press Fit, Metric.” Prepared by Technical Committee No. 16 (Punches and Dies) of the B94 committee with the National Association of Punch Manufacturers (NAPM), it restates the dimensions, tolerances, hole-shape classes, and terminology of the inch-based B94.30-1970 entirely in millimeters, serving metric-tooling die shops, export trade, and international projects that specify the International System of Units (SI). Along with metric head punches and metric ball-lock elements, it forms a coherent metric tooling standardization family.
💡 “Variable press fit” means the interference on the button body diameter is graded upward by size: light interference for small buttons, heavier for large ones. This secures reliable retention while avoiding the cracking risk of heavy interference on small diameters. B94.31 simply delivers this principle as a complete millimeter size system; its structure and logic are identical to B94.30.
1. Subject and Scope 📌
B94.31 covers metric die buttons retained by a variable press fit. Its scope includes:
- Hole shapes: round, oblong, square, and rectangular pierced-hole classes, dimensioned in metric units.
- Constructions: headless and head types, the head providing axial location through its shoulder.
- Dimension system: body diameter D, hole diameter P, land length B, head diameter H, overall length L, all in millimeters.
- Fit feature: interference graded by body size and stated in millimeters (micrometer level).
⚠️ The metric sizes are not a rounded conversion of the B94.30 inch tables; they are an independent set rearranged on metric preferred-number (R) series. Design documents, purchase orders, and die drawings must use one unit system consistently — never mix inch and metric buttons in the same assembly.
2. Terminology and Structural Elements 🔧
The terminology and symbols match the inch-based B94.30; only the unit of the values changes:
| Term | Sym. | Metric definition and engineering role |
|---|---|---|
| Hole Diameter | P | Working (land) bore diameter, mm; square uses across-flats, rectangular/oblong use large P and small W |
| Land Length | B | Length of accurately ground working bore; includes grinding allowance for stable resharpening |
| Body Diameter | D | Cylindrical section engaging the retainer bore, mm; carries the reaction and forms the interference |
| Head Diameter | H | Head-type button head diameter, larger than D, enlarging bearing area and forming the shoulder |
| Head Thickness | T | Head thickness, defines the shoulder bearing face |
| Overall Length | L | Total button length, chosen from retainer thickness |
| Relief Hole Diameter | R | Larger than the working hole so slugs pass freely |
💡 For the retainer bore, ISO limits-and-fits (e.g., H7/h6, H7/js6) are a useful reference for the nominal fit, but the interference bands given by the Standard should take priority — actual retention under pierce reaction depends on interference, not merely the nominal fit symbol.
3. Metric Size System and Press-Fit Interference 📏
Buttons are named by “hole shape × construction” and assigned metric NAPM type numbers. The table below gives representative metric body-diameter bands and typical interference ranges (always use the original tables for official values):
| Body dia. D band (mm) | Typical interference (ref.) | Constructions | Typical characteristics |
|---|---|---|---|
| 6 – 10 | 0.013 – 0.025 mm | Headless / head | Small holes, light loads, easy assembly |
| 10 – 16 | 0.025 – 0.040 mm | Headless / head | Medium loads, common piercing stations |
| 16 – 25 | 0.040 – 0.063 mm | Headless / head | Larger holes, higher push-out resistance |
| 25 – 38 | 0.063 – 0.100 mm | Mostly head type | Heavy duty, thick stock, high reactions |
As in the inch version, rectangular and oblong buttons must respect the limiting circumscribed-circle diameter GMAX: set the small dimension W first, then keep the large dimension P’s circumscribed diameter within the limit so the wall is thick enough to avoid distortion and burst.
⚠️ Interference is stated at micrometer precision and requires precise gauging on the shop floor. Do not scale interference by a fixed ratio with diameter; excessive interference swells or cracks the retainer, while insufficient interference lets a large button loosen or rotate under load.
4. Press-Fit and Assembly Design 🛠️
The metric variable press-fit follows the same logic as the inch version:
- Light interference at small diameters: avoids stress concentration and thin-retainer distortion.
- Heavier interference at large diameters: larger friction resists pierce- and strip-induced push-out and rotation.
- Head shoulder of head-type buttons: preferred for larger sizes, the head carries axial load.
- Lead-in section: enters the retainer bore first to keep the axis aligned during pressing.
🚨 Always align the button axis with the retainer bore before pressing; use a guide rod or arbor-press tool and never hammer the cutting end. Verify land-to-punch concentricity after installation, or you will get one-sided clearance, uneven wear, and excessive burrs.
5. Engineering Practice Essentials 📋
- 🔍 Selection workflow: choose the hole shape class → decide headless or head from load and stock thickness → look up the metric body-diameter interference band → set overall length L from retainer thickness → verify against the retainer bore tolerance.
- 🔧 Clearance setting: die bore = punch diameter + total die clearance, roughly 3%–8% of stock thickness by material, then fine-tuned by grinding.
- 📏 Slug passage: confirm relief bore R clears the slug shape plus margin so slugs drop freely.
- 🔨 Maintenance: replace the whole button when the land wears; check periodically for loosened fit, worn shoulder, and land cracks.
6. Frequently Asked Questions ❓
❓ Q: What is the difference between B94.31 and B94.30?
A: B94.31 is the metric companion to B94.30. The construction, hole shapes, and variable press-fit principle are identical; only the size system is converted to millimeters on a metric preferred-number series. Choose the version matching the project’s unit system and never mix them.
A: B94.31 is the metric companion to B94.30. The construction, hole shapes, and variable press-fit principle are identical; only the size system is converted to millimeters on a metric preferred-number series. Choose the version matching the project’s unit system and never mix them.
❓ Q: Can the metric interference be derived directly from the inch values?
A: No. B94.31’s tables are independently arranged on metric number series; the interference bands do not map one-to-one to the inch version. Always cite the B94.31 tables directly in design and purchasing.
A: No. B94.31’s tables are independently arranged on metric number series; the interference bands do not map one-to-one to the inch version. Always cite the B94.31 tables directly in design and purchasing.
❓ Q: Why must the large dimension P of rectangular/oblong holes be limited?
A: To preserve wall thickness and strength, the circumscribed-circle diameter must not exceed the limiting diameter GMAX; otherwise the wall is too thin, distorts under press fit, and may burst while piercing. Set W first, then compute the largest permissible P.
A: To preserve wall thickness and strength, the circumscribed-circle diameter must not exceed the limiting diameter GMAX; otherwise the wall is too thin, distorts under press fit, and may burst while piercing. Set W first, then compute the largest permissible P.
🔍 Q: How does the Standard fit with the metric punch system?
A: B94.31 aligns dimensionally with metric head punches and metric ball-lock elements, so punch–button–bushing sets are coordinated. Request metric NAPM type numbers and conformance statements from suppliers.
A: B94.31 aligns dimensionally with metric head punches and metric ball-lock elements, so punch–button–bushing sets are coordinated. Request metric NAPM type numbers and conformance statements from suppliers.
