Overview
ISO 6157-3:1988 - Fasteners: Surface discontinuities - Part 3 covers acceptance limits, inspection and sampling for surface discontinuities specifically in bolts, screws and studs for special requirements. It applies to fasteners with nominal thread diameters of 5 mm and larger, product grades A and B, nominal lengths as specified (document cites nominal lengths “1 10 d” or longer if agreed), and to property class 12.9 and, when specified or agreed, 8.8, 9.8 and 10.9. The standard includes an annex sampling plan for acceptance inspection.
Key topics and technical requirements
- Types of discontinuities defined: forging cracks (including socket-head cracks), raw‑material seams and laps, voids, folds, tool marks, laps on threads, and general damages (dents, nicks, gouges).
- Quantitative limits (examples from the text):
- Forging/socket cracks: cracks within 0.3t of the bottom of the socket are not permissible; cracks elsewhere in the socket are permissible up to length 0.25 t and depth 0.03 t (0.13 mm max). Peripheral longitudinal cracks up to 0.06 t (1.6 mm max) may be permissible.
- Seams and laps (raw material): permissible depth = 0.015 d + 0.1 mm; maximum 0.4 mm (d = nominal thread diameter).
- Voids: depth h ≤ 0.02 d (0.25 mm max); combined area of voids on the bearing face ≤ 5% of the total area.
- Tool marks: surface roughness in shank/fillet/bearing areas shall not exceed Ra = 3.2 μm (per ISO 468).
- Laps on threads: laps at thread root or on loaded flank below pitch diameter are prohibited; limited laps on crests allowed (e.g., crest laps ≤ 0.25 H1 or half a turn in specific cases).
- Inspection & evaluation:
- Non‑destructive random sampling per the annex (sample sizes based on lot size and ISO 2859 special inspection level S‑4).
- Destructive testing (sectioning) for suspect items; if defects exceed limits, the lot is rejected.
- Secondary destructive sampling rules apply when defects are found (table‑based sample sizes).
Applications and users
ISO 6157-3 is used by:
- Fastener manufacturers for production quality control and acceptance criteria.
- Purchasing and quality engineers for incoming inspection of bolts, screws and studs with special mechanical/property classes.
- Aerospace, automotive, heavy machinery and other industries requiring stringent fastener integrity where surface discontinuities can affect performance and safety.
- Test labs performing non‑destructive and destructive examinations of fasteners.
Related standards
- ISO 3269 - Acceptance inspection procedures for fasteners (referenced for inspection).
- ISO 468 - Surface texture measurement (used for Ra limits).
- ISO 2859 - Sampling procedures for inspection by attributes (sampling levels referenced).
Keywords: ISO 6157-3, surface discontinuities, fasteners, bolts screws studs, forging cracks, voids, laps, tool marks, sampling plan, inspection, property class 12.9.