ASME B18.18.3M-1987 (1999) — Inspection and Quality Assurance for Special Purpose Fasteners

🔩 ASME B18.18.3M-1987 (1999) is an American National Standard that establishes an inspection and quality assurance plan for special purpose fasteners. The plan is based on the concept of quality assurance through in-process control: it defines the specific inspection functions that must be performed during each process and at each operation involved in the production of fasteners, with the objective of producing finished fasteners that conform to all customer requirements specified in engineering drawings, related standards, and specifications. Reaffirmed by ASME in 1999, it belongs to the B18.18 family of quality assurance documents, complementing B18.18.1 (general purpose fasteners), B18.18.2 (high-volume machine assembly fasteners), and B18.18.4M (fasteners for highly specialized engineered applications).

1. Scope and General Framework 📌

The plan is based on the concept of quality assurance through in-process control. It covers sampling plans, inspection frequencies, control procedures, and record keeping for internally and externally threaded fasteners and their accessories or associated parts. The basic structure applies to fasteners produced by a single manufacturing practice that requires records of in-process and final inspection maintained by the producer.

The scope includes:

• Products covered: internally and externally threaded fasteners, and accessories or associated parts.
• Manufacturing scope: fasteners produced by one manufacturing practice that requires in-process and final inspection records maintained by the producer.
• Companion standards: this Standard is used together with other accepted standards for product, testing, gaging, and material; therefore, performance and packaging provisions are not included herein.

The Standard also emphasizes that adherence to the characteristic requirements of the plan does not release the contractor from the responsibility of exercising due care in the production of all parts, and that records of inspection must be maintained as objective evidence of conformance.

2. Inspection Levels and Characteristic Classification 🔧

Because the importance of various characteristics to the user differs substantially and the degrees of control in manufacture vary, subjecting every characteristic to the same degree of inspection is impractical. The Standard therefore provides four inspection levels — A, B, C, and D — and assigns each characteristic an appropriate level. Any additional characteristics deemed applicable by the user that are not listed in the plan, or characteristics for which a nonstandard inspection level is required, must be designated by the user, preferably on engineering drawings or specifications, by the appropriate code letter at the time of ordering.

Characteristics fall into two groups:

• Transitional (in-process) characteristics: characteristics present at an intermediate manufacturing stage, such as as-quenched hardness, governed by Table 1 (In-Process, Nondestructive) and Table 2 (In-Process, Destructive).
• Final inspection characteristics: finished-product characteristics described by engineering drawings and related specifications, governed by Table 5 (Final Inspection, Nondestructive) and Table 6 (Final Inspection, Destructive).

To determine the most appropriate inspection level, the Standard recommends using the Decision Table in Appendix I (Tables A, B, and C). Appendix II provides the ordering information format so that inspection requirements are expressed accurately at the time of purchase.

3. Lots, Sampling Frequency, and Gage Calibration 📊

Two important sampling provisions apply:

• Fixed sample sizes: when the acceptance number for sampling by attributes is fixed, variation in sample size changes the acceptable quality level. To avoid this inconsistency, fixed sample sizes are applied over the greatest range of lot size.
• Lot size limit: lot sizes are restricted to a maximum of 250,000 pieces; larger quantities must be divided into multiple lots.

Table 3 specifies the frequency of destructive testing, Table 4 the in-process testing frequency, and Table 7 the final inspection sample sizes. Destructive test frequencies are generally lower than nondestructive inspection frequencies to balance assurance strength against manufacturing cost.

All measuring instruments, gages, and testing equipment used for incoming, in-process, and final inspection must be calibrated at planned intervals to National Bureau of Standards (NBS, now NIST) or other applicable equivalent national standards. Dates of calibration and fixture checks must be recorded, and inspection records must be kept for a minimum of 1 year.

4. Engineering Practice Essentials 🛠️

• 🔍 Level selection: use the Appendix I Decision Table to assign each characteristic a level based on failure consequence, process stability, and history — avoid an across-the-board high inspection level that wastes cost.
• 🔧 Transitional characteristics: verify as-quenched hardness and similar intermediate states at the correct in-process stage; too early or too late a check can miss the true process state.
• 📋 Record keeping: maintain lot-correlated inspection records for at least 1 year as objective evidence for customers.
• 📐 Lot control: keep lots within 250,000 pieces and correlate in-process records with lot identity for traceability.
• 📝 Ordering communication: state additional characteristics and their inspection-level code letters on drawings or purchase orders to avoid ambiguity between supplier and purchaser.

5. Frequently Asked Questions ❓