When IQC Says "Just Inspect According to the Drawing," and the Customer Asks for the Inspection Standard but Finds None? —— A Five-Step Method for Writing Incoming Inspection Standards (SIP)

1. An Audit Exposes the "Inspection by Experience" Practice

A monthly customer audit at an electronics manufacturing company. The auditor walked into the IQC inspection area, picked up a recently completed incoming inspection record, and asked the inspector three questions: "What is the tolerance for this dimension?" "±0.05 millimeters." "What tool did you use?" "A caliper." "What is the resolution of the caliper?" "… Probably 0.02 millimeters."

The auditor did not press further and turned to the quality manager: "Where are your incoming inspection standard documents?" The quality manager was taken aback and replied, "We inspect according to the drawings; the drawings are the standard." The auditor flipped through the drawings on the table and asked two more questions: "There are over eighty dimensions on the drawing. Do you inspect all of them?" "No, we pick the important ones." "Which ones, how many, how often, and who decides?" The room fell silent.

The nonconformity issued that day was clear: "Incoming inspection lacks a clear inspection specification. Inspection items, methods, criteria, and sampling plans are not documented in controlled files, making it impossible to ensure consistency in inspection results."

This scenario is extremely common in domestic manufacturing companies. It's not that inspections are not being conducted, but that the inspection process has never been clearly documented. The consequence is: the same batch of materials may be deemed合格 (合格) by the day shift and不合格 (不合格) by the night shift; a supplier may find their shipment合格 (合格) upon self-inspection but不合格 (不合格) upon arrival at the factory, with neither side able to provide a convincing basis; during customer audits, only a few experienced technicians can explain the process verbally. Inspections may be performed ten thousand times, but the standards are never written down.

This article will clarify the matter: what should be included in the incoming inspection standard (SIP), how to write it, and how to ensure it is effectively implemented.

2. Why "Drawings Plus Experience" Cannot Replace Inspection Standards

First, let's clarify the roles of three documents. Much of the confusion arises from mixing them up.

Drawings (and other external technical documents) answer the question "What are the requirements?"—dimensions, tolerances, materials, surface treatments, and symbols. It defines the product, not the inspection process.

Control Plan (CP) answers the question "What methods, at which stage, how often, and what to do in case of anomalies?" It manages process control strategies.

Inspection Standard (SIP) answers the question "How to measure this characteristic, how many points to measure, what measuring tool to use, whether it is合格 (合格), and how to record the results?" It manages the inspection actions themselves.

A drawing may specify "Hole diameter φ8±0.05," which is the requirement. However, the inspector still has a series of unanswered questions: use a caliper or a plug gauge? Measure one hole or three cross-sections? Inspect all items in the first article or sample inspect? How many to sample? Does a slight scratch on the surface count? What about a burr of 0.03mm at the edge?

These questions are never answered by the drawings, but they are answered daily on the shop floor—each person using their own experience.

Not writing standards has three major costs:

First, inconsistent judgments. The same batch of goods, the result entirely depends on who the inspector is. The boundaries for limit samples, the judgment of appearance, and the measurement techniques are all stored in people's minds. When the personnel change, the standards change.

Second, unclear responsibility. If a batch is judged合格 (合格), the basis is unclear; if it is rejected, the supplier demands an explanation, and you can only say, "We don't think it's acceptable." Once a dispute escalates, the company is at a disadvantage—without written criteria, there is no negotiating position.

Third, inevitable audit nonconformities. ISO 9001's 8.4.2 requires the specification of verification activities for purchased products and services; IATF 16949 explicitly requires the development of inspection instructions for incoming inspections, including sampling plans, frequency, and acceptance criteria. During a customer audit, just asking, "Where are the standard documents?" will result in a nonconformity if they are not available, leaving little room for explanation.

In short: the drawing gives you the destination, while the inspection standard gives you the route. Without the route, everyone is driving by instinct.

3. A Five-Step Method for Writing Incoming Inspection Standards

Step 1: Clarify the "Source" of Inspection Items—Create a Source List

Inspection items should not be chosen based on personal feelings. Each item should have a traceable source, commonly falling into six categories:

1. Drawings and Technical Agreements—key dimensions, critical fits, material and appearance requirements.
2. Special Characteristics List (SC/CC)—customer-designated items that must be included and are subject to the highest control intensity.
3. Control Plan—the incoming inspection rows for each material, with the frequency and methods specified in the CP needing to be matched in the SIP.
4. Historical Defects and Customer Complaints—failure modes caused by incoming material issues in the past, which should be addressed by adding corresponding inspection items. This is the easiest to overlook but the most valuable.
5. Customer Special Requirements (CSR) and Regulations—such as RoHS/REACH, safety component requirements, packaging and labeling regulations.
6. Feedback from Downstream Processes—if the workshop reports that "this batch is difficult to install or adjust," this feedback should be incorporated into the incoming inspection.

Organize these six categories into an "Incoming Inspection Item Source Table," clearly indicating the source of each item. When a customer asks, "Why inspect this item?" the answer is in the table, not in someone's memory.

Step 2: Categorize, Do Not Treat Equally

A drawing with over eighty dimensions does not need to be fully inspected, as it is neither economical nor necessary. Categorize incoming materials based on their importance, and adjust the control intensity accordingly:

The significance of categorization is to allocate limited inspection resources to areas where issues are most likely to occur. Categorization rules should be included in the SIP to avoid each inspector having to "pick the important ones"—if the selection criteria vary from person to person, all previous efforts will be negated.

Step 3: Clearly Define "How to Measure"—Measurement Methods and Tools

This step is where the SIP differs most from the drawing and is also the most prone to issues. Each inspection item should clearly define at least five aspects:

Measurement Baseline and Position. From which surface or edge to start measuring, how many points to take, whether to use the average or the maximum value, and whether to measure the most unfavorable point. Measuring three cross-sections of the same shaft versus just the middle section can result in a significant difference in the tolerance band.

Measuring Tools and Gauges. Specify which tool to use and the gauge's identification number. A hard rule here is that the resolution of the measuring tool should not exceed one-tenth of the tolerance. A tolerance of ±0.05mm means a tolerance band of 0.1mm, and using a caliper with a resolution of 0.02mm is barely acceptable; to judge a tolerance of ±0.01mm, a digital micrometer with a resolution of 0.001mm is required, not a caliper. Using a caliper to judge a tolerance of ±0.01mm is a classic case of "measuring inaccurately but confidently."

Measurement Conditions. Temperature, workpiece state (before or after deburring, before or after cleaning). Measuring plastic parts or thin-walled parts without specifying the temperature is equivalent to creating disputes in bulk.

Equipment Status Requirements. Clearly state that measuring tools must be used within their calibration validity period, and specify daily inspection requirements for gauges if necessary.

Retest Rules. How many times to retest, who to retest with, and which result to use. This rule helps prevent the hidden bad habit of "retesting until it passes."

Step 4: Clearly Define "How to Judge"—Quantitative, Qualitative, and Sampling

Quantitative Items should specify the range: upper limit, lower limit, whether the boundaries are included, and the rounding rule (the number of decimal places retained for judgment should match the reading of the measuring tool). Avoid vague statements like "dimension is合格 (合格)" or "meets the drawing," as they are essentially meaningless.

Qualitative Items (appearance, feel, color, packaging) should not be described with adjectives like "no obvious scratches" or "surface is smooth." These are breeding grounds for disputes. The correct approach is "text plus reference samples": include limit samples (physical or photographic, with identification numbers and validity periods), diagrams of the maximum allowable dirt area, and color comparison charts. In the industry, the consistency of appearance judgments is mostly achieved through limit samples, not adjectives.

Sampling Plan should specify three things: how many to sample (plan number, such as GB/T 2828.1 general inspection level II, or a zero-defect c=0 plan), whether to sample each batch or each shift, and the conditions for tightening or relaxing the inspection. Sampling plans and frequencies should not be "set once and unchanged for three years"; transfer rules should be clearly defined.

Combining these categories, a usable inspection standard item should include seven elements:

Step 5: Control, Train, and Update—Keep the Standards Alive

Locking the documents in a cabinet is as good as not writing them. Three things must be done properly.

Controlled Release. The SIP should be included in the document control system, with a number, version, effective date, and distribution record. Only the valid version should be used on the shop floor, and old versions on the inspection table must be recalled. This sounds simple but is often where problems arise.

Training and Assessment. After a new standard is released, conduct a practical measurement training for the inspectors: test their judgment using limit samples and compare consistency using controversial cases. Have the same batch of samples judged by multiple inspectors; if the consistency rate is below 90%, it indicates that the standard or training is not adequate.

Update Triggers. The SIP is not a one-time document. Updates must be triggered by the following four situations: revision of drawings or technical agreements, changes in the control plan, the emergence of new incoming material defect modes, and new requirements from customer complaints or audits. It is recommended to include SIP reviews in the annual document review list and specify that "any change must be accompanied by a review of the corresponding SIP items."

4. Five Common Misconceptions

Misconception 1: Copying and signing the drawing as the inspection standard. Lacks methods, frequency, judgment boundaries, and record formats. The drawing is a requirement document, not an operational document. Using it directly as a standard means handing over all interpretation rights to the inspector's experience, which can easily fall apart under customer scrutiny.

Misconception 2: Judgment clauses written like slogans. "Good appearance," "no obvious defects," "dimension is合格 (合格)"—such statements are actually counterproductive, as they provide everyone with an interpretation space. Any judgment clause without quantified boundaries or reference samples should be rewritten.

Misconception 3: Inspecting despite inadequate measuring tool capability. Insufficient resolution, fixed measurement points, uncalibrated gauges, and unspecified environmental conditions. The result is that the data looks valid, but the capability is actually lacking. The credibility of the inspection system depends first on the measuring tools and then on the people.

Misconception 4: SIP and CP are disconnected. The CP specifies sampling 5 items per batch, while the SIP specifies 2 items; the CP lists a certain dimension as a critical item, but it is not included in the SIP. Customers love to compare the two documents side by side—any mismatch is a nonconformity. A rule should be established during writing: the inspection items and frequencies in the SIP must be traceable back to the CP.

Misconception 5: Writing without training, training without assessment, and assessment without improvement. After writing and training, the standard is archived, and six months later, the shop floor is following a different standard based on someone else's understanding. The effectiveness of the SIP should be measured by consistency data: regularly compare judgment consistency and identify points of disagreement to refine the standard, which is more effective than holding ten promotional meetings.

5. In Summary

The standard for incoming inspection is not on the drawing but in the few pages you write yourself—clarify the source of inspection items, measurement methods, judgment criteria, sampling plans, and update mechanisms. Only then can inspection become a replicable action rather than a personal skill.

The inspection standard is not written on the drawing but in your SIP.

Knowledge code: 9.2.1

Version: v20260911

Author: QTank QTank is dedicated to providing systematic professional knowledge, methodologies, and practical tools for quality management practitioners, helping companies continuously improve their quality capabilities.