Source Inspection

Source inspection is the inspection mode Shigeo Shingo argued was the only one that could actually produce zero defects. Most inspection looks at output and decides what to keep. Source inspection looks at the input conditions that determine output and decides whether to start running at all. The shift sounds small but it changes the economics. A shop that masters source inspection spends almost no time on downstream sorting because the defects never get made in the first place.

"Inspect the conditions that make defects, not the defects themselves."

How source inspection works

Source inspection rests on a specific cause-and-effect logic. For any operation that produces defects, there is a small number of input conditions that determine whether the output will be good or bad. Tool condition. Fixture position. Material variation. Machine parameters. Setup geometry. Identify the conditions, verify them before running, and the output follows.

The process is straightforward.

1. Map the defect causes. For each common defect type on the operation, work backward to the input condition that produces it. A dimensional defect usually traces to tool offset, fixture position, or material variation. A surface defect usually traces to tool condition, speed, or feed. A cosmetic defect on a molded part usually traces to temperature, pressure, or cycle time.

2. Define the verification check. For each cause, define exactly how to verify it before the run. A tool wear check uses a counter and a sample part. A fixture check uses a gauge. A material check uses an incoming hardness or thickness verification.

3. Build the check into the setup. The verifications go into the setup procedure or shift-start checklist. They happen before the first part is made, not after.

4. Stop on a failed verification. Any failed check stops the work until the condition is corrected. The operator does not start running and hope for the best.

The discipline that makes source inspection work is the cause map. A shop that has not done the work to identify which conditions produce which defects ends up with a generic checklist that misses the real causes. The first source inspection effort in a new shop usually requires a FMEA or a focused defect study to identify the real causes.

Where source inspection fits on the shop floor

Picture a 20-person plastics injection shop running parts for a small appliance customer. The shop has historically run a heavy downstream inspection program: every fifth part off each press gets a visual check at a bench station, and every twentieth part gets a dimensional measurement. Despite this, sink-mark and short-shot defects keep appearing in customer shipments.

A source inspection rebuild starts by mapping the causes. Sink marks trace to melt temperature, pack pressure, and cooling time. Short shots trace to material moisture, melt temperature, and injection speed. Each of those conditions can be verified at machine startup with the existing instrumentation. The setup procedure gets a six-line check sheet: melt temp in band, material dried within the last 4 hours, mold temp at setpoint, pack pressure in band, cycle time correct, scrap chute clear.

The downstream visual inspection stays for a few months as a backstop. Within two months, the source-inspection checks are catching the issues before they produce defects, and the downstream visual inspection finds almost nothing. The shop eventually cuts the visual inspection to once per shift as a sanity check rather than a 20 percent sample. Defect rate at the customer drops to under 0.3 percent.

Common mistakes with source inspection

• Generic checklists. A setup checklist that does not target the actual causes of defects produces theater without prevention. The work of mapping causes has to happen first.
• Skipping the checklist when the schedule tightens. Source inspection only works if the checks happen every time. The first skip teaches the team they are optional.
• No response when a check fails. A failed verification has to halt the work and trigger a fix. Otherwise the checklist becomes documentation, not prevention.
• One-time mapping. Causes drift. A source-inspection program built on a 2022 defect study needs to be refreshed every year or two as tools, materials, and operations change.

Source inspection and related Lean tools

Source inspection is one of three core modes of quality at the source, alongside self-inspection by the operator and successive-inspection by the next station. Of the three, source is the earliest and most preventive. The most natural companion technique is poka-yoke, error-proofing fixtures and devices that physically prevent the wrong conditions from being set. Together, source inspection and poka-yoke are the two strongest tools for moving a shop from defect detection toward genuine defect prevention.