A work instruction is a document that tells one person how to perform one task correctly, in enough detail that they can do it with nothing else open. It is the most granular level of process documentation: below the procedure, below the policy.
Where a procedure says the Quality Inspector performs the final inspection, the work instruction tells that inspector which gauge to use, against which tolerance, and what to do when a part is outside it.
What a work instruction contains
- The specific task, and the part, product, or case it applies to, including revision.
- What has to be in place first: tools, materials, calibration status, access.
- The steps in the required sequence, with the actual values. Torque settings, temperatures, timings, tolerances.
- The in-process checks, at the point the feature can still be reached.
- What to do when something is out of specification. Usually stop and escalate, never adjust to fit.
- Images. For station-level work these outperform prose, and their absence is the main reason paper work instructions get ignored.
Their purpose
Consistency at the point of work, and transferability. A good work instruction means the task is performed the same way by someone on their first week as by someone on their tenth year, and it means a trained person can be moved to a different station without a week of shadowing.
In a quality system they serve a second purpose: they are the evidence that the procedure is actually implementable. An auditor reading a procedure that says "inspect to specification" will ask which specification and where it is written down.
Where they sit relative to everything else
Policy, then procedure, then work instruction, with guidelines alongside as advisory material. Each level answers a different question: why, who and in what order, and precisely how.
The most common mistake
Writing one document that tries to be both a procedure and a work instruction. It ends up too general to follow at the bench and too detailed to read as process, so the operator ignores it and the supervisor cannot use it to explain the workflow.
If your document contains a torque value and also describes a handoff between two departments, it is two documents.
Keeping them current
Work instructions change far more often than procedures, because they change whenever tooling, materials, or a drawing revision changes. Review them on change rather than on a calendar, and reference them from the parent procedure so the document hierarchy an auditor expects actually exists.