Definition
An Operation Drawing (工作図, kosakuzu) is a Toyota shop floor document that exists at every step of the machining process, documenting the stock removal, dimensions, tolerances, datum locations, and clamping details for that specific operation. It is not the engineering part drawing and it is not the raw material drawing — it is the intermediate reference that shows exactly what happens to the workpiece at each individual machine.
In machining, a part begins as raw material (素材, sozai, documented in the 素材図 sozaizu / raw material drawing) and ends as a finished part (製品, seihin, documented in the 製品図 seihinzu / final part drawing that matches the design engineer’s requirements). Between start and finish, material is removed step by step — milling, turning, broaching, grinding — operation by operation, to exacting standards at the micron level of precision. The operation drawing documents each of these intermediate steps.
Role in Cutting Point Management
The operation drawing is one of the four key shop floor documents that support Cutting Point Management, Toyota’s third pillar of the 3 Pillar Activity. In the systematic diagnostic sequence for defect investigation:
- Measure the part — compare actual dimensions to the quality standard
- Check the Operation Drawing — verify that stock removal amounts, starting dimensions, and finish dimensions match the documented standard
- Check the Tooling Layout Drawing — verify tooling specifications
- Inspect the machine — check static accuracy
The operation drawing is the second reference point. When a dimensional defect is found on the part, the operation drawing shows exactly how much material should have been removed, what the starting dimensions were, and what the finish dimensions should be. Comparing the actual part to the operation drawing reveals whether the correct amount of material was removed at this step and whether the process produced dimensions within tolerance.
What It Contains
A typical operation drawing includes:
- Part profile showing the workpiece at this operation stage, with the specific surfaces being machined at this step clearly identified
- Stock removal amounts (取代, toridai) — how much material is removed at each cut, specified in millimeters
- Starting dimensions — the incoming condition of the part from the previous operation
- Finish dimensions with tolerances — the target dimensions after this operation, specified with precision tolerances (e.g., φ85 +0.033/-0.027)
- Datum references (基準, kijun) — how the part is located for this operation (e.g., “both center holes”)
- Clamping information — how the part is held during machining
- Surface finish requirements — grind specifications, surface roughness
- Geometric tolerances — circularity, parallelism, perpendicularity, and other form requirements
- Special notes — measurement methods (e.g., “two point measurement”), specific technical requirements for the operation
The drawing also identifies the operation number, operation description (e.g., “Grinding Oil Seal Contact Axis”), the machine type and number, and the part number.
Why Operation Drawings Are Critical
The operation drawing makes plan-vs-actual comparison possible at every intermediate step in a machining line. This is fundamental to effective problem solving.
Consider a crankshaft that passes through dozens of machining operations — rough milling, finish milling, rough turning, finish turning, rough grinding, finish grinding, superfinishing. If the final part is out of tolerance, you need to know where in the process the error occurred. Without operation drawings, you can only compare the final part to the final engineering drawing — you know the part is wrong, but you cannot determine which operation produced the error.
With operation drawings at every step, you can measure the part at each intermediate stage and compare to the documented standard for that operation. This isolates the problem to a specific machine and operation, which then feeds into the next diagnostic steps (checking tooling layout and machine accuracy for that specific operation).
This level of intermediate documentation cannot be done manually or from memory. The precision required — microns of stock removal, tight tolerances on intermediate dimensions — demands formal documentation that is maintained and available at the point of use on the shop floor.
One Per Operation
Each step in the machining process has its own operation drawing. A crankshaft line with 30 operations has 30 operation drawings. The documents can only work if they are step by step and mirror the reality of the production process — each operation drawing corresponds to exactly one machine performing one set of operations on the part.
All operation drawings are created pre-production and checked carefully during the launch process by the responsible engineering departments. They are not created on the fly or after the fact. By the time production begins, every operation in the line has a documented standard showing what happens to the part at that step.
This level of step-by-step documentation is also why companies with this discipline are best positioned to build digital twins between development and production — the complete intermediate documentation already exists.
Connection to Raw Material and Final Part Drawings
The operation drawing sits in a chain of documents:
素材図 (sozaizu) — Raw material drawing. Documents the starting condition: casting or forging dimensions, material specification, incoming tolerances. This is where the machining process begins.
工作図 (kosakuzu) — Operation drawing. One exists at each machining step. Each shows what this operation does to the part — stock removal, resulting dimensions, datum locations.
製品図 (seihinzu) — Final part drawing. The design engineer’s specification. This is where the machining process ends. The final operation’s finish dimensions must match the seihinzu requirements.
The operation drawings form the bridge between raw material and finished part. They decompose the total machining task into documented, verifiable steps.
Common Mistakes
Not having operation drawings at all. In many manufacturing operations outside Japan, intermediate machining steps are not formally documented — the shop floor works from the final part drawing and relies on programmer knowledge or operator experience for intermediate dimensions. This makes systematic problem solving at the process level impossible.
Confusing with the final part drawing. The operation drawing documents what happens at one operation, not the final specification. Using the final part drawing to diagnose an intermediate process step is like using a finished building blueprint to troubleshoot a foundation issue.
Not updating when processes change. When cutting sequences are revised, stock removal amounts are modified, or datum locations change, the operation drawing must be updated. An outdated operation drawing defeats the purpose of the diagnostic system — it provides a reference that no longer matches reality.
Missing stock removal information. An operation drawing that shows finish dimensions but not stock removal amounts is incomplete. Stock removal is critical diagnostic information — if too much or too little material is being removed at an operation, the operation drawing is where this shows up.