9.7 Manufacturing & Service Process Planning: Operation Selection, Sequencing, Route Sheets, Fixturing & Mistake Proofing

9.7 Manufacturing & Service Process Planning

Process planning turns a product design (drawings, tolerances, materials) or a service concept into a defined sequence of work that operators can execute repeatably. The NCEES specification lists the elements: selection of operations, sequence, instructions, tooling and fixturing, and mistake proofing.

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1. Steps in Manufacturing Process Planning

1. Interpret the design: geometry, critical dimensions, tolerances and geometric tolerances (GD&T), surface finish, material, heat treatment, and annual volume.
2. Select processes: choose operations that can meet each requirement, such as casting or forging for the form, turning or milling for features, and grinding for tight tolerance and fine finish.
3. Select equipment: match machine capability (size, power, accuracy, $C_{pk}$) and capacity to the volume.
4. Sequence the operations (Section 2).
5. Specify tooling, fixturing, and parameters: cutting tools, speeds and feeds, gauges, and fixtures.
6. Estimate times: setup and run standards from time study, PMTS, or standard data.
7. Document: route sheet, operation sheets, work instructions, and the control plan.

Selecting Processes

Each process has a range of achievable tolerance, finish, and economical volume. Grinding, for example, holds much tighter tolerances and finer finishes than turning but removes material slowly, so it is normally reserved for finishing. Choose the cheapest process sequence whose capability ($C_{pk}$) meets the specification with margin. A process that can only just hold a tolerance will generate scrap and inspection cost.

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2. Sequencing Rules

Operation order is driven by precedence logic and datum logic:

• Establish datums first. Machine the surfaces that later operations locate from, so every later feature is referenced to an accurate base.
• Rough before finish. Heavy cuts cause heat and stress, which distort finished surfaces.
• Complete soft-state features before heat treatment. Drill, tap, and mill keyways while the part is soft. After hardening, only grinding or other hard-finishing remains.
• Deburr after the operations that create burrs, and before assembly or plating.
• Inspect after critical or costly operations, so defects are not carried into expensive downstream work.
• Minimize setups and handling. Combine operations that use the same setup, which also improves feature-to-feature accuracy.

Example Route Sheet: Hardened Drive Shaft

A route sheet lists the operations and work centers in order. An operation sheet expands one operation with setup, tools, parameters, and inspection. Work instructions show the operator, step by step and with pictures, how to perform it, highlighting key points and quality checks.

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3. Tooling and Fixturing

A fixture locates and holds the workpiece. A jig also guides the tool, such as a drill bushing. Both must locate the part accurately and repeatably, hold it rigidly against cutting forces, load quickly, and be mistake-proof.

The 3-2-1 Locating Principle

A rigid prismatic part has six degrees of freedom: translation along X, Y, and Z and rotation about each axis. The 3-2-1 principle restrains all six with the minimum number of locators:

• 3 locators on the primary datum surface (the largest face) define a plane, restraining Z translation and rotation about X and Y.
• 2 locators on the secondary datum restrain the next translation and the remaining rotation.
• 1 locator on the tertiary datum restrains the last translation.

Clamps then push the part firmly against the locators, never against a clamp on the opposite side of a locator, and clamping forces should act through the locators so the part does not distort. Locate on the datum features named on the drawing so that the measured features relate to the same references the designer used. Cylindrical parts are commonly located with V-blocks, centers, or collets.

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4. Computer-Aided Process Planning (CAPP)

• Variant (retrieval) CAPP: parts are coded into families with a group technology classification code. A standard plan exists for each family, and a new part retrieves its family plan, which the planner edits.
• Generative CAPP: decision logic and process databases build a new plan from part features without starting from a stored plan.

Variant systems are easier to set up; generative systems handle novel parts but need extensive rules and data.

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5. Mistake Proofing in Process Planning

Build poka-yoke devices into the plan rather than relying on inspection:

• Fixtures that accept the part in only the correct orientation, using asymmetric pins.
• Sensors that confirm the part is seated before the cycle can start.
• Kits and counters that stop the cycle if a fastener is missing.
• Source inspection: checking conditions that cause defects, such as the right tool or a seated part, before the operation instead of inspecting the result afterward.

The quality chapter covers contact, fixed-value, and motion-step methods and control versus warning devices.

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6. Service Process Planning: The Service Blueprint

G. Lynn Shostack introduced the service blueprint in 1984. It is a flowchart in horizontal bands:

1. Physical evidence: what the customer sees, such as the lobby, website, or invoice.
2. Customer actions: each step the customer takes.
3. Onstage (visible) contact employee actions, separated from customer actions by the line of interaction.
4. Backstage (invisible) contact employee actions, below the line of visibility.
5. Support processes, below the line of internal interaction.

The planner marks fail points (steps likely to go wrong), wait points, and the time standard for each step. Fail points get service poka-yoke, such as checklists, reminder calls, or swipe cards that verify identity.

Example: At an equipment-rental counter, a fail point is "wrong equipment pulled from the yard" (backstage). A barcode scan matching the reservation removes it. Another is "customer waits for paperwork" (onstage), addressed by pre-filling the contract online before arrival.