3.4 Work Order Execution Model

The Work Order (WO) is the executable container for the product. If the ERP is the "Architect," the WO is the "Contract" issued to the shop floor. It authorizes the consumption of specific materials to produce a specific quantity of goods by a specific date. A unit without a valid, active WO is "Unplanned WIP"—a ghost asset that exists physically but not financially.

The WO State Machine

Do not manage production on vague terms like "started" or "finished." Adopt a rigid State Machine to control cost accumulation and schedule adherence.

1. Created (Planned)

• Owner: ERP / Planner.
• Definition: The demand exists, but resources (material/capacity) are not yet committed.
• Constraint: Invisible to the Shop Floor Operators. Block Execution.

2. Released (Dispatch)

• Owner: Production Manager.
• Definition: Materials are allocated (picked), and the line is scheduled.
• Action: The MES locks the BOM Revision and Route Revision at this moment.
• Logic: If Master Data changes after release → Then WO does not update. (You cannot change the recipe while the cake is in the oven).

3. Running (Active)

• Owner: MES / Operator.
• Definition: The first unit has passed the first operation (Scan-in).
• Metric: WIP Time (Cycle Time) clock starts ticking.

4. Hold (Exception)

• Owner: Quality / Engineering.
• Definition: Production stopped due to shortage, machine breakdown, or quality spill.
• Constraint: Block all transactional movement (Move/Consume) until released.

5. Closed (TECO - Technical Completion)

• Owner: ERP.
• Definition: Target quantity reached or order short-closed.
• Action: Release unused material reservations. Post final Good Receipt. Calculate Variance.

The Digital Traveler (Route Enforcement)

Paper travelers are "Opt-in" compliance. Digital Travelers are "Forced" compliance. The WO dictates the Routing (Sequence of Operations), and the MES enforces it.

Sequential Logic

• Rule: A unit cannot enter Operation N until it has successfully passed Operation N-1.
• If Operator scans unit at Op 30 (Test) but Op 20 (Assembly) is "Incomplete/Fail" → Then MES rejects scan: "Invalid Process Step."

Parallel & Alternative Routes

Sometimes a line splits (e.g., 3 parallel testers).

• Logic: Define Op 30 as a "Operation Group" containing Resource_A, Resource_B, Resource_C.
• Constraint: Any resource in the group satisfies the step.

Pro-Tip: Never hard-code a Work Order to a specific physical Machine ID. Bind it to a "Resource Group" (capability). This allows the Floor Lead to balance the line without IT intervention.

Splitting and Merging

Real production is rarely linear. You must handle partial movements without losing cost traceability.

The Split (Fork)

• Scenario: Order for 1,000 units. 100 units require rework; 900 are ready to ship.
• Action: Split WO-001 (1,000 qty) into:
• • WO-001 (Parent): Reduced to 900 qty. Proceed to Ship.
  • WO-001-R (Child): Created for 100 qty. Rerouted to Rework Loop.
• Costing: The Child WO inherits the accumulated cost of the Parent up to the split point.

The Merge (Join)

• Scenario: Combining multiple partial batches into a full pallet for shipping.
• Constraint: You can only merge WOs of the same Part Number and same Revision.
• Logic: If Rev A mixed with Rev B → Then Block Merge.

Quantity Control & Overproduction

Operators often build "extras" to cover potential fallout. This is unauthorized inventory.

The "Cap" Logic

• Definition: Max_Qty = Ordered_Qty + Yield_Buffer (e.g., 2%).
• Control:
  • If Total_Good_Units >= Max_Qty → Then Disable "Start" button at Operation 10.
  • If Operator attempts to launch new PCB → Then Reject: "Order Fulfilled."

WIP Visibility (Work In Progress)

WIP is cash sitting on the floor. The MES must provide a real-time snapshot of where every dollar is trapped.

The WIP Equation

At any second, the MES must answer:

WIP_Qty = (Started_Qty) - (Scrapped_Qty) - (Finished_Goods_Qty)

Drift Control

• Phantom WIP: Units physically missing but systemically "at Op 20."
• Rule: Implement a "Stale WIP" auto-flush.
• Logic: If Unit inactive > 30 Days → Then Flag for "Lost Audit."

Final Checklist

Category	Metric / Control	Threshold / Rule
Lifecycle	State Enforcement	No execution allowed on "Created" or "Hold" states.
Routing	Step Lock	Cannot skip Op N-1. "Backward" movement requires authorized rework.
Versions	Frozen Configuration	Released WO locks BOM/Route revisions. Updates require new WO.
Quantity	Overproduction Cap	Block Input scans when Good_Qty >= Order_Qty + Allowance.
Splitting	Cost Inheritance	Child WOs inherit accrued costs of the Parent at the split point.
Hygiene	Stale WIP	Auto-report any unit inactive for > 14 days.
Completion	Auto-Close	If Qty = Target → Then Auto-trigger "TECO" message to ERP.