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 on the floor but not financially.

The WO state machine

Production must not be managed on vague terms like “started” or “finished.” A rigid State Machine must be adopted 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: This state is invisible to Shop Floor Operators. Execution is blocked.

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: When Master Data changes after release, the Work Order should not automatically 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: Work In Progress (WIP) Time (Cycle Time) clock starts ticking.

4. Hold (exception)

Owner: Quality / Engineering.
Definition: Production stopped due to a material shortage, machine breakdown, or quality issue.
Constraint: Block all transactional movement (Move/Consume) until the hold is formally 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 Work Order (WO) dictates the Routing (Sequence of Operations), and the Manufacturing Execution System (MES) enforces it.

Sequential logic

Rule: A unit cannot enter Operation N until it has successfully passed Operation N-1.
When an operator scans a unit at Operation 30 (Test), but Operation 20 (Assembly) registers as “Incomplete/Fail,” the MES should reject the scan with an “Invalid Process Step” flag.

Parallel & alternative routes

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

Logic: Op 30 must be defined as an “Operation Group” containing Resource_A, Resource_B, Resource_C.
Constraint: Any resource in the group satisfies the step.

Pro-Tip: A Work Order must never be hard-coded to a specific physical Machine ID. It must be bound 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: WO-001 (1,000 qty) must be split 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: When Revision A is loaded with Revision B, the system should actively block the merge transaction to prevent a mixed-model pallet.

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:
- When the total good units reach or exceed the maximum quantity, the system should disable the “Start” button at Operation 10.
- When an operator attempts to launch a new PCB after this point, the system should reject the action with an “Order Fulfilled” notice.

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 given moment, the MES should be able to answer:

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

Drift control

Phantom WIP: Units visibly missing but systemically “at Op 20.”
Rule: A “Stale WIP” auto-flush must be implemented.
Logic: When a unit remains inactive for an extended period (e.g. more than 30 days), the system should automatically flag it for a “Lost Audit.”

Final Checkout: Work order execution model

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 (or defined threshold).
Completion	Auto-Close	When Quantity equals Target, auto-trigger a “TECO” message to the ERP.