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”—unaccounted-for inventory that exists physically on the floor but is not accounted for in the financial system.

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 rely on operator adherence, while digital travelers enforce 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.

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 an attempt is made to merge Revision A 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.”

Recap: Work Order State Management and Control Actions

State / Condition	Requirement / Constraint	Action / Control	Owner / System
Created	Execution blocked; invisible to shop floor.	No transactional movement permitted.	ERP / Planner
Released	BOM & Route Revision locked; materials allocated.	MES enforces locked revision; authorizes production start.	Production Manager
Running	WIP Time clock starts at first operation scan-in.	Track cycle time; units follow enforced routing.	MES / Operator
Hold	All transactional movement (Move/Consume) blocked.	Block scans until hold is formally released.	Quality / Engineering
Closed (TECO)	Target quantity reached or order short-closed.	Release unused material; post final Good Receipt; calculate variance.	ERP
Routing Enforcement	Unit cannot enter Operation N until Operation N-1 is complete/passed.	MES rejects scan with “Invalid Process Step” flag on sequence violation.	MES
Resource Binding	WO must be bound to a Resource Group (capability), not a specific Machine ID.	Floor lead can balance line within group without IT intervention.	MES / Floor Lead
Split (Fork)	Partial movement for rework/scrap while maintaining cost traceability.	Create Child WO for split quantity; Child inherits Parent’s accumulated cost.	MES
Merge (Join)	Merge only permitted for identical Part Number and Revision.	System actively blocks merge transaction for mixed revisions/models.	MES
Quantity Cap	Max_Qty = Ordered_Qty + Yield_Buffer (e.g., 2%).	System disables “Start” button and rejects new unit scans when Max_Qty is reached.	MES
Stale WIP	Unit inactive for extended period (e.g., >30 days).	System automatically flags unit for “Lost Audit”.	MES