12.5 Sequencing Rules, Lot Splitting, and Overlapped Scheduling

Time Standards and Priority Control

Every technique in this section runs on two numbers from the routing. The standard setup time is the hours needed to change a resource over, independent of order quantity; the standard run time is hours per piece. Together these time standards convert an order quantity into standard hours, which is what lets you load a work center, offset a lead time, and schedule an operation. A stale standard does not cause one small error - it corrupts loading, sequencing, capacity reporting, and labor variance at the same time, which is why ECM Domain VII lists time standards as a scheduling tool and not an accounting detail.

The ECM separates setting priority from priority control, and so should you. Setting priority is a planning act: material requirements planning (MRP) and the master schedule give each order a due date, and operation due dates fall out of the lead-time offsets in the routing. Priority control is an execution act - the ongoing process of communicating current start and completion dates to the floor and keeping those dates valid as demand and supply move. MRP produces a due date, the customer then pulls the order in two weeks, and unless somebody acts on the reschedule-in message the shop keeps working to a date nobody needs.

That is the ceiling on any dispatch list. A dispatch list is only as good as the priority data behind it: if operation due dates are not maintained, the list ranks jobs precisely against stale information, and supervisors quietly abandon it for an informal hot list. The failure sequence on the exam is always the same - priorities not maintained, dispatch list ignored, informal expediting, due-date performance collapses even though capacity was adequate.

What a dispatch list contains and how often it is issued

A dispatch list is published by work center, typically daily or by shift, over a very short horizon. It normally carries:

• Order or job number, item number, and operation number
• Order quantity and quantity remaining at that operation
• Standard setup hours and standard run hours for the operation
• Priority sequence and the operation due date
• Jobs in queue now, plus jobs scheduled to arrive in the next day or two so the supervisor can group setups ahead

A list issued weekly is a work list, not a dispatch list: by Wednesday it describes a plant that no longer exists.

Sequence-Dependent Setup and the Setup Matrix

The priority rules taught in section 12.3 - first come first served, shortest processing time, earliest due date, and critical ratio - all treat setup as a fixed cost per job. On a paint line, plating tank, extruder, or reactor requiring cleanout, that assumption is false: setup time depends on which job ran immediately before. This is a sequence-dependent setup, and it is documented in a setup matrix giving changeover hours from each family to each other family.

A paint line runs four colours this week:

Arrival order puts the jobs on the floor as black, red, white, yellow. Changeovers: black to red 2.4 + red to white 2.5 + white to yellow 0.5 = 5.4 hours.

Resequencing light-to-dark gives white, yellow, red, black: 0.5 + 0.6 + 0.7 = 1.8 hours.

Same four jobs, same run time, 3.6 hours of capacity recovered - a 67% cut in changeover hours - purely from sequence. Identical logic applies to tooling families (all parts sharing one die), fixtures, temperature ramps, and allergen or grade sequencing in food and chemical plants.

The trap: setup-family sequencing is a capacity tool and due dates are a priority tool. A pure colour wheel will happily leave a job due tomorrow at the back because it is white. Mature practice sequences by setup family within a due-date bucket - group setups among the jobs due this week, not across the entire queue.

Lot Splitting

Lot splitting divides an order into two or more sublots so part of the quantity moves ahead while the rest waits or runs later. Use it when the downstream operation or the customer needs a partial quantity now, when a gap in a resource's schedule can be filled, or when a quality hold traps part of a lot.

Take a 600-piece order at 0.05 hours per piece with a 1.0-hour setup: 1.0 + 30.0 = 31.0 hours before anything ships. Split it 200/400 and the first 200 are complete after 1.0 + 10.0 = 11.0 hours. The cost is explicit: the second sublot needs its own setup, so total resource hours rise from 31.0 to 32.0 and the full order finishes later than it otherwise would. Lot splitting buys time-to-first-good-piece with capacity - never split at the constraint without checking that trade.

Overlapped Scheduling

Overlapped scheduling (operation overlapping, or send-ahead) starts the downstream operation before the whole lot finishes upstream, moving a transfer quantity as soon as it is ready.

A 100-piece lot runs at two operations. Operation 10: 1.0-hour setup, 0.10 hours per piece. Operation 20: 0.5-hour setup, 0.06 hours per piece. Move time is 0.5 hours; ignore queue.

Sequential: 1.0 + (100 x 0.10) + 0.5 move + 0.5 setup + (100 x 0.06) = 1.0 + 10.0 + 0.5 + 0.5 + 6.0 = 18.0 hours.

Overlapped, transfer quantity 20: operation 10 finishes its first 20 pieces at 1.0 + 2.0 = 3.0 hours; they arrive at operation 20 at 3.5 and its setup completes at 4.0. Operation 20 is the faster process (0.06 against 0.10), so it runs dry between transfers and completion is governed by the last transfer batch. Operation 10 finishes all 100 at 1.0 + 10.0 = 11.0 hours; move to 11.5; run the final 20 at 20 x 0.06 = 1.2 hours. Order complete at 12.7 hours - a 5.3-hour, 29% lead-time reduction.

The cost is real and examinable. You made five moves instead of one, so material handling, move transactions, and work-in-process tracking all multiply. Operation 20 is now occupied from hour 3.5 to hour 12.7 - 9.2 hours of resource time to deliver 6.0 hours of run - because it waits for feed. Overlapping through a constraint therefore costs throughput; overlap into a non-constraint that has protective capacity to spare.

Hold the distinction: lot splitting adds a setup and splits the order; overlapping adds moves and keeps one setup per operation.

Alternate Operations and Routings

An alternate routing produces the same item through a different sequence of work centers; an alternate operation substitutes a single step. Reach for either when the primary resource is down, overloaded, or has lost tooling and the due date will not survive the wait.

Three conditions come first. The alternate must be qualified for the part (quality approval, tooling, program, operator certification); it must carry its own time standards so the schedule and the cost are right; and it must actually have free capacity. The penalty is almost always higher cost - longer run times, extra setup, lower yield, or an outside-processing charge - and the overload does not disappear, it moves. If the alternate feeds your constraint, you have saved one due date and created a worse problem.

Sequencing Rules Versus Priority Rules

Sequencing rules are the whole family of methods for deciding the order in which queued jobs run at a resource. Priority rules - first come first served, shortest processing time, earliest due date, and critical ratio, all worked numerically in section 12.3 - are the subset that ranks jobs by due date or processing time. The rest of the family is what this section adds: setup-matrix and setup-family sequencing, lot splitting, overlapping, and alternate routings.

Read every exam stem for what is being optimised. Average flow time, lateness, and number of late jobs point at the priority rules. Changeover hours, time to first good piece, and lead-time compression point at the sequencing and lot-sizing techniques taught here.