Infinite capacity scheduling answers a narrow question: if this machine were free, when would the job finish? Finite capacity scheduling answers the question a customer actually hears: when is there an open slot?
Most planning systems still do the first one, then print the date. The overload shows up later, on a load report nobody quoted.
What infinite capacity does
An infinite scheduler takes the routing — setup, run, and move — and lays the operation on the work-center calendar. It does not look at the other work already sitting there. Ten jobs can be planned on one mill in the same shift. The calendar knows the shifts. It does not know the queue.
That is useful. It is how material requirements planning has worked for decades, and it is the right tool when you want to see unconstrained demand. A load report that climbs past the day is the point. The report is a signal that the date is not real yet.
It is a bad promise. The date assumes the resource was waiting. On a busy cell, it was not.
What finite capacity does
A finite scheduler refuses to load a resource past the hours it actually has. Those hours are the shift pattern, minus breaks, times the efficiency you are willing to stand behind, and only for the crew and tools that will really be there. When the slot is full, the operation moves to the next open hour. Every downstream operation moves with it.
The promise date gets longer. That is the schedule telling the truth.
Finite does not mean every work center. A heat-treat oven, a paint line, a five-axis cell, and the one fixture you own twice are constraints. A bench with spare hours is not. Scheduling every resource as finite turns the plan into a puzzle the planner will stop trusting. Schedule the constraints finite. Let the rest stay infinite, and watch their load.
The same three jobs
A mill cell runs two shifts, 16 hours on Friday. Three jobs are due Friday. Each is 8 standard hours: a housing, a bracket, and a shaft.
The infinite schedule places all three on Friday. Planned load is 24 hours. The promise is Friday for all three. Eight of those hours have no machine.
The finite schedule fills Friday with the housing and the bracket. The shaft moves to Monday, and Monday still has 8 hours open. The promise is Friday for two jobs and Monday for the third.
Where the extra hours go
Scale that up to a week. The same cell has 16 hours available each weekday, and 94 hours of released work. On an infinite schedule the load is:
- Monday, 14 hours
- Tuesday, 16 hours
- Wednesday, 24 hours
- Thursday, 22 hours
- Friday, 18 hours
Sixteen hours sit above the line: 8 on Wednesday, 6 on Thursday, and 2 on Friday. Monday is under the line, and that slack cannot absorb the overflow if those jobs cannot start early.
A finite pass keeps Monday at 14 and holds Tuesday through Friday at 16. The 16 hours with no machine move to the following Monday.
What to run each method on
- Where will demand exceed the building? Infinite, then read the load.
- What date can we tell the customer? Finite, on the constraint.
- Can we quote a new job without reshuffling the floor? Finite, against the current load.
- What material do we need, and when? Infinite MRP is enough for most parts.
- Which order should run next on the constraint? Finite, with setup family and due date.
Rough-cut capacity planning sits with the infinite side. It checks a few key resources in weekly buckets before you promise a program. It will not sequence Thursday afternoon.
Where a finite schedule still lies
Finite is only as honest as the capacity model.
- A calendar that still shows a shift you dropped in March will sell hours you do not have.
- Run standards that ignore setup will pack a day that dies at the second changeover.
- One crew covering two machines is one resource. Modeling them as two independent centers double-counts the people.
- An efficiency of 100 percent is a wish. Use the number the cell has actually held.
- Alternate routes that exist on paper and not on the floor will clear an overload by sending work somewhere it cannot go.
If those are wrong, a finite date is an infinite date with extra steps. Fix the constraint data before you ask the software for a promise.
What to tell the customer
Quote the finite date. Keep the infinite date in the planner's load review, where an overload is a useful alarm and not a commitment.
When sales is holding a Friday the board cannot build, the conversation is not about software. It is about which of the three jobs moves, and who calls the customer. The finite schedule makes that choice visible on Thursday, instead of on the dock on Friday night.
