One Fixture, Two Mills: How the Schedule Resolves the Clash

When two jobs on two different machines need the same fixture, they draw from one shared pool of fixture hours, so the first job takes what it needs and the second takes what is left, with the remainder spilling to the next day. Both machines can show free hours the entire time. The fixture is what ran out. EDGEBIC by User Solutions resolves the clash by moving work, never by inventing tooling that does not exist.

This is the headline case for the tool constraint, and it is worth walking end to end with numbers, because the resolution is not intuitive the first time you see it.

The setup

Two work centers, one machine each, both on a single day shift running 08:00 to 16:00, Monday through Friday. One fixture, quantity 1. Two jobs, each five hours of work.

Monday offers sixteen machine-hours across the two mills, and there are ten hours of work. Machine capacity is not remotely tight.

Before the fixture is modeled: the deceptive plan

With no tool requirement on either step, the two jobs are independent. Each finds its own machine free from 08:00 and takes it.

• JOB-0141 on CNC-Mill-1: Monday 08:00 to 13:00
• JOB-0142 on CNC-Mill-2: Monday 08:00 to 13:00

Both bars start together, both machines sit idle all afternoon, and the plan looks comfortable. Every capacity number in it is arithmetically correct.

On the floor, the second job never starts. The fixture is across the aisle, bolted to the other mill, and it will be there until 13:00. The plan was fiction from the moment it was printed, and nothing on the Gantt could have told you.

After: one pool, and the handover

Create the fixture as a tool with a quantity of 1, set Required tool: on both routing steps, and reschedule. The fixture now has a pool for each day and shift: quantity multiplied by the window's working hours. Monday's day shift is eight hours long, so Monday's pool is eight fixture-hours, and it is shared by both mills.

Every allocated machine-hour books exactly one fixture-hour. Watch the pool drain:

Monday day shift, FIX-EM47 pool = 1 unit x 8 hours = 8 fixture-hours

JOB-0141 on CNC-Mill-1
  pool 8h available, needs 5h
  books 5 fixture-hours              pool remaining: 3h

JOB-0142 on CNC-Mill-2
  pool 3h available, needs 5h
  books 3 fixture-hours              pool remaining: 0h
  2h still required                  spills forward

Tuesday day shift, pool resets to 8 fixture-hours
JOB-0142 books its final 2 hours     pool remaining: 6h

The resulting plan:

• JOB-0141 on CNC-Mill-1: Monday 08:00 to 13:00
• JOB-0142 on CNC-Mill-2: Monday 13:00 to 16:00, then Tuesday 08:00 to 10:00

The fixture is fully consumed on Monday, five hours to the first job and three to the second, and the second job's last two hours land Tuesday morning.

Read the plan carefully: what actually changed

Three things are worth noticing, because each one is a common misreading.

Nothing got busier. No machine gained work, no shift was shortened, no calendar changed. The only new information in the model is that the fixture exists and both steps need it.

Total hours are identical. JOB-0142 is still five hours: three on Monday, two on Tuesday. The constraint moved work; it did not create or destroy any. If a tool change ever alters a job's total hours, something else changed at the same time.

CNC-Mill-2 is idle Monday morning while its job waits. This is the part planners push back on, and it is the correct answer. The machine is genuinely free. The operation still cannot run, because the thing it needs is on the other machine. A plan that showed both jobs starting at 08:00 was hiding exactly this.

The handover happens at 13:00, and the plan says so

The most useful way to describe the after plan to a supervisor is as a handover. At 13:00 Monday the fixture comes off CNC-Mill-1 and goes onto CNC-Mill-2. That sentence is a physical instruction someone can act on, and it is what the two adjacent bars mean.

The before plan had no such moment in it, which is why the fixture handover used to be a conversation that happened at the machine, at 08:05, with two operators and one fixture.

Reschedule keeps the handover honest

There is a second-order behavior worth knowing. If JOB-0141 is already scheduled and not yet started, and you run a reschedule that brings in JOB-0142, the surviving job re-reserves its fixture hours before any new work is placed. The new job therefore sees a Monday pool of three hours, not eight, and lands exactly where it did in the walkthrough above.

Without that step a reschedule would happily plan new work straight over hours the existing plan is still holding, which is the most insidious way a tooling model can quietly stop being true. How surviving work is preserved generally is covered in how a job resumes from where actuals left off.

The three levers on this outcome

If the Tuesday spill is unacceptable, there are exactly three honest moves.

1. Raise the quantity, if you genuinely own or can free a second unit. The pool doubles to sixteen hours and both jobs finish Monday at 13:00, which is the before plan made true rather than assumed.
2. Resequence, so the job that most needs an uninterrupted block takes the fixture first.
3. Accept it, and let the plan carry a date the floor can hit.

What is not a lever is entering a quantity you do not own. That removes the split from the schedule and from nowhere else. Quantity and what a second unit really buys is the subject of what changes when you own a second fixture.

Over 35-plus years, User Solutions has seen this exact clash cost shops a shift a week before anyone named it. The rest of the placement logic is in the scheduling engine guide, and you can run your own before-and-after on a shared fixture in EDGEBIC.