Flow and Schedule

Our High-Mix Shop Keeps Missing Dates

A high-mix shop is one of the hardest places to schedule. Every part takes its own path through shared machines, setups eat capacity, and the bottleneck moves with the mix. Expediting the late job fixes today and makes tomorrow worse.

What Is Usually Going On

  • Load is not checked against machine hours before work is released.
  • Setups are not counted as capacity, so setup-heavy machines are overloaded on paper and on the floor.
  • Jobs run in arrival order or by who asks loudest, not by due date at the constraint.
  • Expediting breaks into running setups and loses more hours.
  • Quoted lead times are fixed numbers that ignore the queue.

How to Fix It

  1. Load every machineFor each machine: sum of (demand x run time) plus setup hours for the period. Compare to available hours by shift.
  2. Find the overloaded onesA machine loaded past its available hours will miss dates. That is a capacity call: add a shift, use an alternate routing, outsource or move the date.
  3. Release to the constraintRelease jobs only as fast as the most loaded machine can take them. Hold the rest at the door, not on the floor.
  4. Sequence at the constraintSet the order by due date at the constraint machine and let the others follow. Group like setups where dates allow.
  5. Cut setups where they blockRun SMED on machines where setup is a large share of the load.
  6. Quote from the queueQuoted lead time = work ahead of the job / throughput, plus the job's own run time. Update it weekly.

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Common Questions

Should we buy another machine?

Load the shop with setups first. If one machine is over, see what alternate routings, a second shift or setup reduction give back. Buy when the hours still do not fit.

Why does expediting make it worse?

Breaking into a setup to run a hot job adds a second setup and pushes the whole queue. The constraint loses hours it cannot get back.

How do I schedule so many part numbers?

Schedule the constraint, not every machine. Set the sequence there and run the rest first in, first out under a WIP cap.

What lead time should we quote?

Work already in the queue divided by what the shop actually finishes per day, plus the job's run time. Measure it; do not use a fixed number.