One missing screw stops the line: materials and promise dates

Thursday, an assembly cell. The order is the biggest one on the board this month: machined housings, a motor, a controller, all of it on the shelf and kitted since Tuesday. The team starts at six. At half past eight the line leader walks to the stores with an empty bin in his hand.

The bin was for a self-tapping screw. A standard part, bought by the thousand, so cheap that nobody has ever looked at its stock level on purpose. The last box went into a different order last week. There is an open purchase order for more, placed on time, with a delivery date the supplier confirmed, and the supplier’s lorry is not coming until Monday.

The promise date for the big order was calculated carefully. It allowed for the machining, the assembly, the test, and a margin. It did not allow for the screw, because a date is built from capacity, and capacity assumes the material is there.

That is the cause. A promise date in most plants is a statement about time on machines. Whether the parts exist, are in the building, and are not already spoken for by another order is a different question, held in a different system, checked by a different person, usually on the day the kit is picked. The two questions meet only when the bin comes back empty.

Why the cheapest part stops the dearest order

Stock is a number, availability is a question. The system shows forty on hand. Twelve are in a kit for another order, eight are on the rework bench with the assembly they came off, and the remaining twenty were counted in March. A date that reads the stock figure is reading a number that answers nothing about Thursday.

An open purchase order is a hope with a date on it. The planning run treats confirmed purchase orders as if the goods were already in the door. Suppliers slip, lorries miss slots, a delivery turns up short. The purchase order’s date is the supplier’s promise date, built the same optimistic way as yours, and it deserves the same scepticism.

Attention goes where the money is. Buyers chase the castings and the motors, because those are expensive and long-lead. The fasteners, labels, packaging and adhesive live on min-max levels set years ago, and a min-max level does not know that a large order is about to consume three months of usage in a week.

The shortage is discovered at the latest possible moment. Kitting happens the day before, or the morning of. By then every other order that could have used that part has already been picked, and the only options left are to stop, to borrow from another kit and move the problem, or to drive to a supplier with a credit card.

Nobody owns the collision. The buyer placed the order. The planner had the capacity. Stores counted correctly at the last count. The missing screw is not anybody’s mistake, which is exactly why nobody saw it coming.

What a date should know about materials

Availability is an input to the promise date, not a check after it. Before a date goes to a customer, the system should ask the same question the line leader asks on Thursday: for every line of the bill, is the material here, free, and enough. The answer belongs in the date, not in a separate report.

Open purchase orders carry their own real ETA. Not the date on the order, but what this supplier actually does: the last ten deliveries against their confirmed dates, and whether this one has shipped. A supplier who routinely lands three days late has a three-day-late ETA, whatever the paperwork says.

Allocation, not just stock. Forty on hand means nothing until the system knows which orders have already claimed them. Material that is reserved for a kit is not available, and the date calculation should see the difference.

Substitutes are a decision, recorded. Often there is an equivalent part on the shelf, a slightly longer screw, the same adhesive from another supplier. Whether it may be used is an engineering and quality question, and it should be answered once and held on the product, not rediscovered under pressure at half past eight.

The warning comes early and goes to a person. The useful moment is not Thursday. It is the day the big order is scheduled, when an agent that can see the bill, the stock, the allocations and the purchase orders can say: at the current plan, this part runs out two days before this order needs it, and the open purchase order will not close the gap. That message, with its reasoning, goes to the buyer and the planner while there is still time to expedite, substitute or resequence.

The person decides who gets the scarce part. When two orders want the last twenty screws, the system can show both, with their dates, their customers and their margins. Which one waits is a commercial decision, and it belongs to somebody who will answer for it, not to a rule in a planning run. The system prepares the choice; it does not make it.

A test you can run today

Take the largest order on your board for next month. Print its bill of materials, every line, including the ones nobody thinks about. For each line, write three things next to it: on hand, already allocated to other orders, and the real expected date of the next delivery if there is one, based on what that supplier actually did last time.

Then find the first line where on hand minus allocated is less than the order needs, and the delivery is not certain to arrive first. If there is no such line, your date is standing on something. If there is one, you have found the screw weeks before the empty bin would have, and you have also found how long that exercise takes by hand.

MESCATOR builds the promise date from the works order, with materials, allocations and purchase orders read alongside the real throughput of the line, and raises the shortage to a buyer and a planner as a draft they decide on. It reads from the ERP and the floor; it writes to no machine and places no order on its own. The companion question of what the plan cannot see on the machine side is in the changeover the plan cannot see, and the case for dates built from real data rather than standards is in a promise date from standard times. More on the MESCATOR homepage.