What is MES? The Layer That Illuminates the Dark Side of Work Orders
Yesterday we talked about what ERP cannot do. It is powerful for finance, sales, procurement, and inventory, but it cannot see what happens inside the shop floor. When a work order goes to the shop floor, who records what happens there?
The answer is MES.
MES (Manufacturing Execution System) is the layer that captures every second, every stoppage, and every scrap loss in real time from the moment a work order reaches the shop floor until it closes. It illuminates the “dark area” that ERP cannot see.
The Real Face of the Shop Floor
A production manager’s morning starts like this: “Last night the customer ordered 200 units of Product B. Deadline is 3 days. Let’s open a work order immediately.”
The work order is created. It goes to the shop floor. And from this point on, ERP’s eye closes. Because ERP estimates the time a work order spends on the shop floor using “standard time.” It does not know the actual time.
MES’s eye opens exactly at this point.
What Data Does MES Capture?
Let’s follow a work order’s journey on the shop floor:
08:00 — The work order arrives at the shop floor. The operator picks up the card. But the machine is finishing another job. The operator waits. MES records this as “queue time.”
08:30 — The machine becomes available. The operator changes the die, adjusts the machine, and runs a trial. The first part comes out wrong; readjustments are made. 45 minutes pass. MES records this as “setup time.”
09:15 — Series production begins. The machine produces 40 parts per hour. The operator inspects the parts.
10:30 — The machine suddenly stops. A tool has broken. Maintenance is called. 40 minutes of repair. The operator waits. MES records this as “downtime / tool breakage.”
11:10 — The machine runs again. But the operator notices the material is running low. 20 minutes of waiting until new material arrives. MES records this as “standby / material shortage.”
11:30 — Material arrives; production continues.
13:00 — The work order closes. Target was 200 units; 185 units passed; 15 units are scrap. MES records this as “scrap.”
According to ERP, this work order should have taken 5 hours. Actual production time is 3 hours 45 minutes. But total elapsed time is 5 hours. Where is the difference? Setup, downtime, standby. ERP either does not account for these at all or fills them with averages. MES records each one separately.
What Happens Without MES?
In a shop floor without MES, the operator finishes the job, drops the card at the shop office, and says “done.” But:
- How long did it really take? Unknown. Stuck in the supervisor’s memory.
- Why did it take that long? Unknown.
- How long did each machine stop? Unknown.
- What is the scrap rate? Left to estimates.
The result: when a similar job comes again, estimates are wrong again. Costs spiral out of control. The customer asks, “Why were you late?” There is no concrete answer.
What Changes with MES?
MES makes the shop floor visible:
- The actual time for every work order is known.
- Reasons for stoppages are recorded: breakdown, material shortage, operator waiting.
- Scrap rates are visible in real time.
- The real capacity of machines emerges.
- Operator performance can be measured.
- The gap between standard cost and actual cost can be analyzed.
A shop floor manager can look at the screen at lunch and see: “Machine 3 was down for 2 hours today. Cause: tool breakage. Is the tool at end of life, or did the operator misuse it?” Without MES, these questions can only be answered by guesswork.
The Limit of MES
MES captures shop floor operations. But MES has a limit: it is only the “execution” layer. If the work order design is wrong, the recipe is incomplete, the route is not defined, MES does not fix it. It only records what happened.
In demand-driven manufacturing, the alignment between work order design and execution is critical. If the recipe changes but the shop floor does not know, MES says “scrap rate is high” but does not say whose fault it is. When a quality issue arises, should we rework, or issue a credit to the customer? These decisions require coordination beyond MES.
This is why MES alone is not enough. An orchestration layer must be built on top of it. This layer is called MOM (Manufacturing Operations Management).
Tomorrow we will discuss MOM.
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