What SMED is
SMED stands for Single-Minute Exchange of Die: a changeover in under ten minutes, that is, a single-digit number of minutes. The method was developed in Japan by Shigeo Shingo and is one of the pillars of lean manufacturing.
The core idea is simple: split the changeover into internal tasks, which can only be done with the machine stopped, and external tasks, which can be done while the machine is still running the previous batch. Every task moved from internal to external is machine time gained.
Why it concerns the tool room
Watch a changeover on a machining centre and count the time spent on tools: finding them, mounting them in the holder, measuring them at the machine, entering offsets, finding out one is missing. Nearly all of it can be done beforehand, off the machine.
That is exactly the tool room's work. SMED is not a production project the tool room takes part in: for the tooling side, the tool room is SMED.
Step 1 · Watch a real changeover
Start from a real changeover, not from how it should go. Film it or follow it with a stopwatch and write down every task with its duration, including walking, waiting and searching. That is usually where the surprise is: the time is not eaten by the hard operations, but by the ones nobody counts as work.
Step 2 · Separate internal and external
For every task ask one question: does the machine really need to be stopped? Fetching the tools, no. Mounting the end mill in the holder, no. Measuring it, not if there is a presetting bench. This separation alone, without changing anything else, shortens the changeover: just do beforehand what used to be done during.
Step 3 · Move it off the machine
- Presetting: tools measured at the bench, with length and radius ready to load into the control instead of being taken at the machine.
- Pre-mounted assemblies: tool and holder prepared and checked in advance.
- Next-job cart: everything needed, and only that, ready next to the machine before the batch ends.
- Checked tool list: whoever prepares checks the list against the program, so nobody finds a missing tool with the machine stopped.
Step 4 · Streamline what is left
Some tasks stay internal by nature: unloading and loading the machine, the first part, the inspection. These can be shortened too:
- Standard holders: fewer different interfaces, fewer wrenches to look for.
- Sister tools: the same tool left in the machine for several jobs is never changed at all.
- Readable labels and codes: the right tool goes in first time.
- Transferred offsets instead of handwritten ones, where presetter and control allow it.
An invented example
A machining centre, a batch change with twelve tools, eight of which must be swapped. Before: the operator stops the machine, walks to the tool room, mounts the eight tools and measures them with the probe in the machine. After SMED: the eight tools arrive on a cart, already mounted and measured at the bench, with the values ready. With the machine stopped, all that is left is unloading the old ones, loading the new ones and entering the offsets.
The gain is not a faster operation: it is what is no longer done with the machine stopped. Multiplied by every changeover in a year, it is nearly always the biggest item.
The most common mistakes
- Buying a presetter and stopping there: if measured tools still wait in a cabinet, the changeover does not change.
- Preparing from memory: without a list tied to the program, sooner or later a tool is missing just when the machine is stopped.
- Measuring the changeover once: SMED only holds if changeover time keeps being watched.
SMED and Kaizen
After the changeover, the next step is improving what happens while the machine runs: Kaizen in the tool room.
SMED and Kaizen work together: SMED shortens the changeover once, Kaizen keeps shortening it. To see what all this is worth in money, there is the guide Why a tool room, with the calculator.