What is stack light monitoring?
Stack light monitoring means reading the coloured lamps on top of a machine with a small device, and recording what they show. The lamps tell you if the machine is running or stopped. The device keeps a time stamped record, so you see status without walking the floor.
By Lalit FauzdarPublished: Last updated:
What is a stack light?
A stack light is a vertical tower of coloured lamps fitted on a machine to show its status at a glance. It is also called a tower light or a signal tower.
Most stacks have two to five lamps, and often a buzzer. The idea is simple. A supervisor standing at the end of the shed can see which machines need attention without going to each one.
You may also hear the word andon. An andon is a visual signal or board that calls attention to a problem on the line. A stack light is the most common andon on an Indian shop floor.
What do red, amber and green mean?
On many machines green means running, amber means waiting or warning, and red means stopped or fault. This is common practice and not a fixed rule.
| Lamp | Common meaning | Can differ to |
|---|---|---|
| Green | Running in cycle | Machine ready but not cycling |
| Amber | Waiting, warning, cycle finished | Setup in progress, tool life warning |
| Red | Stopped, alarm, fault | Emergency stop only |
Meanings change between machine makers, and sometimes between two machines of the same make after a repair. That is why the meaning of each lamp is agreed per machine before any data is trusted. Do not assume.
How does a device read the lamps?
The device senses whether each lamp is on or off, from outside the stack. It records the pattern with a time stamp.
LiveMach has a built in profile called stack_light_3 for a three lamp stack. The profile tells the device how to read three lamp signals and how to record them.
The device is fitted outside the machine. Nothing is cut or rewired inside, and it is electrically isolated from the machine. If the internet drops, it keeps logging and syncs when the connection returns.
How the lamps are wired differs from machine to machine, so the exact way a device reads a given stack is set up per machine at the pilot.
How do lamp signals become running, stopped, idle and off?
Each lamp pattern is mapped to one of four machine states. The mapping is agreed per machine, then applied to every reading.
An example of a mapping (agreed per machine, yours may differ)
| Lamp pattern | Machine state |
|---|---|
| Green on | Running |
| Red on | Stopped |
| Amber on | Idle |
| No lamp on, or no power | Off |
Once the mapping is set, the dashboard shows the live state of each machine. The time spent in each state is added up into run time, downtime and shift reports.
How do you start with stack light monitoring on your floor?
Start with one machine that has a working stack light, and confirm what each lamp means there before you add more.
- 1Walk to the machine and note which lamps are fitted, and which ones light up when the machine is cutting, waiting, in alarm and switched off.
- 2Ask the operator and the maintenance team to confirm what each lamp means, and write it down.
- 3Agree the mapping to the four machine states with us at the pilot.
- 4Compare the dashboard with what you see on the machine for a few shifts.
- 5Repair any dead or bypassed lamp before you trust the numbers.
Only then add the next machine. Machines of the same make often share a mapping, but check each one.
What can stack light monitoring not tell you?
A lamp shows status. It does not show the reason for a stop or how many pieces were made.
- No reason. Red says stopped, not whether it was a tool break or a wait for material. The optional downtime reasons module lets an operator add it.
- No piece count. A green lamp does not say how many parts came out.
- Wiring varies. Lamps wired by the machine maker may follow their own logic, and a lamp can stay lit in cases where the machine is not cutting.
- Lamps can be broken or bypassed. A dead bulb or a stack that someone disabled gives a wrong picture.
What if the machine has no stack light?
Use a current clamp instead. It senses the current the machine draws and reads state from that.
A clamp sits around a power cable, so nothing is opened. A running machine draws a different current from one that is switched on and idle, and the device uses that difference.
The same signal model supports other sensor combinations too. Which one suits a machine is decided at the survey, machine by machine.
For a step by step view on existing CNC machines, read the retrofit guide. How to retrofit old CNC machines