At a Polish plant, the bottleneck has become a horizontal boring mill with no way to connect to its controller. The plant has to produce more, but there is nobody to staff an extra shift: in Podkarpackie, the region where it is, machine tool operators are a shortage occupation in the forecast for 2026. So more has to come from the boring mill the plant already has.
The production manager knows the mill often stands idle, but not for how long or why. The causes vary: the crane is busy, cutting inserts have to be fetched from the store, or the part is measured again and again. The ERP has a downtime form, but the operator fills it in by hand. When nobody does, the log stays empty and the causes of the losses remain hidden.
This post compares three methods of machine downtime tracking on machines without a PLC: a current sensor, a stack light signal and a camera. It covers what each one records and how the log of stoppages feeds availability, OEE and the cost of downtime.
Why downtime records matter
Availability rests on downtime: operating time is loading time minus downtime. Without a record of stops, availability and OEE are only estimates. A machine with no PLC records nothing by itself, and hand-written records are often incomplete or inaccurate. That is the case in the plant from the story: the downtime tables in the ERP are empty or incomplete, so it is hard to choose which line improvements to make, measure their effect and estimate the savings.
Three methods for recording running and idle time
1. Current sensor on the power supply
A clamp-on current transformer goes around the machine's power cable. The current draw gives three states: high draw means the machine is running under load, low draw means it is switched on but not producing, and no draw means it is off. Fitting it needs no change to the controller and no machine downtime. The sensor shows whether the machine runs, but not why it stopped: the operator has to enter the reason.
2. Stack light signal
If the machine has a stack light, an extra transmitter can be fitted into it. The transmitter reads the state of the lights and sends it wirelessly to software that shows which machines are running, idle or in fault, whatever their make. The reasons for stops, such as a material shortage, still have to be defined and commented by hand.
3. Camera
A camera aimed at the machine sees its moving parts, so it records running and idle time with nothing connected to the machine. Each stop is kept on video. The same image shows where the moving parts are and what the analogue gauges and indicator lights read. It can be a camera already in the hall, if the machine is in its view.
Choosing a method
The table sets the three methods side by side: what they record, what they need and whether they show the reason for a stop.
| No. | Method | What it records | What it needs | Reason for a stop |
|---|---|---|---|---|
| 1 | Current sensor | Running under load, switched on but not producing, switched off | A current transformer on the power cable; no change to the controller and no machine downtime | Not shown; the operator enters it |
| 2 | Stack light | Running, idle and fault, as the lights show them | A stack light on the machine and a transmitter fitted into it | Not shown; defined and commented by hand |
| 3 | Camera | Running and idle time, movement of the machine's parts, gauge and indicator light readings | A camera that sees the machine, including one already installed; no wiring into the machine | The video shows what happened during each stop |
If only running and idle time is needed, a current sensor or a stack light is enough. If the reason for each stop is needed as well, and the operator does not enter it, only the camera of the three records the stop itself.
What a camera shows at the boring mill
At the plant in the story, a camera aimed at the boring mill records when it runs and when it stands idle. Each stop goes into a log with its time, duration and a video clip. The clip shows what the mill was waiting for: the crane, cutting inserts from the store, or a measurement of the part.
From the log of stoppages to the cost of downtime
A log of stoppages does not yet say what the downtime costs. Two figures are needed: how much time the machine lost, and what one hour of the machine costs.
The time comes from the log. The OEE calculator turns shift length, planned breaks, stops and part counts into availability, performance, quality and OEE. Its last step prices the lost time: the cost of losses per shift and per year, and the good parts the machine did not make in that time.
The hourly cost comes from the machine hour rate calculator. It works out what one hour of a machine costs whether it runs or not: depreciation, interest, insurance, floor space, operators and overheads, divided by the planned hours in a year. It leaves out costs that grow with running hours: energy, repairs and tools. The OEE calculator takes its result as the hourly cost.
For the boring mill in the story, that means the downtime from the camera's log, the hourly cost from the machine hour rate calculator, and the cost of downtime per shift and per year from the OEE calculator.
For machines without a data interface, our machine monitoring provides running and idle time for each shift and a log of stoppages with time, duration and a video clip.
Sources
- Muchiri, P., Pintelon, L. (2008). Performance measurement using overall equipment effectiveness (OEE): literature review and practical application discussion. International Journal of Production Research, 46(13), 3517-3535. https://doi.org/10.1080/00207540601142645
- Lahri, V., Pathak, P. (2015). A case study of implementation of overall equipment effectiveness on CNC table type boring & milling machine of a heavy machinery manufacturing industry. IOSR Journal of Mechanical and Civil Engineering, 12(5), 63-70. https://doi.org/10.9790/1684-12556370
- Peakboard (2026). How to Monitor Older Machines Without a PLC. https://www.peakboard.com/en/blog/how-to-monitor-older-machines
- WERMA Signaltechnik. SmartMONITOR (brochure). https://www.wolfautomation.com/media/pub/media/pdf/comm/werma-smartmonitor-brochure.pdf
- Voivodeship Labour Office in Rzeszów, Ministry of Family, Labour and Social Policy (2025). Barometr zawodów 2026 (occupational barometer), województwo podkarpackie. https://barometrzawodow.pl/forecast-card-zip/2026/poster_pl/BAROMETR_ZAWODOW_wojewodztwo_podkarpackie_2026.pdf