Unplanned downtime rarely arrives without warning. Machines almost always signal trouble first — the signals are just easy to normalise when you are busy.
1. Surface finish drifts on a proven programme
A programme that produced an acceptable finish last month and does not today has told you something changed, and it is not the programme. Look at spindle bearings, tool holding and way wear before editing feeds.
2. The spindle sounds different at the same RPM
Operators know their machines by ear. When someone says a spindle "sounds off", that is data. Log it, then check vibration and temperature at a fixed speed so you have a number to trend rather than an opinion.
3. Positioning repeatability has widened
Cut a test part, measure the same feature across ten pieces, and record the spread. If that spread is wider than it was six months ago, backlash or ballscrew wear is developing. Catching it here is a service call; ignoring it is a rebuild.
4. Coolant is dirty, low or the wrong concentration
Coolant is treated as a consumable and behaves like a system component. Poor concentration reduces tool life, corrodes slideways and turns into a genuine hygiene problem for the operators standing over it.
5. Chips and swarf are not clearing
Recutting chips destroys edges and finish, and packed swarf traps heat exactly where you do not want it. If the conveyor or flush is not keeping up, that is a maintenance issue, not an operator issue.
Build the smallest schedule you will actually follow
A one-page daily checklist that gets completed beats a detailed plan that gets skipped. Start with levels, pressures, filters and a listening check. Add to it only when the basics are habitual.