What do spindle hours really tell me?

How long the spindle ran, not how it was treated. Check runout, noise, heat and rebuild records too.

Spindle hours tell you how long the spindle has run. They don’t tell you how it was treated. Use them as one clue among several, not a verdict.

What the number hides

Light finishing work versus heavy roughing
Clean, filtered coolant versus contaminated coolant
Proper warm-ups versus cold starts at full speed
Crashes, which can damage bearings in seconds
Whether the spindle has already been rebuilt

Other numbers to note

Many controls also show power-on time and cutting time. A machine with high power-on hours and low spindle hours sat idle a lot, which brings its own issues with seals and lubrication.

What to check alongside

Runout on a test bar
Noise across the speed range after warm-up
Temperature after a run
Invoices showing any rebuild

A rebuilt spindle with records can be better than an original one with an unknown past.