If you check only one thing on a used CNC machine, check the spindle. It is the heart of the machine — it holds the tool and drives the cut — and it is the single most expensive thing to get wrong. A spindle rebuild runs $8,000–$30,000, enough to wipe out the savings on a used purchase and then some. The good news: a structured inspection takes fifteen minutes and tells you almost everything.
Why the spindle matters most
Every other worn component on a CNC — ways, ballscrews, tooling — is repairable at manageable cost. A worn spindle is different: it degrades your accuracy on every part and its rebuild cost can dwarf the price of the machine. Inspect it first, and let what you find set the price.
Step 1: Listen through the RPM range
Start the spindle and run it slowly up through its full RPM range, then back down. Listen for bearing growl, whine, rumble or vibration that changes with speed — healthy spindle bearings run smooth and quiet. Any grinding, knocking or a rising vibration as RPM climbs points to worn bearings and a rebuild in your future.