What To Look For When Buying Used CNC Machines

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  Key takeaways
• 2
  Used machine condition: what is acceptable
• 3
  Start with the spindle, not the paint
• 4
  Ball screws, guides and the backlash test
• 5
  Geometry, leveling and the foundation question
• 6
  Controller, drives and the support horizon
• 7
  Envelope, tooling and what you actually need
• 8
  Step by step: how to inspect a used machine

The spindle is the most expensive single component to replace, and it is the one most sellers hide. Pull a test bar or a ground pin into a clean holder and indicate it at 50 mm and 300 mm from the gauge line. On a healthy 40-taper spindle you want radial runout at or below 0.005 mm, and taper contact above 80% on bluing. Anything past 0.015 mm means a grind, a rebuild, or a new cartridge.

Taper condition matters more than runout in some shops. Look into the taper with a light. Fretting, a ring of dark polish near the gauge line, or a score mark from a spun tool all point to tool pull-out events. A re-grind removes 0.1-0.2 mm of material and shortens the holder's axial position, so you must re-set every tool offset after the repair.

Run the spindle through its speed range and listen. A faint whine at 12,000 rpm is normal on a greased bearing pack. A rhythmic knock, a squeal that appears only when warm, or vibration that shows up in a tap test at the nose are bearing or preload problems. Ask for the last spindle service record. If there is none and the machine is over 10 years old, assume a rebuild within 18 months.

Check the drawbar last. Measure retention force with a gauge if the seller allows it. Weak retention causes tool pull-out during heavy cuts, and it is a common reason a used machine arrives and immediately starts scrapping parts on the first roughing pass.

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  Bring your own indicatorA 0.001 mm resolution dial or lever indicator and a known-good test bar.
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  Check the taper cold and hotRun 20 minutes at working speed, then re-indicate.
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  Ask for the retention force numberWritten value, not a verbal yes.

Backlash tells you how much the axis moves before the table actually responds. Mount an indicator against the table and jog the axis in 0.010 mm increments in one direction, then reverse. The reading where the table starts moving is your backlash. On a linear-guide machine, 0.008 mm or less is healthy. Above 0.020 mm you are looking at ball screw re-balling, a new nut, or a full screw replacement, which on a large machine can run into five figures.

Linear guides and box ways fail differently. Roller guides develop a rumble that you can feel by hand when you push the table along its travel. Box ways show scoring, pitting, or a wiped oil film. Check the way covers and bellows for tears, because a torn cover lets chips into the guide and that damage is usually already done by the time you see it.

Thrust bearings and couplings are the cheap parts that cause expensive symptoms. Grab the ball screw end and try to move it axially. Any perceptible end play shows up as a surface finish problem on facing cuts. Listen for a click at direction reversal, which often means a worn coupling or a loose lock nut.

Ask the seller to run a circular interpolation test, ideally a 100 mm circle at feed 500 mm/min. Out-of-roundness above 0.015 mm usually points to backlash or servo tuning, not to the mechanical parts alone. That distinction changes your repair budget a lot.

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  Reverse the axis slowly0.010 mm jog steps reveal backlash you will never see at rapid.
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  Feel the guides by handPush the table along its full travel with the servo off.
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  Run a circle test100 mm circle, feed 500 mm/min, check roundness.

Squareness drifts when a machine sits on a bad floor or has been moved without re-leveling. Check X-to-Y squareness with a granite square and indicator, or with a Renishaw-style ballbar if the seller has one. A target of 0.010 mm per 300 mm is realistic for a used machine in good shape. Beyond 0.025 mm per 300 mm, you are correcting with cutter comp on every job, and that gets old fast.

Ask how the machine was anchored and when it was last leveled. A machine that ran on a 200 mm reinforced slab in a temperature-controlled bay has a very different risk profile than one that sat on a 100 mm slab next to a stamping press. Vibration history shows up in the electronics as much as in the castings.

Spindle-to-table parallelism and Z-axis perpendicularity matter for mold work and for any job with tight flatness. If you plan to run parts at ±0.005 mm, budget for a full geometry re-scrape or a laser calibration after the move. That is a normal cost, not a red flag, but it belongs in your offer calculation.

Thermal behavior is the last geometry item. Run the machine for two hours and re-check a critical dimension on a test part. A machine that drifts more than 0.020 mm over that window in a 20 °C shop needs ball screw cooling or a different duty cycle than you planned.

The controller decides how long the machine stays economical. Fanuc, Siemens, Heidenhain, Mitsubishi and Mazak controls all have different spare-part horizons. Before you bid, call the builder's local distributor with the serial number and ask three questions: are boards still stocked, is the software still supported, and what is the lead time on a replacement drive.

Check the hours meter and the servo load. Most controls store axis load and spindle load history. A machine that idles at 40-60% servo load on a light cut is fighting friction, and that usually means guide or ball screw wear. Also read the alarm history if the seller will show it. Repeated overtravel or servo overload alarms tell a story the paint does not.

Look at the electrical cabinet. Clean, dry, and dust-free is what you want. Blown cooling fans, scorched terminal blocks, or a nest of field wiring added over the years all raise your commissioning cost. Take photos of the drive nameplates so you can price replacements before you commit.

Ask about the parameter and ladder backup. If the seller cannot provide a backup file, you are one dead battery away from a very expensive recovery. That single file is often worth more than a 5% discount on the machine.

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  Call the distributorSerial number in hand, ask about board and drive availability.
• 2
  Read the alarm historyServo overload and overtravel entries reveal wear.
• 3
  Demand the backup fileParameters, ladder, and servo data on a USB stick.

A used machine is only a bargain if its work envelope matches your parts. A 4,000 mm travel machine ties up floor space and rigging money that a 750 mm machine would not. Write down the largest part you expect to run in the next three years, add 100 mm clearance on each axis, and buy to that number. Oversizing used iron is one of the most common mistakes in shop expansion.

Tool magazine capacity and taper size drive your cycle time. A 20-tool CAT 40 magazine covers most job-shop work. A 60-tool HSK-A63 magazine is worth the premium only if you run lights-out or high-mix production. Count the pockets and check that the ATC arm indexes without a hesitation or a double-clutch sound.

Look at the coolant system as a wear indicator. A machine with a full chip conveyor, a paper filter, and a clean tank has been maintained. A machine with a 20 L tank and a hand scoop has not. Through-spindle coolant is a major plus for deep-hole work, but only if the rotary union holds pressure. Test it at 50-70 bar and watch for drips at the union.

Finally, price the tooling separately. Used machines often come with holders that are worn or mismatched. Assume you will replace 30-50% of the holders within the first year, and factor that into your offer.