How to Appraise Used CNC Machines
A used machining center can add capacity for less capital, but only if the iron, the motion system and the control all still hold tolerance. This guide shows engineers how to appraise used CNC machines in seven checks, with numeric pass limits. Read it before you book a flight to see the machine.
Key takeaways
What appraisal actually covers
Appraisal is not a price negotiation trick. It is a measurement exercise that answers one question: can this machine still produce the parts you sell? Everything else, paint, cabinet layout, brand name, comes after that answer.
Work in a fixed order. Static structure first, then motion, then spindle, then control, then documentation. If the frame fails, stop. You do not need to inspect a ballscrew on a machine that cannot hold squareness.
Bring three things: a 0.01 mm dial test indicator with magnetic base, a machined test block, and a written list of the tolerances your own drawings call out. Without your own tolerance list, every number you measure has no context.
Set a walk-away threshold before you travel. For most job-shop work a machine that holds ±0.02 mm across its working volume is usable. If your prints call ±0.005 mm, used equipment rarely gets there without rebuild, and the rebuild usually costs more than the delta to a new machine.
- 1Static structureCastings, linear guide or box way condition, leveling, foundation.
- 2Motion systemBallscrews, thrust bearings, couplings, servo health, backlash.
- 3SpindleTaper condition, runout, drawbar force, bearing noise under load.
- 4Control and documentationSoftware version, parameter backup, board availability, service history.
Iron, leveling and way surfaces
Start with the base and column. Look for cracks near casting ribs, welded repairs on the saddle, and fresh paint over machined surfaces. Fresh paint on a 15-year-old machine is a warning, not a selling point.
Check leveling with a precision level of 0.02 mm/m before anything else. Place it on the table in X and Y and record both readings. A machine that has been sitting unleveled on a bad floor for years may still align, but you need to know before you quote the move.
On linear guide machines, run a finger along the rail and look at the ball track. Grey streaks or a visible wear band on one rail only means uneven preload or a misaligned installation. On box way machines, check the Turcite or liner thickness at the ends of travel against the middle.
When you appraise used CNC machines, ask how long the machine has been idle. A spindle that has not turned in 18 months may show rust in the coolant passages and dried grease in the ways. It can be recovered, but only with a teardown you should price in.
- 1PassNo cracks, no weld repairs, level within 0.02 mm/m.
- 2CautionLight wear band on one rail; fresh paint on machined faces.
- 3Walk awayCast iron cracks, structural welds, or a table that rocks when bolted.
Ballscrews, thrust bearings and backlash
Backlash is the single most common reason a used machine misses tolerance. Measure it, do not trust the compensation table in the control. A controller can hide 0.05 mm of mechanical lost motion, and it will change again when the machine warms up.
Mount a dial indicator on the table against the spindle nose. Command 0.05 mm of movement in one direction, then reverse. The difference between commanded and actual is your backlash on that axis. Repeat five times and take the worst reading. Under 0.01 mm is good. Between 0.01 and 0.02 mm is workable for general milling. Over 0.02 mm needs investigation.
Then check the thrust bearing end. Push the table by hand along the axis with the servo disabled and feel for a click. A click that repeats at the same spot is usually a worn angular contact bearing pair, not the screw nut. That is a cheaper repair, but it still needs a teardown.
Listen to the servo during a rapid move. A low growl that changes pitch along the travel points to a dry or pitted screw. A high whine at one position is often a cable or a resonance setting, which is cheaper.
- 1Under 0.01 mmNormal for a well-kept machine; no action needed.
- 20.01–0.02 mmAcceptable for ±0.02 mm work; re-check after a warm-up cycle.
- 3Over 0.02 mmPrice in a screw, nut or thrust bearing replacement.
Spindle, taper and drawbar force
The spindle is the most expensive single component. Check the taper for scoring, fretting and blue-black marks from a spun tool holder. A light polish is fine. Deep circumferential grooves mean the taper needs regrinding or replacement.
Measure taper runout with an indicator on a clean test bar, not on the taper itself. Standard CAT40 or BT40 tooling should read within 0.005 mm at 100 mm from the gauge line. Above 0.01 mm, expect poor surface finish and short tool life. Our own shop holds ±0.005 mm on production parts, and a used spindle that cannot get near that number will not run precision work.
Check drawbar force with a gauge if the seller allows it. A 40-taper spindle typically needs around 8 to 10 kN to hold a holder against a heavy cut. If force is 30 percent below spec, springs or the Belleville stack are tired, and pull-out under load will scrap parts.
Run the spindle at maximum speed for 15 minutes and listen. Bearing noise that grows with temperature is a rebuild. Buyers often skip the warm-up run, then find the problem on the first production shift.
- 1TaperLight polish only; no scoring, no fretting, no heat discoloration.
- 2RunoutWithin 0.005 mm at 100 mm from the gauge line.
- 3DrawbarWithin 10 percent of the builder's spec for the taper size.
Control, drives and documentation
A mechanically sound machine with a dead control is a parts donor. Check the control model, the software version, and whether the builder still sells replacement boards. Some older controls have no supported spares path, and a retrofit can cost as much as the machine.
Ask for a full parameter backup on a USB stick and confirm it loads. Also ask for the ladder logic, the servo drive parameters and any option codes. Without these, a single battery failure can turn the machine into a paperweight.
Look at the alarm history. A machine with 40 overtravel alarms is normal. A machine with repeated servo overload or spindle orientation faults is telling you something. Ask for the maintenance log and compare dates against the hour meter.
Finally, list what is missing. Tool holders, chucks, chip conveyor, coolant system, probe, transformer. Tooling and accessories often add 15 to 30 percent to the real cost of a used machine, and sellers rarely include them in the asking price.
- 1Ask forParameter backup, ladder logic, drive parameters, option codes, maintenance log.
- 2VerifyThe backup actually loads on the machine before you pay.
- 3Budget forSpares, tooling, freight, foundation and commissioning hours.
Seven steps to appraise a used machine
Each step has a pass limit. Stop when a step fails.
- 11. Ask for documents before you travelRequest the maintenance log, hour meter photo, parameter backup list, alarm history and any geometric inspection report. If the seller will not send a parameter list, the trip is probably wasted.
- 22. Level check on arrivalPut a 0.02 mm/m precision level on the table in X and Y. Record both. Re-leveling a machine is cheap, but a machine that will not hold level after adjustment points at a bent frame.
- 33. Geometry run with a test barCheck squareness between X and Y over 500 mm, then Z perpendicularity to the table. Within 0.02 mm over 500 mm is normal for a used VMC. Over 0.03 mm needs scraping or a rebuild quote.
- 44. Backlash measurement on every axisCommand 0.05 mm, reverse, read the indicator. Repeat five times per axis and keep the worst number. Under 0.01 mm passes. Over 0.02 mm means screw or thrust bearing work.
- 55. Spindle checkInspect the taper, measure runout with a test bar at 100 mm, and check drawbar force with a gauge. Run at maximum rpm for 15 minutes and listen for bearing noise that grows with heat.
- 66. Thermal drift testRun a 30-minute warm-up cycle, then re-measure a known feature on the test block. Drift over 0.02 mm between cold and warm tells you the spindle or the ballscrew cooling is tired.
- 77. Test cut and part measurementCut a representative part, not a cube. Measure every print tolerance on a CMM or with a height gauge. If the machine cannot hold your real part, no discount fixes that.
Pass, caution and walk-away limits
Use these numbers as the first filter, then apply your own print tolerances.
| Item | Pass | Caution | Walk away |
|---|---|---|---|
| Level and frame | Within 0.02 mm/m | Re-levels, drifts again | Cracks or weld repairs |
| Squareness over 500 mm | Under 0.02 mm | 0.02–0.03 mm | Over 0.03 mm |
| Backlash per axis | Under 0.01 mm | 0.01–0.02 mm | Over 0.02 mm |
| Taper runout at 100 mm | Under 0.005 mm | 0.005–0.01 mm | Over 0.01 mm or scored taper |
| Drawbar force | Within 10% of spec | 10–20% low | Over 30% low |
| Thermal drift, cold to warm | Under 0.01 mm | 0.01–0.02 mm | Over 0.02 mm |
| Control spares | Boards and backup available | Boards only from brokers | No backup, no spares path |
Questions buyers ask after the inspection
How many hours on the spindle is too many?
Hour meters are easy to reset and rarely tell the whole story. A 40-taper spindle that has run 20,000 hours at 8,000 rpm in aluminium is often healthier than one that ran 6,000 hours in cast iron at low speed with heavy radial load.
Judge by taper condition, runout, drawbar force and bearing noise under a warm-up run. Those four measurements predict remaining spindle life better than the counter does.
Should I trust the backlash compensation in the control?
No. Compensation hides mechanical wear without removing it, and the value shifts as the machine heats up. Measure real backlash with an indicator, then compare it against the compensation value stored in the parameters.
If the stored value is much larger than the mechanical reading, someone has been chasing a problem with numbers instead of parts.
Is a ballbar test worth paying for?
Yes, if the machine is going into production work with circular interpolation or contouring. A ballbar run in X-Y and X-Z shows squareness, backlash, servo mismatch and reversal spikes in one test.
A written ballbar report also gives you a baseline. When the machine drifts six months later, you know exactly what changed.
What documentation should be in the sale?
Parameter backup that loads on the machine, ladder logic, servo drive parameters, option code list, maintenance log, and the electrical and hydraulic drawings. Ask for the original builder manuals too.
If the seller cannot produce a parameter backup, treat the control as unsupported and price a retrofit into your offer.
When is a used machine the wrong purchase?
When your prints call tolerances tighter than the machine can hold after a proper warm-up, or when the control has no supported spares path. Also when the machine footprint, spindle taper or travel does not match the parts you actually sell.
A cheap machine that sits idle is more expensive than a new one that runs. Do the numbers on floor space and operator time before you bid.
Should we buy as-is or ask for a rebuild?
As-is is fair when the measurement data is clean and you accept the wear. A rebuild only makes sense if the frame is sound and the cost still leaves you below a comparable new machine.
Get the rebuild quoted by a third party before you commit. Seller rebuild quotes often exclude spindle, control and paint.
Need a second opinion on a machine?
Send us the inspection data or the part drawings. We will tell you whether the machine can hold the tolerance, and we run 127 high-precision CNC machines across three plants in Dongguan and Singapore.
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