What to Ask Before Buying CNC Machine

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  Key takeaways
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  Buying CNC machine: what to check against your part
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  What to ask before buying cnc machine: define the part first
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  Tolerance: what the machine holds on your part
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  Spindle torque, speed and the material you cut
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  Axis travel, table size and the parts you will really run
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  Spindle hours, wear and the maintenance history
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  Total cost: what the sticker price leaves out
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  Buying cnc machine vs outsourcing: when each one wins
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  Step by step: how to screen a machine in one visit

Most bad machine purchases start with a spec sheet instead of a part drawing. Before you call a dealer, print the three parts you expect to run most often. Write down the tightest tolerance, the hardest material, the largest envelope and the annual quantity. That single page answers half the questions below.

A shop that runs aluminum brackets at ±0.05 mm needs a very different machine from one cutting Inconel housings at ±0.01 mm. Aluminum cuts fast with light rigidity demands. Titanium, 17-4PH and tool steel push the spindle, the frame and the coolant system much harder.

Be honest about future work too. If 20% of your pipeline is 5-axis geometry, a 3-axis mill with a rotary table will not replace it. You will end up outsourcing those parts anyway, which is fine, but you should plan for it rather than discover it.

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  Tightest toleranceTake it from the drawing, not from a target you hope to reach.
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  Hardest materialThis drives spindle torque and coolant pressure.
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  Largest envelopeAdd fixture and tool length before comparing travel.
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  Annual volumeLow volume favors flexibility; high volume favors cycle time.

A machine rated at ±0.005 mm is measured under ideal conditions: stable temperature, light cuts, new bearings, no fixture error. Your part sees thermal growth, tool deflection, clamping stress and chip load. The practical number is usually two to three times looser than the rating.

Ask how the tolerance was verified. A ballbar test shows circular interpolation error. A laser interferometer shows positioning error across the full stroke. Both matter, and both should be recent. If the seller cannot show either, you are buying hope.

Temperature is the quiet variable. A shop that swings 8 °C between morning and afternoon will struggle to hold ±0.005 mm on a 300 mm steel part, no matter how good the machine is. Ask about the room, not just the iron.

For most structural parts, ±0.05 mm is plenty. Do not pay for ±0.005 mm capability if your drawing allows ±0.1 mm. The money is better spent on a probe, a better fixture or a second shift.

Spindle speed sells machines. Spindle torque cuts metal. A 15,000 rpm spindle with 20 Nm at low rpm will stall in a 50 mm face mill on 4140 steel. A 8,000 rpm spindle with 120 Nm will walk through it.

Ask for the torque curve, not a single peak figure. You want to know torque at the rpm range you will actually use. Aluminum at 10,000 rpm needs speed. Titanium at 800 rpm needs torque and rigidity.

Coolant matters as much as the spindle. Through-spindle coolant at 70 bar clears chips from deep holes and controls heat at the cutting edge. Flood coolant alone struggles past 5× diameter depth in stainless.

If you cut mostly aluminum and plastics, a high-speed spindle with modest torque is the right call. If you cut steel and superalloys daily, prioritize torque, frame mass and coolant pressure over maximum rpm.

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  Aluminum and plasticsSpeed and fast feed win; torque is secondary.
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  Steel and stainlessTorque at 500–2,000 rpm plus rigid fixturing.
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  Titanium and InconelHigh torque, high coolant pressure, heavy frame.

Quoted travel is measured with no fixture, no vise and a short tool. In practice, a vise adds 50–80 mm of Z height, and a long reach tool eats another 40 mm. Subtract both before you compare the machine to your part.

Table size matters as much as travel. A 4,000 × 400 × 150 mm travel machine may only accept a part that fits between the clamping zones. Ask for the table drawing with T-slot spacing and clamping area marked.

Think about part access too. A 5-axis machine with a trunnion limits how close you can get with a face mill. A C-frame 3-axis mill gives better access but cannot reach five sides in one setup.

If 80% of your parts fit in a 500 × 500 × 450 mm envelope, buying a 4,000 mm machine wastes floor space and power. Buy the envelope you use weekly, and outsource the occasional large part.

The year of build tells you very little. Spindle hours and cutting hours tell you a lot. A 2015 machine with 8,000 spindle hours is often in better shape than a 2021 machine that ran three shifts on cast iron.

Ask for the maintenance log. Ballscrew replacement, spindle rebuild, way cover replacement and lubrication records all show how the machine was treated. A machine with no log is a machine with unknown risk.

Check the ballscrews and linear guides for backlash with a dial indicator. Push the table by hand and watch the reading. More than 0.01 mm of lost motion means a rebuild is coming soon.

Run a test cut if you can. Bring your own material and program, and inspect the result. A 30-minute test cut reveals more than any inspection report.

The machine is roughly half the real cost. Toolholders, a probe, fixtures, coolant, chip conveyor, transformer and installation add up quickly. A 40-taper machine with 30 holders can absorb another 15–20% of the purchase price.

Power and floor prep are easy to forget. A large VMC may need a 30–40 kVA supply, a dedicated breaker and a leveled concrete pad. If your building cannot supply it, the machine is not the only cost.

Then there is the ongoing part: spindle rebuilds at 10,000–15,000 hours, way lubrication, coolant changes and calibration. Budget roughly 3–5% of the machine price per year for maintenance.

Compare the full cost against outsourcing. For 200 parts a year, an outside shop with a 5-axis cell may deliver lower total cost and better accuracy than a machine sitting idle between jobs.

Buy a machine when you have steady volume, tight lead times and part families that repeat. Roughly 1,500 hours of spindle time per year is a common break-even point against outsourcing, though the exact number depends on your hourly rate.

Outsource when volumes are low, geometry is complex, or the parts need certifications you do not hold. A supplier with ISO 9001:2015, IATF 16949, ISO 13485 and ISO 27001 already carries the audit burden.

Outsourcing also gives you access to capability you cannot justify in-house. A 5-axis trunnion machine with a Ø400 mm rotary table handles undercuts and compound angles that a 3-axis mill cannot reach in one setup.

Many shops run a hybrid model: one or two 3-axis mills in-house for simple work, and a qualified partner for 5-axis, large envelope or regulated parts.