Can a CNC Machine Magnetize Metal?

A CNC machine does not have a magnet in the spindle, so the field has to come from somewhere else. The usual source is current. Variable frequency drive cables, spindle motor leads, and servo power cables carry high-frequency current, and any loop of that current radiates a magnetic field. If that field crosses a steel workpiece sitting in the fixture, the domains inside the steel rotate toward it. The part does not need to touch anything magnetic.

Shielding matters more than most shops assume. Double-braided, grounded cable on VFD and spindle runs cuts radiated EMI substantially compared with unshielded cable. IEC 61000-6-4 is the industrial emission standard that governs this, and it is worth checking cable routing against it when a shop has repeat magnetic complaints on one machine.

Workholding is the other common path. A magnetic chuck transfers its field into the part directly, and steel parallels, steel vise jaws, and steel fixture plates can retain a small field after the current is switched off. If the same fixture holds a part for a long cycle, the part sits in that field the whole time.

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  VFD and spindle cableRadiated field, worst on long unshielded runs
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  Magnetic chuck or magnetic viseDirect transfer into ferromagnetic parts
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  Fixture plates and parallelsSmall residual field after the cycle, builds up over shifts

Mechanical working rearranges magnetic domains. Drilling, turning, milling, and especially tapping and thread rolling strain the crystal lattice, and in ferromagnetic steel that strain can leave a net magnetic moment. A heavily cold-worked surface often reads higher than the bulk of the part. The effect is strongest on thin sections and on the last few passes where the tool is dull.

Heat does the same thing from the other direction. When a part heats past its Curie point and cools in a magnetic field, or cools unevenly, domains freeze in a partially aligned state. That Curie temperature is roughly 770 °C for iron and around 1,400 °C for some cobalt alloys, well above normal cutting temperatures. So ordinary milling heat alone rarely causes it. Welding, grinding burns, and EDM sparks are the processes that reach these temperatures.

EDM is interesting because it both creates and erases magnetism. The spark temperatures exceed 8,000 °C locally and the rapid thermal cycling randomizes domains rather than aligning them, so parts often come off an EDM below 0.5 Gauss. That helps only if the field came from earlier machining. EDM will not stop EMI-induced fields during its own cut.

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  Tapping and thread rollingHigh local strain, common on fastener threads
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  Dull toolingMore rubbing, more cold work on the surface layer
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  GrindingLocal heat plus strain, worst on thin walls

Only ferromagnetic metals retain a meaningful field. That group is iron, most carbon and alloy steels, martensitic and ferritic stainless like 420, 430, 431, and 440C, and precipitation-hardening grades such as 17-4PH. Cast iron behaves the same way and holds a field well.

Austenitic stainless is different. Grades 303, 304, 316, and 316L are non-magnetic in the annealed condition, so a finished part is not attracted to a magnet. That changes after heavy cold work. Drawing, severe forming, or aggressive machining can convert some austenite to martensite and make a normally non-magnetic grade weakly magnetic at the surface. A 304 part that reads magnetic is usually telling you about how it was formed, not about the CNC cycle.

Aluminium, copper, brass, titanium, and magnesium are not ferromagnetic. A 6061 or Ti-6Al-4V part will not hold a field no matter what the spindle does. If someone reports a magnetized aluminium part, the attraction is coming from something attached to it, such as a steel insert or a bonded steel backing plate.

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  Holds a field1018, 1045, 4130, 4140, 4340, 420, 430, 431, 440C, 17-4PH, cast iron
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  Usually non-magnetic303, 304, 316, 316L in the annealed state
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  Never ferromagnetic6061, 7075, C36000, Ti-6Al-4V, AZ31B

You cannot fix what you have not measured, and a paper clip is not a measurement. A handheld gauss meter or a Hall-effect probe gives a number in Gauss or millitesla. For most precision work a limit of 2 Gauss or lower at the part surface is a reasonable target, and some sensor and medical assemblies will call out tighter numbers on the drawing.

Measure in the right places. The strongest reading is usually not in the middle of a flat face. Check the ends of the part, the cut edges, around tapped holes, and any thin section. Zero the probe away from the machine first, then hold it at a consistent distance, typically right against the surface, and log the highest value rather than the average.

Also record the machine state. A reading taken with the spindle stopped and the drives off is the part's own field. A reading taken with the drives live includes whatever the cables are radiating, and that difference tells you whether the problem is the part or the machine. Log a few parts across a shift before you decide anything; a single reading is noise.

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  InstrumentHandheld gauss meter or Hall-effect probe
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  Target2 Gauss or lower at the part surface unless the drawing says otherwise
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  Measurement pointsPart ends, cut edges, tapped holes, thin sections
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  BaselineRead once with drives off, once with drives on

Prevention starts with the machine. Route VFD and spindle cable away from the work zone, use double-braided grounded cable, and keep the shield terminated at both ends per the drive manufacturer's instructions. Replace magnetic chucks and steel fixture plates with non-magnetic workholding where the part is ferromagnetic and the drawing has a magnetic limit. Ceramic-coated and austenitic stainless workholding is the usual answer.

Then the process. Sharper tooling and lighter finishing passes reduce the cold work left in the surface. On a part that is prone to holding a field, demagnetize the fixture and the probe between cycles so residual field does not build up shift after shift. This is cheap and it removes a slow drift that is hard to diagnose later.

If a part still comes out magnetized, demagnetize it as a finishing step. A pass-through or tunnel demagnetizer applies a reversing field that decays to zero and brings most steel parts well under 1 Gauss in a few seconds. For parts that cannot go near a demagnetizer, a controlled thermal cycle above the Curie point works, but that brings scale, distortion, and hardness risk, so it is rarely the first choice.

One caution on sequence. If you demagnetize before grinding, tapping, or any heavy cold-work operation, the field comes back. Put demagnetizing last in the routing, after the final cut and before packing, or the operation buys you nothing.

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  Machine sideShielded, grounded VFD and spindle cable; non-magnetic workholding
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  Process sideSharp tooling, light finishing passes, demagnetize fixtures between cycles
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  Part sidePass-through demagnetizer as the last operation before packing