CNC machining magnesium: key tips for safe, accurate parts

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  Key takeaways
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  What makes CNC machining magnesium different
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  Pick the right magnesium alloy before you cut
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  Tool geometry, speeds and feeds that work
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  Fire prevention is a housekeeping discipline
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  Finishing magnesium parts and protecting them
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  Step by step: setting up a magnesium job
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  AZ31B vs AZ91D for CNC machining magnesium
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  Magnesium vs aluminum: when to choose which

Magnesium is the lightest structural metal in common use, about 35% lighter than aluminum and roughly two-thirds lighter than steel. Density sits near 1.74 g/cm³ for AZ31B and 1.81 g/cm³ for AZ91D. That is the whole reason the material shows up in aircraft brackets, drone frames, camera housings and handheld medical devices. Every gram saved upstream is payload or battery life downstream.

The machining behavior follows from the same low density. Magnesium has a low specific heat and high thermal conductivity, so heat leaves the cut quickly and the workpiece stays cool. It machines with low cutting forces, which means less chatter, less tool deflection and better surface finish on thin walls. On a well-set-up machine, magnesium can hold the same ±0.005 mm tolerance as aluminum without special fixturing.

The catch is ignition. Solid magnesium is not easy to light because the bulk metal conducts heat away from any local hot spot. The problem starts with fines. A 0.1 mm chip has a large surface-to-volume ratio, heats up in milliseconds, and can ignite at a temperature well below the melting point of the workpiece. Once burning, magnesium supplies its own oxygen and burns hot enough to crack concrete and set off water-based coolant fires.

That single fact drives every rule in this article. Tool geometry, feed rate, chip evacuation, coolant choice and housekeeping all exist to keep the cut cool and the chips out of the machine bed. If your shop already machines aluminum well, you are close. The fire discipline is what you have to add.

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  Density1.74–1.81 g/cm³, the lightest structural option in most shops
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  Cutting forceLow, so thin walls and small tools behave better than in steel
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  Real hazardFine chips and dust, not the solid workpiece

AZ31B and AZ91D cover most machining work, and they behave differently. AZ31B is a wrought alloy: rolled or extruded plate, good ductility, better surface finish, easier to form. It is the default for prototypes, brackets and housings that will be machined from plate. AZ91D is a die-casting alloy, harder and stronger in the as-cast state, but it contains more aluminum and can be more abrasive on tool edges over long runs.

If the drawing calls out a casting, you may be finishing a die-cast or thixomolded blank rather than cutting from solid. That changes the setup: cast skins carry porosity and hard spots, and you want to take a light first pass to get under the skin before you trust your tool life numbers. For a machined-from-solid prototype, AZ31B plate at 25–50 mm thickness is the usual choice.

Ask what the part will do after machining, because the alloy choice usually follows the environment. AZ91D offers better corrosion resistance and strength at room temperature, which suits housings that see handling and humidity. AZ31B is more ductile and weldable, which suits brackets that need to bend or take impact without cracking.

One caution on mixing: never machine magnesium on a machine that just ran steel, cast iron or aluminium with water-based coolant, without a full clean-down first. Swarf from the previous job carries moisture and fine metal dust into the chip pile. That is a small thing that has started real fires.

Use sharp, uncoated carbide with a high rake angle. For end mills, 2 flutes or 3 flutes with 10–15° helix and a polished flute surface work well; a 15–20° rake reduces cutting pressure and keeps the chip curling away from the wall. Coatings like TiAlN are not required for most magnesium jobs and can round the cutting edge, which raises force and heat. Aluminium-specific geometry is a reasonable starting point.

Run fast and feed hard. Peripheral speeds of 300–600 m/min are normal on magnesium, and many shops push higher on rigid setups. The rule that matters more than the number is chip load: keep it heavy enough that the tool cuts rather than rubs. A rub generates fine dust instead of a chip, and dust is exactly what you are trying to avoid. If the chips come off as powder, increase feed per tooth before you touch spindle speed.

Avoid dwell and hesitation. Any pause with the cutter touching the work converts spindle energy into heat at one point. Ramp into cuts, use helical entry, and keep the tool moving through corners. On finishing passes with small stepovers, raise the feed to keep the chip thickness at the cutting edge up in a healthy range.

Rigidity pays off twice on magnesium. It reduces chatter, and it lets you use aggressive parameters that shorten cycle time. Thin walls down to 1.0–1.5 mm are achievable with light radial passes and a stable fixture. Watch for chips packing into pockets; a high-pressure air blast clears them better than coolant.

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  ToolSharp uncoated carbide, 2–3 flutes, 10–15° rake, polished flutes
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  Surface speed300–600 m/min, higher on rigid setups
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  Chip loadHeavy enough to make a chip, never a powder
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  EntryHelical or ramp, no plunge, no dwell

Most magnesium fires in machine shops start with accumulated fines, not with a cutting pass. The fix is boring and effective: keep the machine clean, keep the swarf bin empty, and never let dust build up on ledges, in the base casting or inside the extraction duct. A single dry cut can throw a spark into a pile of fines that has been sitting for a week.

If a small fire starts, smother it. Dry sand, dry graphite powder or a Class D extinguisher are the correct agents. Water makes it worse: it reacts with burning magnesium to release hydrogen, which can explode. A water-based coolant flood on a magnesium fire is the classic way a small incident becomes a serious one.

Tool wear matters for safety too. A dull cutter rubs, generates fines and heat, and increases the chance of ignition. Change tools on a schedule rather than waiting for a poor finish. On long production runs, log tool life by part count and replace before the edge breaks down.

Post-processing has its own rules. Bead blasting and tumbling create fine dust, so use a dedicated cabinet with extraction and never blast magnesium in a cabinet that also handles steel or aluminum without a full clean. Laser marking is safe because it does not generate airborne fines, but it must be done on a clean part with no oil film.

Bare magnesium corrodes quickly in humid air, and chloride exposure accelerates it. Any part that leaves the shop needs a protective finish. For machined housings, the common routes are a chromate-free conversion coating followed by powder coat or paint, chemical conversion plus an epoxy primer, or a hard anodizing-type treatment where wear resistance matters.

GreatLight runs anodizing, electroless nickel, zinc and silver plating, powder coating, black oxide, bead blasting, tumbling, brushing and polishing in-house. Not every finish suits magnesium, and the ones that do need care: plating baths can attack magnesium if the pre-treatment is wrong, so the sequence matters more than the finish name.

Machined surfaces should be free of embedded iron before coating. If a part was blasted in a cabinet shared with steel, fine iron contamination will show up later as rust spots under the paint. Use a dedicated magnesium cabinet or switch to chemical deoxidation.

Laser marking works well for part numbers on magnesium, with a minimum character height of 1.5 mm for reliable legibility. Engraved marks should sit outside high-stress areas and away from thin-wall sections.