Machining Magnesium: Speed, Weight, and Fire Safety
The lightest structural metal is also the easiest to cut — and the only one whose chips can burn your shop down. Grades, speeds, safety discipline, and when magnesium beats aluminum.
IsoCNC Engineering Team
CNC Machining & DFM Engineers · Updated · 9 min read
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Quick Answer
Magnesium is the easiest structural metal to machine — typical CNC speeds run 350–600 SFM with low cutting force and short chips, and AZ31B (1.77 g/cm³) covers most parts. The trade-offs are fire discipline and corrosion: fine magnesium chips are flammable and demand continuous evacuation, sharp tools, and Class D fire response (never water), while finished parts need a conversion coating, anodize, or paint to survive service. Choose magnesium when weight rules; choose aluminum when you want the cheaper all-rounder.
Why Machine Magnesium at All?
One word: weight. At roughly 1.74–1.77 g/cm³, magnesium is the lightest structural metal in production use — about 35% lighter than aluminum and 75% lighter than steel for the same geometry. It also damps vibration better than aluminum, which is why it lives in optical mounts, instrument structures, and camera bodies as well as aerospace brackets and racing hardware. The reason it is not everywhere is that the weight savings come with handling requirements — flammable chips, reactive surfaces — that not every shop is set up for. This guide covers what those requirements actually look like.
Magnesium Grades for Machined Parts
Four alloys cover most machined magnesium work. AZ31B is the default; the others answer specific problems — cast housings, higher strength, or elevated temperature.
| Grade | Type | Key Properties | Best For |
|---|---|---|---|
| AZ31B | Wrought (sheet, plate, bar) | Density 1.77 g/cm³, yield 200 MPa, UTS 260 MPa, excellent machinability | The workhorse: brackets, housings, fixtures, tool plate — most machined magnesium parts |
| AZ91D | Cast (high-pressure die cast) | Good strength and castability; machined as cast-then-finish parts | Avionics housings, covers, and high-volume cast parts with machined interfaces |
| ZK60 | Wrought, high strength | Higher strength than AZ31B via zinc-zirconium | Stressed structural parts where AZ31B is not strong enough |
| WE43 | High-temperature rare-earth alloy | Retains properties at elevated temperature, premium price | Aerospace and motorsport parts that see heat |
If the drawing just says “magnesium,” expect the shop to quote AZ31B. Specifying anything else should come with a reason — and a material cert requirement to match.
Machinability: The Easy Part
Magnesium is widely rated the most machinable structural metal — several times easier than free-machining steel. In practice that means typical CNC cutting speeds of 350–600 SFM, low cutting forces, minimal tool wear, and short, well-broken chips. Deep pockets and thin walls that would be slow in aluminum come off quickly in magnesium, and surface finishes arrive almost for free. Two thermal notes: magnesium conducts heat at only 50–60 W/m·K (aluminum runs 150–200), so heat management leans on sharp tools and chip evacuation rather than the workpiece carrying heat away; and dry machining with air blast is common precisely because it keeps the flammable chips clean and dry.
Fire Safety: The Part Everyone Remembers
Solid magnesium parts do not burn. Fine chips, turnings, and dust do — and once burning, magnesium fires burn extremely hot and cannot be fought with water, which reacts with the burning metal and intensifies the fire. So the entire safety case is chip management: keep chips short, evacuate them continuously, never let fines accumulate in the machine, conveyors, or ducts, and keep tools sharp so the cut stays cool. Suppression is Class D dry powder or dry sand, staged at the machine. Chips go to licensed handling as flammable scrap, not the general bin.
A shop that machines magnesium regularly treats all of this as written procedure, not tribal knowledge. When you vet a supplier for magnesium work, ask to see the chip-handling procedure and the Class D extinguishers — the answer tells you whether they actually run the material. Our supplier vetting guide has the full audit checklist.
Corrosion and Finishing
Bare magnesium is reactive: it corrodes in humid or salt environments and suffers galvanic attack wherever it touches a dissimilar metal. Machined magnesium parts are therefore almost never shipped bare. The standard stack is a chemical conversion coating (chromate or a modern chrome-free equivalent) as a base, with anodize, paint, or powder coat over it. In assemblies, isolate fastener interfaces with washers or coatings. This is a drawing-stage decision — call out the finish on the print, because the choice affects both cost and lead time. Our surface finish guide covers the finishing vocabulary.
Where Machined Magnesium Earns Its Keep
The applications follow the weight: aerospace and defense brackets and avionics housings, motorsport components, camera and optical structures, laptop and electronics enclosures, and portable equipment where every gram is felt. If the part also sees heat, that is WE43 territory; if it is a high-volume housing, cast AZ91D with machined interfaces usually beats hogging from solid. And if the weight saving is nice-to-have rather than mission-critical, run the aluminum version of the numbers first — aluminum wins on material cost, finishing simplicity, and shop availability. Our aluminum CNC machining page covers the comparison baseline, and titanium vs aluminum maps the next step up in strength-to-weight.
FAQ: Machining Magnesium
Is magnesium easy to machine?
Yes — magnesium is widely rated the most machinable of all structural metals, several times easier than free-machining steel. It cuts at high speed (typical CNC running is 350–600 SFM), with low cutting force, minimal tool wear, and short, well-broken chips. Cycle times are short and surface finishes come easily. The catch is not the cutting, it is the housekeeping: the chips are flammable, so the shop’s chip management and fire discipline matter more than its spindle speed.
Is machining magnesium dangerous?
It is safe with discipline and dangerous without it. Solid magnesium parts do not burn — the fire risk is fine chips, turnings, and dust, which can ignite from a spark or excessive heat and then burn extremely hot. The rules: keep chips short and evacuate them continuously, never let fines accumulate in the machine or ducts, keep tools sharp so the cut stays cool, and never use water on a magnesium fire — water reacts with burning magnesium and makes it worse. Class D dry-powder extinguishers and dry sand are the correct suppressants. A shop that machines magnesium regularly treats chip management as a procedure, not a suggestion.
What coolant do you use for machining magnesium?
Many shops machine magnesium dry precisely to avoid wet, contaminated chip sludge, using air blast for chip evacuation. Where coolant is used, it must be a suitable non-water-reactive formulation and the chips must be handled as flammable waste. What you never do is reach for water to fight a magnesium fire. Because magnesium’s thermal conductivity is only 50–60 W/m·K (versus 150–200 for aluminum), heat management in the cut is about sharp tools and chip evacuation more than flood cooling.
Magnesium vs aluminum for machined parts — which should I choose?
Choose magnesium when weight is the dominant requirement and the budget supports it: at 1.74–1.77 g/cm³ it is about 35% lighter than aluminum and 75% lighter than steel, and it machines faster than either. Choose aluminum when you want the better all-rounder: stronger common alloys, better corrosion resistance without coating, easier finishing, lower material cost, and no fire-handling overhead. Magnesium also damps vibration better than aluminum, which is why it shows up in optical and instrument structures. For the aluminum side of the comparison, see our titanium vs aluminum and steel vs aluminum guides.
What is AZ31B magnesium?
AZ31B is the most widely used wrought magnesium alloy — roughly 3% aluminum and 1% zinc, density 1.77 g/cm³, yield strength around 200 MPa, tensile 260 MPa, elastic modulus 45 GPa, hardness 49 HB. It is not heat-treatable, welds well, and machines superbly, which makes it the default for machined brackets, housings, and fixture plate. When a drawing just says “magnesium,” AZ31B is usually what the shop will quote.
Does magnesium corrode?
Yes — bare magnesium is chemically reactive and corrodes readily, especially in salt environments, and it suffers galvanic corrosion when bolted directly to dissimilar metals. That is why machined magnesium parts are almost always finished: chemical conversion coating (chromate or modern chrome-free equivalents), anodizing, or paint/powder over a conversion layer. Design assemblies with isolating washers or coatings at fastener interfaces. Plan the finish at the drawing stage, not after machining.
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