Magnesium CNC Machining Service: What to Check Before You Order
Magnesium cuts fast and weighs about two-thirds of aluminum, but the chips burn. This guide gives engineers and buyers the checks that separate a capable magnesium CNC machining service from one that only quotes the alloy. Read it and you can judge alloy choice, tooling, fire control, tolerance and lead time in one pass.
In this article
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Key takeaways
AZ31B vs AZ91D vs WE43: Which Alloy Fits Your Part
Density is roughly 1.77–1.84 g/cm³ for all three.
| Alloy | Best for | Watch out for |
|---|---|---|
| AZ31B | Sheet-like housings, brackets, ducting | Lower strength; dents in thin sections under clamp load |
| AZ91D | Die-cast-style housings, covers, high-volume parts | Better castability than machinability; harder on edges |
| WE43 | High-temperature aerospace and medical parts | Higher cost; tighter supply; needs experienced tooling |
| All three | Weight-critical parts where aluminum is too heavy | Fine swarf ignites easily; corrosion without coating |
Supplier Checklist: Five Judging Criteria
| Criterion | What good looks like | Red flag |
|---|---|---|
| Alloy knowledge | Names grade and temper, asks about coating | Quotes generic magnesium |
| Chip and fire control | Dry or mist cutting, class D extinguishers, clean-down | Flood coolant, shared swarf bins |
| Tolerance discipline | States ±0.005 mm and where it applies | Promises the same on every feature |
| Capacity match | Right machine class for the part size | One machine for everything |
| Quote quality | DFM notes, inspection plan, packaging | Price and days only |
When Magnesium CNC Machining Makes Sense, and When It Does Not
Magnesium is the lightest structural metal you can machine in production. Density sits near 1.74–1.84 g/cm³, about two-thirds of aluminum and roughly a quarter of steel. If a part's job is to hold stiffness while shaving mass, magnesium is usually the first metal worth quoting. That covers camera gimbals, drone frames, EV battery brackets, handheld tool bodies, laptop and instrument housings, and some aerospace secondary structures.
The trade is not subtle. Magnesium has a lower modulus than aluminum, so a wall that felt stiff in 6061 may flex in AZ31B. It also corrodes quickly in salt air if left bare, and its fine chips can ignite at temperatures that would not bother aluminum. Those three points decide most projects. If your part is a heavy machine base, a wear surface, or a part that sits outdoors unpainted for years, magnesium is usually the wrong answer.
A magnesium CNC machining service should tell you this in the first reply. If the quote arrives without a single question about wall thickness, coating, or chip handling, treat it as a warning sign rather than a bargain.
One more boundary: magnesium is not a drop-in replacement for aluminum in every geometry. Threads need more engagement depth, sharp internal corners concentrate stress, and thin floors vibrate during finishing. Designers who plan for those details get good parts. Designers who copy an aluminum drawing and swap the material name usually get scrap.
Tolerances and Surface Finish You Can Realistically Hold
Magnesium machines freely. Cutting forces are low, spindle speeds can run high, and tool wear is gentle compared with stainless or titanium. That means tight tolerance is mainly a thermal and fixturing problem, not a cutting-force problem. On well-supported features we hold ±0.005 mm (±0.0002 in) and reach Ra 0.2–0.8 μm on sealing faces when the part is drawn that way.
Where magnesium fights you is thin walls and long unsupported spans. A 0.8 mm wall in a 100 mm long housing will move when the tool passes, no matter how sharp the cutter is. The usual fix is a two-stage cut: rough with light depth of cut, let the part settle, then finish. Chilling the workpiece or running a cool-down between stages also helps hold flatness.
Finish choice is a cost lever. As-machined faces land around Ra 1.6–3.2 μm, which is fine for most brackets and internal covers. Ra 0.8–1.6 μm covers visible housings and mating faces. Below that you are paying for a sealing or optical surface, so specify it only where the drawing needs it.
Do not specify a tolerance tighter than the datum chain supports. If the part is located from a coated face, the coating thickness enters the stack. Anodizing, conversion coating and paint all add material, and hardcoat can add several micrometres per side. Put the coating callout on the drawing and let the shop hold the pre-coat dimension.
Chip Control, Fire Safety and the Questions Buyers Forget to Ask
Magnesium fires are a chip problem. A solid block is hard to ignite. Fine swarf, dust from grinding, and the fluffy turnings that come off a dull tool are what burn, and they can burn hot enough to be hard to put out with water alone. A serious shop controls this with three habits: cutting tools that break chips short, air or mist instead of flood coolant where possible, and swarf collection that never lets fines pile up on the machine bed.
Specific process rules matter. Keep the depth of cut low and the feed per tooth high enough to make thick, broken chips rather than long strings. Keep the tool sharp; a dull edge rubs and makes dust. Never let magnesium chips mix with steel or aluminum swarf in the same bin, and never run magnesium on a machine still wet with water-based coolant from a previous steel job. Dry cutting with a controlled mist is the common approach.
Ask about the extinguishing plan. The correct media for a magnesium chip fire is a dry powder rated for class D metals, plus dry sand as a backup. A water hose on the wall is not a plan. Ask where the bins are stored, how often they are emptied, and who checks the machine at the end of a shift.
One more question buyers forget: who cleans the machine between jobs. Magnesium fines left in a coolant tray or under the way covers are the classic cause of a fire that starts after the operator has gone home. A shop that schedules a full clean-down between magnesium runs has thought about this. One that does not, has not.
What a Complete Quote Should Contain
A quotation for a magnesium part should be more than a number and a lead time. The useful ones list the alloy grade and its temper, the stock form, the machine class that will run the part, the finish callout, the inspection plan, and the packaging. If the alloy grade is missing, you cannot compare two quotes, because AZ31B and AZ91D behave differently on the same drawing.
Look for a DFM note. A shop that reads geometry will flag things like a 0.5 mm wall in a high-vibration area, a thread that is too shallow for magnesium, or a sharp corner that will crack during coating. Our DFM analysis and quotation come back within 12 hours, and production can start within 24 hours once the drawing is frozen.
Inspection scope should match the part's function. We inspect 100% of parts before shipment, covering raw material check, in-process monitoring and final inspection, with reports on request. For a magnesium bracket that is mostly a weight-saving shell, that is enough. For a sealed housing or a medical component, ask for the specific dimensional report you need and check that the shop's certifications cover your industry.
Finally, ask about packaging. Magnesium is soft on edges and scratches easily in transit. Parts should be separated, not loose in a bin, and coated parts need protection until the coating has fully cured. It sounds minor until you open a crate of scratched housings.
Step by Step: Ordering Magnesium Parts Without Surprises
Follow these in order. Each step removes a common failure mode.
- 11. Fix the alloy and the coating before you request a quotePick AZ31B for formed or machined housings, AZ91D for cast-style covers, WE43 only when temperature or medical rules demand it. Name the coating on the drawing. Alloy and coating change both the price and the tolerance stack.
- 22. Send a 3D model plus a drawing with the critical features markedMark datums, mating faces and sealing surfaces. A model alone hides which features carry the tolerance. Shops quote faster when the critical list is explicit.
- 33. Ask for the DFM note in writingRead it for wall thickness, thread depth, sharp corners and coating clearance. Reply with your decisions. This exchange is where scrap is usually avoided.
- 44. Confirm the cutting strategy and the safety planAsk for high spindle speed with low depth of cut, sharp polished-flute tooling, and chip breaking rather than long strings. Confirm class D extinguishers and a clean-down between jobs.
- 55. Agree the inspection report before the first cutDecide which dimensions get measured, at what frequency, and in what format. Raw material check, in-process monitoring and final inspection should all appear in the plan.
- 66. Plan the packaging and the coating cure timeCoated parts need separation, protection and enough cure time before packing. Say so in the purchase order, not in an email after the parts ship.
Magnesium CNC Machining Questions Buyers Ask
Is magnesium CNC machining dangerous?
The metal itself is not the hazard. Fine chips and grinding dust are. A shop that breaks chips short, keeps swarf out of mixed bins, uses dry or mist cutting, and keeps class D powder extinguishers on the floor runs magnesium safely every day.
Ask to see the chip handling routine before you place an order. The answer is usually more informative than any certificate.
Can you hold ±0.005 mm on a thin magnesium wall?
Yes, on supported features. Thin walls move under cutting pressure and thermal growth, so the shop needs light depth of cut, a settling stage between rough and finish, and fixtures that support the wall rather than clamp it.
If a feature is a 0.8 mm wall with no support, expect the tolerance to loosen. Tell us the function and we will say what is realistic before you commit.
Which magnesium alloy should I specify?
AZ31B for machined and formed housings where weight matters most. AZ91D for parts that would otherwise be die cast, since it casts and machines well at volume. WE43 for high-temperature or medical work where the extra cost is justified.
If you are unsure, send the drawing and the service temperature. The alloy choice usually becomes obvious from those two inputs.
How fast can parts ship?
Quotation and DFM analysis come back within 12 hours. Production can start within 24 hours after the drawing is frozen, and parts typically ship in 3–5 days depending on quantity and finish.
Our historical late-delivery probability is below 2%. We do not quote a delivery date we cannot hold.
Do you have a minimum order quantity?
No. We run from one prototype to 10,000+ part runs on the same process. Prototype and production parts come off the same machine class, so the geometry you approve is the geometry you get at volume.
Uploads are secure and confidential, and an NDA is available on request.
Can magnesium parts be anodized?
Yes, but the process differs from aluminum anodizing and the coating is usually a conversion coating or a paint system rather than a decorative anodic layer. Hardcoat options exist for wear surfaces.
We also offer electroless nickel, zinc plating, powder coating, bead blasting and laser marking, with a minimum character height of 1.5 mm for marking.
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