Magnesium CNC Processing Service: How to Pick the Right Shop

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  Key takeaways
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  Alloy and job fit at a glance
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  Why the alloy decides the quote
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  Chip control, coolant and the fire question
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  Tolerance and surface finish you can hold
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  What a magnesium CNC processing service should have
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  Order size, tooling cost and lead time
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  Seven checks before you place the order

Magnesium is the lightest structural metal in common use, and that is the reason most projects start here. Density sits around two-thirds of aluminum, so a housing that weighs 300 g in 6061 lands near 200 g in AZ31B. Weight is not the only argument. The material also damps vibration well, which helps on robot arms and camera mounts where ringing shows up in the data.

The grades are not interchangeable. AZ31B is the one most shops keep on the shelf, and it machines with clean, short chips at moderate speeds. AZ91D is a die-casting alloy that also machines well, but it is harder and abrasive on tooling. WE43 and Elektron 21 carry rare-earth additions for strength at temperature, and they cost several times more per kilogram.

A quote that says "magnesium" without a temper is not a quote. Whether the stock is AZ31B-H24 or annealed O changes the springback and the final wall thickness on a thin rib. Ask for the grade and temper in writing, then compare the price against the same grade from another shop.

One more point that buyers miss: magnesium is almost always specified for a reason the drawing does not state. It might be inertia in a moving axis, or heat spread in a sealed enclosure. If you tell the shop that reason, they can suggest a geometry change that saves weight instead of just cutting the part you drew.

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  Grade and temper in writingAZ31B-H24 and O temper do not machine the same.
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  Density checkRoughly one-third lighter than steel, two-thirds the weight of aluminum.
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  DampingUseful on parts that carry vibration, not just on weight-critical ones.

Magnesium fines ignite. That single fact separates a shop that runs magnesium every week from one that has read about it. The risk is not the solid part on the table. It is the fine swarf that piles up in the chip tray, under the fixture and inside the enclosure, where a spark from a broken tool can start a fire that burns hot and is hard to reach.

Ask how chips leave the cutting zone. High-pressure through-spindle coolant or a strong air blast keeps the pocket clear on deep features. Flood coolant with a high water content is common on turning and on sawing. Some shops cut magnesium dry with air and a mist, which is workable on light finishing passes but risky on heavy roughing where the swarf gets hot.

Tooling choice follows the same logic. Sharp, polished flutes and high positive rake angles cut magnesium with less heat and less smearing. Two-flute and three-flute carbide end mills in the 8–12 mm range cover most pocket work in AZ31B. Dull tools are the main cause of built-up edge, and built-up edge on magnesium burns the surface finish.

Housekeeping is a process parameter, not a chore. Swarf should be removed from the machine at intervals during the run, not at the end of the shift, and stored in a covered steel container. If a shop cannot describe its chip routine in two sentences, treat the low price with suspicion.

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  Through-spindle coolantKeeps deep pockets clear and limits heat at the tool tip.
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  Sharp geometryHigh positive rake, polished flutes, no built-up edge.
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  Swarf routineCleared during the run, stored in a covered metal bin.

Magnesium cuts freely, so the machine is rarely the limit on small parts. Our general working tolerance is ±0.005 mm on critical features, which is achievable in AZ31B on bores, bosses and mating faces that sit close to the fixture. Thin ribs and long unsupported walls are a different story. They deflect under cutting load and move again after the part cools.

Heat is the reason. Magnesium conducts heat away from the cut quickly, so the part itself warms up during a long roughing pass. A wall that measures on size at 10:00 may be out of tolerance an hour later. Shops that understand this rough, let the part stabilize, then finish. If your drawing has a wall under 1.5 mm, say so up front and expect a conversation about fixturing.

Surface finish lands in three bands. As-machined work sits around Ra 1.6–3.2 μm, which suits most internal brackets. A high-quality finish of Ra 0.8–1.6 μm covers visible covers and mating faces. Fine finishes of Ra 0.2–0.8 μm are reserved for seal faces, optical mounts and sliding surfaces where friction matters.

Inspection should match the tolerance callout. A ±0.005 mm feature needs a CMM report, not a caliper check. We run raw material verification, in-process monitoring and a final inspection on 100% of parts before shipment, and we share reports on request.

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  ±0.005 mmRealistic on supported features; verify thin walls separately.
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  Ra 1.6–3.2 μmStandard as-machined finish for internal parts.
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  CMM reportAsk for it when the drawing holds ±0.005 mm.

Capacity is the first filter. Magnesium parts often carry compound angles, deep pockets and five-sided access, so a shop with only three-axis mills will ask you to split the part or accept extra setups. Extra setups add stack-up error and cost. Sixteen simultaneous 5-axis centers and 16 mill-turn centers give us room to finish a part in one or two setups instead of five.

Size matters too. Our largest travel reaches 4,000 × 400 × 150 mm, with medium and compact machines covering 750 × 1,150 × 550 mm down to 500 × 310 × 200 mm. That range matters for magnesium because a lot of the work is long, thin housings and frame rails that do not fit a small envelope.

Certification is the third filter, and it should match your industry. ISO 9001:2015 covers general industrial work. IATF 16949:2016 is what automotive and EV programs expect. ISO 13485:2016 applies to medical devices. ISO 27001:2022 matters if you are sending CAD files that cannot leave a controlled environment.

The practical test is not a certificate wall. Ask the shop to name the last three magnesium jobs it ran, the grades involved and how it handled the swarf. A supplier that can answer in specific terms is a supplier that will not learn on your part.

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  5-axis countFewer setups means tighter stack-up on angled features.
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  Envelope rangeCheck both the largest and the smallest machine travel.
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  Certificates by industryISO 9001, IATF 16949, ISO 13485, ISO 27001 each answer a different audit.

Magnesium does not need a minimum order. We quote from a single prototype up to 10,000-piece runs, and the price break comes from setup amortization rather than from a hard floor. That matters on development programs where you want one machined housing to test before committing to a die-cast tool.

Tooling is usually simple. Magnesium cuts easily, so a soft jaw set or a vacuum plate often replaces a dedicated fixture. Complex geometry may need a machined nest, and that cost should appear as a separate line on the quote. If a supplier buries tooling inside the unit price, you cannot compare quotes honestly.

Lead time on the metal is the hidden variable. AZ31B bar and plate are widely stocked. WE43 and Elektron 21 usually are not, and the mill lead time lands on your schedule. Ask whether the quoted lead time starts from receipt of material or from receipt of the purchase order. Those are different dates.

For planning, we quote and return a free DFM analysis within 12 hours, production can start within 24 hours, and parts ship in 3–5 days once material is on hand. Historical late-delivery probability sits below 2%. Treat those numbers as the shop's own track record, not a guarantee for a grade that has to be mill-ordered.

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  No MOQOne prototype and a 10,000-piece run use the same process window.
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  Tooling as a line itemSeparate fixture cost makes quotes comparable.
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  Material lead timeRare-earth grades can dominate the schedule.