Aluminum and stainless steel carry the majority of micro CNC machined parts, and the choice between them matters more at small scale than at large. A thin wall is thinner, a tapped hole is smaller, a deflection is a larger fraction of the feature, and the material's behavior under the cutter shows up first in the smallest features. Both families hold the same tolerances in a capable shop, so the decision is about the application: what the part must survive, what it may weigh, and which grade behaves best at the micro scale. This guide walks the decision by requirement, by grade, and by how each family machines, finishes, and measures at small size.
What should you settle before choosing the material for a micro part?
Start with the function, not the metal. Write down what the part must survive — the load it carries, the environment it lives in, the temperature it sees, the weight it is allowed — and score each family against those requirements at the micro scale. Weight-critical micro parts point to aluminum, at roughly a third of steel's density. Parts that need corrosion resistance, strength at a small section, or temperature capability point to stainless. The environment and the load decide the family, and the small geometry decides how carefully the grade has to behave under the cutter.
Writing the requirements down also keeps the decision honest against the cost. Aluminum is the economical answer on material and on cycle time, which makes it the default for micro parts that do not need stainless properties; stainless is the correct answer where the application demands it. Choosing the cheaper grade when the part needs the other family is not a saving — on a micro part the failure simply shows up in a smaller feature. If you score the requirements first, you make the decision once; if you pick by price first, you make it again when the part fails in service.
When does aluminum make the better micro machined part?
Aluminum wins wherever the application does not demand what stainless provides. It weighs roughly a third of steel, so a micro bracket, housing, or cover serves weight-critical designs directly. It machines fast, cutting the cost of small features that would take longer in steel. It anodizes to a hard, colored, corrosion-resistant surface, and it conducts heat well, which matters in micro electronics housings and heat spreaders. For micro parts in controlled indoor environments, aluminum is the standard answer.
Aluminum's conditions are sharpened at small scale. Its strength is about a third of steel's, so a structural micro part either thickens its sections or moves to another family, and there is less room to thicken a micro section. Its corrosion resistance depends on the coating, which means an uncoated aluminum part in a wet environment is a design waiting to fail. And its softness shows first in the details: thin walls flex, small tapped holes strip, and a burr on a micro edge is proportionally larger. The drawing that names the grade, the temper, and the finish is the drawing that gets the aluminum micro part it intended.
When should stainless win, and what does it cost on a micro part?
Stainless wins when the service environment or the load rules out aluminum. The corrosion case is the most common: micro parts that see salt, washdown, chemicals, or bodily fluids need the molybdenum-bearing grades such as 316 and 316L. The strength case is next: at a small fixed section stainless carries far more load, and 17-4PH extends that to structural micro hardware. The temperature case settles parts that run hot. Each of those properties is why stainless micro parts exist, and none of them is negotiable for a lower price.
The cost of the win shows at the cutter. Stainless stock costs more and machines slower, and on small features the work-hardening that punishes a hesitant feed is harder to manage because there is less room to recover. That is not a reason to avoid stainless — it is the price of the property the part needs, and the grade choice matters more on micro parts, where 303's free machining makes small features controllable while 316's toughness makes them demanding. Choose stainless for the application, then the grade for the micro-scale behavior, and put both on the drawing.
Which grades belong in the specification, and what does each deliver?
The grade is the second half of the decision, and on a micro part it is judged by behavior at small size as much as by properties. The table below maps the common grades to the micro applications they serve.
| Grade | Best for | Micro machining behavior | Finish pairing |
|---|---|---|---|
| 6061-T6 / -T651 aluminum | General micro housings, brackets, covers | Fast and clean; good for fine detail | Anodize by class, or conversion coating |
| 7075-T6 / -T7351 aluminum | High-strength micro fittings, thin sections | Stiffer under small tools; -T7351 holds thin walls | Anodize; design out sharp edges |
| 303 stainless | Free-machining micro parts, light corrosion duty | Controllable on small features; economical | Passivation |
| 304 / 304L stainless | General corrosion-resistant micro service | Tougher; consistent feed needed on small sections | Passivation |
| 316 / 316L stainless | Salt, chemical, food, and medical micro parts | Work-hardens on small features; careful cutting | Passivation; electropolish where required |
| 17-4PH stainless | High-strength structural micro parts | Machined annealed, aged, then finish-machined | Passivation after aging |
The pattern inside each family is the same trade between value and performance, judged at micro scale. Within aluminum, 6061 is the economy and 7075 the strength; within stainless, 303 is the controllability, 316 the protection, and 17-4PH the strength. Name the family for the application and the grade for the specific demand, with the temper and the age condition where they exist, because the certificate and the cutting strategy both follow the full callout. A micro part specification that stops at the family leaves the most important decisions to the shop.
How do the two families machine at the micro scale?
Both families hold the same tolerance bands in micro machining — milling and turning hold fit and alignment features to ±0.003 mm (milling) / ±0.002 mm (turning), with Cpk of 1.67 or higher on key characteristics and Cpk ≥ 1.33 on general characteristics. Capability is reported per characteristic: every report names the feature, its tolerance band, the sample size, the sigma method, and the study window., and precision grinding reaches ±0.002 mm, but the path differs at small size. Aluminum cuts fast with sharp tools and coolant that flushes chips, which keeps micro detail economical and predictable. Stainless needs tough carbide, heat-resistant coatings, and a consistent feed so it does not work-harden on a small section, and a work-hardened micro feature is difficult to recover because the stock is thin and the tolerances are tight.
Deflection and chip control matter more at micro scale. A small tool reaching into a micro feature bends under cutting pressure, and a thin wall in aluminum or a small section in stainless moves with it, so the finishing pass has to be planned with light engagement. Chips that clog a micro cut in aluminum are manageable with coolant; chips that weld to a small tool in stainless are a real problem. When you review a micro part quote, ask how the shop manages tooling and chips on the small features — the answers show whether the routing was designed for the material at that scale or simply applied from a larger part.
Tolerance and measurement close the micro-scale loop. Both families hold fit and alignment features to ±0.003 mm (milling) / ±0.002 mm (turning), with Cpk of 1.67 or higher on key characteristics and Cpk ≥ 1.33 on general characteristics. Capability is reported per characteristic: every report names the feature, its tolerance band, the sample size, the sigma method, and the study window., and precision grinding reaches ±0.002 mm, but on a micro feature the measurement is as demanding as the cut, because a position tolerance against a datum and a cylindricity on a small journal are resolved by a CMM probing features measured in fractions of a millimetre. Ask how the shop verifies the geometric controls on the drawing and what the inspection report will carry, because the answer shows whether the metrology plan was built for the part's actual features. A micro part is only as good as the measurement that proves it, the measurement is only as good as the report that ships with it, and the report is the evidence an auditor or a receiving inspector actually reads.
What finish should a micro part carry in each family?
Finish belongs to its family, and at micro scale it is specified by class on the drawing. Aluminum micro parts are anodized — standard for corrosion and color, hard anodize for wear — or conversion-coated where electrical conductivity must survive. Stainless micro parts are passivated to remove the free iron that machining embeds, with electropolish where a lower finish is needed on a functional surface. Applying the wrong family's treatment spends money and protects nothing, and the treatment's certificate joins the lot record beside the material certificate.
The micro scale sharpens the finish decision in two ways. First, the ratio of surface to feature is high, so a finish applied unevenly is proportionally more visible and more damaging. Second, the dimensional effect of a coating is a larger fraction of a small feature, so an anodized or plated micro part has to have its dimensions adjusted for the coating thickness before it is cut. Name the finish and its class at quoting, state whether dimensions are before or after treatment, and let the supplier plan the sequence — on a micro part, the finish is part of the geometry, not an afterthought.
How do you validate and verify the choice on a micro part?
Validation starts with the specification and flows through the first article. Name the grade with its condition — 6061-T651, 7075-T7351, 316L annealed, 17-4PH in its age condition — because the certificate can only confirm what the drawing names, and the micro machining can only be planned for the state the part will arrive in. Run the DFM review with the material visible, and when the family choice is genuinely open, machine one micro part in each and let the application test decide — the cost of two parts is trivial against the cost of choosing wrong at volume.
Verification is the two-step chain that runs on every lot: the material certificate names the grade, the condition, and the heat or lot number, and the incoming stock is XRF-verified against it before machining. The check matters more on a micro part because a part machined beautifully from the wrong grade fails every test that assumed the right one, and the failure shows first in the smallest features. When the drawing, the certificate, and the physical material agree, the material decision is closed. The parts guide on this site shows how material, tolerance, and finish land on a quoted micro part, and the quality page explains how material verification and CMM inspection are handled.
Cost follows the family but lands at the micro scale. Aluminum is cheaper on the material and the cycle lines, and the saving shows across the many small parts that make up a micro program; stainless adds cost on both lines, with the grade spread from the economical 303 to the tougher 316L and the aged 17-4PH. Compare the two families at the same quantity, because a micro quote at a tiny batch for one family and at volume for the other compares the batch rather than the material. When the application allows either family, aluminum is the economical answer; when it demands stainless, the extra cost is the price of the property, and no cheaper grade that lacks it is a substitute. Where the drawing is genuinely undecided, prototype both at the same quantity and let the first-article numbers settle the debate. And because a micro part is judged by the smallest features it carries, every document in the chain — the certificate, the XRF record, the CMM report — should trace to the same lot, so the material story holds unbroken from the bar to the box.
What this shop actually runs
If you ask for capability evidence, you deserve a specific answer. This facility operates 100+ CNC machines across 6,800 m2 in Dongguan, China, staffed by 100+ people. The machines, listed by category:
- 5-axis machining centres (10+) - Mazak VARIAXIS I-600 (510/910/510 mm travel, 12,000 rpm, 22 kW, 71.6 N·m, 30-tool BT40) and Mazak INTEGREX I-250H S (B-axis 0.0001°, 24 kW, 124 N·m, 38-tool HSK-A63). Chosen for compound-angle parts, single-setup multi-face prismatic work, and aerospace fittings where datum transfer loss is unacceptable.
- 3-axis / 4-axis machining centres (50+) - Brother SPEEDIO S500Z2N / S700Z2N (10,000 rpm, 14/21-tool ATC, 0.7 s tool-to-tool, Z-axis 2.2 G) and Mazak VCE-430A L / VCE-570C L (10,000 rpm, 118 N·m, 24-tool ATC). Chosen for prismatic parts, plates, brackets, housings.
- Swiss-type turning centres (10+) - Citizen Cincom A20-3F7 (Φ20 mm, 10,000 rpm), TSUGAMI B0206-III / B0205-III (Φ20 mm, 0.1 µm), STAR SR-32JII / SR-32J Type N (Φ32 mm, 8,000 rpm). Chosen for long, slender rotational parts at ±0.002 mm.
- CNC turning centres (20+) - HEADMAN T70M/1250 (720 mm swing, 117 mm bore, 3,000 rpm, BMT75 12-station turret). Chosen for shafts, bushings, flanges up to ±0.002 mm.
- Wire EDM (5+) - Sodick AQ535L (550/350/300 mm travel, ±20° taper, 800 kg workpiece). Chosen for hardened tool steel, internal splines, sharp internal corners that milling cannot reach.
- Inspection: Hexagon CMM + 20+ gauges - Hexagon CMM for coordinate measurement, plus vision system, profilometer, hardness tester, coating thickness gauge, XRF for material verification. The 10:1 rule (gauge 10x more accurate than the band) is applied to every feature.
- Machine highlights - VARIAXIS I-600: one-setup multi-face 5-axis + MAZA-CHECK auto-calibration; INTEGREX I-250H S: DONE-IN-ONE mill-turn, B-axis 0.0001° + Ai thermal shield; SPEEDIO S500Z2N / S700Z2N: Z-axis 2.2 G + 0.7 s ATC; Swiss-type: guide-bush switchable + 0.1 µm resolution.
Tolerance bands: ±0.003 mm milled, ±0.002 mm turned, ±0.002 mm ground or wire-cut. Capability indices: Ppk >= 1.67 at the initial study; Cpk >= 1.33 ongoing in production.
Certifications: ISO 9001:2015 (116024/A/0001/UK/En) · IATF 16949:2016 (131941/A/0001/SM/En, design clause 8.3 excluded) · ISO 14001:2015 (116024/B/0001/UK/En), held by Dongguan Zhuohang Ltd (USCC 91441900MA4UWT6X39, since 2005).
Service area: we supply Austria, Belgium, the Czech Republic, Denmark, France, Germany, Greece, Hungary, Ireland, Italy, the Netherlands, Spain, Sweden and the United Kingdom in Europe; Brazil, Canada, Mexico and the United States in the Americas; China, Israel, Japan, Singapore and Sri Lanka in Asia; Australia and New Zealand in Oceania; and South Africa in Africa, with worldwide delivery from China.
Contact: Felipe Iglesias, [email protected], +86 13126536955.
Unsure whether your micro part should be aluminum or stainless?
Email [email protected] or call +86 13126536955 for a material recommendation tied to the application and the micro scale, with the grade, the finish, and the verification spelled out.
Frequently Asked Questions
How do I choose between aluminum and stainless for a micro machined part?
In short, Score the application: weight, load, corrosion, and temperature decide the family. Weight-critical or cost-sensitive micro parts point to aluminum; parts needing corrosion resistance, strength at a small section, or temperature capability point to stainless.
Which grades work best for small aluminum features?
In short, 6061-T6 is the general default, cutting cleanly and holding fine threads in moderate sections. 7075-T6 and -T7351 step up in strength, with the temper stress-relieved for thin sections.
Which stainless grades suit micro parts?
In short, 303 is the free-machining grade for lighter corrosion duty; 304 and 304L cover general service; 316 and 316L are the corrosion workhorses; 17-4PH adds strength. On a micro part the small section makes machinability matter, because a work-hardening grade in a tiny feature is harder to control.
Do the two families hold different tolerances on micro features?
In short, No — both machine to the same bands: milling and turning hold fit and alignment features to ±0.003 mm (milling) / ±0.002 mm (turning), with Cpk of 1.67 or higher, and precision grinding reaches ±0.002 mm. What differs is the effort, not the band: aluminum reaches it fast, stainless needs more careful cutting on small features.
Which family handles thin walls and micro threads better?
In short, Aluminum is soft, so thin walls flex and very small tapped holes strip, though it machines fast. Stainless is tougher and holds fine features under load, but it cuts slower and work-hardens on small sections.
What finish should a micro machined part carry in each family?
In short, Aluminum micro parts are anodized by class or conversion-coated; stainless micro parts are passivated, with electropolish where a lower finish is needed. On a micro part the coating's dimensional effect is a larger fraction of the feature, so name the finish and state whether dimensions are before or after treatment.
How do I verify a micro part is the grade I specified?
In short, Require the material certificate with the heat or lot number, and confirm the supplier XRF-checks the incoming stock against it before machining. A part machined from the wrong grade fails every test that assumed the right one, and the failure shows first in the smallest features.
Standards referenced on this page
- ASTM A276/A276M-17 — Standard Specification for Stainless Steel Bars and Shapes
- ASTM B211/B211M-19 — Standard Specification for Aluminum-Alloy Rolled or Cold Finished Bar, Rod, and Wire
- ISO 9001 — Quality management systems — Requirements