Materials for mil-spec CNC machined parts

The alloys and plastics that the CNC suppliers we match typically machine for defense-adjacent, ground support, training and dual-use parts. For each grade: the designations you may see on a drawing, where it is usually used and what it means for machining cost and finish. Figures are typical; the specification on your drawing governs.

Aluminum

Aluminum alloys

Most enclosures, brackets, mounts and optics housings start as aluminum bar or plate. Aluminum machines fast, takes MIL-PRF-8625 anodize and MIL-DTL-5541 chem film well and is not a DFARS specialty metal.

Alloy / temper	UNS	EN / JIS	Typical spec on drawing	Typical use and notes
6061-T6 / T651	A96061	EN AW-6061 / A6061	AMS-QQ-A-225/8 bar, AMS-QQ-A-250/11 plate, ASTM B221	Enclosures, brackets, panels. Good corrosion resistance and weldability; moderate strength
7075-T6 / T651 / T7351	A97075	EN AW-7075 / A7075	AMS-QQ-A-225/9 bar, AMS-QQ-A-250/12 plate	Mounts, rails, load-bearing brackets. High strength; T73 tempers trade some strength for stress-corrosion resistance
2024-T351	A92024	EN AW-2024 / A2024	AMS-QQ-A-225/6 bar, AMS-QQ-A-250/4 plate	Fatigue-loaded fittings. Poor corrosion resistance bare: anodize or chem film plus paint
5083-H116 / H321	A95083	EN AW-5083 / A5083	ASTM B928, ASTM B209	Marine and vehicle panels. Non-heat-treatable; gummy in machining

Background: 7075 aluminium alloy (Wikipedia).

Steels

Stainless, alloy and tool steels

Steels cover parts that see wear, impact or high clamp loads: pins, shafts, latches, hinge hardware and fixture components. Heat treatment is usually subcontracted to a local heat-treat house with a certificate per lot.

Grade	UNS	EN / JIS	Typical spec	Typical use and notes
303 / 304 / 316L	S30300 / S30400 / S31603	1.4305, 1.4301, 1.4404 / SUS303, SUS304, SUS316L	ASTM A276, A479, AMS 5639 (304)	Fittings, fasteners, marine hardware. 303 machines best; passivate per AMS 2700 or ASTM A967
17-4 PH	S17400	1.4542 / SUS630	AMS 5643, ASTM A564	Shafts, latches, high-strength fittings. Machined in condition A, aged H900 to H1150 per drawing
15-5 PH	S15500	1.4545	AMS 5659	Like 17-4 PH with better transverse toughness
4140 / 4130	G41400 / G41300	1.7225 (42CrMo4) / SCM440, SCM430	ASTM A29, AMS 6370 (4130 bar)	Pins, shafts, brackets. Quench and temper; plate or black oxide
4340	G43400	SNCM439 (close)	AMS 6414 (VAR), AMS 6415	High-strength shafts and fittings. Through-hardens in large sections; hydrogen embrittlement relief after plating
1018 / 1045	G10180 / G10450	C45 (1.0503) / S45C	ASTM A108	Low-stress brackets, spacers, fixture plates
D2 / H13	T30402 / T20813	1.2379 / 1.2344, SKD11 / SKD61	ASTM A681	Wear plates, test fixtures, tooling. Rough, harden, then grind or hard-mill

4140 vs 4340 steel: heat treat and cost · 17-4 PH from H900 to H1150

Titanium

Ti-6Al-4V (Grade 5, UNS R56400, EN 3.7165) is used where weight matters and aluminum is not strong or stiff enough: UAV and unmanned platform structural parts outside export control, lightweight mounts and fittings. Common specs are AMS 4928 for bar and AMS 4911 for plate and sheet. CP titanium Grade 2 (UNS R50400) appears on corrosion-critical, low-load parts.

Titanium is a DFARS specialty metal. It machines at lower surface speeds than steel with high-pressure coolant and rigid fixturing, so expect a higher cost per part than the same geometry in 7075. See AMS 4911 vs AMS 4928.

Copper alloys and nickel

Brass, copper, bronze and Monel

Free-cutting brass C36000 (ASTM B16) machines fastest of all metals and suits connectors, fittings and knobs. Copper C11000 (ASTM B187) is used for electrical bus parts and grounding hardware. Aluminum bronze C63000 (ASTM B150) and Monel 400 (UNS N04400, ASTM B164) cover marine and seawater-exposed hardware where stainless is not enough. Nickel alloys are specialty metals under DFARS.

Aluminum bronze and Monel for marine hardware

Plastics

Engineering plastics

Plastics are used for insulators, wear pads, cable guides, light housings and RF-transparent covers. They move with heat and clamping, so typical achievable tolerance is looser than for metals, around ±0.02 to ±0.05 mm depending on size, confirmed per drawing.

Plastic	Typical use	Notes
Acetal (POM, Delrin type)	Bushings, rollers, latches, knobs	Machines cleanly, low friction, low moisture uptake
PEI (Ultem type)	Electrical insulators, housings near heat	High heat resistance, inherently flame resistant grades available
PEEK (unfilled / 30% GF)	High-temperature insulators, wear parts	Expensive stock; anneal between roughing and finishing on precision parts
Nylon 6/6	Spacers, guides, low-cost wear parts	Absorbs moisture: allow for dimensional change
PTFE (virgin / filled)	Seals, low-friction pads, insulators	Creeps under load; avoid tight press fits

Ultem, Delrin and nylon for mil-spec plastic parts

Selection

Choosing a material and writing the callout

Name the spec, not just the alloy

"7075" alone leaves temper, product form and test requirements open. Write the alloy, temper and spec, for example 7075-T7351 per AMS-QQ-A-250/12, so the mill certificate can be checked line by line.

Plan for dissimilar metals

Steel or stainless fasteners in aluminum housings corrode in wet service unless they are isolated or coated. MIL-STD-889 lists compatible couples; call out sealant, wet-install or coatings on the drawing.

Match finish to alloy

Type III hardcoat on 7075 and 2024 behaves differently from 6061 and grows into the part. State whether dimensions apply before or after finishing, and mask threads and bonding surfaces.

If the part needs	Usual first candidates	Trade-off to check
Light enclosure, sealed and anodized	6061-T6	Lower strength than 7075
Stiff, strong mount or rail	7075-T651 / T7351	Stress corrosion in T6; poor weldability
Wear or impact surfaces	4140 / 4340, 17-4 PH, D2	Heat treat cost and distortion
Weight-critical structure	Ti-6Al-4V	Material and machining cost; DFARS melt source
Saltwater exposure	316L, aluminum bronze, Monel 400	Cost and availability of bar sizes
Electrical insulation	PEI, PEEK, POM	Tolerance on thin walls

Related: capability range and finishes · applications · quality and mill certificate review · mill certs and traceability · DFARS clause text at acquisition.gov.

FAQ

Material questions

Do matched suppliers buy material to AMS specifications?

Yes, when the drawing calls for it. Bar and plate to AMS, ASTM or AMS-QQ specifications are sourced through mill or distributor stock with a mill certificate traceable to heat number. Some AMS products, for example vacuum-remelted 4340 or titanium plate in thick sections, are not always stocked in Vietnam and may add lead time. The supplier confirms availability in its quote.

What happens if my contract carries the DFARS specialty metals clause?

Say so in the RFQ. DFARS 252.225-7009 restricts the melt source of specialty metals such as titanium, many alloy and stainless steels and nickel and cobalt alloys. The supplier then quotes bar with melt-source certification that you can review. Aluminum is not a specialty metal. You remain responsible for deciding whether the clause applies to your part.

Can a supplier substitute an equivalent grade?

Only with your written approval. EN, JIS and Chinese equivalents are close to their US counterparts but not identical in chemistry limits, temper designations or test requirements. If the specified grade is hard to source, the supplier proposes an alternative in the quote and you decide.

Does the material choice change the export classification of my part?

It can. Classification depends on the part, its design and its end use, not only on the material, and it is your responsibility. We do not accept ITAR-controlled technical data or EAR-controlled data without your licence. Confirm classification with your compliance officer before sending files.

Have a drawing with a material callout?

Send it with material spec, finish and quantity. We reply within 24 business hours with your matched suppliers.

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