Guide
What should you specify for an aluminum CNC machining job?
Aluminum CNC machining is the process of turning or milling a wrought bar, or a die-cast blank, down into a finished part, and every choice on the drawing changes what that part can do. 2011-T3 is a free-machining alloy suited to small turned parts made in volume, 6061-T6 is the general-purpose default for most machined shapes, and 7075-T6 is reserved for parts that are limited by strength rather than by stiffness, corrosion resistance or weldability.
By Saola Manufacturing engineering team · Published · Updated
This guide is for mechanical design engineers and sourcing managers who are about to fill in the material block of a machined-aluminum drawing. It works from the registered chemistry of three wrought alloys and from the coating rules that a common anodizing specification places on that chemistry, then walks through the practical questions: alloy, stock form, achievable tolerance and finish, and when a die-cast-then-machined blank is the better starting point than bar.
On this page
Which aluminum alloy should you specify — 2011-T3, 6061-T6 or 7075-T6?
2011-T3 is the free-machining choice for small turned parts, 6061-T6 is the general-purpose default for most machined shapes, and 7075-T6 is for parts a stress calculation shows need the extra strength.
For a side-by-side of the two wrought alloys most drawings choose between, see 6061 vs 7075 aluminum for CNC parts.
All three sit in different registered alloy families. 2011 is an aluminum-copper alloy with bismuth and lead added specifically to break chips and cut cleanly; 6061 is hardened by magnesium silicide; 7075 is hardened by zinc and magnesium with a copper addition, which is what gives it the highest strength of the three and also costs it corrosion resistance and weldability. The registered international symbol for 2011 is AlCu6BiPb, reflecting that copper-bismuth-lead chemistry (The Aluminum Association, International Alloy Designations and Chemical Composition Limits for Wrought Aluminum and Wrought Aluminum Alloys, revised August 2018).
| Element or property | 2011 | 6061 | 7075 |
|---|---|---|---|
| Copper (Cu) | 5.0–6.0 | 0.15–0.40 | 1.2–2.0 |
| Magnesium (Mg) | — | 0.8–1.2 | 2.1–2.9 |
| Zinc (Zn) | 0.30 | 0.25 | 5.1–6.1 |
| Silicon (Si) | 0.40 | 0.40–0.8 | 0.40 |
| Iron (Fe) | 0.7 | 0.7 | 0.50 |
| Chromium (Cr) | — | 0.04–0.35 | 0.18–0.28 |
| Bismuth (Bi) | 0.20–0.6 | — | — |
| Lead (Pb) | 0.20–0.6 | — | — |
| Others, each / total | 0.05 / 0.15 | 0.05 / 0.15 | 0.05 / 0.15 |
| Aluminum | Remainder | Remainder | Remainder |
| Nominal density, lb/in³ (g/cm³) | 0.102 (2.83) | 0.098 (2.70) | 0.101 (2.81) |
What does 2011-T3’s free-machining chemistry cost you?
Speed and surface finish, at the cost of corrosion resistance and weldability. A producer technical data sheet for 2011-T3 rod and bar rates its resistance welding, general corrosion resistance and anodize response as only fair, and says other forms of welding are not recommended.
The bismuth and lead in the registered chemistry are there to make the chip break short instead of stringing, which is why 2011 is the standard choice when a part is round, small in diameter, and made in volume on a sliding-headstock (Swiss-type) lathe: those machines feed bar automatically and depend on a clean-breaking chip to keep running unattended. The trade-off shows up away from the cutting tool. Per a producer technical data sheet for cold-finished 2011-T3 rod and bar, corrosion resistance, resistance welding and anodize response are each rated only fair, and other welding processes are not recommended at all (Kaiser Aluminum, Rod & Bar Alloy 2011 technical data, accessed September 2026). The same sheet notes that the registered lead range runs 0.20 to 0.6 percent, but RoHS-compliant 2011 bar has shipped with a 0.4 percent lead maximum since May 2009, which matters for a part going to a market that screens for restricted substances.
| Property | 2011-T3 |
|---|---|
| Ultimate tensile strength, ksi (MPa) | 55 (379) |
| Tensile yield strength, ksi (MPa) | 43 (296) |
| Elongation, % in 4D | 15 |
| Brinell hardness, 500 kg / 10 mm | 95 |
| Elastic modulus, 10³ ksi (GPa) | 10.2 (70) |
| Nominal density, lb/in³ (g/cm³) | 0.102 (2.82) |
Should a part be machined from bar stock or from a die-cast blank?
Bar stock gives full subtractive freedom for round, turned geometry; a die-cast blank arrives already close to the finished shape, so only bores, threads and datum faces need to be machined afterward.
A part cut entirely from bar or plate pays for every cubic inch of chip it removes, and it can carry whatever internal or external geometry the tool can reach. A die-cast blank is different: the casting process sets most of the outer shape, the wall thickness and any ribs or bosses in one shot, and machining is a second pass that touches only the features a cast surface cannot hold to print — a bore diameter, a thread, a flatness or true-position callout against a datum. Which route is cheaper depends on quantity and geometry, and the break-even guide works through that math in detail.
This is also where a shop’s actual equipment matters more than a generic capability list. Saola’s CNC turning runs 2011-T3 and 6061-T6 aluminum bar, along with steel and silicon bronze bar, at its lead CNC plant on conventional CNC lathes. CNC milling, at the same lead plant, is mainly a second operation on ADC12 aluminum and Zamak 3 zinc die castings sourced from partner foundries — it is not a general billet-machining job shop, and a large part that must be milled entirely from a solid block of bar or plate is not a fit here. Neither the turning side nor the milling side runs simultaneous five-axis machining, so a design that depends on simultaneous multi-axis contouring should be discussed before it is quoted, on either process. See the CNC turning and CNC milling pages for what each process actually covers.
When does a die-cast-then-machined blank beat machining from bar?
When the part is a housing, cover or bracket needed in enough volume that the tooling cost amortizes, its strength requirement is modest, and most of its shape can come straight out of the die.
A wrought 6061 or 7075 part machined from bar or plate has no tooling cost, so it wins at low quantity and on parts whose geometry a die simply cannot form. A die-cast aluminum blank pays once for the tool and then comes out close to net shape, so it wins once quantity is high enough to spread that cost, and especially on parts that are mostly hollow or ribbed rather than solid. Die-casting alloys such as ADC12 are not a substitute for 6061 or 7075 in every respect. They run lower strength and ductility than the wrought alloys in their peak tempers and are normally used as cast rather than heat treated, so a part that needs 7075-level strength should stay wrought. ADC12 is also not a good anodizing alloy, so a part that needs a bright decorative finish should stay wrought too. The ADC12 vs A380 guide covers that alloy chemistry, the break-even guide works the volume math with a worked example, and the thread insert guide covers how threads are usually handled in a cast-then-machined part. If a part is a stiffness-limited enclosure needed in quantity, it is worth sending the drawing for a die casting review alongside the machined version.
How tight can an aluminum CNC tolerance realistically be held?
It depends heavily on whether the feature was turned from bar or machined onto a cast blank. A feature Saola turns from aluminum bar is cut in one setup, and the quote confirms the band it can hold on your drawing; a bore or face machined onto an ADC12 casting is limited by how consistently the raw casting locates in the fixture.
A turned feature is cut directly into solid bar on a single setup, which is why a tight, repeatable tolerance band is realistic for diameters, lengths and shoulder positions on 2011-T3 or 6061-T6 round parts. A feature machined onto a die-cast blank is only as good as the datum surfaces the casting presents to the fixture; if the casting itself carries draft, parting-line offset or minor warpage, the machined feature inherits some of that variation no matter how precise the cutting operation is. General tolerance classes for both machined and cast features, and how ISO 2768 and NADCA classes relate to each other on one drawing, are covered in the tolerance guide; use that guide, not a single number here, to set the class for features that are not held to a specific true-position or profile callout.
Which finish should you specify for a machined aluminum part?
For 6061 and 7075 wrought parts, anodizing, powder coating or plating are all realistic options. For 2011 bar and for any die-cast-then-machined blank, plating or powder coating are the more predictable choices.
A common anodic-coating specification sets hard chemistry limits on which alloys can take which type of coating. Per that specification, chromic-acid (Type I) coatings are not applied, unless the drawing says otherwise, to aluminum alloys with a nominal copper content above 5.0 percent, a nominal silicon content above 7.0 percent, or a total nominal alloying content above 7.5 percent; hard anodic (Type III) coatings are not applied to alloys with a nominal copper content above 5.0 percent or a nominal silicon content above 8.0 percent (MIL-A-8625F, Anodic Coatings for Aluminum and Aluminum Alloys, Department of Defense, September 1993). Measured against the registered chemistry above, 2011’s copper content alone, 5.0 to 6.0 percent, already crosses the 5-percent line those two coating types share, and 7075’s zinc, magnesium and copper together already add up to 8.4 to 11.0 percent across the registered ranges, over the 7.5-percent combined limit for Type I. 6061 runs far lower on every column in the table above, which is why it is the easiest of the three to specify for a uniform, bright chromic or sulfuric anodized finish.
| Coating type | 2011-T3 | 6061-T6 | 7075-T6 |
|---|---|---|---|
| Type I, chromic acid | Excluded (Cu > 5%) | Permitted | Excluded (total alloying > 7.5%) |
| Type II, sulfuric acid | Permitted | Permitted | Permitted |
| Type III, hard anodic | Excluded (Cu > 5%) | Permitted | Permitted |
A die-cast ADC12 blank is a different case again. Its silicon and copper content make it not a good anodizing alloy, regardless of which wrought alloy the rest of the assembly uses. Powder coating, electroplating, zinc plating or RoHS blue trivalent chromate are specified on that blank instead. Pad printing and an oil-dip finish cover cosmetic marking and light corrosion protection on either the wrought or the cast parts. The finish comparison guide and the surface finishing page cover how to choose between them, and the threads-after-coating guide explains why a threaded hole needs a size allowance before it goes into any of these baths.
How should the drawing call out alloy, temper, stock form and finish?
Name the alloy, temper and product-form specification together, state whether the stock is bar or a casting, and call out the finish by name rather than by a general note.
A drawing that just says “aluminum” leaves the alloy, the temper and the stock form open to the supplier, and any of those choices can change strength, corrosion behavior or which anodize type is even legal to apply. A workable material block does four things:
- Names the alloy and temper together, for example 2011-T3, 6061-T6 or 7075-T6, rather than the alloy alone.
- States the product-form specification: rolled or cold-finished round or hex bar to ASTM B211/B211M, or extruded bar to ASTM B221, for a turned part; or names the die-casting alloy directly for a part that starts as a blank.
- Calls out the finish by name and, where it matters, by coating type — Type II sulfuric anodize reads differently from Type III hard anodize on a print, and only one of them may be realistic for the alloy chosen.
- Asks for a mill or foundry certificate with the shipment that names the alloy, temper and specification, matched to the heat or lot on the parts.
The RFQ guide covers where these notes belong in a full quote package, alongside drawing views, GD&T and inspection requirements.
Frequently asked
Can the same drawing be quoted in 2011, 6061 and 7075?
Only if the geometry does not depend on welding, a bright anodized finish or sustained corrosive exposure, since those three questions can rule an alloy out regardless of strength. State the alloy as fixed, or ask for a proposal, so the quote reflects the real constraint.
Is 2011 available as extruded bar as well as cold-finished bar?
Cold-finished round and hex bar to ASTM B211/B211M is the common form for 2011 because most 2011 parts are turned, small-diameter round shapes. State the product form on the drawing rather than assuming it.
Can an ADC12 die-cast blank be machined to the same tolerance as a turned bar part?
Not on every feature. A bore or face machined directly into the casting can hold a tight tolerance, but a dimension that spans two as-cast surfaces inherits the casting’s own tolerance and any draft or parting-line offset between them.
Is 7075 ever anodized?
Yes, with sulfuric-acid (Type II) or hard (Type III) anodizing; chromic-acid (Type I) anodizing is the type its chemistry rules out by default, unless the drawing specifically calls for it.
Where do I send a drawing in any of these alloys?
Send it through the quote form or to rfq@saolamfg.com as drawn, with the temper, product-form specification and finish stated, and note whether the alloy is fixed or open to a proposal.
Sources
- International Alloy Designations and Chemical Composition Limits for Wrought Aluminum and Wrought Aluminum Alloys, The Aluminum Association, revised August 2018
- MIL-A-8625F, Anodic Coatings for Aluminum and Aluminum Alloys, Department of Defense, September 1993
- ASTM B211/B211M-19, Aluminum and Aluminum-Alloy Rolled or Cold Finished Bar, Rod, and Wire, ASTM International, 2019 (scope)
- ASTM B221-21, Aluminum and Aluminum-Alloy Extruded Bars, Rods, Wire, Profiles, and Tubes, ASTM International, 2021 (scope)
- Rod & Bar Alloy 2011 Technical Data, Kaiser Aluminum, accessed September 2026