If you walk through our shop right now, about 65% of the machines are cutting aluminum. The rest are split between stainless, plastics, and a couple of titanium jobs that always seem to be running late.

Aluminum is what keeps the lights on around here. It's fast to machine. It's predictable. It doesn't eat end mills for breakfast the way stainless does. And when you design for it properly, you get parts that cost less, ship faster, and work better than anything else you could specify.

But aluminum is also easy to get wrong. Wrong alloy. Wrong heat treat. Wrong assumption about anodizing. I see these mistakes regularly, and they're almost always avoidable.

This is the guide I wish engineers had before they hit send on their first aluminum RFQ.

Aluminum Alloys We Actually Use — And Which Ones We Don't

People think "aluminum" is one material. It isn't. The difference between 6061 and 2024 is about as large as the difference between mild steel and 4140 chrome-moly. Picking the wrong one means your part costs more than it should, takes longer to machine, or fails in service.

Here are the alloys we machine regularly and what they're actually good for:

Alloy Tensile Strength Machinability Corrosion Resistance Anodize Quality Typical Cost Index
6061-T6 310 MPa Excellent Very good Excellent (clear, black, color) 1.0x (baseline)
6061-T651 310 MPa Excellent Very good Excellent 1.1x
7075-T6 572 MPa Good Below average — will corrode Poor — dark, uneven 1.4x
7075-T651 572 MPa Good Below average Poor 1.5x
5052-H32 228 MPa Good Superior (marine grade) Good (satin matte) 1.1x
6063-T6 240 MPa Excellent Good Best — architectural grade 1.05x
2024-T3 470 MPa Fair Poor — needs alclad Not recommended 1.8x
MIC 6 165 MPa Best — cast tooling plate Good Not for structural 1.6x

A few notes on this table that matter more than the numbers:

6061-T6 is the default for a reason. It machines like butter, anodizes beautifully, and costs the least. If you don't know what alloy to pick, start here. You can rule it out later if the strength or environment doesn't work.

7075-T6 is about twice the strength of 6061 and roughly the strength of mild steel at one third the weight. But it corrodes. If your 7075 part sees moisture, you need a coating. And it anodizes ugly — dark gray, uneven. Don't spec clear anodize on 7075 expecting it to look like 6061. It won't.

2024-T3 is the classic aerospace alloy. Excellent fatigue resistance. Terrible corrosion resistance. Almost always used with an alclad layer (pure aluminum cladding) for protection. We machine it less than the others because fewer applications need what it offers.

5052 is for boats. Salt spray, marine environments, chemical exposure. It forms well, welds well, and machines adequately. Lower strength but the corrosion numbers are what sell it.

6063 is 6061's prettier sibling. Slightly lower strength, but anodizes to a gorgeous finish. If the part is visible — architectural trim, consumer product housing, display hardware — 6063 is worth the switch.

6061 vs 7075: The Decision That Drives Cost

I wrote a detailed comparison of these two, but here's the short version that covers 90% of decisions:

Pick 6061 when you care about cost, corrosion resistance, or appearance. Most CNC parts are 6061, and most of them work fine.

Pick 7075 when strength is the primary requirement and corrosion isn't an issue. Aerospace brackets, high-stress bicycle components, rock climbing hardware — these are 7075 territory.

The cost difference is real. A bracket milled from 7075 costs about 30-40% more than the same part in 6061. Part of that is material cost. Part is slower machining speeds. Part is the fact that 7075 is harder on tools.

If you're not sure, buy 6061. Read the full comparison if you need the deep data — 6061 vs 7075: which alloy for your CNC project.

What Makes Aluminum Great for CNC Machining

Three things make aluminum the best material for CNC work:

First, cutting speed. You run aluminum at 2-3x the SFM of steel and 6-8x the SFM of titanium. A part that takes 30 minutes in aluminum might take 90 in stainless and 3 hours in titanium. That's direct cost to you.

Second, heat dissipation. Aluminum pulls heat into the chip instead of into the tool. That means tools last longer and you don't pay for thermal distortion in your tolerances.

Third, availability. Every metal supplier on earth stocks 6061 plate and bar. Exotic grades of stainless or titanium can take weeks to source. 6061 ships same day.

The catch: aluminum is soft. It scratches, dents, and galls. Threaded holes in aluminum need care — helicoils for anything that gets disassembled frequently, careful torque specs, and anodizing before threading for the cleanest results.

Design Rules That Matter for Aluminum Parts

Aluminum is forgiving. You can break these rules and still get parts. They'll just cost more.

Internal radii: 3mm minimum for cost, no smaller than your end mill diameter. A 3mm internal corner radius lets us use a 6mm end mill — fast, rigid, good tool life. A 1mm internal radius forces a 2mm end mill — slow, fragile, expensive.

Wall thickness: 1mm minimum for aluminum, 1.5mm for anything structural. Below 1mm, the wall vibrates during cutting and you get chatter marks and tolerance problems. If you must go thin, add ribs.

Pocket depth: no more than 4x the diameter of the smallest tool that fits. A 10mm wide pocket can be about 40mm deep with a standard 10mm end mill. Deeper than that, you need extended reach tooling, which costs more and deflects more.

Threaded holes: M3 minimum. Below M3, taps break too easily in production. For anything smaller, consider thread inserts or redesign the assembly.

Anodize growth: add 0.025mm per surface to your tolerance stack. Type II anodizing builds up about 0.012-0.025mm on each surface. If you have a precision bore at 10.000mm before anodize, it will be about 9.975mm after. Account for this in your drawing or your parts won't fit.

Full design guide is here: CNC Aluminum Part Design Guide.

Surface Finishing: Anodizing and Beyond

90% of the aluminum parts we ship get either as-machined finish or Type II anodize. Here's what each option actually gives you:

As-machined. Visible tool paths, Ra 0.8-3.2µm. Good for functional parts that don't need cosmetics. Zero cost, zero lead time impact.

Type II anodize (clear). 5-25µm oxide layer. Protects against corrosion, improves wear resistance, gives a uniform satin appearance. Adds $15-40 per batch, one day to lead time. Can be dyed black, blue, red, gold — black is most common for industrial parts.

Type III anodize (hardcoat). 25-150µm oxide layer. Dark gray-green color, Rockwell C 60-70 surface hardness. For wear surfaces and high-stress applications. Costs 2-3x Type II, adds 2-3 days.

Bead blast + anodize. Blasting before anodize gives a uniform matte texture that hides tool marks. For consumer-facing parts, this is the standard finish. Adds $10-20 per batch.

Alodine (chromate conversion). Chemical film, golden or clear. No dimensional change (unlike anodize). Good corrosion protection, minimal cost. Used a lot in aerospace for parts that need corrosion resistance without thickness buildup.

Powder coat. Thick (50-150µm), durable, available in any color. Not for precision surfaces — the thickness variation is too large. Good for brackets, enclosures, and visual parts that don't have tight fits.

For more detail: CNC Machining Surface Finishes Guide.

Aluminum Machining Cost: What You're Actually Paying For

A simple aluminum bracket in 6061, quantity 10, will run about $15-35 per part from a Chinese machine shop. Same part in the US might be $45-90. These are real numbers from quotes I've compared.

What drives the price:

Factor Low Impact Medium Impact High Impact
Quantity 1-5 pcs 10-50 pcs 100+ pcs (setup amortized)
Tolerances ±0.1mm ±0.05mm ±0.01mm (CMM verification needed)
Surfaces to machine 1-2 sides 3-4 sides (multiple setups) 5+ sides (needs 5-axis or many setups)
Internal corners R3mm+ R1.5mm R1mm or less
Threaded holes 1-5 holes 6-15 holes 15+ holes or small threads (<M4)
Surface finish As-machined Type II anodize Type III or electropolish

The two biggest cost drivers in aluminum: number of setups and tolerances. Every time we have to flip the part and re-indicate, you pay for labor and a new setup. Every time you go from ±0.1mm to ±0.01mm, you pay for slower feeds and CMM time.

I wrote a longer piece on this: CNC Machining Cost Reduction Guide.

When Aluminum Is Not the Right Call

Aluminum is great for a lot of things. Here's when to pick something else:

Get an Aluminum CNC Quote — We'll Check Your Alloy Choice for Free

Aluminum shows up across every industry we serve, but the requirements differ. Automotive CNC parts — sensor brackets, EV battery mounts, brake adapters — need production repeatability and material cert traceability. Robotics parts — cobot joint housings, motor flanges, LIDAR mounts — need aggressive lightweighting and bearing bore precision. For industry-specific deep dives, see our guides on robotics and automation CNC machining and automotive CNC machining.

We machine more aluminum than anything else. 6061, 7075, 5052, 6063, 2024. From a single prototype to 10,000 production parts. Type II and Type III anodizing, bead blasting, alodine, powder coat — all in-house or through partners we've worked with for years.

Send your drawing. I'll give you a quote within 24 hours, and if your alloy choice doesn't make sense for what you're building, I'll tell you. That DFM feedback is free whether you order from us or not.