Aluminum is the most commonly CNC-machined metal on the planet. Because of that, almost every CNC shop claims to be great at aluminum. But sending the same aluminum part drawing to five different shops will reliably produce five different prices, five different lead times, and — most importantly — five different levels of quality.

The cheapest quote on aluminum parts is rarely the cheapest once you account for rejected batches, delayed shipments, and the engineering time spent chasing a supplier who does not communicate. This guide walks through ten factors that matter more than the bottom-line number on a quote — factors I have learned to check the hard way after machining aluminum parts for over a decade.

1. How Fast Do They Machine Aluminum Specifically?

Aluminum cuts fast. That is the whole point. A shop that runs aluminum efficiently should be able to remove material at 500-1,000 surface feet per minute (SFM) on a modern CNC mill with carbide tooling. A shop that runs aluminum conservatively — at 300 SFM because their machines are underpowered or their programmers are cautious — will quote longer cycle times and higher prices for the same part.

Ask this question: "What feed rate and depth of cut would you run on a 6061-T6 pocket 50mm wide and 20mm deep?"

A shop that actually knows aluminum will give you a specific answer involving a 12-16mm end mill, 8,000-12,000 RPM, and 2,000-4,000 mm/min feed rate. A shop that hesitates or gives a vague answer probably machines more steel than aluminum — and your parts will pay for their learning curve.

2. What Aluminum Alloys Do They Stock?

A shop that stocks 6061-T6 and nothing else is a generalist. There is nothing wrong with being a generalist, but if your parts call for 7075-T6, 2024-T3, 5083-H321, or MIC-6 cast tooling plate, a generalist will need to source material from a distributor — adding 2-5 days and a material markup.

Here is what different aluminum alloys tell you about a shop's capabilities:

Alloy When It Appears What It Signals About the Shop
6061-T6 70% of aluminum parts Baseline. Every shop stocks this.
7075-T6 Aerospace, high-stress fittings Shop likely has aerospace or defense work
2024-T3 Aircraft skins, structural Shop definitely has aerospace customers
5083-H321 Marine, chemical tanks Shop serves marine or chemical processing industries
6061-T651 (plate) Large plates, fixture bases Shop has large-bed machines (1m+ travel)
MIC-6 cast plate Vacuum chambers, optical bases Shop understands flatness and stress-relieved material
7050 / 7150 Aerospace forgings Shop is deep in the aerospace supply chain

If your parts are 6061 and always will be, any competent shop works. If you anticipate needing 7075, 2024, or specialized alloys, ask the supplier what non-6061 aluminum jobs they ran last month — and ask for a photo.

For a deeper dive on alloy selection, see our aluminum CNC machining complete guide and our 6061 vs 7075 comparison.

3. What Is Their Anodizing Situation?

Most aluminum parts need anodizing. The question is whether the shop handles it in-house or outsources it — and whether their outsourcing partner is reliable.

In-house anodizing means the shop controls the full timeline. The parts come off the machine, go to the anodize line, and ship. Outsourced anodizing means the parts leave the shop, go to a third-party finisher, come back to the shop for inspection, and then ship. Each handoff adds 1-2 days and a risk of miscommunication about color, thickness, or masking requirements.

Ask: "Do you anodize in-house? If not, what is the turnaround time for Type II and Type III anodizing through your partner?"

For Type III hard anodize specifically, confirm that the shop has experience with it. Hard anodize grows the part dimension by roughly 50% of the coating thickness — a 50-micron coating adds about 25 microns per surface. A shop that does not account for this in their machining will deliver parts that do not fit.

For a complete guide on aluminum anodizing, see our aluminum anodizing guide for CNC parts.

4. How Do They Handle Thin-Wall Aluminum Parts?

Thin-wall aluminum machining separates competent shops from the rest. A wall thickness under 1.5mm in aluminum will vibrate during machining, producing chatter marks, dimensional inaccuracy, and sometimes a scrapped part. A shop that knows aluminum uses specific strategies to manage this: light radial engagement (under 10% of tool diameter), high-speed toolpaths that maintain constant tool engagement, and sometimes fixturing tricks like packing thin sections with machinable wax.

Ask: "What is the thinnest aluminum wall you have successfully machined in a production part, and what was the wall height?"

A shop that says "0.5mm wall, 15mm tall" and can show you the part is an aluminum specialist. A shop that says "2mm minimum wall" is a generalist being honest — which is better than a shop that says "any wall thickness" without qualification.

5. What Tolerances Do They Guarantee on Aluminum?

Aluminum is thermally active — it expands roughly 23 microns per meter per degree Celsius. A part machined at 30 degrees in a hot shop will measure differently at 20 degrees in your inspection room. A shop that understands aluminum accounts for this.

Standard commercial tolerance for aluminum CNC parts is plus/minus 0.1mm. A good aluminum shop routinely holds plus/minus 0.025mm on critical features. Tolerances tighter than plus/minus 0.01mm require temperature-controlled inspection, and the shop should be upfront about whether they have that capability.

Red flag: a shop that claims "plus/minus 0.005mm on all features" without asking about your part geometry, the feature size, and the inspection temperature. That tolerance is achievable on a turned diameter in a temperature-controlled lab. It is not achievable on a 200mm-long milled pocket in an un-air-conditioned shop in July.

For a complete tolerances reference, see our CNC machining tolerances guide.

6. How Many Aluminum Parts Do They Run Per Month?

Volume experience matters. A shop that runs 50 aluminum parts a month has different process maturity than a shop that runs 5,000. The high-volume shop has learned — through painful experience — what tooling lasts, what feeds and speeds are actually optimal (not just what the tooling catalog says), and how to manage batch consistency.

Ask for a rough number: "How many aluminum parts did you ship last month?"

You are not looking for a specific number. You are looking for whether they know the number. A shop that can answer immediately — "roughly 800 aluminum parts, mostly enclosures and brackets" — tracks their work. A shop that has to think about it or says "many" does not.

7. Do They Understand Aluminum's Burr Behavior?

Aluminum burrs differently than steel. Steel burrs tend to be small and brittle. Aluminum burrs can be large, ductile, and tenacious — especially on 6061 in the T6 condition. A shop that primarily machines steel will often under-deburr aluminum parts because they are used to steel's burr behavior.

This sounds minor. It is not. I have seen entire batches of aluminum parts rejected because sharp burrs on threaded holes and cross-drilled intersections damaged mating components during assembly. A shop that takes aluminum seriously has a defined deburring process: vibratory tumbling for external surfaces, manual deburring of thread starts and cross-hole intersections under magnification, and a final inspection step that specifically checks for burrs.

Ask: "What is your deburring process for aluminum parts with threaded holes?"

The answer should mention specific tools ( carbide deburring blades, vibratory bowls with specific media) and a defined inspection step. "We deburr everything" without details means "the operator runs a file over the part if they remember."

8. Can They Handle Large Aluminum Plates Without Warping?

Aluminum plate — particularly 6061-T651 in thicknesses under 10mm — contains residual stress from the rolling process. When you machine away material from one side, the stress imbalance causes the plate to warp. This is one of the most common causes of out-of-tolerance aluminum parts.

A shop that knows aluminum manages this in several ways:

Ask: "If I send you a 300mm x 200mm x 8mm 6061 plate with pockets on one side, how do you prevent warping?"

If they cannot answer this question specifically, do not send them large aluminum plate work.

9. What Is Their Aluminum Scrap Rate?

Every shop scraps parts occasionally. The question is the rate and the transparency. A shop that claims a 0% scrap rate on aluminum is either lying or doing such low-complexity work that the claim is meaningless.

For aluminum, a realistic internal scrap rate on production work is 0.5-2% for simple parts and 2-5% for complex, tight-tolerance parts. A shop that admits to a 2% scrap rate and can explain what causes most of their scrap (broken tools, setup errors, material defects) is running an honest operation. A shop that deflects the question is hiding something.

Ask: "What is your typical internal scrap rate on aluminum production runs, and what causes most of it?"

The follow-up question — "What causes most of it?" — is the real test. It tells you whether the shop does root-cause analysis or just re-runs scrapped parts and hopes for better luck.

10. Can They Show You a Recent Aluminum Job — Right Now?

This is the single most effective supplier evaluation technique I know. During a video call, ask: "Can you show me an aluminum job that is running on your shop floor right now?"

A direct manufacturer walks to a machine, shows you the part being cut, and explains what it is and who it is for. A trading company or broker makes excuses — the shop floor is closed for cleaning, the camera does not work, the production manager is not available today.

This test filters out brokers, who add 15-30% markup and a communication delay, from actual manufacturers. It takes 60 seconds and is more informative than any amount of email back-and-forth.

For more supplier verification techniques, see our guide on how to verify a Chinese CNC supplier remotely.

The Decision Framework

Here is how to weight these ten factors when evaluating an aluminum CNC machining supplier:

Factor Weight Why It Matters
Aluminum-specific speed (factor 1) 15% Directly determines your part cost
Alloy range and stock (factor 2) 10% Determines whether they can handle your future work
Anodizing integration (factor 3) 15% Affects lead time and finish quality
Thin-wall capability (factor 4) 5% Matters only if your parts have thin walls
Tolerance reliability (factor 5) 20% Directly determines whether parts fit
Volume experience (factor 6) 10% Predicts process maturity
Burr management (factor 7) 5% Affects assembly and end-user experience
Plate warping control (factor 8) 5% Critical for large flat parts, irrelevant for small blocks
Scrap rate transparency (factor 9) 5% Honesty indicator
Live shop floor verification (factor 10) 10% Filters out brokers

The most heavily weighted factors — tolerance reliability, aluminum-specific speed, and anodizing integration — are where the real cost difference between a good shop and an average shop shows up. An average shop quotes a lower price but delivers late because they are slow, fails inspection because they cannot hold tolerance on aluminum reliably, and makes you manage the anodizer yourself. A good shop costs more on the quote and less in real life.

Get a Quote for Your Aluminum Parts

Choosing an aluminum CNC machining supplier is not about finding the lowest quote. It is about finding the shop whose process discipline, aluminum-specific expertise, and communication practices mean your parts arrive on time, in spec, and ready to use — the first time.

Send your aluminum part drawing for a free DFM review. Our engineering team will evaluate your design for aluminum manufacturability, identify cost-saving opportunities (thinner walls where possible, standard radii, anodize-ready geometry), and provide an aluminum 6061 CNC parts quote within 24 hours. We run aluminum parts every day — 6061, 7075, 5083, and MIC-6 — and we have been doing it for over a decade.

Upload your STEP file today. We will tell you honestly whether your part is a good fit for our shop, and if it is, exactly what it will cost and when it will ship.