Guides / Buying
What to Settle Before You Order Aluminum CNC Parts
September 20, 2026 by The Alu CNC Parts editorial desk / how this is checked
Aluminum is the default material for machined parts, and for good reason. It cuts fast, holds tight tolerances, takes a hard protective coating and weighs a third of what steel does. That also makes it the material most people buy badly the first time, because a part can be quoted five different ways depending on four lines that are missing from the drawing.
This guide covers what to settle before the request goes out.
Start with the alloy, not the shape
There is no such thing as ordering a part in “aluminum”. Shops stock specific alloys in specific tempers, and they behave differently enough that the choice changes the price, the strength and how the part looks when it comes back.
| Alloy | Why it gets chosen |
|---|---|
| 6061-T6 | The default. Cuts fast, welds, anodizes evenly, widely stocked. |
| 7075-T6 | Roughly twice the yield of 6061. Does not weld, costs more, anodizes darker. |
| 2024-T351 | Fatigue life on parts that flex repeatedly. Needs a coating outdoors. |
| 5052 / 5083 | Corrosion resistance, marine service, formed and welded assemblies. |
| MIC-6 cast plate | Large plates that have to stay flat after material comes off. |
If you are unsure, write 6061-T6. The full comparison with 7075 covers when to move up. It is right for most brackets, housings, mounts and plates, and a shop will tell you when the part needs something else. What you should avoid is leaving the alloy blank and hoping the quotes are comparable, because they will not be.
The temper matters as much as the alloy. 6061 in the T6 temper is heat treated and stable. The same alloy in T0 is soft and will not hold a thread the same way. Write both, and see what each temper code means if the letters are unfamiliar.
Tolerance only what has to be tolerated
The single most expensive habit in machined parts is putting a tight tolerance on every dimension because it looks rigorous. Each band tighter than necessary pulls in a slower finish pass, a reamer instead of a drill, an extra inspection step and sometimes a temperature controlled room.
A workable rule:
- ±0.13 mm for outside profiles, clearance holes and anything that just has to fit in a space.
- ±0.05 mm for locating features, counterbores and pockets that carry a bought-in part.
- ±0.025 mm for bearing bores, dowel holes and shaft fits. These get bored or reamed, not drilled.
- ±0.013 mm only when you can explain why. On aluminum this is rare and it is measurably more expensive.
Name the two or three critical features on the drawing and let the rest sit at a general tolerance note. Shops read that as a buyer who knows what they are doing, and they quote it accordingly. There is more on which bands are worth paying for.
Surface finish follows the same logic. Ra 1.6 um is what a normal finish pass produces. Asking for Ra 0.8 um or better means a separate operation at a slower feed, so only ask for it on sealing faces and sliding surfaces.
Decide the finish before you fix the fits
Aluminum corrodes in air, so nearly every part gets coated. This is where first-time buyers get caught, because the coating lands on the part after it has been machined to size.
A Type II anodize builds around 5 to 25 um. Roughly half of that thickness grows outward from the original surface, so a 20 um coating closes a bore by about 20 um across the diameter and adds the same to an outside dimension. Type III hardcoat is thicker again, 25 to 50 um, and can easily take a slip fit into an interference fit.
You have two ways to handle this and both are fine, as long as you say which one you want:
- Dimension after coating. State that the tolerance applies post-anodize and let the shop machine slightly oversize or undersize to land on it.
- Mask the feature. Bores, threads and datum faces are plugged or taped so no coating reaches them.
Masking costs a little in handling. Discovering the problem after 200 parts have been coated costs a great deal more. The dimensional growth guide has the numbers, and the Type II against Type III comparison covers which coating you actually want.
One more practical point: anodizing is usually sent out to a specialist with a minimum batch charge. On ten pieces that minimum can be a bigger line on the quote than the machining. If cost matters more than looks on a low quantity part, chem film or plain bead blast will save real money.
Understand what the quote is actually made of
A machined part price is not driven by the weight of the finished part. It is driven by:
The block it starts from. You pay for the stock size, not the finished size. A part drawn from 80 mm plate when it would clear 60 mm carries the extra material and the extra time spent cutting it away. Stock form is one of the few real cost levers on your drawing.
The number of setups. Every time a part is unclamped, re-fixtured and re-indicated, someone is paid for it. The same five features cost less on two faces than spread across five.
The alloy. 7075 and 2024 run two to three times the price of 6061 per kilo. On a part that is mostly swarf, that gap is most of the quote.
Quantity. Programming, fixturing and the first proving cut are paid once. On a single piece they are almost the whole price. Past roughly fifty pieces the unit price stops dropping quickly and settles.
That last point is worth planning around. If you expect to need forty parts this year, quoting forty now is usually cheaper than quoting ten four times, because you pay for the setup once. The full cost breakdown walks through every line.
What to send with the request
Shops quote fastest and most accurately from a STEP file plus a PDF drawing. The model carries the geometry. The drawing carries what the model cannot: alloy and temper, tolerances, surface finish, thread callouts, the finish and any masking, plus a note on which dimensions are critical.
If you have no CAD at all, photographs with a caliper or a steel rule in frame are enough to start a conversation. Say what the part bolts to and which dimensions have to stay exactly as they are. A shop that reverse engineers from a sample will tell you what it needs to measure or scan.
Finally, say how many you want and when you need them. A quantity and a date turn an enquiry into something a shop can actually price, and that alone tends to halve the time it takes to get a number back. When the quotes come in, check they are quoting the same part before you compare the numbers.
Sources
Figures in this guide are checked against the following. Published values are typical ranges for test material, not a guarantee for your part.
- ASTM B209, aluminum sheet and plate specification
- MIL-A-8625, anodic coatings for aluminum
- ISO 2768, general tolerances
Frequently asked questions
How long does a first aluminum part take?
For a simple milled part in 6061 with no coating, one to two weeks is normal once the drawing is settled. Anodizing usually adds three to five working days because it goes to a second vendor in batches. Tight tolerances, five sided work and heat treatment all add days before anything ships.
Is aluminum cheaper than steel to machine?
Per part, usually yes, even though aluminum costs more per kilo than mild steel. It cuts several times faster, tools last longer and parts come off the machine in a fraction of the time. Machine time is the larger share of most quotes, so the faster material wins.
Do I need a 3D model, or is a drawing enough?
A STEP file plus a PDF drawing is the combination shops want. The model gives them the geometry to program from and the drawing carries what the model cannot hold: tolerances, surface finish, alloy, temper, finish and which features are critical. A drawing on its own is workable. A model on its own leaves the shop guessing at what matters.
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