When a prototype has to survive real loads, 7075-T6 aluminum is usually the first material an engineer asks for. It is stiff, machines cleanly and offers a strength-to-weight ratio that makes ABS or PLA look like toy plastic. At Protom, we machine a lot of 7075 for aerospace fixtures, drone arms, automotive test brackets and robotics end-effectors. This guide is what we tell customers who want the part right the first time: which tolerances are realistic, what surface finish to expect, and where the money goes.
Why 7075-T6 Wins for Functional Prototypes
7075-T6 sits at the high-strength end of aluminum alloys. Typical yield strength is around 503 MPa (73 ksi), roughly double that of 6061-T6. That means a thinner 7075 wall can carry the same load as a thicker 6061 wall, which matters when you are trying to keep mass down. It also machines well with the right tooling, although it is harder on cutters than 6061 because of the zinc content.
The catch is corrosion resistance. Bare 7075 does not like salt or moisture, so outdoor or marine prototypes usually need a clear anodize or a conversion coating. If you only need strength in a dry environment, leave it bare and save a few days of finishing.
Tolerances a CNC Cell Can Actually Hold
Tight tolerances are where CNC machining earns its keep. On a modern 3-axis mill with calibrated ball screws, we routinely hold positional tolerances of ±0.005 mm (±0.0002 in) on small features. For general prototype work, ±0.025 mm (±0.001 in) is the sweet spot: easy to verify with standard tooling, fast to machine and unlikely to fight thermal expansion on the shop floor.
- Hole diameters: reamed holes ±0.013 mm (±0.0005 in); drilled holes ±0.05 mm (±0.002 in).
- Threaded holes: the thread itself buys you tolerance, but the positional tolerance of the hole center should be held to ±0.05 mm or better.
- Flatness on a milled face: 0.05 mm over 100 mm is comfortable; 0.025 mm needs a finish pass and careful fixturing.
The fastest way to blow a budget is to tolerance every dimension to ±0.005 mm. Only tolerance the features that mate with something else. Everything else gets a liberal general tolerance, and the machinist stops chasing microns.
Surface Finish: What Ra Numbers Mean on the Floor
Surface finish is where drawings often promise more than physics allows. A face-milled aluminum part typically lands around Ra 1.6 µm to 3.2 µm. That looks fine to the eye but feels slightly directional. If you need a cosmetic show piece, plan for a secondary finish: bead blast drops the visual tool marks, anodize evens out the texture, or a light polish can get you toward Ra 0.4 µm.
Internal corners and pockets are harder. A standard end mill leaves a radius equal to half its diameter. If the drawing calls for a sharp internal corner, someone has to sink an electrode and EDM it, which adds setup, cost and lead time. For prototypes, we usually ask the customer whether a small radius, down to 0.5 mm, will still work. It almost always does.
Where the Cost Lives
7075 material cost is higher than 6061, but on prototypes the material is rarely the dominant line item. The cost drivers are:
- Number of setups: Each time the part comes out of the vise and is flipped, the operator risks a tiny stack-up error. A single-setup design costs less and repeats better.
- Tool access: Deep pockets, thin walls and undercuts force custom tooling or longer cycle times.
- Volume: One part is expensive because programming and setup are amortized over a single piece. At five to ten pieces, the per-part price drops sharply.
- Inspection: Every tight tolerance is another measurement on a CMM or bore gauge. If you spec it, we have to check it.
For functional prototypes, we usually quote a 3-5 day lead time after drawing approval. That is enough time for one or two setups, standard inspection and a quick deburr. If you need next-day parts, be ready to pay a premium and accept slightly looser finishing.
Common DFM Mistakes and How to Fix Them
The best prototype is the one that teaches you something without eating your schedule. The mistakes we see most often on 7075 parts:
- Sharp internal corners: Replace with a 0.5 mm radius unless the joint truly has to be square.
- Deep, narrow holes: A hole deeper than four times its diameter is hard to clean and easy to break a drill in. Use through-holes or increase diameter.
- Thin walls: Below 0.8 mm, 7075 tends to chatter and distort. Aim for 1.5 mm or more if the design allows.
- Tight tolerances on cosmetic features: Only tolerance mating surfaces. A logo pocket does not need ±0.01 mm.
Send a STEP file plus a PDF with callouts, and we will flag the risky features before cutting metal. If your project needs production metal parts later, our die casting team can review the same CAD for high-volume feasibility.
Bottom Line
CNC machining 7075 aluminum gives you prototype parts that behave like production parts. Hold tolerances where they matter, let the shop choose standard tooling elsewhere, and plan surface finish by function rather than cosmetics. If you need stronger parts or just want to compare metal options, Protom's CNC machining service covers 7075, 6061, steel and engineering plastics from one-off prototypes to low-volume production.
