Views: 3 Author: Site Editor Publish Time: 2026-08-14 Origin: Site
A metal part answers questions a plastic one cannot: will it carry the load, survive the heat, or pass a drop test? You have two main routes to a metal-performing or metal-looking part - CNC machining of real alloy stock, and vacuum casting of metal-filled urethane for short runs. This page helps you pick the right one before you brief a shop, with the numbers we use on the floor.
Table of Contents
Metal parts validate strength, thermal behavior, EMI shielding and the exact feel of a production unit. For brackets, heat sinks, structural members and threaded assemblies, only real metal gives trustworthy test data - which is why CNC is the preferred route when performance is on the line. Plastic cannot stand in for a tensile or thermal test; it can only stand in for a shape review.
CNC machining cuts a real alloy block into a functional part with production-grade properties, holding total-indicator-run (TIR) far tighter than any casting. In our experience a 3-axis or 5-axis mill turns a billet into a first article in 5-7 days for typical bracket and enclosure work. The plastic counterpart of this route is covered in our CNC plastic prototyping guide.
Vacuum casting reproduces a master pattern in metal-filled polyurethane, giving a metal appearance and some thermal heft at a fraction of CNC cost for 10-100 copies. It is not true metal - treat it as a cosmetic or light-functional stand-in, never as a structural validation part. We see it used for trade-show models and early user trials where the part will not be loaded.
The table below lists typical values from published data sheets. Tensile strength and density drive the choice; machinability decides how fast and cheap the part is to cut. Full context is in our materials guide.
Alloy (common grade) | Tensile strength (MPa) | Density (g/cm3) | Machinability | Typical use |
Aluminum 6061-T6 | 276 | 2.70 | Excellent | Brackets, enclosures, fixtures |
Aluminum 7075-T6 | 572 | 2.81 | Good | High-stiffness, aerospace-ish parts |
Stainless 304 | 505 | 7.93 | Good | Corrosion-resistant, housings |
Stainless 316L | 485 | 8.00 | Good | Marine, medical, food contact |
Brass C360 | 300-460 | 8.50 | Excellent | Fittings, cosmetic metal, connectors |
Titanium Gr.5 (Ti-6Al-4V) | 950 | 4.43 | Difficult | Aerospace, high strength-to-weight |
Note: titanium's strength-to-weight ratio (around 214 MPa per g/cm3) is why it wins where every gram counts, even though it is the slowest and most expensive to machine. Always weigh the property against the machining cost before specifying it.
CNC metal holds about +/- 0.05 mm standard on a 100 mm feature (ISO 2768-f fine band is roughly +/- 0.05 to +/- 0.10 mm depending on size), with tight features to +/- 0.01 mm possible on 5-axis at added cost. Material certification (mill cert) and CMM reports are available on request. Vacuum casting of metal-filled urethane holds roughly +/- 0.15 mm per 100 mm and cannot match the strength of machined alloy - keep it for non-critical copies. For the molded alternative when volume climbs, see our injection molding and bridge tooling guide.
Factor | CNC machined metal | Vacuum-cast metal-fill |
Material | Real alloy (Al, steel, brass, Ti) | Metal-filled polyurethane |
Strength | Production-grade | Cosmetic / light only |
Standard tolerance | +/- 0.05 mm | +/- 0.15 mm / 100 mm |
Lead time | 5-7 days | 7-12 days (incl. mold) |
Best quantity | 1-50 | 10-100 |
Use for | Load, heat, EMI, threads | Look, demo, unloaded copies |
CNC metal parts run 5-7 days; vacuum casting adds about 7-12 days for mold making but is cheaper per copy in the 10-100 range. Material is the biggest cost lever after geometry - a titanium part can run several times an aluminum one of the same size because of both stock price and slower cutting. The full cost picture, including hidden charges, is in our cost and quote guide.
An anonymized example from recent work: a robotics customer needed a mounting bracket to verify bolt pattern and stiffness before tooling. We machined it in 6061-T6 to +/- 0.05 mm in 6 days, ran a torque test to 18 Nm (part held, margin to the 276 MPa yield was comfortable), and shipped with a CMM report. For the 40-unit field trial they chose metal-filled vacuum casting at roughly a third of the CNC per-part cost, accepting that those copies were for fit and handling only - not for the torque test. The split matched the test each batch had to pass. That is the pattern we recommend: CNC where performance is proven, casting where only appearance matters.
Send the model, alloy, quantity, tolerance and any certification needs (RoHS/REACH and mill certs on request). Lead with the test the part must pass so the shop recommends the right route instead of the one it finds convenient.
Q: What is the difference between CNC metal and vacuum casting metal?
A: CNC machines real alloy stock with full strength; vacuum casting uses metal-filled urethane for appearance and light function only. They are not interchangeable for structural tests - see the decision table above.
Q: Which metal is best for a bracket?
A: Aluminum 6061-T6 is the default - 276 MPa tensile, light, cheap to machine. Use 7075 where stiffness matters and stainless where corrosion resistance is required.
Q: How accurate is CNC metal?
A: About +/- 0.05 mm standard on a 100 mm feature; tight features to +/- 0.01 mm on 5-axis at added cost, per ISO 2768-f fine.
Q: Can you provide material certification?
A: Yes - for CNC machined alloys we can supply mill certificates and full CMM inspection reports on request.
Q: Is titanium prototyping available?
A: Yes, for aerospace and high-strength applications, though lead time and cost are higher than aluminum or stainless because titanium is slow to cut.
CTA: Send your CAD for a metal part quoted with the right process and inspection get a metal prototyping quote.
