CNC Prototype Savannah GA: How a Design File Becomes a Working Part

• 1
  What a CNC prototype Savannah GA job actually removes
• 2
  Choosing 3, 4 or 5 axis for a Savannah prototype
• 3
  How material choice changes the prototype
• 4
  Tolerance, inspection and what the numbers mean
• 5
  Which process fits the prototype

Every CNC prototype starts as a solid block, bar or casting. The tool removes material until what is left matches the CAD model. Nothing is added, so the geometry you can reach with a cutter defines what you can build. That single constraint drives most of the decisions that follow, and it is the first thing to check before you send a file for quotation.

A three-axis mill moves the tool in X, Y and Z while the part stays still. Holes, pockets, faces and slots on five or six sides of a part are reachable, but each new face needs a new setup. The part is unclamped, rotated and re-zeroed. Every setup adds a small positional error, and those errors stack.

A five-axis machine tilts the tool or the table, so the cutter approaches the part from an angle instead of straight down. Undercuts, deep pockets and contoured faces are cut in one setup. On our 16 simultaneous five-axis machining centers we hold ±0.005 mm on features that would need three or four separate fixtures on a three-axis machine.

Stock removal also sets the floor on wall thickness. A thin floor or a tall thin rib will chatter or deflect before the cutter finishes. As a rule, keep unsupported walls above 0.8 mm in aluminium and above 1.0 mm in stainless steel unless the part is supported by a fixture. Below that, the part may survive inspection but fail in a drop test.

Axis count is a cost and accuracy decision, not a prestige decision. A part with all features reachable from one direction, such as a flat mounting plate or a simple bracket, runs faster and cheaper on a three-axis machine. Adding axes only pays off when it removes a setup or reaches geometry a straight tool cannot.

Four-axis machining adds rotation around one axis, usually A. This suits parts that are cylindrical or that carry features on several faces around a common centerline: shafts, bushings, valve bodies, rotary manifolds. The part turns while the tool works, so a bolt circle or a cross-hole pattern stays concentric instead of being dialed in four times.

Five-axis earns its place on impellers, turbine blades, medical instrument housings and any part with a compound angle. The tool stays normal to the surface, which keeps the step-over even and the surface finish predictable. It also lets a short, stiff tool reach deep cavities that would need a long, flexible tool on a three-axis machine.

The trade-off is programming and verification time. A five-axis toolpath takes longer to prove out, and the first article usually costs more than the same geometry cut in three setups. For one or two pieces of a simple part, three-axis is often the better answer. For a complex part that will be tested and then produced, five-axis protects the geometry you validated.

Aluminium 6061-T6 is the default for functional prototypes. It machines fast, holds ±0.005 mm without much effort, and takes anodizing well. When the prototype has to match a production part in strength, 7075 gives roughly twice the yield strength but cuts slower and is harder to anodize evenly. 2024 sits between the two and is common in aerospace brackets.

Stainless 303 is the free-machining grade and the easiest to turn on a lathe. 304 and 316L are tougher, gummy at low speeds and prone to work hardening if the tool rubs instead of cuts. 17-4PH (SUS630) can be machined in the annealed condition and then aged to reach high strength, which makes it useful for parts that see real load.

Titanium TC4 (Ti-6Al-4V) cuts at roughly one third the speed of aluminium and needs sharp tooling and steady coolant. Grade 2 is softer and easier for brackets and spacers. Both hold tolerance well but spring back, so light finishing passes matter more than on steel.

Engineering plastics behave differently again. PEEK and Ultem hold their shape at high temperature but are abrasive and expensive. POM (Delrin) is dimensionally stable and the usual choice for gears and sliding parts. ABS and PC are cheap but move with humidity and heat, so they suit fit checks rather than loaded testing. The material you pick decides what the prototype can prove.

±0.005 mm (roughly ±0.0002 in) is the tightest general tolerance we hold across a batch, and it applies to critical features, not to every dimension on the drawing. A part with a 0.005 mm bore and a 0.5 mm cosmetic step does not need the same tolerance on both. Marking which features matter keeps the price down and the process honest.

Surface finish follows the same logic. As-machined surfaces sit around Ra 1.6–3.2 μm. A finishing pass brings that to Ra 0.8–1.6 μm, and fine finishing reaches Ra 0.2–0.8 μm on sealing faces and bearing bores. Each step adds time, so specify the finish only where a gasket, O-ring or sliding contact needs it.

Inspection is where the numbers are proven. We check raw material on arrival, monitor dimensions during the run, and inspect 100% of parts before shipment. Reports are available on request. For a prototype, ask for the report on the features you marked critical rather than a full first-article package.

One trap catches engineers new to prototyping: a tolerance that is easy to hold on a lathe can be impossible on a milled pocket, and vice versa. A round bore is turned to size with a boring bar. The same diameter inside a milled cavity depends on tool deflection and interpolation. The process, not the drawing, decides which features stay tight.