Savannah GA of CNC: How Machined Parts Get Made

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  What the Savannah GA of CNC Process Really Covers
• 2
  Machine Types and What Each One Is Good For
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  Material Choice Drives Tolerance and Finish
• 4
  How Surface Finish and Inspection Fit Together
• 5
  What to Check Before You Place an Order
• 6
  When CNC Is the Wrong Choice
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  Step by Step: From Model to Shipped Part
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  Choosing a Process and Tolerance by Part Type

The savannah ga of cnc phrase gets used loosely. In practice it means a chain of steps: a 3D model is converted into toolpaths, a machine moves a cutter along those paths, and the finished part is measured against the drawing. Nothing about the city changes the physics. What changes is which shop you pick and how well it controls the variables.

Start with the drawing. A part that calls out ±0.005 mm everywhere is not the same as one that reserves that tolerance for a bearing bore and leaves the rest at ±0.1 mm. Tight tolerances on a long thin wall will drive cost, scrap rate and lead time up. Mark only the features that carry function.

Next comes fixturing. A 5-axis machine can reach five faces in one setup, which removes stacking error from repeated re-clamping. For a housing with bores on three sides, that single setup is often the difference between a true position of 0.02 mm and one that drifts past 0.05 mm.

Then toolpath strategy. Roughing clears material with a large cutter, semi-finishing leaves a uniform stock allowance, and finishing cuts the final surface. If the shop skips semi-finishing on a deep pocket, the finishing tool deflects and the wall tapers. You will see it in the CMM report before you see it on the bench.

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  Drawing firstTolerance callouts decide cost more than material choice.
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  Setup count mattersFewer re-clamps means less stacked error.
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  Semi-finishing is not optionalIt controls deflection before the finish pass.

A 3-axis mill cuts from one direction. It is fast, rigid and cheap per hour. Use it for plates, brackets, manifolds with features on a single face, and any part where the back side is flat. If your part needs holes on two perpendicular faces, a 3-axis machine forces a second setup, and the second setup adds error.

A 4-axis mill adds a rotary table, usually Ø400 mm. That lets the part index around one axis while the tool stays perpendicular. Good for shafts with cross-holes, cylindrical housings, and parts with repeating features every 90°. It is still a single rotary axis, so compound angles stay out of reach.

A 5-axis machine moves the tool or table on two rotary axes at once. Simultaneous 5-axis means the cutter stays normal to a curved surface while it moves. That is how you cut an impeller blade, a turbine vane or an organic bracket without leaving witness lines. It also lets you reach undercuts that no 3-axis setup can touch.

Turning centers handle round parts. A mill-turn center combines turning and milling in one machine, so a part with a turned OD and milled flats does not need to move between machines. That removes a whole setup and the concentricity error that comes with it.

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  3-axisFlat plates, single-face features, lowest cost per hour.
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  4-axisIndexed round and prismatic parts, repeating features.
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  5-axisCurved surfaces, undercuts, one-setup complex geometry.
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  Mill-turnTurned OD plus milled features, concentric by design.

Aluminum 6061-T6 is the default for prototypes and fixtures. It machines fast, holds ±0.005 mm on a rigid setup, and takes anodizing well. 7075 is stronger but galls more easily and costs more per kilo. If you need stiffness without weight, 7075 is the pick. If you need corrosion resistance and weldability, stay with 6061.

Stainless 303 machines freely and is the easiest of the 300 series to cut. 304 and 316 work-harden, so the cutter must keep moving and the feed must stay high enough to stay under the hardened layer. 17-4PH holds high strength after heat treatment and is common in aerospace fittings. Expect slower cycle times and more tool wear.

Titanium TC4 (Ti-6Al-4V) has a low thermal conductivity, so heat goes into the cutter instead of the chip. That means lower surface speed, more coolant, and a shorter tool life. It is the right material when you need a high strength-to-weight ratio, and the wrong one when a 6061 part would pass the load case.

Plastics behave differently. POM and PA are stable and machine cleanly. PEEK holds properties at high temperature but is expensive and abrasive on tooling. ABS and PC are fine for non-structural covers. For any plastic, watch clamping pressure: it deforms the part and the measured size will spring back after unclamping.

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  6061-T6Default aluminum. Fast, stable, anodizes well.
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  303 vs 316303 cuts freely; 316 resists corrosion but work-hardens.
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  TC4High strength-to-weight, slow to cut, hard on tools.
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  PEEKHigh-temperature plastic, abrasive, costly.

Surface finish is written as Ra, the arithmetic mean roughness. Ra 1.6–3.2 μm is a normal as-machined finish. Ra 0.8–1.6 μm needs a finer finishing pass with a smaller stepover. Ra 0.2–0.8 μm usually means a polished or lapped step after machining, and it adds cost and handling risk.

Finish and tolerance interact. A bore held to ±0.005 mm with a rough finish will wear faster in service, and a smooth finish on a loose bore does not help function. Specify both only where they matter. A sealing face needs both; a mounting boss rarely does.

Inspection closes the loop. A shop should check raw material on arrival, monitor dimensions during the run, and inspect the finished part before shipment. Reports are available on request. For a first article, ask for the CMM numbers on the features that carry the tolerance, not just a pass or fail stamp.

Full inspection before shipment is the standard here. If a feature is out of tolerance, it is better to catch it at the machine than at your assembly line. That is the whole point of measuring against the drawing rather than against the last part off the machine.

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  Ra 1.6–3.2 μmStandard as-machined surface.
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  Ra 0.2–0.8 μmPolished or lapped, higher cost.
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  InspectionMaterial, in-process and final checks; reports on request.

Certifications tell you what system the shop runs, not what part it will make for you. ISO 9001:2015 covers general quality management. IATF 16949:2016 applies to automotive work. ISO 13485:2016 is the medical device standard. ISO 27001:2022 covers information security, which matters when you send proprietary CAD files.

Ask about machine count and size range. A shop with 127 high-precision CNC machines, including 16 simultaneous 5-axis centers and 16 mill-turn centers, can absorb a rush job without pushing your part aside. Maximum processing size of 4,000 mm covers large frames that many shops cannot hold.

Ask about minimum order quantity. If a shop has no MOQ, you can run one prototype and then a 10,000-part production batch on the same process. That keeps the first article and the production part comparable, which is what you want when you are validating a design.

Confidentiality is a real concern. Uploads should be secure, and an NDA should be available on request. If a shop hesitates on an NDA for a new design, that is a signal about how it treats your files.

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  CertificationsMatch the standard to your industry, not to a logo wall.
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  CapacityMachine count and max size decide whether your job fits.
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  MOQNo minimum lets you validate before you scale.
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  NDAAvailable on request for proprietary designs.

CNC is subtractive. It removes material from a solid block, so a part with a hollow internal cavity and no access opening cannot be machined in one piece. That part needs casting, 3D printing or a split design with a bolted or welded joint.

Thin walls are another limit. A wall under 0.5 mm on aluminum will chatter and deflect during cutting. You can machine it, but the yield drops and the finish suffers. If the design allows 1 mm, take the 1 mm.

Very large flat parts with a single shallow pocket are often cheaper as sheet metal. If the part is a cover with a few holes and a bend, laser cutting and forming beat milling on cost and lead time. CNC wins when you need tight tolerances, complex 3D geometry or a machined surface finish.

Finally, quantity. For a simple part at 100,000 pieces, die casting or injection molding wins on unit cost. CNC wins from one piece to a few thousand, and it wins on iteration speed when the design is still moving.

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  Closed cavitiesUse casting or additive, not milling.
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  Walls under 0.5 mmExpect chatter and low yield.
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  Flat coversSheet metal is usually cheaper.
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  High volumeDie casting or molding beats CNC on unit cost.