Precision CNC machining in Savannah: how tolerances decide the process

• 1
  What CNC machining in Savannah actually buys you
• 2
  How 3-axis, 4-axis and 5-axis change the part
• 3
  Where ±0.005 mm is reasonable and where it is not
• 4
  Surface finish is a separate decision from tolerance
• 5
  Material choice moves the process window
• 6
  What the inspection report should contain
• 7
  Lead time, quantity and the Savannah supply chain
• 8
  Which process fits your geometry

Savannah sits on the I-16 and I-95 corridors with the Port of Savannah next door, so a large share of the region's work is aerospace structures, automotive and EV components, medical instruments and heavy machinery parts. Those buyers rarely need a machine tool for its own sake. They need a part that fits the assembly on the first try, arrives inside a build schedule, and can be documented when an auditor asks.

CNC machining is subtractive: a rotating cutter removes material from a solid billet under program control. The process holds its accuracy because the tool path is fixed in a file and the machine repeats it. What changes between shops is not the idea, it is the number of setups, the rigidity of the fixture and how carefully the drawing was written.

Three numbers decide almost everything on a quote: the tightest tolerance on the print, the smallest internal corner radius, and the number of faces that carry a tolerance. A part with one tight bore on one face is a different job from a part with tight bores on six faces. The second one may still be cheap, but only if the shop owns the right equipment.

So the useful question is not "who is nearby". It is which process the geometry forces you into, and whether the shop you pick has that process under one roof. The rest of this page walks through that decision.

A 3-axis mill moves X, Y and Z only. The tool always points straight down. Every face that needs work has to be presented to the spindle, which means re-fixturing the part. Each re-fixture adds setup time and adds a small positional error that stacks onto the tolerance you already asked for.

Manual 3+2 positioning adds a rotary trunnion. The part is tilted to a fixed angle, locked, then cut. This is how most prismatic parts with features on four or five faces are made. It is fast and it is stable, but the tool still approaches along one direction per operation, so deep side pockets with curved walls stay difficult.

Simultaneous 5-axis moves all five axes at once, so the cutter tip stays normal to a curved surface while it travels. That is what makes impellers, turbine blades, medical bone plates and organic-looking brackets machinable in one setup. The trade is programming time and machine cost per hour. On a simple flat plate, 5-axis buys you nothing.

A useful rule: if more than one tight feature sits on a non-orthogonal face, price the job both ways. If the 3-axis version needs three or more fixtures, the 5-axis version usually wins on total cost and on tolerance stack.

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  3-axisFlat plates, pockets, slots, housings open on one side
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  4-axisShafts, bushings, parts with features indexed around a bore
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  5-axisCurved surfaces, undercuts, one-setup complex geometry

±0.005 mm (±0.0002 in) is achievable on a rigid machine with a warm spindle, sharp tooling and a temperature-stable shop. It is not achievable on every feature of every part. The tolerance applies to the dimension you call out, at the datum you define, in the material you specify.

A 200 mm long aluminium bracket will move more than 0.005 mm between a 20 °C inspection room and a 30 °C loading dock. Thermal expansion of aluminium is roughly 23 × 10⁻⁶ per °C, so 200 mm grows about 0.046 mm over that 10 °C swing. If the print does not state a reference temperature, the shop has to guess, and guessing costs money.

Thin walls behave the same way. A 0.8 mm wall on a 100 mm part will deflect under cutting force and under its own residual stress after the material is removed. We usually rough, stress-relieve or let the part rest, then finish. That adds a day to the schedule but it is the only way the final dimension holds.

The practical call: put the tight tolerance on the features that mate, and loosen everything else. A print where every dimension is ±0.005 mm tells the shop nothing about which two features actually matter. It also raises the price, because every dimension now needs a measured report.

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  Tight, keep itBearing bores, seal faces, dowel holes, mating spigots
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  Loosen itClearance holes, outer profiles, non-mating faces
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  Add to the printDatum scheme, reference temperature, critical-to-quality list

Ra and dimensional tolerance are independent. A bearing bore can be ±0.005 mm and still need Ra 0.2–0.8 μm so the seal does not weep. A cosmetic cover can be Ra 1.6–3.2 μm and carry a loose ±0.1 mm profile. Mixing the two on one drawing line is a common mistake.

As-machined finish lands around Ra 1.6–3.2 μm with a sharp cutter and a normal feed. Getting to Ra 0.8–1.6 μm usually means a finishing pass with a smaller stepover, or a wiper insert. Below Ra 0.8 μm you start adding operations: fine boring, lapping or polishing, and the cost curve turns up steeply.

Feature size limits what is possible. A Ø3 mm deep pocket cannot be polished by hand. A Ø1 mm cross-hole cannot be honed. If the function needs a mirror bore, design the bore big enough for the tooling and say so on the print, or accept the finish the process can actually deliver.

Anodizing, plating and powder coating change dimensions. Hardcoat anodizing builds roughly 25–50 μm per surface, which matters on a ±0.005 mm bore. Call out pre-plate dimensions or mask the tight features. Otherwise the parts will be in tolerance at the machine and out of tolerance at the assembly bench.

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  Ra 1.6–3.2 μmGeneral machined surfaces, non-sealing faces
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  Ra 0.8–1.6 μmSliding fits, seal counterfaces, wiper bores
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  Ra 0.2–0.8 μmBearing journals, hydraulic sealing surfaces

Aluminium 6061-T6 and 7075 machine fast and hold tight dimensions well. 7075 is stronger but more prone to residual stress movement after heavy stock removal, so it often needs a rough-and-rest cycle on thin sections. 2024 behaves similarly and is common in aerospace brackets.

Stainless 303 and 304 cut cleanly, though 304 work-hardens if the feed is too light. 17-4PH (SUS630) in the H900 condition is common for medical and aerospace shafts, but it should be machined in the annealed or H1150 state where possible and heat treated after, because the hardened condition wears tooling quickly.

Titanium TC4 (Ti-6Al-4V) and Inconel are where 5-axis pays off. Both hold heat at the cutting edge, so the tool needs a tilted approach to keep the contact patch small and the heat in the chip. Rigid setups and high-pressure coolant matter more here than on aluminium. Thin titanium ribs will chatter if the fixture is not supported underneath.

Plastics need their own rules. POM and PEEK cut well but move with temperature; ABS and PC can gum up if the feed is low. Carbon fibre is abrasive and needs diamond-coated tooling. The tolerance you can hold on a plastic part is usually wider than on the same part in aluminium.

A machined part is only as good as the evidence attached to it. For a medical or aerospace order, the drawing is not the deliverable, the drawing plus the measured data is. Ask what will be measured, on which instrument, and against which datum before the job starts.

A CMM report on the critical-to-quality dimensions is normal. For a bearing bore, a bore gauge or an air gauge reading is often more useful than a CMM point because it captures the full diameter rather than a few touches. For a flatness call, a surface plate and indicator still beat a CMM in a drafty room.

Material certificates and finish certificates should travel with the parts. If the part is heat treated or anodized outside, the certificate chain has to stay intact, or the traceability breaks at the subcontractor. That is a paperwork problem that shows up as a rejected shipment.

GreatLight inspects 100% of parts before shipment, with raw material checks, in-process monitoring and a final inspection. Reports are issued on request. The four certificates held are ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022, which covers quality, automotive, medical device and information security scope respectively.

Savannah's advantage is logistics, not machining capacity. The port and the interstate crossings make it a good inbound and outbound point, which matters when a part has to reach an assembly line in Greenville, Charleston or Atlanta on a known date. Freight time is predictable on that corridor.

Engineering time is the variable. A quote that arrives in a week has already cost you a week. GreatLight returns a quotation and a free DFM analysis within 12 hours, and production can start within 24 hours once the drawing and material are confirmed. Typical parts ship in 3–5 days.

Quantity rarely changes the process. There is no minimum order quantity, so one prototype and a 10,000-part run go through the same first-article route. What changes is the fixture: a prototype may run on a soft jaw, while a production run gets a dedicated fixture that cuts cycle time and tightens the repeatability.

Confidentiality is part of sourcing. Uploads are handled as secure and confidential, and an NDA is available on request. For defense-adjacent or medical work, agree the NDA before the drawing is sent, not after the first article.