Prototype CNC machining in Georgia

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  What a 5-axis setup changes about prototype geometry
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  Tolerances that hold on a first article
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  Material choice for a machined prototype
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  Prototype fit, finish and assembly checks
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  When machining is the wrong process for a prototype
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  Machined prototype vs molded prototype
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  Envelope and capacity at a glance

A 3-axis mill moves the tool in X, Y and Z while the part stays clamped. Every new face means a new fixturing operation. Each refixture adds setup error, and the errors stack. On a prototype with five or six angled faces, the stack can exceed the feature tolerance you care about.

A simultaneous 5-axis center adds two rotary axes, so the tool can reach the part from almost any direction in one setup. For prototypes the practical effect is fewer work holdings, which means fewer datum shifts and less chance that a hole pattern drifts relative to a mating face.

The geometry that benefits most is not the most complex shape. It is the part with several tight relationships between features on different faces: a valve body with an angled port, a bracket with a machined boss that must stay concentric to a bore.

Undercuts and pockets with a depth-to-diameter ratio above roughly 4:1 still need a long-reach tool. Five axes let you tilt the tool to shorten the effective overhang, but the tool is still thin. Chatter remains the limit, not the machine.

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  One setup, one datumAngled faces and cross-drilled holes come off the same zero point.
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  Shorter tool overhangTilting the spindle lets a stubby cutter reach deep pockets.
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  Not a fix for thin wallsBelow about 0.8 mm, deflection and heat govern, not axes.

A general tolerance of ±0.005 mm is achievable on a machined prototype when the feature is a bored hole, a turned diameter, or a flat face that can be measured directly. It is not realistic across every dimension on a drawing, and a quote that promises it everywhere is telling you the drawing was not read.

What matters is the datum scheme. If the drawing calls out a primary datum, a secondary and a tertiary, and the inspection report references the same scheme, the numbers mean something. If the drawing is a pile of bilateral dimensions with no datums, the machinist has to guess which face is the reference.

Surface finish couples to tolerance. A sealing face at Ra 0.2–0.8 μm usually needs a finishing pass with a small stepover, which adds cycle time. A non-critical cover at Ra 1.6–3.2 μm comes off in one pass. Tell us which faces seal and which are cosmetic.

Wall thickness is the usual surprise. A 0.5 mm aluminum wall will deflect under normal clamping pressure even if the cutter path is perfect. If the part needs walls under 0.8 mm, expect to discuss support material, light passes, and possibly a redesign.

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  Bores and turned diameters±0.005 mm is routine with a boring head or a finishing pass.
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  Long thin featuresA 100 mm shaft at Ø6 mm will not hold ±0.005 mm easily.
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  Freeform surfacesProfile tolerance is driven by the CAD model, not the machine.

Aluminum 6061-T6 is the default for prototypes that need to be handled, assembled and tested quickly. It machines fast, takes anodizing, and its strength is close enough to a production 6061 part that handling behavior is realistic. Use 7075 when the prototype sees real load, and 2024 when fatigue matters.

Stainless 303 is the easy one to machine; 304 and 316L are tougher and tend to work-harden if the feed is too light. For a medical or food-contact prototype, 316L is the usual pick and the finish matters as much as the alloy. 17-4PH gives you strength after heat treatment.

Titanium TC4 (Ti-6Al-4V) and Inconel are slow and expensive in prototyping. They are justified when the part will run hot, sit in a corrosive line, or be weight-critical. Otherwise a titanium prototype is an expensive way to learn nothing new about the design.

Plastics behave differently. POM and PEEK hold tolerance well; ABS and PP flex and can be gummy. For a snap-fit or a living hinge, the material governs whether the prototype tells you anything useful.

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  6061-T6Fast, stable, anodizes well. Default for most enclosures and brackets.
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  316LCorrosion resistance and cleanability. Common for medical and food parts.
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  PEEKHigh temperature and chemical resistance, but abrasive on tooling.

A prototype is usually judged by whether it assembles with the parts around it. That means the critical dimensions are the interface dimensions: hole positions for a mating connector, a bore that receives a bearing, a face that seals against a gasket. Machine those to the drawing and leave cosmetic surfaces looser.

Threads deserve a note. Cut threads are stronger and more accurate than rolled threads on a prototype, but a rolled thread is faster. For a prototype that will be torqued repeatedly during testing, say so and we will cut the threads and check them with a gauge.

Finishing changes dimensions. Anodizing adds roughly 5–15 μm per surface depending on the type; hardcoat adds more. If a bore must stay at size after coating, mask it or plan the pre-coat dimension. The same applies to electroless nickel and plating.

Marking is often an afterthought. Laser marking needs a minimum character height of about 1.5 mm to stay legible. If your part number has to fit on a 4 mm boss, plan the layout before the geometry is frozen.

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  Interface dimensions firstHold the fits; relax the cosmetics.
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  Mask before coatingAnodizing and plating shift bore sizes.
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  Laser markingMinimum character height around 1.5 mm.

Machining wins when the part is functional, the geometry is prismatic or turned, and you need real material properties. It loses when the part is a thin shell with complex internal ribs, when the surface must be optically smooth over a large area, or when you need twenty identical units in a week.

A thin-wall enclosure with snap features and internal bosses is often better as a molded or cast part. Machining it means removing most of the material from a solid block, which is slow and can distort the walls. A vacuum cast or 3D printed shell may answer the same question faster.

Very large parts hit a travel limit. Our largest envelope is 4,000 × 400 × 150 mm, and the medium machines cover 750 × 1,150 × 550 mm. A part beyond those envelopes has to be split or made another way.

Quantity is the other limit. One to a few hundred units is machining territory. Above that, die casting or molding usually wins on unit cost, and the prototype should be used to validate the design before tooling is cut.

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  Good fitFunctional brackets, housings, manifolds, fixtures, test rigs.
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  Poor fitThin cosmetic shells and large optical surfaces.
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  Travel limit4,000 mm is the longest single envelope we run.