Small batch CNC processing explained

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  Key takeaways
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  What small batch CNC processing actually means
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  How the cutting process works, step by step
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  Which machine geometry suits your part
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  Material behavior and the tolerance you can hold
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  Where the cost sits in a small run
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  Small batch CNC against the alternatives

Small batch CNC processing is subtractive machining of a limited quantity of parts, usually between 1 and a few thousand, cut from solid stock on a CNC mill or lathe. There is no pattern, no mold and no die. The cutting path comes from a CAD/CAM model, and the machine follows it tool by tool.

The batch size is not the defining feature. What defines it is that the tooling cost stays near zero while the per-part cost stays high relative to mass production. You pay for machine time and setup, not for a mold. That trade is what makes low volumes viable at all.

For an engineer, the practical consequence is freedom to change the design between runs. Revise a wall thickness, re-post the program, cut twenty more. Nothing is committed until you decide to invest in tooling for a different process.

It also means the first article and the last article should be identical if the setup is stable. A good shop proves that with an inspection report, not with a promise.

A rotating cutter with defined edges removes material in passes. The CAM software decides step-over, step-down, feed rate and spindle speed from the material and the tool geometry. Aluminum 6061 will run at a much higher surface speed than Ti-6Al-4V, and Inconel slower still.

Heat is the constraint that sets most of those numbers. Too much heat dulls the edge and pushes the part out of tolerance through thermal growth. Too little and the tool rubs instead of shearing, which work-hardens stainless and burns the edge.

Roughing removes the bulk with larger tools and leaves stock for finishing. Finishing takes light radial cuts to hit the surface and the dimensional target. On a turned part, the same logic applies to the OD and the bore.

Between operations the part may be flipped, re-fixtured or moved to a second machine for a different face. Each move adds a small positional error. That error, not the machine's own accuracy, is usually what limits the finished tolerance.

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  RoughingHigh material removal, generous stock left for finishing.
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  Semi-finishBrings the surface close, equalizes the stock for the finish pass.
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  FinishingLight cuts set the final size and the Ra value.
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  InspectionCMM or hand gauges confirm the drawing before the run continues.

Three-axis milling cuts from one direction. It suits plates, brackets and housings with features open to the top face. If every face you need can be reached by rotating the part by hand and re-clamping, three axes is enough and it is the cheapest route.

Four-axis adds a rotary table, so the part can be indexed to a new face without being removed. Shafts, connectors and parts with features on four sides benefit. The setup stays rigid because the part is clamped once.

Simultaneous 5-axis moves the tool and the table at the same time. It reaches undercuts, blends angled faces in one pass, and can keep the cutter normal to a curved surface for a better finish. Impellers, turbine blades and complex housings are the classic cases.

Mill-turn combines a lathe spindle and a milling head. Parts with a turned body plus milled flats or cross-holes finish in one setup with no re-chucking error. GreatLight runs 16 simultaneous 5-axis centers, 16 mill-turn centers and a range of 3-axis and 4-axis machines.

Aluminum is the default for low-volume functional parts. It cuts fast, holds tight tolerances and takes anodizing well. 6061-T6 and 7075 are common; 2024 and 5052 cover other strength and corrosion needs.

Stainless 303 machines freely, while 304 and 316 work-harden if the feed is too light. Titanium and Inconel cut slowly and burn tools, so the cost per part climbs. The geometry does not change, but the cycle time does.

Plastics such as POM, PEEK and PA machine cleanly but move with temperature and clamp pressure. A plastic part measured on the machine may not match the same part measured an hour later in a 20 °C room.

Standard tolerance here is ±0.005 mm, with tighter bands achievable on specific features. Surface finish ranges from Ra 1.6–3.2 μm as machined down to Ra 0.2–0.8 μm when a fine finish is required.

The quote for a small batch splits into setup and cycle time. Setup covers programming, workholding and first-article inspection. Cycle time covers the actual cutting and the finishing passes.

At 10 parts, setup can be half the invoice. At 200 parts, it is a rounding error. This is why the same part looks expensive in a prototype quantity and reasonable at a few hundred.

Buyers who understand this can save money by simplifying one feature instead of asking for a discount. A deep pocket that needs a long reach tool may cost more than the rest of the part combined.

There is no minimum order quantity at GreatLight. Runs go from a single prototype to 10,000-piece lots, and the quotation with a free DFM analysis comes back within 12 hours.