CNC Processing Store Overview: How the Work Actually Flows

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  Key takeaways
• 2
  What a CNC processing store actually is
• 3
  Machine types and the geometry they fit
• 4
  Tolerance, finish, and what drives the price
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  Materials and how they change the cut
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  From upload to shipped part: the store workflow
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  Step by step: how a part moves through the store
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  Which machine type fits which part
• 9
  Material choice and its machining impact

A CNC processing store is the front end of a machine shop. It is where a CAD file, a material spec, and a tolerance callout get turned into a routing: which machine, which fixture, which tools, which inspection steps. The customer sees a quote and a delivery date. The store sees a sequence of operations that have to fit inside a tolerance band.

That distinction matters when you compare suppliers. A store that only quotes a price per part is guessing. A store that quotes a process is telling you how it will hold the dimension. GreatLight runs 127 high-precision CNC machines across 3 wholly-owned plants, so the routing decision is usually about picking the right machine, not about whether one exists.

The physical output is simple: a metal or plastic part within a stated tolerance. The engineering work behind it is not. A single bracket might need three setups on a 3-axis mill, or one setup on a 5-axis center. Both can be correct. Only one is cheaper for the quantity you are ordering.

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  InputSTEP, IGES, or native CAD plus a 2D drawing with tolerances and finish callouts.
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  RoutingMachine selection, workholding, tool list, and inspection plan.
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  OutputFinished parts, inspection report on request, and a repeatable process for the next run.

The number of axes is not a quality label. It is a statement about how many sides of the part can be cut without re-fixturing. A 3-axis mill cuts from one direction. A 4-axis mill adds rotation around one axis, so you can cut slots and flats around a cylinder. A 5-axis center adds a second rotary axis, which lets the tool reach undercuts and angled faces in one setup.

For a part with features on five faces, a 5-axis center removes four setups and the position errors that come with them. For a flat plate with holes, a 3-axis machine is faster and cheaper. GreatLight runs 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines, and 16 mill-turn centers. The mix exists because no single machine type wins on every part.

Mill-turn centers are worth a separate note. They combine turning and milling in one machine, so a shaft with cross-holes and milled flats can be finished without moving it. That eliminates the concentricity error you get when a part is re-chucked between a lathe and a mill. If your drawing has a tight coaxial tolerance between a turned diameter and a milled feature, mill-turn is usually the answer.

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  3-axisPrismatic parts, plates, pockets, and holes from one or two sides.
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  4-axisCylindrical parts with flats, slots, or holes around the circumference.
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  5-axisComplex contours, undercuts, and angled faces in a single setup.
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  Mill-turnShafts and housings where turning and milling share a datum.

Tolerance is a process cost, not a wish. A general machining tolerance of ±0.05 mm is routine on most machines. Moving to ±0.005 mm changes the equation: you need a machine in good condition, temperature control, sharp tooling, and more frequent in-process checks. GreatLight holds ±0.005 mm (±0.0002 in) on qualified features, but not every feature on a drawing needs that band.

The practical advice is to tolerance only what the function requires. A mounting hole that locates a bearing needs a tight band. A clearance hole for an M6 bolt does not. When a drawing carries a single tight tolerance across every dimension, the quote goes up and the inspection time goes up, with no gain in how the part works.

Surface finish follows the same logic. As-machined surfaces sit around Ra 1.6–3.2 μm. A high-finish cut gets to Ra 0.8–1.6 μm. Fine finishing reaches Ra 0.2–0.8 μm, and that usually means a slower feed, a smaller stepover, or a separate finishing pass. Each step adds machine time. If the surface is not sealing, sliding, or visible, the coarser finish is the right call.

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  As-machinedRa 1.6–3.2 μm. Fine for brackets, covers, and internal parts.
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  High finishRa 0.8–1.6 μm. Typical for mating faces and visible surfaces.
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  Fine finishRa 0.2–0.8 μm. Sealing faces, bearing bores, and sliding contacts.

Aluminium is the default for prototypes and most enclosures. Grades 6061 and 6061-T6 machine fast, hold a good finish, and take anodizing well. Grade 7075 is stronger but more prone to distortion on thin walls, so it needs lighter passes. ADC12 is a die-casting alloy, and it behaves differently again when machined from billet.

Stainless steels are where tool wear shows up. Grades 303 and 304 cut cleanly. Grade 316 and 17-4PH are tougher, generate more heat, and need slower speeds and more coolant. Grade 440C is hard and abrasive. On a cost-per-part basis, stainless usually runs 1.5 to 3 times the price of aluminium for the same geometry, mostly from longer cycle times and shorter tool life.

Titanium and Inconel sit at the top of the difficulty scale. Ti-6Al-4V (TC4) has low thermal conductivity, so heat stays in the cutting zone and tool life drops fast. Inconel is worse. These materials demand rigid setups, sharp tooling, and conservative parameters. They are also where a 5-axis center earns its keep, because fewer setups mean fewer chances to scrap an expensive part.

Plastics are their own category. POM and PA cut cleanly. PEEK is expensive and abrasive. ABS and PC can be machined but tend to burnish rather than cut if the feed is too low. Carbon fibre is abrasive and requires tooling that can handle the wear.

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  Easy to machine6061, 6063, 6082, 303 stainless, brass, POM.
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  Moderate304, 316, 17-4PH, 4140, titanium TA1 and TA2.
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  DifficultInconel, Ti-6Al-4V, 440C, PEEK, carbon fibre.

The workflow starts with a file review. An engineer checks the model against the drawing, looks for features the tool cannot reach, and flags any tolerance that will be hard to hold. This is a DFM pass, and it is where most problems get caught. At GreatLight, the quotation and DFM analysis come back within 12 hours.

Once the routing is agreed, production can start within 24 hours. Programming and fixturing run in parallel with material preparation. The first article is inspected before the run continues. In-process checks keep the part inside the band as tools wear. Final inspection is done on 100% of parts before shipment, with reports available on request.

For simple parts, shipping happens in 3–5 days. Complex 5-axis work or parts needing outside finishing take longer. Finishing is a separate step: anodizing, plating, powder coating, and laser marking all happen after machining. Laser marking has a minimum character height of 1.5 mm, which is worth knowing if you plan to put a part number on a small face.

Confidentiality runs alongside the workflow. Files are handled as confidential, and an NDA can be put in place before drawings are shared. For defense-adjacent or medical work, that step is usually the first conversation, not the last.

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  Quote and DFMWithin 12 hours, including manufacturability feedback.
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  Production startWithin 24 hours of approval.
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  InspectionFirst article plus 100% final inspection before shipment.
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  Shipping3–5 days for standard parts.