How to Choose an Appropriate CNC Machining Service

Axis count is the first filter. A prismatic bracket with holes on one face belongs on a 3-axis mill. Add a second face and you need a 4-axis mill or a vise flip, which adds setup error. Undercuts, compound angles, and contoured pockets push you toward simultaneous 5-axis work, where the tool reaches the feature in one setup instead of three.

The mistake we see most often is quoting a 5-axis part on a 3-axis machine to save cost. It can work for a prototype if the shop has good fixturing, but the stack-up of re-fixturing usually eats the savings. If your part has features on five sides plus a curved surface, ask for simultaneous 5-axis machining and check the machine travel against your part envelope.

Size also decides the shop. A 900 mm housing needs a machine with enough travel, and not every shop has one. GreatLight runs 16 simultaneous 5-axis centers, 16 mill-turn centers, and travels up to 4,000 × 400 × 150 mm, so long shafts and large plates stay on one machine.

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  3-axisFlat plates, single-face pockets, simple holes.
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  4-axisCylindrical parts, features on two or more sides.
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  5-axisContoured surfaces, undercuts, tight true position between faces.

Machining 6061 aluminium and machining Inconel 718 are different jobs. Aluminium cuts fast with high spindle speed and generous feed. Inconel work-hardens at the cut, so the shop needs rigid tooling, low surface speed, and a cutting strategy that keeps the tool in the cut. A shop with aluminium experience but no nickel-alloy experience will burn tools and scrap parts.

Ask what material your supplier machines most often. Then ask for the cutting parameters they would use on your alloy, or at least the tool coating and speed range. Vague answers are a warning sign. The appropriate cnc machining service for a Ti-6Al-4V implant is not the same shop that runs 10,000 aluminium enclosures.

Plastics add their own rules. PEEK and POM move with heat, so clamping force and coolant choice matter as much as the cutter. Carbon fibre wears edges quickly and needs dust control. For any of these, the shop should be able to name the grade they have run before.

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  Aluminium6061-T6, 7075, 2024, 6082, ADC12
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  Stainless steel303, 304, 316L, 17-4PH, 440C
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  Titanium and nickelTC4 (Ti-6Al-4V), Inconel, TA2
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  PlasticsPOM, PEEK, PC, PA, carbon fibre

Tolerance drives cost faster than any other line on the drawing. A general tolerance of ±0.1 mm is routine. Moving to ±0.005 mm means the shop must control tool wear, thermal growth, and fixturing deflection. Only apply tight tolerance to the features that need it. A cosmetic face at ±0.05 mm costs far less than the same face at ±0.005 mm.

Surface finish follows the same logic. As-machined at Ra 1.6–3.2 μm suits most structural parts. Sealing faces and bearing bores often call for Ra 0.8–1.6 μm. Optical and vacuum surfaces can need Ra 0.2–0.8 μm, which usually means a secondary operation, not a different cutter.

Inspection is where you separate claims from evidence. Ask whether the shop inspects 100% of parts or samples by lot. Ask for first article inspection reports on the critical dimensions. GreatLight checks raw material on receipt, monitors in-process, and inspects 100% before shipment, with reports on request. If a supplier cannot show you a report format before you order, that is your answer.

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  As-machinedRa 1.6–3.2 μm for general surfaces
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  FineRa 0.8–1.6 μm for seals and bores
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  OpticalRa 0.2–0.8 μm, often a second op

A part is rarely finished when the spindle stops. Anodizing, plating, powder coating, and laser marking all change the final dimension or appearance. If the shop handles finishing in-house or with a controlled partner, the transition is smoother and the accountability stays in one place. Mixing vendors for machining and anodizing is where color and fit problems usually start.

Volume changes the right process. One to a few hundred parts suit CNC machining. Past that, die casting or dedicated lines may cut cost per part, but only after a DFM review confirms wall thickness and draft. A shop that can run both lets you prototype on a mill and scale without redesigning the whole part.

There is no minimum order quantity at GreatLight. We run a single prototype and 10,000+ part runs on the same floor, which means the process, fixturing, and inspection plan carry over from first article to production. That continuity is often worth more than a lower unit price.

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  Prototype1 to 50 parts, fast turnaround
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  Low volume50 to 1,000 parts, soft tooling
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  Production10,000+ parts, dedicated cell