CNC metal processing expertise: how metal parts actually get made

• 1
  Key takeaways
• 2
  What CNC metal processing expertise actually means
• 3
  How tolerance and surface finish are actually produced
• 4
  Why the number of setups decides the result
• 5
  How material choice changes the cut
• 6
  When a part should not be CNC machined
• 7
  Finishing steps and the inspection that closes the loop
• 8
  Choosing a machining route by part type

CNC metal processing is subtractive. A cutter spins, the workpiece moves under program control, and material leaves as chips. What remains is the part. There is no mould and no forming die, so the geometry is defined by tool paths and by how rigidly the workpiece is held while those paths run.

That is the whole idea, and it explains most of the practical limits. Anything the cutter can reach, it can cut. Anything it cannot reach has to be cut from another direction, or left to a secondary process such as EDM or hand finishing. When an engineer asks whether a feature is machinable, the real question is whether a tool of reasonable diameter and length can reach it without chattering.

CNC metal processing expertise is therefore mostly about decisions made before the first chip: which machine, which fixture, which tool, which order of operations. Two shops can quote the same drawing and produce very different results if their setup plans differ. The drawing sets the requirement. The setup decides whether the requirement is met.

GreatLight has run this process since 2011 across three wholly-owned plants, with 127 high-precision CNC machines and 150 technicians. That is the scale behind the explanations below, and it shapes which jobs fit and which are better routed elsewhere.

• 1
  Subtractive, not formingGeometry comes from tool motion, so changes are a program edit rather than a new die.
• 2
  Reach limits the designTool length and diameter set the smallest internal feature you can cut.
• 3
  Setup drives accuracyThe machine repeats well; the fixture and clamping decide the absolute position.

A stated tolerance such as ±0.005 mm is a capability, not a default. It applies to specific features on a specific machine under specific conditions. When a drawing applies one blanket tolerance to every dimension, the shop has to decide which features genuinely need it and which can be opened up. That decision affects cost more than any other single choice on the quote.

Surface finish follows the same logic. Ra 0.2–0.8 μm usually needs a finishing pass with a sharp tool, light depth of cut, and a stable setup. Ra 0.8–1.6 μm is a normal fine-machined result. Ra 1.6–3.2 μm is as-machined and is where most structural parts live. Asking for a mirror finish on a mounting face adds time without adding function.

Thermal behavior matters more than most people expect. Aluminium expands roughly twice as fast as steel for the same temperature change. A part measured hot off the machine can drift outside tolerance once it reaches room temperature. Shops that hold tight numbers let parts stabilize, or measure at a controlled temperature, before the final report is written.

Tool wear is the slow variable. A cutter that was on size at the start of a run is not on size after several hundred parts. In-process monitoring catches that drift. GreatLight runs raw material checks, in-process monitoring, and final inspection, with 100% inspection before shipment and reports available on request.

• 1
  Blanket tolerances cost moneyTightening every dimension is not the same as tightening the ones that matter.
• 2
  Finish is a finishing passRa 0.8–1.6 μm comes from a light final cut, not from the roughing pass.
• 3
  Heat moves the numberLet parts stabilize before final measurement on tight work.
• 4
  Wear drifts mid-runIn-process checks catch the cutter going off size before the batch is finished.

Every time a part is unclamped and turned, it is located again. Each locating event carries its own small error. Stack three or four of them and the accumulated position error can exceed the machining tolerance itself. This is why five-axis work is not only about reaching angled faces; it is about cutting several faces in one continuous setup.

A three-axis machine cuts one face at a time and needs a fixture per orientation. A four-axis mill adds rotation around one axis and handles cylinders and multi-face parts. A simultaneous five-axis center tilts the tool and rotates the table together, so undercuts, compound angles, and deep cavities are reachable without re-clamping. GreatLight runs 16 simultaneous five-axis machining centers, 12 four-axis mills, 27 three-axis machines, and 16 mill-turn centers.

The size of the work also picks the machine. Large frames up to 4,000 mm go on the gantry-type machines with travel of 4,000 × 400 × 150 mm. Medium housings fit the 750 × 1,150 × 550 mm and 600 × 600 × 600 mm envelopes. Compact parts run on the 500 × 500 × 450 mm and 500 × 310 × 200 mm machines. A rotary table of Ø400 mm handles round work that would otherwise need a fourth setup.

Mill-turn centers matter for parts that are mostly round with milled features, such as shafts with flats, cross-holes, or keyways. Doing the turning and milling on one machine removes a whole re-clamping step and keeps the round and the milled features concentric.

• 1
  Each setup adds errorRe-clamping is a locating event, and locating is never perfect.
• 2
  Five-axis reduces setupsAngled and undercut features can be cut without turning the part.
• 3
  Match machine to envelopeFrame, housing, and compact parts each have a natural machine class.
• 4
  Mill-turn for round-plus-milledOne machine keeps turned and milled features aligned.

Aluminium is the default for prototypes and many production parts. Grades such as 6061, 6061-T6, 7075, 2024, 5052, 5083, 6063, 6082, and ADC12 cover most needs. The 6061 family machines cleanly and anodizes well. The 7075 family is stronger but more prone to stress movement when a lot of material is removed. A part that bows after roughing usually had residual stress, not a bad program.

Stainless steel behaves differently. Grades 303 and 304 cut reasonably well; 316 and 316L are tougher and gummier; 17-4PH (SUS630) can be machined in the solution-treated state and then aged to higher strength. In all of them, heat builds at the cutting edge and work hardening is a real risk. Light, steady feeds with adequate coolant beat aggressive passes that glaze the surface.

Titanium and nickel alloys are the slow group. TA1, TA2, TC4 (Ti-6Al-4V), and Inconel cut at low surface speeds and generate heat that stays in the tool. Tool life is short and cycle times are long. These materials are chosen for temperature and strength, not for ease of machining, and the quote should reflect that. Magnesium AZ31B and AZ91D machine very fast but need chip handling discipline.

Plastics and composites run on the same machines with different feeds and sharper tools. ABS, PC, PMMA, POM, PA, PEEK, PP, HDPE, and carbon fibre all appear in prototype work. PEEK and carbon fibre are abrasive and wear tools quickly. POM machines cleanly but moves with temperature.

• 1
  Aluminium moves after roughingResidual stress in 7075 shows up as bowing on thin sections.
• 2
  Stainless work hardensSteady feed and coolant prevent a glazed, hard surface layer.
• 3
  Titanium and Inconel run slowHeat goes into the tool, so tool life and cycle time suffer.
• 4
  Plastics need sharp toolsPEEK and carbon fibre wear edges fast; POM shifts with heat.

CNC is not always the right answer, and saying so early saves everyone money. A thin-walled box with a wall under about 0.8 mm in aluminium will deflect under cutting force no matter how the program is written. The part may be possible, but it will be slow, and the yield will be poor. Sheet metal fabrication or die casting usually wins there.

Sharp internal corners are another limit. A rotating cutter leaves the radius of the tool. A true 90° internal corner requires EDM, or a relief notch, or a design change to a larger radius. If a drawing shows a square internal corner with a tight tolerance, the honest response is to ask whether the corner is functional or just drawn that way.

Very deep, narrow pockets are hard for a different reason. Reaching the bottom needs a long, thin tool, and long thin tools chatter. The depth-to-diameter ratio is the number to watch. Past roughly 4:1 in steel, the cut becomes delicate. Past 6:1, expect to slow down a lot or step the pocket wider.

Finally, quantity changes the math. For a few prototypes, machining is usually the fastest route. At 10,000+ parts, die casting or another forming process may be cheaper per piece. GreatLight runs from one prototype to 10,000+ part runs with no minimum order quantity, so the crossover point is a real conversation rather than a policy line.

• 1
  Thin walls deflectUnder about 0.8 mm in aluminium, cutting force wins over rigidity.
• 2
  Corners carry tool radiusA square internal corner needs EDM or a relief.
• 3
  Deep pockets need long toolsBeyond roughly 4:1 depth-to-diameter, chatter risk climbs.
• 4
  Quantity changes the routeMachining for prototypes, forming for very large runs.

Machining leaves a functional part, but most parts need a surface treatment before they are usable. Anodizing in clear, colour, hardcoat, or conductive form is common on aluminium. Electroless nickel, zinc, silver, and gold plating cover wear, corrosion, and conductivity needs. Powder coating and black oxide handle appearance and mild corrosion protection.

Mechanical finishing changes the surface without adding a coating. Bead blasting gives a uniform matte look and hides tool marks. Tumbling deburrs edges in volume. Brushing and polishing produce directional or bright finishes. Laser marking and engraving add part numbers and logos, with a minimum character height of 1.5 mm so the mark stays legible.

Post-processing done in-house matters because it removes a shipping step and a scheduling gap. When machining, finishing, and inspection sit in the same plant, a rejected finish is a same-week correction rather than a return shipment. GreatLight provides one-stop post-processing and completion so parts arrive finished.

Inspection is what turns a claim into evidence. Raw material check confirms the grade before cutting. In-process monitoring catches drift during the run. Final inspection measures the finished part against the drawing. Reports are available on request, and the qualification rate across this flow is 99.99%. For regulated industries, the certification set is ISO 9001:2015, IATF 16949:2016, ISO 13485:2016, and ISO 27001:2022.

• 1
  Coating or mechanical finishAnodizing and plating add properties; blasting and polishing change appearance.
• 2
  Marking has a size floorLaser marking stays legible at a minimum character height of 1.5 mm.
• 3
  In-house finishing saves timeCorrections happen in the same plant instead of across a shipment.
• 4
  Three inspection stagesMaterial, in-process, and final, with reports on request.