CNC Processing Riverside: How the Process Actually Works

• 1
  What CNC processing removes, and why the setup decides the result
• 2
  3-axis, 4-axis and 5-axis: what each one is for
• 3
  What ±0.005 mm really means on a drawing
• 4
  Material behaviour changes the cutting plan
• 5
  When CNC processing is the wrong answer
• 6
  Machine setup and tolerance by part type

CNC processing is subtractive. A rotating cutter is driven along a programmed path and shears material away in chips. Nothing is shaped by a mold or a die, so the first part and the thousandth part come off the same code. That is the whole appeal: geometry is defined by numbers, not by tooling wear or operator feel.

The catch is that the cutter has to reach the surface. Every feature you design imposes a question of access. A pocket 40 mm deep and 6 mm wide at the bottom cannot be cut by a tool rigid enough to hold tolerance, because the tool must be at least as long as the pocket is deep and at least as thin as the corner is small. Long and thin means deflection.

Deflection is the quiet error in CNC processing. A 6 mm end mill hanging 60 mm out of its holder will bend under cutting force, and the bend shows up as taper in the wall, chatter marks on the floor, and a dimension that drifts from the first pass to the last. The machine is not at fault. The tool assembly is.

So when a shop quotes a tight tolerance, it is really quoting a setup: which machine, which holder, how many passes, how much material left for a finishing cut. Tolerance is not a property of the machine alone. It is a property of the plan.

• 1
  Access beats powerA reachable feature on a small machine beats an unreachable one on a large machine.
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  Tool length sets accuracyEvery extra 10 mm of gauge length costs stiffness.
• 3
  Finishing passes define finishRoughing clears metal; the last 0.2–0.5 mm sets Ra.

A 3-axis mill moves X, Y and Z. The part sits still and the tool approaches from one direction, usually from above. This is the fastest and cheapest way to cut a prismatic part: plates, housings, brackets, manifolds with features on one face. If your part is mostly flat with pockets and holes on two or three faces, 3-axis work plus two or three re-fixturings is often the right call.

A 4-axis machine adds rotation about one axis, typically A. The part turns like a cylinder while the tool cuts. Shafts, cam profiles, splined hubs and parts with holes on a bolt circle all become one-setup jobs. The gain is not only speed. Every re-fixturing on a 3-axis machine reintroduces datum error, and a 4-axis setup removes that class of error entirely.

A 5-axis machine adds a second rotary axis, so the tool can tilt relative to the part. This is what lets a shop cut undercuts, deep cavities with steep walls, impeller blades and contoured surfaces without splitting the job across multiple fixtures. GreatLight runs 16 simultaneous 5-axis machining centers alongside 12 four-axis mills and 27 three-axis machines, so the routing decision is made on geometry rather than on what happens to be free.

Five-axis is not automatically better. It is slower to program, needs more verification, and on a simple plate it buys nothing. The honest rule: use the fewest axes that reach every feature in one or two setups. Beyond that you are paying for motion you do not need.

• 1
  3-axisPrismatic parts, one dominant approach direction, lowest cost per part.
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  4-axisRotational parts and bolt-circle features in a single setup.
• 3
  5-axisUndercuts, blades, steep walls, contoured surfaces, hard-to-reach faces.

A tolerance of ±0.005 mm is achievable, but not on every dimension of every part. It depends on the feature, the material and the temperature. Aluminum expands about 23 μm per meter per degree Celsius. A 300 mm aluminum part measured at 25 °C but machined at 30 °C has already moved 0.03 mm before any cutting error is counted.

That is why tight tolerances belong on specific, functional dimensions, not on the whole drawing. A bearing bore, a dowel hole, a mating face and a seal groove deserve the tight callout. An outside profile that bolts to a bracket does not. When every dimension carries the same tight tolerance, the shop either charges for it or quietly ignores the ones that do not matter.

Surface finish follows a similar logic. As-machined surfaces land around Ra 1.6–3.2 μm. A fine finishing pass gets you Ra 0.8–1.6 μm. Below that you are usually into Ra 0.2–0.8 μm territory, and that often means a different process: finer stepovers, a smaller cutter, sometimes a lap or a polish. Say which surfaces need it.

Inspection closes the loop. GreatLight checks incoming material, monitors dimensions in process, and inspects 100% of parts before shipment, with reports available on request. The point of the report is not paperwork. It is knowing which dimensions were actually measured and with what instrument.

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  Call out functional fits onlyBores, dowels, mating faces, seal grooves.
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  Note the measuring temperatureThermal drift can exceed the tolerance itself.
• 3
  Match finish to functionSealing surfaces need finer Ra than clearance faces.

Aluminum 6061 and 7075 cut freely and hold a good finish, which makes them the default for prototypes and enclosures. 7075 is stronger but more prone to distortion when a lot of material is removed from one side, so sequence matters. Rough, stress-relieve if needed, then finish.

Stainless 304 and 316 work-harden. If the cutter rubs instead of biting, the surface gets harder and the next pass wears the tool faster. The fix is a positive feed that keeps the cutting edge engaged, sharp tooling, and no dwelling in the cut. 17-4PH in the H900 condition is machinable but abrasive; expect shorter tool life and plan for it.

Titanium Ti-6Al-4V (TC4) has low thermal conductivity, so heat goes into the tool rather than the chip. Roughing at conservative parameters with high-pressure coolant is normal. Inconel is worse. Both are cut regularly here, but they are never the cheap option, and a design that uses titanium where aluminum would work is a design that costs more than it needs to.

Plastics behave differently again. POM and PEEK hold dimension well; ABS and PP are softer and can burr or melt at the edge. Carbon fibre is abrasive and needs carbide or diamond tooling. The material choice and the finishing choice are linked, because anodizing only applies to aluminum, and hardcoat changes the bore size by tens of microns.

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  AluminumFast, stable, anodizes well; watch distortion on thin sections.
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  StainlessWork-hardens; keep the edge engaged and the feed positive.
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  Titanium and InconelHeat stays in the tool; coolant strategy drives tool life.
• 4
  PlasticsSharp tooling, light finishing passes, watch edge burrs.

CNC processing removes metal one part at a time. When you need 50,000 identical small parts, that is a die casting or an injection molding job, and machining them would be slow and expensive. Machining still has a role there, but it is the tool or the first article, not the production run.

Very large, very thin, or very flexible parts are also a poor fit. A 2 mm aluminum panel 1,500 mm long will move when the clamps come off, no matter how carefully it is cut. Sometimes the answer is to machine a fixture first, or to change the design so stiffness comes from a rib rather than from thickness.

Sharp internal corners are another boundary. A pocket with a true square internal corner cannot be milled, because the cutter is round. The corner will carry the tool radius, typically 1–3 mm on a finishing cutter. If the mating part needs a square corner, you either change the mating part or accept a relief groove, which is an extra operation and an extra cost.

Deep small holes, deep narrow slots and features with a depth-to-width ratio beyond about 4:1 push against tool stiffness. They can be done. They will be slower, measured more often, and occasionally scrapped. If the design can be opened up by a millimetre, that is usually the cheapest change on the drawing.

• 1
  High volumeMove to casting or molding; keep machining for the tool and first article.
• 2
  Thin and flexibleClamping release changes the shape; add ribs instead of thickness.
• 3
  Square internal cornersThe tool radius is unavoidable; design a relief or a round mating feature.