CNC machining in Adelaide: a guide to precision manufacturing

• 1
  What CNC machining actually does to metal
• 2
  Why setup count decides tolerance and cost
• 3
  Material choice sets the tolerance you can hold
• 4
  From machined surface to inspected part
• 5
  Which machining approach fits which part
• 6
  What tolerance and finish you can expect

CNC machining is subtractive. A rotating cutter follows toolpaths generated from a CAD model and removes material until the remaining shape matches the drawing. The machine does not know what the part is for. It only knows coordinates, feed rates and spindle speeds, which is why the quality of the result depends so heavily on how the part was modeled and how it is held.

The two core operations are milling and turning. Milling spins the tool and moves it in X, Y and Z while the workpiece stays put, which suits pockets, slots, flats, bosses and contoured surfaces. Turning spins the workpiece against a single-point tool, which suits shafts, bushings, threads and any rotationally symmetric feature. A mill-turn center does both in one setup.

Material removal creates heat, and heat moves metal. A 6061 aluminium bracket can be cut aggressively because the chips carry heat away and the material is stable. A 17-4PH stainless housing at the same depth of cut will work-harden at the surface, push the tool away and drift out of tolerance. The difference is not machine quality. It is how the alloy behaves under the cutter.

Every operation leaves a tool mark. Surface finish is reported as Ra, the arithmetic mean roughness of the profile. Ra 1.6–3.2 μm is a normal as-machined surface. Ra 0.8–1.6 μm needs a finishing pass with a sharp tool and light depth of cut. Ra 0.2–0.8 μm usually means a separate finishing operation, slower feeds and more inspection time.

For engineers, the useful question is not whether a feature can be cut. Almost any geometry can be cut if you accept enough setups and enough cost. The useful question is which features must be cut in the same setup to hold the relationship between them.

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  MillingPockets, slots, flats, contours, 2.5D and full 3D surfaces
• 2
  TurningShafts, threads, bores, grooves, rotational symmetry
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  Mill-turnBoth operations in one setup, fewer datum shifts
• 4
  Finish passDetermines Ra, not the roughing pass

A setup is one fixturing of the workpiece. Each time a part is unclamped and turned over, a small position error enters the stack. On a good vise and a clean machine, that error is often 0.02–0.05 mm. Tighten the tolerance on a cross-face relationship to ±0.01 mm and the part may need to be cut in one setup instead of two.

This is where 5-axis machining changes the economics. A trunnion table rotates the part under the spindle, so five faces can be reached without re-clamping. Datum relationships stay intact because the part never moves. The trade is that 5-axis programming takes longer and the machine is slower per cubic centimeter of removed metal.

For a flat plate with holes on one face, 3-axis milling is the right answer. For a housing with angled ports, a deep cavity and features on four sides, 5-axis usually wins on total cost even though the hourly rate is higher. The setup you eliminate is often more expensive than the spindle time you add.

Thin walls are a separate problem. A wall under about 1 mm deflects under cutting force, chatters, and comes out tapered. Adding a finishing pass with a 0.1–0.2 mm radial step helps, but the real fix is to leave more material around the wall and remove it in a later, lighter operation.

Deep pockets have the same issue from the other direction. As tool length grows, stiffness falls with the cube of the length-to-diameter ratio. A pocket 6× deeper than the cutter diameter will chatter unless you step down in small increments or switch to a smaller tool with a shorter reach.

So the honest answer on tolerance is conditional. ±0.005 mm is achievable on a well-supported feature with a short tool and a stable material. It is not achievable across a thin, tall wall on the far side of a long part, no matter which machine cuts it.

• 1
  One setupBest for tight cross-face relationships
• 2
  Two or more setupsAdds roughly 0.02–0.05 mm position error
• 3
  Thin wallsDeflect under load; plan light finishing passes
• 4
  Deep pocketsLong tools chatter; step down and reduce radial engagement

Aluminium is the default for prototypes and most enclosures. 6061 and 6061-T6 cut cleanly, hold ±0.005 mm on supported features and take anodizing well. 7075 is stronger but gummier and needs sharper tools and more coolant. 2024 machines well but corrodes faster, so it usually gets a coating.

Stainless behaves differently. 303 is the free-machining grade and the easiest to run. 304 and 316 work-harden quickly, so the cutter must keep moving and never rub. 17-4PH in the solution-treated condition is tough on tools but holds dimensions after aging, which is why it shows up in pump and valve parts.

Steel grades split by carbon content. 1018 and 1045 are straightforward. 4130, 4140 and 4340 need lower speeds and more rigid setups, and they move when you remove material, so a stress-relief step before finishing is often worth the time. Tool steel is usually machined soft and then hardened and ground.

Titanium and Inconel sit at the difficult end. Ti-6Al-4V has low thermal conductivity, so heat stays in the cutting edge. Inconel is worse. Both need reduced surface speed, high-pressure coolant, fresh inserts and patience. Tolerances stay achievable, but cycle times and tool cost rise sharply.

Plastics are not automatically easier. POM and PEEK hold dimensions well and machine to a good finish. ABS and PP soften with heat and can smear. PMMA chips and cracks at sharp corners. Carbon fibre wears tools fast and creates dust that needs extraction.

One rule covers most of this. The harder and more heat-resistant the material, the more the tolerance depends on tool condition and coolant delivery, not on the control system.

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  Easy group6061 aluminium, 303 stainless, 1018 steel, POM
• 2
  Moderate group7075, 304, 4140, PEEK
• 3
  Difficult groupTi-6Al-4V, Inconel, hardened tool steel
• 4
  Watch forWork hardening, residual stress, thermal growth

As-machined surfaces carry visible tool marks. Bead blasting removes them and gives a uniform matte look. Tumbling softens edges on small parts. Brushing leaves a directional grain that hides light scratches on covers and panels. Polishing reaches a reflective finish but adds hand work and cost.

Anodizing is the most common aluminium finish. Clear and coloured types build a thin oxide layer, hardcoat builds a thicker and more wear-resistant one, and conductive anodizing keeps selected areas electrically grounded. Type and thickness change the part dimensions by a few micrometres, so mask critical bores or plan the allowance.

Plating covers electroless nickel, zinc, silver and gold. Electroless nickel gives uniform coverage on complex shapes and good corrosion resistance. Gold plating appears on electronics contacts where contact resistance matters. Powder coating and black oxide cover larger steel parts and frames.

Laser marking handles part numbers, logos and traceability codes. Minimum character height is 1.5 mm, otherwise the mark fills in and becomes unreadable. It does not cut deep, so it should not be used where the mark has to survive heavy abrasion.

Inspection is where the tolerance claim gets verified. The workflow is raw material check, in-process monitoring and final inspection before shipment, with reports on request. For a ±0.005 mm feature, that means a controlled temperature room and a CMM or optical comparator, not calipers.

If a drawing calls for a tolerance that cannot be measured with the equipment available, the drawing is not really specifying anything. Specify what you can verify.

• 1
  Bead blastingUniform matte, hides tool marks
• 2
  AnodizingClear, colour, hardcoat, conductive
• 3
  Laser markingMinimum character height 1.5 mm
• 4
  Inspection100% before shipment, reports on request