CNC machining Sydney: how the process actually works

• 1
  What CNC machining does to a metal block
• 2
  Why 5-axis changes the setup, not just the geometry
• 3
  Tolerance and surface finish: where the real limits sit
• 4
  How material choice drives cost and lead time
• 5
  Inspection: what gets measured before parts ship
• 6
  Lead time and the steps that actually take time
• 7
  Which process fits which part

CNC machining is subtractive. A rotating cutter removes material along a path defined by G-code, and the finished geometry is whatever the tool leaves behind. Nothing about the process is additive, so every feature has to be reachable by a tool of some diameter, from some direction. That single constraint explains most design and cost problems engineers hit after the first quote.

Start with the part in your head. A pocket with a 0.5 mm internal radius cannot be cut by a 12 mm end mill. The tool has to be small enough to enter the corner, and small tools deflect more, so they must run slower with lighter passes. Wall height matters too. A deep, narrow pocket needs a long tool with a high length-to-diameter ratio, and that tool will chatter before it cuts cleanly.

The material you choose changes everything downstream. Aluminium 6061 and 7075 cut fast and hold tight tolerances with little tool wear. Stainless 316L work-hardens at the cut, so feeds and speeds must stay aggressive enough to stay under the hardened layer. Titanium TC4 (Ti-6Al-4V) conducts heat poorly, so most of the heat goes into the tool, not the chip. Inconel is worse still.

This is why the same drawing can cost three different amounts depending on alloy. A part in 6061-T6 that takes 20 minutes might take 90 minutes in 17-4PH stainless, and the tooling bill is higher. Anyone quoting CNC machining Sydney work should ask about alloy and heat treatment before quoting, not after.

A 3-axis mill moves the tool in X, Y and Z while the part stays still. A 5-axis machine adds two rotary axes, so the cutter can approach the part from almost any angle without the operator unclamping and re-fixturing it. That is the real gain, fewer setups, not simply more complex shapes.

Each re-fixture introduces a new datum error. If a part has features on five faces and you machine it on a 3-axis machine, you might set it up four times. Stack four setups at ±0.02 mm each and you can lose 0.05 mm before any cutting error appears. On a 5-axis machine the same part often needs one setup and one datum.

Simultaneous 5-axis is not the same as 3+2 positioning. In 3+2, the rotary axes index to an angle and lock, then the tool cuts a normal 3-axis path. In simultaneous mode all five axes move together, which is what you need for a contoured impeller blade or a sculpted mold surface. Simultaneous paths need CAM verification to avoid collisions.

The trade-off is access and rigidity. A 5-axis trunnion table tilts the part, and at extreme angles the setup becomes less rigid than a solid 3-axis vise. Deep bores in a large block can still be faster on a 3-axis machine with a long reach tool. Choose the machine that matches the feature, not the brochure.

Tolerance is not a single number you apply to a whole drawing. A ±0.005 mm callout on a 20 mm bore is achievable on a good machine with the right tool and a temperature-stable shop. The same callout on a 500 mm long aluminum extrusion is a different job entirely, because thermal expansion alone can move the part more than the tolerance band.

Surface finish follows the same logic. As-machined surfaces land around Ra 1.6–3.2 μm. A finish pass with a sharp tool and a light radial step gets you to Ra 0.8–1.6 μm. Below that, you are usually looking at a secondary operation, not a machining parameter. Ra 0.2–0.8 μm often means lapping, polishing or a fine grinding step after the CNC work.

Feature size limits tolerance too. A 0.8 mm wide slot in stainless will deflect as it is cut. A thin wall under 1 mm will move when the clamping pressure releases. Deep holes beyond 5× diameter tend to drift, so a reamed or gun-drilled hole may be the honest answer.

The practical rule: apply tight tolerances only where the function needs them. A bolt clearance hole does not need ±0.005 mm. A bearing seat does. Marking the functional surfaces on the drawing saves money and prevents the shop from over-inspecting features that do not matter.

Material availability decides the start date more often than machine capacity does. Aluminium 6061 and 7075 plate is stocked in most sizes. Titanium TC4 and Inconel are often mill-run material with a minimum buy and a mill cert on the way. That can add days before a single chip is cut.

Machinability ratings give a rough order. Free-machining brass C36000 cuts at 100 on most scales. Aluminium 6061 sits near 90. Stainless 303 is around 78, while 316L drops to roughly 45. Titanium and nickel alloys sit far lower and wear tools much faster. The number is only a guide, but it tells you the relative cycle time before you quote.

Heat treatment adds a step that cannot be skipped. A 17-4PH part that needs H900 condition has to be machined, solution treated, aged, then finish machined if the tolerance is tight enough. That is a schedule, not a single operation. Plan for it early or the deadline will move.

Plastics behave differently again. POM and PEEK cut cleanly but move with temperature and moisture. A PEEK part measured hot off the machine will not match the same part measured the next morning. For tight plastic work, let the part stabilize before final inspection and state the measuring temperature on the drawing.

Inspection is a process, not a final gate. It starts with the raw material. A mill certificate confirms the alloy and heat lot, and a quick hardness check catches mix-ups before machining. Cutting the wrong alloy wastes the whole cycle.

In-process checks catch drift. A first-article inspection on the first part sets the baseline. Then operators check critical dimensions at set intervals, typically every few parts on a tight tolerance feature. If the tool wears and the bore grows, the correction happens before the whole batch is out of spec.

Final inspection happens on 100% of parts before shipment at our shop. That does not mean every dimension on every part. It means the drawing's critical dimensions are verified with the right instrument: a micrometer for a shaft, a bore gauge for a hole, a CMM for a position callout. Reports are available on request.

The measurement itself can be the problem. A part checked on a warm shop floor with a cold gauge can read differently than the same part on a granite table at 20 °C. For tolerances under ±0.01 mm, the measuring environment matters as much as the machine that cut the part.

Quoting is fast. A DFM review and quotation typically come back within 12 hours, and production can start within 24 hours on standard materials. Parts usually ship in 3–5 days for straightforward work. Those numbers assume the drawing is complete and the material is in stock.

The steps that stretch a schedule are usually outside the machine. A missing callout triggers a question. A titanium plate order adds days. A heat treatment step adds a furnace cycle. Anodizing and plating add a day or two at a vendor. None of these are machining problems, but they all land on the delivery date.

Batching matters too. Running 500 parts as one lot is faster per part than running them as five lots of 100 with different finishes. If the parts are identical, keep them in one batch. If they need different anodize colors, split them and expect two finishing cycles.

Ordering one prototype and 10,000 parts uses the same process but not the same plan. With no minimum order quantity, we can start with one part to validate the design, then scale the same program to a production run without re-quoting the geometry.