Precision CNC machining Sydney: what actually decides part quality

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  Tolerance is a system, not a number on a drawing
• 2
  Why 5-axis setup changes the tolerance stack
• 3
  Which features actually need five axes
• 4
  Material choice sets the realistic tolerance and finish
• 5
  Inspection is the part of the quote you cannot see
• 6
  How lead time is really built
• 7
  Choosing the process by feature, not by habit

A print that says ±0.005 mm does not tell you whether the part can be made. That number is the end of a chain that starts with the machine, runs through the fixture, and finishes at the probe. Break any link and the tolerance is gone, even if the machine is capable on paper.

The first link is thermal. Aluminium 6061 expands roughly 23 µm per metre per degree C. A 300 mm part that warms 5 °C between roughing and finishing moves about 35 µm before the cutter touches it. On a ±0.020 mm feature that is most of the budget. Shops that hold tight tolerances rough, let the part cool, then finish.

The second link is the fixture. A vise clamping a thin wall deflects it. The cut is dimensionally correct at the moment of measurement and wrong once the jaws open. Soft jaws bored to the part profile, or a vacuum plate on a flat face, remove most of that error.

The third link is measurement. A caliper reads to 0.02 mm on a good day. If the drawing calls for ±0.005 mm, the inspection has to run on a CMM or a micrometer with a known reference, and the report has to state the temperature the part was measured at.

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  Thermal driftRough, cool, finish. Do not finish a hot part.
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  Fixture deflectionMatch the clamp to the geometry, not to the vise.
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  Measurement floorCaliper work cannot verify a ±0.005 mm callout.

On a three-axis machine, every new face means a new setup. Each setup adds a locating error, and the errors add up across the part. A bracket with features on five sides might need four setups on a three-axis mill. The same bracket on a five-axis center is one setup.

That matters for more than labor. Position tolerance between a bore on the top face and a slot on the side face is governed by how accurately the part was re-located, not by how accurately the machine cuts. Eliminating the re-location removes that whole error term.

Five-axis also lets the tool tilt. A ball nose cutter held at an angle to a curved surface cuts with the side of the tip instead of the dead center, where surface speed drops to zero. The result is a better finish and longer tool life on contoured surfaces.

The trade-off is setup complexity. Five-axis work needs a post-processor that matches the machine, a fixture that holds the part without blocking the rotary axes, and a programmer who understands both. A part with all features on one face gains nothing from five axes and may cost more.

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  One setup, one datumFewer re-locations means fewer stacked errors.
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  Tool tiltBetter finish on contoured surfaces and deep cavities.
• 3
  Not always worth itSingle-face prismatic parts run cheaper on three axes.

Undercuts, deep cavities and organic contours are the clear cases. If the tool has to reach behind a feature, or if the surface curvature changes direction in three dimensions, a three-axis machine will either miss it or need a custom cutter that costs more than the part.

Impellers, turbine blades, medical implant geometries and sculpted housings fall here. So do parts with tight true-position callouts between faces that sit at odd angles to each other.

Parts with deep pockets and a high depth-to-width ratio are a different problem. A pocket 5 mm wide and 40 mm deep needs a long, thin tool. The tool deflects, the wall tapers, and the finish suffers. Five-axis does not fix that; a smaller depth or a redesigned corner radius does.

Sharp internal corners are another limit. A cutter has a radius. If the drawing shows a square internal corner, the shop either adds a corner relief or the corner comes out with a radius equal to the tool. This is a design decision, not a machining one, and it is cheaper to make it before the part is cut.

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  Good fitUndercuts, compound angles, contoured surfaces, multi-face datums.
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  Poor fitFlat plates, simple turned parts, single-face prismatic blocks.
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  Design fixesCorner reliefs, larger radii, shorter pocket depth.

Aluminium 6061-T6 machines fast and holds ±0.005 mm on features that are not thin walls. It also moves when you remove material, because the residual stress inside the plate redistributes. A part milled from both sides of a 25 mm plate can bow 0.1 mm or more if the stock was not stress-relieved.

Stainless 304 work-hardens. Light passes at the wrong feed speed rub the surface instead of cutting it, and the next pass cuts through a harder skin. 303 machines cleaner. 17-4PH in the H900 condition is hard enough that tool wear shows up in the finish before it shows up in the dimension.

Titanium Ti-6Al-4V and Inconel are heat-limited, not hardness-limited. The heat goes into the tool because the chip carries less of it away. Feeds and speeds drop, cycle time rises, and the tolerance budget has to account for more tool deflection over a longer cut.

Plastics behave differently again. POM and PEEK hold dimensions well but scratch. ABS and PP deflect under clamping pressure. A plastic part designed with the same wall thickness as an aluminium one will often fail on the fixture, not on the cutter.

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  Stress reliefAsk whether the stock was relieved before thin-plate work.
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  Work hardening304 needs a feed that cuts under the hardened skin.
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  Heat-limited alloysTi and Inconel trade cycle time for tool life.

A shop can cut a part to ±0.005 mm and still ship a bad one if the inspection does not match the drawing. The check that matters is the one tied to the critical dimension, measured with a tool whose resolution is at least four times finer than the tolerance.

In-process monitoring catches drift before the part is finished. A probe cycle between roughing and finishing tells the operator whether the stock moved, and lets the finishing pass compensate. This is standard on complex parts and unnecessary on a simple bracket.

Final inspection before shipment is where the report comes from. A dimensional report with the measured values, the instrument used and the reference temperature is worth more than a certificate that only says the part conforms. Ask for it on the critical dimensions only; a full report on every feature slows the order.

Material traceability is the other half. The heat number on the stock should appear on the inspection record, especially for aerospace and medical work where the alloy condition is part of the specification.

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  Gauge ruleInstrument resolution at least 4× finer than the tolerance.
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  Probe cyclesCatch stock movement before the finish pass.
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  TraceabilityHeat number on the record for regulated industries.

Lead time is not one number. It is quoting, material, programming, machining, finishing and inspection, and each step has its own clock. A part that needs anodizing will not ship in the same window as a bare aluminium part, because the finishing step is a separate queue.

Programming time scales with complexity, not with quantity. A five-axis part with twelve contoured surfaces may take longer to program than to cut. That cost is fixed for the first part and amortized over the run, which is why a 1-piece order and a 100-piece order can quote very differently per unit.

Material availability is the wildcard. 7075 and 17-4PH are common. Inconel in a specific condition, or a cast aluminium grade like ADC12, may need to be ordered. If the drawing names a grade, ask what stock is on the shelf before assuming the schedule.

A quotation that includes a DFM review is worth reading carefully. The review often finds a feature that cannot be cut as drawn, and fixing it before programming saves a full cycle of setup and inspection.

• 1
  Finishing is a separate queueAnodizing and plating add their own lead time.
• 2
  Programming is fixed costComplex geometry costs the same for 1 part or 100.
• 3
  Check stock firstUncommon grades can set the whole schedule.