CNC machining CT: what the process can and cannot hold

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  How CNC machining CT removes material
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  What tolerance and finish are realistic
• 3
  Why 5-axis changes the part you can design
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  Material choice and how it cuts
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  From drawing to finished part
• 6
  Which setup and process fits the part

A CNC machine does one thing: it moves a spinning cutter along a path defined by numbers. The controller reads G-code, drives ball screws and servos, and repeats the same motion thousands of times. Nothing about the process is random, which is why the same program on the same machine produces the same part.

The cutting edge is where the physics happens. Aluminium 6061 cuts clean at 200–400 m/min surface speed with a two- or three-flute carbide tool. Titanium TC4 runs at 40–60 m/min and sheds heat poorly, so it needs flood coolant and lighter radial engagement. Pick the wrong speed and the tool rubs instead of cutting.

Heat is the main enemy of tolerance. Most of the heat leaves with the chip, but enough stays in the workpiece to move it. A 200 mm aluminium plate can grow 0.05 mm when it warms by 10 °C. That is ten times the ±0.005 mm tolerance we hold on a finished feature.

So the sequence matters. Rough the part, let it cool, then finish. On thin walls we leave 0.3–0.5 mm of stock for the finishing pass and take it in two light cuts. Skip the cool-down and the finish cut removes metal that has already moved back.

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  Rough, cool, finishNever combine roughing and finishing in one continuous pass on tight parts.
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  Chip evacuation firstRecutting a chip doubles the load on the edge and ruins the finish.

Tolerance is a number with conditions attached. We hold ±0.005 mm (±0.0002 in) on features that are reachable, rigid and measured at 20 °C. That figure does not apply to a 900 mm long bore, a 0.5 mm wall, or a face that has to be checked with a caliper.

As a rule, the tighter the tolerance, the shorter the feature. A 10 mm bore at ±0.005 mm is routine on a mill-turn center. A 500 mm bore at the same tolerance needs a boring head, a temperature-controlled room and a discussion about who pays for the scrap.

Surface finish follows the same logic. Ra 0.8–1.6 μm comes straight off the machine on most aluminium and steel parts. Ra 0.2–0.8 μm needs a finer stepover, a fresh tool and often a second operation. Ra 1.6–3.2 μm is the as-machined default when the drawing says nothing.

If a surface is only cosmetic, say so. Marking a non-functional face as Ra 0.4 μm adds cost for no benefit. Marking a sealing face as as-machined gets you a leak.

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  20 °C ruleTolerance claims assume the part and the gauge sit at the same temperature.
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  Aspect ratioDeep pockets beyond 4× diameter need a long-reach tool and lose accuracy.

A three-axis machine moves the cutter in X, Y and Z. Every new face means a new setup: unclamp, rotate, re-indicate, reclamp. Each setup adds a datum error, and four setups stack four errors.

A simultaneous 5-axis center tilts the tool or the table while cutting. Undercuts, angled ports, impeller blades and deep cavities get machined in one setup. The part stays clamped, so the only error left is the machine's own geometry.

We run 16 simultaneous 5-axis machining centers with a Ø400 mm rotary table, alongside 12 four-axis mills and 27 three-axis machines. Not every part needs five axes. A flat bracket with holes on one face is faster and cheaper on a three-axis mill.

Use five axes when the geometry forces it: features on five sides, contoured surfaces, or a tolerance stack that a second setup would break. The test is simple. Count the setups. If the answer is three or more, ask for a 5-axis quote.

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  Setup count drives costEach extra setup adds fixturing time and a new datum error.
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  One setup, one datumFive-axis work removes the stack-up that comes from re-clamping.

Aluminium is the default for prototypes and housings. 6061 and 6061-T6 machine fast and take anodizing well. 7075 is stronger but gummier, and it will not anodize to the same cosmetic standard. 2024 cuts cleanly and is common in aerospace brackets.

Stainless 303 is the free-machining grade and the easiest to turn. 304 and 316L work-harden if the tool dwells, so keep the feed up and never let the cutter rub. 17-4PH (SUS630) goes to high strength after aging, which means machine it soft and expect slight movement in heat treat.

Titanium TC4 (Ti-6Al-4V) and Inconel sit at the difficult end. Tool life drops, cycle time triples, and thin sections can deflect during the cut. We machine them, but the quote reflects the time. Magnesium AZ31B and AZ91D cut easily and need chip control for fire safety.

Plastics behave differently again. POM and PEEK hold tolerance well. ABS and PP move with temperature and are better treated as prototype materials. Carbon fibre eats tools and needs diamond coating.

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  Free-machining grades303 stainless, 6061 aluminium and C36000 brass cut fastest.
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  Heat treat after machining17-4PH and 4140 will shift slightly, so leave stock for a finish pass.

A quote needs three things: a 3D model or a dimensioned drawing, a material callout, and a finish spec. Send all three and we return a quotation with a free DFM analysis within 12 hours. The DFM note lists the features that will be slow, the tolerances that cannot be held, and the cheaper alternative where one exists.

Once the design is locked, production can start within 24 hours. Raw material is checked on arrival, and the first article is inspected before the run continues. In-process checks catch drift during long cycles rather than at the end.

Parts ship in 3–5 days for most jobs. There is no minimum order quantity, so a single prototype and a 10,000-part run go through the same inspection route. Historical late-delivery probability sits below 2%.

Finishing runs after machining and before final inspection. Anodizing, plating, powder coating, black oxide, bead blasting and laser marking are all handled in house, which removes the transport and handling risk of sending parts out.

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  DFM before cuttingA 12-hour DFM note is cheaper than a scrapped first article.
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  Inspection recordsRaw material, in-process and final reports are available on request.