CNC machining Johannesburg: how the process works and where it stops

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  What CNC machining Johannesburg shops actually do to metal
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  When 5-axis pays off and when 3-axis is enough
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  Reading a tolerance callout before you quote
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  Material choice changes the cut, not just the finish
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  Why setup count controls your delivery date
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  CNC machining Johannesburg process selection

CNC machining removes material with a rotating cutting edge that follows a programmed path. The tool turns, the part or the tool moves along controlled axes, and chips carry heat away. Nothing is formed or cast. The final geometry comes from stock that was slightly larger than the finished part.

That matters because every surface you design has to be reachable by a tool. A pocket needs a corner radius at least as large as the cutter radius. A deep bore needs clearance for the holder behind the tool. Engineers who model with those rules early get quotes in hours instead of days of back-and-forth.

Tolerance follows the same logic. Holding ±0.005 mm on a 20 mm bore is routine on a rigid machine with the right boring head. Holding it across a 600 mm thin wall is a different problem. The wall deflects under cutting force, so the feature moves when you unclamp it.

So the useful question is not whether a shop can machine your part. It is which features set the process, and whether the shop can reach them in one setup. That is where cost and lead time are decided.

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  Stock removalMaterial leaves as chips; geometry is cut, not formed
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  Tool reachEvery surface needs a clear path for cutter and holder
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  Setup countFewer setups means fewer datum shifts and less error

A 3-axis machine moves X, Y and Z. The part sits on the table and you reach the top face. Most plates, brackets and housings are done this way, and it is the cheapest route per part. If all your critical features face one direction, stop reading about 5-axis.

A 4-axis machine adds rotation about one axis. Shafts, pins, and parts with features on several sides around a bore fit here. You machine four faces without resetting the part, so the angular relationship between them stays true.

A 5-axis machine tilts the tool as well as rotating the part. That lets a short, stiff cutter reach an angled face instead of a long thin one reaching over a wall. Two things improve at once: surface finish and dimensional control on deep or contoured features.

The hard limit is size and access. A 5-axis center with 500 × 500 × 450 mm travel cannot take a 2 m frame. For long parts, a 3-axis machine with 4,000 × 400 × 150 mm travel or a mill-turn center is the realistic option. Pick the process by part envelope, not by machine prestige.

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  3-axisFlat plates, single-face features, tight budget
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  4-axisShafts and parts with features indexed around an axis
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  5-axisContoured surfaces, undercuts, angled holes, deep pockets

A drawing that says ±0.005 mm everywhere is not a tight drawing. It is an expensive one. Most of those dimensions do not need it, and a shop that honors the blanket callout will inspect every one of them.

Split tolerances by function. Mark the bearing bore, the mating face, and the alignment feature as critical. Let the cosmetic surfaces and clearance holes sit at ±0.1 mm or looser. That single change often removes an entire inspection step and shortens the schedule.

Surface finish behaves the same way. Ra 0.2–0.8 μm needs a finishing pass with a small stepover. Ra 0.8–1.6 μm is a normal machined finish for a mating surface. Ra 1.6–3.2 μm is fine for anything that only needs to look clean.

Give the shop a datum scheme you actually use in assembly. If the drawing datums do not match how the part mounts, the machined feature may be correct to the print and still not fit the fixture.

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  Blanket tight toleranceRaises cost and inspection time with little benefit
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  Functional toleranceCost sits where the fit actually matters
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  Datum choiceDatums should match the assembly joints

Aluminum 6061 machines fast and holds tolerance well. It is the default for brackets, housings and prototypes. 7075 is stronger and machines cleanly too, but it costs more and is less weldable. For die-cast style parts, ADC12 is the casting grade, not a machining grade.

Stainless 303 is the free-machining grade and turns well. 304 and 316 gum up more, work-harden at the cut, and need slower speeds and heavier feed to stay under the hardened layer. 17-4PH machines in the annealed state and gains strength after heat treatment.

Steel 1018 and 1045 are straightforward. 4140 and 4340 are used where strength matters and still cut predictably with the right insert. Tool steel needs more care and usually a pre-hardened grade if you want to avoid distortion after machining.

Titanium TC4 (Ti-6Al-4V) and Inconel are the hard cases. Both hold heat at the cutting edge, so tool life drops and the cycle time rises. They are machinable, but design them with fewer tight features and expect a longer schedule.

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  Aluminum6061, 6061-T6, 2024, 7075, ADC12
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  Stainless303, 304, 316L, 17-4PH, 440C
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  Steel1018, 1045, 4130, 4140, 4340, A36
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  Hard alloysTC4 titanium, Inconel, magnesium AZ31B

Every extra setup costs two things: time and accuracy. The part is unclamped, moved and clamped again. The new position is not identical to the old one, even with a good fixture. That small shift shows up in any feature machined across the two setups.

A 5-axis machine solves this by tilting the tool instead of moving the part. A part with thirty holes on five faces can be finished in one setup. The angular relationship between all thirty holes comes from the machine table, not from a human re-clamping the part.

Inspection follows the same setup. A first-article check on a coordinate measuring machine confirms the datums and the critical dimensions. In-process checks catch drift on a long run. Final inspection before shipment confirms the parts are within the print tolerance.

Ask the shop for the inspection plan, not just the report. A report tells you what was measured. The plan tells you whether the right features were measured, and whether the measuring method matches the tolerance grade.

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  One setupBest accuracy, fewer datum shifts
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  Multiple setupsAdds cost and stack-up error
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  Inspection planDefines what gets measured and how