CNC Cutting Machine Guide

• 1
  What a CNC cutting machine actually does
• 2
  Milling as the default CNC cutting method
• 3
  Turning and mill-turn for round parts
• 4
  Laser, plasma and waterjet cutting compared
• 5
  How material choice changes the cut
• 6
  Measuring the cut and holding tolerance
• 7
  Cutting process selection table

Every CNC cutting machine follows the same loop: a CAD model becomes toolpath instructions, the controller moves an axis, and a cutting edge or beam meets the workpiece. The difference between processes is the energy source doing the cutting. A rotating cutter shears material. A laser vaporizes it. Plasma melts it. Water and abrasive erode it.

That single difference sets the tolerance, the heat input and the part geometry you can hold. A 3-axis mill holds ±0.005 mm on a flat pocket because the cutter is rigid and the force path is short. A laser cuts a 3 mm stainless sheet at high speed but leaves a heat-affected zone on the cut face. Neither is better. They answer different questions.

For most machined parts the real question is not which machine, but how many setups. Every time a part is unclamped and rotated, you add fixture error. A mill-turn center or a 5-axis machine cuts that error out because the part stays in one grip. That is usually worth more than a marginally finer tool.

Material removal rate depends on spindle power, tool material and the rigidity of the setup. On aluminium 6061 we run carbide at 3,000–10,000 rpm and hold Ra 0.8–1.6 μm without a secondary operation. On 17-4PH stainless the same geometry runs at roughly a third of that speed, and tool wear becomes the controlling cost.

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  Subtractive onlyAll CNC cutting removes material; nothing is added back.
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  Rigidity winsA stiff setup beats a faster spindle on accuracy.
• 3
  Setups drive costFewer grips usually means lower unit price.

Milling covers the widest range of part shapes: pockets, slots, bosses, threads, flat faces and contoured walls. An end mill rotates while the workpiece stays fixed on the table, so the achievable geometry is limited mainly by tool reach and the number of accessible sides.

Three-axis milling handles prismatic parts well. Once a part has features on five faces, or a wall that tapers in two directions, a 5-axis machine removes the part from three or four separate fixtures and does it in one. We hold ±0.005 mm on 5-axis work and use 16 simultaneous 5-axis machining centers for that reason.

Tool reach matters more than people expect. An end mill that sticks out 4× its diameter will deflect under load and leave chatter marks. If a pocket is deeper than about 3× the tool diameter, expect to step down in multiple passes or switch to a smaller cutter with a slower feed.

For thin walls, watch the cutting force. A 0.8 mm aluminium wall will bow under a heavy radial cut even if the toolpath is correct. Take lighter radial passes and leave a finishing pass of 0.2–0.3 mm. That is usually enough to bring the wall back to nominal.

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  Good fitPrismatic housings, plates, brackets, mould cavities.
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  Poor fitVery deep narrow slots under 2 mm wide.
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  MaterialsAluminium, stainless, steel, titanium, brass, plastics.

Turning rotates the workpiece against a stationary tool. It is the right process for shafts, bushings, fasteners, hydraulic fittings and anything dominated by a rotational axis. Diameter control is strong because the tool stays on center and the machine compensates for wear in the X axis.

The limit is off-axis features. A cross hole, a flat or a slot on a turned part normally means a second operation on a mill. Mill-turn centers combine both: in our shop, 16 mill-turn centers turn and mill in the same cycle, so concentricity between a bore and its bolt pattern stays inside one setup.

Long slender parts need support. A shaft with a length-to-diameter ratio above about 8:1 will deflect under cutting force and produce a barrel-shaped profile. A tailstock or steady rest fixes this. Below that ratio, a standard chuck holds well.

Surface finish on turned parts is easier to control than on milled surfaces. A sharp insert at the right feed rate leaves Ra 0.8–1.6 μm on most steels, and Ra 0.2–0.8 μm when we deliberately finish with a wiper insert or a slow feed.

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  Good fitShafts, bushings, connectors, valve bodies, spacers.
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  Watch L/D ratioAbove 8:1 add a tailstock or steady rest.
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  Mill-turn valueConcentric features stay in one setup.

These three processes cut sheet and plate rather than solid blocks. Laser melting or vaporizes metal with a focused beam. It gives a narrow kerf and a clean edge on thin material, and it handles stainless, carbon steel, aluminium and plastics. The cut face carries a small heat-affected zone that may need removal if the part will be welded or fatigue-loaded.

Plasma cuts conductive metal with an ionized gas jet. It is faster than laser on thick plate, roughly above 20 mm carbon steel, but the kerf is wider and the heat input higher. That means more distortion and a rougher edge. Use plasma when plate thickness or speed matters more than edge quality.

Waterjet erodes material with high-pressure water and abrasive. It adds no heat, so there is no heat-affected zone and no distortion, which suits titanium, thick aluminium and heat-sensitive alloys. It is slower and the kerf tapers slightly with depth. If a part needs a welded edge or a fatigue-critical edge, waterjet is often the safer choice.

None of the three holds tight tolerances on thickness or perpendicularity the way milling does. They produce a blank or a finished sheet part, not a machined feature with a ±0.005 mm fit.

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  LaserThin sheet, narrow kerf, fast on stainless and steel.
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  PlasmaThick conductive plate, higher speed, wider kerf.
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  WaterjetNo heat, no distortion, good for titanium and thick plate.

Aluminium 6061 and 7075 cut freely with carbide. They allow high spindle speeds and long tool life, which is why prototypes are often quoted in aluminium first. Plastics such as POM, ABS and PEEK cut even faster but need sharp tools and air blast, because chips melt and weld to the cutter if heat builds up.

Stainless 304 and 316 work-harden. If the tool rubs instead of cutting, the surface hardens and the next pass is harder still. The fix is a positive feed that stays under the hardened layer and a rigid setup. 17-4PH adds another variable: in the solution-treated condition it machines like mild steel, and after aging it is much harder.

Titanium Ti-6Al-4V and Inconel sit at the difficult end. They conduct heat poorly, so the cutting edge absorbs most of the temperature, and they are chemically reactive with many tool coatings. Low speeds, high feed per tooth, generous coolant and sharp tools are the only reliable route.

Carbon fibre is a special case. It abrades carbide quickly and produces conductive dust that must be captured. We cut it on machines set up for composite work with extraction, not on a general-purpose mill.

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  EasyAluminium, brass, ABS, POM, PMMA.
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  Work-hardening303, 304, 316, 17-4PH in aged condition.
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  DifficultTi-6Al-4V, Inconel, magnesium, carbon fibre.

A tolerance on a drawing only means something if it can be measured. A ±0.005 mm callout needs a controlled temperature, a calibrated machine and a measurement method that matches. On the shop floor that usually means a CMM or a micrometer in a temperature-stable room, not a caliper at the machine.

In-process monitoring catches drift before the part is finished. We check raw material on arrival, monitor dimensions during the run and inspect 100% of parts before shipment, with reports available on request. That sequence matters most on long runs, where a tool wears gradually and the last parts differ from the first.

Surface finish and tolerance interact. A tight tolerance on a rough surface is hard to verify because the contact point of the gauge moves. If a drawing calls for ±0.005 mm, the surface should be Ra 1.6 μm or finer so the measurement is repeatable.

For regulated work, the process has to be documented, not just performed. We hold ISO 9001:2015, IATF 16949:2016, ISO 13485:2016 and ISO 27001:2022, which covers automotive, medical device and information security requirements that buyers in those industries ask about early.

• 1
  Measure with the right toolCMM or micrometer, not a caliper, for ±0.005 mm.
• 2
  Inspect in sequenceRaw material, in-process, final; reports on request.
• 3
  Finish supports toleranceRa 1.6 μm or finer for tight dimensional control.