CNC cutting treatment: how a cut forms and where it stops working

• 1
  What actually happens during CNC cutting treatment
• 2
  How axis count changes the cutting treatment
• 3
  Material decides the cutting window
• 4
  Tolerance, finish and where the limits sit
• 5
  Choosing the cutting route for a feature

CNC cutting treatment is a subtractive process. A rotating or stationary cutting edge is driven along a programmed path, and each pass removes a controlled layer of material. The tool does not melt or burn the workpiece away. It shears it. The quality of the result comes from the balance between cutting speed, feed per tooth, and the depth of that layer.

Three variables set the cut. Surface speed is how fast the edge slides across the material, in m/min. Feed per tooth is how far the tool advances per cutting edge, usually 0.02–0.15 mm. Axial and radial depth of cut decide how much of the flute is engaged at once. Change any one of them and the others must follow, or the edge rubs instead of cutting.

Heat is the visible consequence. Almost all of the mechanical energy that goes into the shear zone becomes heat, and most of that heat leaves with the chip. When the chip is too thin, the heat stays in the workpiece and the tool. That is why a light finishing pass at the wrong feed can burn a surface that a heavier roughing pass left clean.

Chip formation also tells you whether the parameters are right. Aluminium 6061 produces long, bright, curled chips at the right feed. Steel 1045 produces short grey chips that break and fall away. Titanium TC4 produces thin segmented chips and needs slower surface speed plus flood coolant, or the edge work-hardens the surface it just cut.

A three-axis machine moves the tool in X, Y and Z only. The workpiece stays still. This is the fastest and cheapest way to cut flat faces, pockets, slots and drilled holes, and it covers a large share of real parts. On our three-axis machines the working envelope runs from 500 × 500 × 450 mm up to 4,000 × 400 × 150 mm for long parts.

A fourth axis adds rotation about one axis, usually the X or Y. The part turns while the tool cuts, so features on four sides can be reached in a single setup. That matters for accuracy. Every extra setup adds a re-clamp and a new datum, and each one gives back some of the tolerance you paid for.

Five-axis machining tilts the tool as well as the table. The benefit is not only access to undercuts. It is that a ball nose cutter can be held at an angle to the surface, so the tip is no longer cutting at zero speed. That single change improves finish on curved surfaces and lets shorter, stiffer tools reach deep pockets. Our 16 simultaneous five-axis centers handle this work, with a Ø400 mm rotary table available for round parts.

Not every part gains from five axes. A prismatic bracket with holes on three faces is usually cheaper on a three-axis machine with two setups. Five-axis programming costs more, and the machine hour costs more. The right question is whether the geometry or the tolerance truly demands the extra motion.

Aluminium is the forgiving end of the range. Grades like 6061-T6, 7075 and 6082 cut at high surface speed with sharp, uncoated or polished tools. They also move after machining, because the residual stress from the plate is released as material is removed. Thin walls can bow by more than the drawing tolerance unless you take light finishing cuts on both sides.

Stainless steel 304 and 316 work-harden. If the tool rubs rather than bites, the surface gets harder and the next pass cuts worse. The fix is constant feed and a depth of cut that stays under the hardened layer. Grades such as 17-4PH and 440C add abrasive carbides, so tool life drops and you should plan for more than one cutter per batch.

Titanium TC4 and Inconel take this further. They conduct heat poorly, so the edge runs hot while the part stays cool. Surface speed drops to a fraction of what aluminium allows, coolant must reach the cut, and the tool path should avoid stopping in the cut. Magnesium AZ31B and AZ91D cut easily but need chip control, because fine magnesium chips ignite.

Plastics behave differently again. POM and ABS cut cleanly but melt if the feed is too slow. PEEK and carbon fibre are abrasive and need sharp carbide. Carbon fibre also produces dust that must be extracted, and the cut edge can fray if the tool is dull.

Our standard machining tolerance is ±0.005 mm ( ±0.0002 in ) on features we can reach and measure. That number is not automatic across a whole part. It holds on a bore you can probe, less reliably on a 0.8 mm tall rib that deflects under the cutter. Tolerances should be attached to the features that need them, not applied to the whole drawing.

Surface finish follows the same logic. As-machined surfaces land around Ra 1.6–3.2 μm. A controlled finishing pass reaches Ra 0.8–1.6 μm. Fine finishes at Ra 0.2–0.8 μm are possible on the right material with a light pass and a sharp tool. Chasing a fine finish on a deep pocket wall is slower, because the tool has to be long and the long tool vibrates.

Deburring is part of the treatment, not an extra. A cut edge carries a burr whose size depends on material and feed. On aluminium the burr rolls and is easy to remove. On steel and stainless it can be hard and sharp. We inspect 100% of parts before shipment, and reports are available on request for critical dimensions.

Some features simply should not be cut. A hole 6 mm deep at Ø1 mm calls for a tool that will snap. A square internal corner cannot be milled square; it will always carry the cutter radius. A wall thinner than about 0.5 mm will chatter and cannot be trusted to hold a tolerance. Those are good moments to change the design, not the machine.