CNC Milling Basics: How the Cut Actually Works

• 1
  What matters most
• 2
  How the tool meets the workpiece
• 3
  3-axis, 4-axis and 5-axis in practice
• 4
  Matching the alloy to the cut
• 5
  What tolerance and finish really cost
• 6
  Design choices that change the price
• 7
  Choosing the right setup

CNC milling starts with a solid block clamped to a table. A spindle spins a multi-flute cutter, and the machine moves that cutter along programmed paths. Every pass shears away a chip of material. Nothing is formed or bent. The final shape is whatever remains after the unwanted volume is gone.

The cutting edge does the real work. Each tooth enters the material, compresses it until it fractures, then exits with a chip riding up the flute. If the chip is too thin, the edge rubs instead of cutting and work-hardens the surface. If it is too deep, the tool deflects and the wall tapers. Feed per tooth is the number that keeps this balance.

Heat is the other limit. Aluminum pulls heat into the chip and cuts cool. Stainless steel and titanium push heat back into the tool nose, so speeds drop and coolant matters more. That is why the same cutter that runs 8,000 rpm in 6061 may run 1,200 rpm in 316L without losing tool life.

Rigidity decides how close you can hold a dimension. The loop runs from spindle to tool holder to cutter to fixture to table. Any weak link shows up as chatter, a poor finish, or a size that drifts across a batch. On our 127 machines, the first fix for a tight callout is usually a stiffer setup, not a slower feed.

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  Climb millingTool rotates into the material; better finish on most modern machines.
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  Conventional millingStill used on older machines or rough castings with hard skin.
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  Chip evacuationRecutting a chip dulls the tool fast; air blast or flood coolant clears it.

A 3-axis mill moves X, Y and Z. The tool always approaches from the top. It suits plates, housings, brackets and any part where the features sit on a few accessible faces. Fixtures hold the part while the spindle does the work. For flat parts with moderate depth, this is the cheapest way to cut metal.

A 4-axis mill adds a rotary table, usually turning around the X axis. The part can index to new faces without being unclamped. Shafts with flats, cylinders with cross-drilled holes and parts with features on four sides fit here. The rotary table on our floor is Ø400 mm, which sets a practical limit on part diameter for this class.

A 5-axis mill adds a second rotary axis, so the tool can tilt relative to the part. Two ways exist: a trunnion that tilts the table, or a head that swivels the spindle. Either way, the cutter reaches undercuts, angled holes and contoured surfaces in one setup. We run 16 simultaneous 5-axis centers for exactly these shapes.

Five axes is not automatically better. Programming takes longer, and the machine is slower to position than a plain 3-axis. For a flat cover plate with six holes, 5-axis adds cost and no benefit. Use it when the geometry cannot be reached otherwise, or when one setup removes enough error to justify the rate.

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  Choose 3-axisPrismatic parts, open faces, high volume, tight budget.
• 2
  Choose 4-axisCylindrical parts, indexing to multiple sides, moderate complexity.
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  Choose 5-axisUndercuts, compound angles, contoured surfaces, fewer setups.

Aluminum is the default for prototypes and housings. 6061-T6 machines cleanly, takes anodizing well and holds ±0.005 mm on stable geometry. 7075 is stronger but tougher on tools and more prone to stress movement after heavy stock removal. 2024 cuts fast but needs care with corrosion and finishing.

Stainless steels trade speed for corrosion resistance. 303 is the free-machining grade and the easiest to turn or mill. 304 and 316L gum up more, so feeds stay light and coolant stays heavy. 17-4PH holds strength after heat treatment and appears often in medical and aerospace work. All of these are in our standard stock list.

Steel grades like 1018, 1045 and 4140 behave predictably and are common for shafts, plates and tooling. Titanium TC4 (Ti-6Al-4V) and Inconel are a different class. They hold heat at the edge, so speeds drop hard and tool wear climbs. These alloys need sharp tooling, rigid setups and realistic cycle estimates.

Plastics range from easy to difficult. POM and ABS cut fast with sharp cutters and air blast. PEEK and carbon fibre are abrasive and expensive, so they run slower with carbide or diamond tooling. Part geometry still matters: a deep thin wall in PEEK will deflect no matter how good the program is.

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  Best all-rounder6061-T6 for prototypes, fixtures and enclosures.
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  Hardest common alloyInconel and Ti-6Al-4V; plan for slower speeds and more tool changes.
• 3
  Watch stress7075 and 2024 can move after removing a lot of stock.

A general tolerance of ±0.1 mm costs little. Tightening to ±0.005 mm changes the process: more setups are controlled, tools are measured, temperature matters, and inspection time grows. We hold ±0.005 mm on our 5-axis centers, but only on features that justify it. Blanket tight tolerances on a drawing raise the price without improving function.

Surface finish follows the same logic. As-machined at Ra 1.6–3.2 μm is normal for most brackets and covers. Ra 0.8–1.6 μm needs a finer stepover or a finishing pass. Ra 0.2–0.8 μm often means a separate operation, slower feeds and a risk of visible tool marks. Specify finish by function, not by habit.

Datums matter as much as numbers. A hole located from a machined face will repeat. The same hole located from a raw casting surface will vary. If a drawing calls out a tight position, the datum should be a surface the machine can actually touch and measure.

Inspection closes the loop. We check raw material, monitor in-process, and inspect 100% before shipment. Reports are available on request. If a feature is critical, say so early. It is easier to plan a measurement than to argue about one after the parts are made.

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  Loose is cheap±0.1 mm suits most non-mating features.
• 2
  Only tighten what matesApply ±0.005 mm to bores, pilots and alignment features.
• 3
  Finish by functionSealing faces need fine Ra; hidden pockets do not.

Corner radii are the first thing to check. A cutter is round. If a pocket corner is drawn square, the shop must use a smaller tool, slow down, or leave material for EDM. A radius slightly larger than the cutter diameter lets the machine clear the corner at full speed.

Pocket depth is the second. A pocket deeper than about four times the cutter diameter forces a long, thin tool that deflects. The shop then reduces stepdown, adds passes and loses time. Shallow pockets with generous radii cut fast and hold size well.

Thin walls are the third. A wall under about 1 mm in aluminum or 0.5 mm in steel will move during and after the cut. Support it with tabs, leave extra stock, or accept that final dimensions will drift. If the wall is a sealing surface, plan a finishing pass with light engagement.

Threads, holes and marking follow simple rules. Standard metric and imperial threads are easy. Very fine threads in soft alloy strip easily. Laser marking needs a character height of at least 1.5 mm to read cleanly. None of these are hard limits, but each one adds a step if it is pushed.

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  Inside corner radiusKeep it larger than the cutter radius to avoid a second operation.
• 2
  Pocket depthAim for depth under four times tool diameter.
• 3
  Wall thicknessKeep aluminum above 1 mm and steel above 0.5 mm where possible.
• 4
  Marking heightMinimum 1.5 mm character height for laser engraving.