Internal CNC machining explained

• 1
  What happens inside the workpiece
• 2
  Which internal features suit CNC milling
• 3
  Why five-axis changes the internal problem
• 4
  How material shifts the internal limits
• 5
  Measuring what you cannot reach
• 6
  Where the cost actually sits
• 7
  Internal feature: which process fits

Internal machining removes material from a cavity, bore, slot, or undercut that sits inside the part envelope. The cutting edge is hidden from the operator, so every decision has to be made before the tool enters the material: how far it reaches, how it clears chips, and how it exits without rubbing the wall it just finished.

The tool is the limiting factor, not the machine. A Ø6 mm end mill needs a shank and holder behind it, so the practical reach-to-diameter ratio sits near 3:1 in aluminum and drops to about 2:1 in stainless or titanium. Push past that and the tool deflects, the wall tapers, and the floor chatters.

Reach is not the only ceiling. A cutter running at 3× diameter depth has far less room for chips than one cutting at 0.5× depth. Recutting those chips raises temperature, dulls edges fast, and leaves a torn surface that no finishing pass will fully repair. Coolant through the tool helps, but it does not create space that is not there.

That is why internal work is planned around access. If a boring bar can enter from both ends, the effective depth halves. If a corner can be reached with a ball nose instead of a square corner, the load on the tool drops sharply. Small geometry changes upstream remove most internal machining problems.

• 1
  Reach over rigidityA long, thin tool cuts, but not to the tolerance you drew.
• 2
  Chip room firstDepth-to-diameter above 3:1 usually needs through-tool coolant.
• 3
  Design the exitWhere the tool leaves the cut decides the surface you get.

A mill wins when the internal feature has a flat floor, a square shoulder, a cross-hole, or a shape that a single-point tool cannot generate. Cross-drilled oil galleries, bolt patterns around a bore, keyways, and internal pockets with radiused corners all fall into this group. Multi-axis control lets the tool approach at an angle instead of straight down.

A mill struggles with long, straight, round holes. A Ø10 mm bore at 100 mm deep is a 10:1 ratio. No end mill holds that. A drilled, reamed, or honed hole does it in one pass with far better straightness, and the cost per part is lower once volumes climb.

Blind cavities with sharp internal corners are another mismatch. The corner radius can never be smaller than the tool radius. If the drawing calls for R0.5 mm at the bottom of a 40 mm deep pocket, the cutter needed to reach that corner will chatter before it gets there.

The practical split is simple. Complex shape, few parts, tight positional tolerance: mill it. Simple round geometry, deep, high volume: drill and finish by another method, then bring the part back for the faces and cross-features.

On a three-axis machine, every new internal face usually means a new setup. Each re-clamp adds stack-up error and handling time. For a part with internal features on four sides, that is four chances to lose position.

A simultaneous five-axis center tilts the tool and the table together, so the cutter can follow a curved internal wall or reach an undercut in one continuous path. At GreatLight, 16 simultaneous five-axis machining centers handle this work, alongside 12 four-axis mills, 27 three-axis machines, and 16 mill-turn centers.

Tilting also solves reach. A tool angled into a pocket uses its side instead of its tip, which shortens the unsupported length and raises the depth it can cut without chatter. The same Ø6 mm cutter that stalls at 18 mm straight down may reach 30 mm when the head tilts 30°.

The trade-off is programming time and verification. Five-axis internal paths need full simulation, because a tilted holder can collide with the part wall in ways a three-axis path never can. That cost is paid once per part number, not once per part.

• 1
  Fewer setupsInternal faces on several sides cut in one clamping.
• 2
  Shorter effective reachTilted tools use side cutting edges.
• 3
  Simulation requiredHolder and wall clearance must be verified.

Aluminum 6061 and 7075 cut cleanly at deep reach. Chips are light, break easily, and evacuate with modest air or coolant pressure. A 4:1 depth-to-diameter ratio is realistic in a pocket, and 6061-T6 holds a fine Ra 0.8–1.6 μm wall without a separate finishing operation.

Stainless 304 and 316L work-harden. If the tool rubs instead of cutting, the surface gets harder and the next pass is worse. Internal work in these grades needs a sharp edge, a positive rake, and a feed rate that stays above the rubbing zone. 17-4PH in the H900 condition is harder again and usually calls for a smaller depth of cut.

Titanium TC4 (Ti-6Al-4V) and Inconel hold heat at the cutting edge. Deep internal pockets in these materials need high-pressure through-tool coolant and conservative stepovers. The material removal rate drops, and the internal feature often becomes the cost driver on the part.

Plastics behave differently again. POM and PEEK cut fast but move under clamping load, so a deep internal bore can close in after the vise releases. Light finishing passes and stress relief between roughing and finishing keep the bore round.

An internal feature is hard to inspect because the probe has the same access problem as the cutter. A Ø8 mm bore cannot be checked with a touch probe that needs 15 mm of clearance. Measurement has to be planned with the same care as the toolpath.

For bores, an air gauge or a bore micrometer covers most sizes down to Ø6 mm. Below that, pin gauges give a go/no-go answer rather than a number. Deep bores are checked at three depths, not one, because taper is the failure mode that matters.

Position of cross-holes and internal slots is usually verified on a coordinate measuring machine with a star probe or an extension stylus. The stylus deflection is calibrated, not guessed. Where a feature is genuinely unreachable, we cut a witness section on a first article and measure that.

GreatLight inspects 100% of parts before shipment, with raw material checks, in-process monitoring, and a final inspection. Reports are available on request. The qualification rate on this work runs at 99.99%.

Internal machining cost is driven by time under the surface, not by the outer envelope. A part that looks small can carry an hour of internal roughing if the pocket is deep and the material is tough. Quoting from a bounding box misses this entirely.

Tool life is the second cost. A long, thin cutter in stainless may last one or two parts before it breaks. That risk is priced into the process, and it is why an internal feature with a 5:1 ratio costs noticeably more than the same feature at 2:1.

Programming and simulation add a fixed cost. Multi-axis internal paths take longer to prove out, so a one-off prototype carries more of that burden per unit than a 500-piece run. Volume spreads it out.

There is no minimum order quantity at GreatLight, so a single prototype is workable. Quotation and a free DFM analysis come back within 12 hours, production can start within 24 hours, and parts typically ship in 3–5 days.

• 1
  Depth ratioCost climbs sharply past 3:1 depth-to-diameter.
• 2
  Tool breakage riskThin cutters in tough alloys are priced for replacement.
• 3
  Fixed programmingMulti-axis internal paths cost the same at any volume.