CNC Machining Parts 2: How the Cutting Process Shapes the Part

• 1
  What actually happens when a CNC machine cuts CNC machining parts 2
• 2
  Tool reach and how it caps your part geometry
• 3
  Where ±0.005 mm comes from and when you do not need it
• 4
  How material choice changes the cutting process
• 5
  From single prototypes to 10,000-part runs
• 6
  When CNC machining fits and when it does not

Every part starts as a solid block, bar or plate. The machine removes material until what is left matches the CAD model. There is no mold and no forming die. The shape comes from the path of a spinning cutter, so the tool has to physically reach every surface it creates.

That single fact explains most of the design rules. A cutter is a cylinder with a defined diameter and length. It cannot cut a square internal corner, it cannot reach behind a lip, and it deflects when it hangs too far out of the holder. Deep pockets with small corner radii are where those limits meet.

The cut is also a controlled collision. Each tooth of the cutter hits the workpiece and shears off a chip. Heat leaves with the chip, which is why feed rate matters as much as spindle speed. Push too light and the tool rubs instead of cutting. Push too hard and the tool bends or breaks.

Programming sits on top of all this. The CAM system decides tool entry, step-over and retract moves. A clean toolpath keeps load steady. A poor one hammers the cutter on entry and leaves witness marks on the finished surface.

A Ø6 mm end mill cutting 40 mm deep has a length-to-diameter ratio of about 7:1. At that ratio the tool bends under cutting force. The result is chatter, an out-of-tolerance wall, or a broken cutter. The usual fix is to open the pocket, shorten the depth, or accept a larger corner radius.

Internal corners are the other hard limit. A cutter leaves the radius of its own diameter at every internal corner. If your drawing calls for a sharp internal corner, the shop has three choices: leave the tool radius, add an undercut with a different tool, or move to EDM. All three change cost and lead time.

Thin walls behave in the opposite way. A wall under about 0.8 mm will flex away from the cutter and spring back after the pass. Roughing first and finishing with light passes helps, but very thin sections often need support material or a change in design.

Undercuts, internal threads and cross-holes all need tool access from a specific direction. Five-axis machines reduce the number of setups by tilting the part, but they do not remove the need for clearance. If the tool cannot see the feature, the feature cannot be cut in one setup.

GreatLight holds ±0.005 mm on machined features when the drawing calls for it. That number is not a default. It comes from a stable setup, a rigid tool, temperature control and 100% inspection before shipment. Tightening a tolerance costs money at every step, from programming to metrology.

Most functional features do not need it. A mounting hole pattern at ±0.05 mm is fine for bolts. A bearing bore at ±0.01 mm is usually tight enough. Only sealing faces, mating spigots and precision locating features tend to justify ±0.005 mm.

Surface finish and tolerance are linked. A Ra 1.6–3.2 μm as-machined surface is normal for general parts. Ra 0.8–1.6 μm needs a finishing pass with a sharp tool and a light cut. Ra 0.2–0.8 μm needs careful feed control and often a different tool geometry.

Geometry matters too. A long slender part will move under cutting force no matter how tight the tolerance callout is. The shop can plan around this with supports and light passes, but the practical limit for a long shaft will always be looser than for a compact block.

Aluminium 6061-T6 cuts fast and holds a good finish. It is the default for prototypes and many production parts. 7075 is stronger but more brittle, so it needs sharper tools and lighter cuts to avoid chipping at edges.

Stainless 304 work-hardens. If the cutter rubs instead of cutting, the surface gets harder and the next pass is worse. The fix is a positive feed that stays under the hardened layer. 316L behaves similarly and is common in medical and marine parts.

Titanium TC4 (Ti-6Al-4V) conducts heat poorly, so the cutting edge runs hot. Speeds drop and coolant flow matters more. Inconel is harder again and is usually reserved for features that genuinely need high-temperature strength.

Plastics behave differently. POM and ABS cut cleanly but can melt if the feed is too slow. PEEK is stable and strong but abrasive on tooling. Carbon fibre wears tools fast and needs dust extraction, so it is often a separate cell from metal work.

The same machine can make one part or ten thousand. What changes is the setup and the fixturing. A one-off part is programmed once and cut from a vise or a simple fixture. A production run gets a dedicated fixture, a proven tool list and a first-article inspection.

GreatLight runs no minimum order quantity, so a single prototype and a 10,000+ part run both go through the same shop. Quotation and free DFM analysis come back within 12 hours, and production can start within 24 hours of approval. Parts normally ship in 3–5 days.

Volume does change cost per part. Tool wear, fixture cost and inspection time spread across more pieces. At low volume most of the price is setup and programming. At high volume material and cycle time dominate.

It also changes the right process. A part that is machined at 50 pieces may be better die cast or vacuum cast at 5,000. The geometry has to allow it, and the tolerance has to survive the change. That trade is worth checking before you commit to a tool.