CNC machining of metal parts: a beginner's guide

• 1
  How CNC machining of metal parts removes material
• 2
  Milling, turning and drilling: which one your part needs
• 3
  Which metals are practical, and which are not
• 4
  What tolerance and surface finish really cost
• 5
  When CNC machining of metal parts is the wrong call
• 6
  Process fit for common metal part shapes

CNC machining of metal parts starts with a solid block, bar or casting. A cutting tool spins and moves along programmed paths, and each pass peels away a thin layer of material. Nothing is molded or bent. The final shape comes from what is left after thousands of small cuts.

The program is written in G-code. Every line tells the machine a position, a feed rate and a spindle speed. Feed rate is how fast the tool travels through the material, usually given in mm/min. Spindle speed is how fast the tool rotates, given in rpm. Surface speed, the speed of the cutting edge against the metal, ties the two together.

Tool choice sets the practical limit. A 12 mm carbide end mill can take a deeper cut in 6061 aluminium than in 316 stainless steel, because stainless work-hardens at the cut and pushes back harder on the edge. Chatter, heat and tool wear all show up first at the wrong combination of speed and feed, not at the wrong drawing.

So the design stage and the cutting stage are not separate worlds. A radius the tool cannot reach, a pocket deeper than three times the tool diameter, or a wall thin enough to deflect under clamping force will all be decided on the shop floor, not in the CAD file.

Milling holds the workpiece still and moves a rotating tool around it. It suits pockets, slots, faces, bosses and any shape where the material has to be carved from the outside in. Three-axis milling covers flat and stepped geometry. Adding a fourth or fifth axis lets the tool reach angled faces in one setup instead of three or four.

Turning holds the tool still and spins the workpiece. It suits round parts: shafts, bushings, connectors, valve bodies, anything with a dominant axis of rotation. A lathe with live tooling can also drill and mill cross-features, so a part with a turned body and a few side holes does not need a second machine.

Drilling is the third basic move, and it is usually folded into the other two. Hole diameter, depth and entry angle decide whether a standard twist drill works or whether the shop needs a pilot, a spot drill or a peck cycle to clear chips. Deep holes past five times the diameter are where most quotes change.

Many parts need more than one route. A mill-turn center combines turning and milling in one spindle, which removes a re-clamping step and protects the relationship between a bore and the face it sits on. That relationship is often the real tolerance that matters, not the individual dimensions.

Aluminium is the default for prototypes and low-volume parts. Grades 6061 and 6061-T6 machine fast, hold a good finish and take anodizing well. Grade 7075 is stronger but cuts less cleanly and is harder to weld. Grade 2024 behaves well on a machine and poorly in a corrosive environment unless it is coated.

Stainless steel 303 is the free-machining grade and the easiest of the family to run. Grades 304 and 316 are tougher, gummier and more prone to work hardening. Grade 17-4PH gives high strength after heat treatment and is common in aerospace and medical work. Expect slower feeds, shorter tool life and a higher price per part.

Steel grades 1018 and 1045 are straightforward. Alloy steels 4130, 4140 and 4340 are machinable in the annealed state and often heat treated afterwards, which means the shop cuts soft and the part gets hard later. Tool steel is machinable but abrasive, so tool wear drives cost more than cycle time.

Titanium TC4 (Ti-6Al-4V) and Inconel sit at the difficult end. Both generate heat at the cutting edge, both work harden, and both need rigid setups and generous coolant. They are rarely the cheapest choice, but for high-temperature or high-strength brackets there is often no substitute. Copper, brass and bronze machine easily and are chosen for conductivity or wear resistance, not strength.

A general machining tolerance of ±0.005 mm is achievable on a rigid machine with the right fixture, but it is not free. Every dimension you tighten adds inspection time, slower feeds and sometimes a second operation. The usual mistake is to tolerance every dimension on the drawing to the same tight value instead of marking the two or three that actually control function.

Surface finish follows the same logic. As-machined surfaces sit around Ra 1.6–3.2 μm. A high-quality finish of Ra 0.8–1.6 μm is normal for mating faces. A fine finish of Ra 0.2–0.8 μm usually means a finishing pass with a smaller stepover or a secondary operation such as polishing or lapping.

Feature geometry matters as much as the numbers. Internal corners take the radius of the cutter, so a sharp inside corner is not machinable without electrical discharge machining or a broach. Threads smaller than M2, slots narrower than 1 mm, and holes with a depth-to-diameter ratio above 10 all push a part out of the standard range.

The practical rule is to tolerance what touches something else and leave the rest at the general block tolerance. That single decision often removes one operation and one inspection step from the quote without changing how the part works.

CNC machining wins when you need a small number of accurate parts, when the geometry has tight features, or when the material has to be a specific alloy. It loses when the part is a thin shell, a large flat panel with lightening ribs, or a hollow box with uniform wall thickness. Those shapes waste material and machine time because the tool removes most of the stock.

Casting and die casting make sense once volume is high and the geometry is stable. Sheet metal fabrication handles brackets, enclosures and panels faster and cheaper than milling them from plate. 3D printing is often better for a first fit check, because it skips fixturing and tool path decisions entirely.

The crossover is not a fixed number. A part with three tight bores on one axis may stay economical as a machined part at 5,000 pieces, while a simple flat plate with two holes moves to stamping at a few hundred. Part count matters less than how much material has to be removed and how many setups the features demand.

A useful test: if the finished part weighs less than 20 percent of the stock it came from, ask whether another process makes the near-net shape first. Machining a casting or a forging to final size is common practice, and it can cut cycle time sharply while keeping the critical faces machined.