CNC Machine Basic Guide: How Metal Gets Cut

• 1
  What a CNC Machine Actually Does
• 2
  From CAD Model to Finished Part
• 3
  Milling, Turning, and When Each Fits
• 4
  What Tolerance and Finish Really Cost
• 5
  Material Choice Changes the Cut
• 6
  Which Machine for Which Part

At its core, a machining center is a tool holder that moves on programmed axes while a spindle spins the cutter. The controller reads G-code and drives ball screws, linear guides, and servomotors to positions measured in microns. Nothing about the shape comes from a human hand. The shape comes from a toolpath file that a programmer built from your CAD model.

Subtractive machining starts with a solid block or bar. The cutter removes material in passes until what remains is the part. That is the opposite of casting or 3D printing, where material is added or formed into a cavity. Because material is removed, internal corners always carry the radius of the tool that cut them.

Three numbers describe most of what the machine can do. Axis count sets which faces you can reach in one setup. Spindle speed and torque set which materials cut cleanly. Work envelope sets the largest part that fits. Everything else, from coolant to chip conveyors, supports those three.

Repeatability matters more than peak accuracy on a production run. A machine that holds ±0.005 mm on part one and part five hundred is worth more than one that hits ±0.002 mm once. Thermal growth, tool wear, and fixture stiffness drive that drift far more than the controller does.

The chain starts with a solid model and a drawing with tolerances. A programmer imports the model into CAM software and defines the stock, the work coordinate system, and the tools. Feeds and speeds come from material, cutter diameter, and depth of cut. A 6 mm carbide end mill in 6061 aluminum runs very differently from the same cutter in 17-4PH stainless.

Next comes the setup sheet. Which face goes down, where the vise jaws clamp, and how many setups the part needs. Every extra setup adds a re-clamp error and labor time. Good DFM work tries to reach the critical features from one or two sides. That single decision often moves cost more than toolpath tweaks.

On the floor, the operator loads the stock, touches off tools, and runs a first article. In-process checks catch drift before a full batch is scrap. At the end, the part is deburred, cleaned, and inspected against the drawing. If a feature cannot be measured easily, it will be hard to control in production.

Programming quality shows up in cycle time and tool life. A clean toolpath keeps radial engagement steady, avoids full-width slotting in hard metals, and retracts clear of the fixture. Sloppy paths cause chatter, broken cutters, and scrap. None of that is visible in a CAD review.

Milling spins the tool and feeds the workpiece past it. Prismatic parts, pockets, slots, and flat faces are mill work. A three-axis mill cuts from one direction, so features on five sides need multiple setups or a re-fixture. Adding a fourth axis rotates the part around one axis, which lets you machine a cylinder's flats without unclamping.

Turning spins the workpiece and feeds a single-point tool along it. Any part that is mostly round belongs on a lathe: shafts, bushings, fittings, and threaded bodies. Turned diameters hold tight tolerance easily because the tool never leaves the cut. Off-axis holes and flats still need a mill, or a mill-turn center that does both.

Five-axis machining tilts either the tool or the table. It reaches undercuts, steep walls, and complex contoured surfaces in one setup. The trade-off is programming effort and machine time. Five-axis is not automatically more accurate; it is more reach. Use it when the geometry demands it, not as a default.

Pick the machine by the dominant geometry, not by the material. A round part with a cross hole is still a lathe job with a secondary operation. A flat plate with pockets is mill work even in titanium. Getting this wrong adds setups, cost, and tolerance stack-up.

A general machining tolerance of ±0.1 mm is easy on most features and keeps cost low. Tightening to ±0.005 mm changes the process. The machine needs thermal stability, the fixture needs rigidity, and the operator needs to measure the feature in-process. That applies only to the dimensions that need it, never to the whole drawing.

Surface finish follows the same logic. As-machined surfaces sit around Ra 1.6–3.2 μm. A finer Ra 0.8–1.6 μm needs a lighter finishing pass, and Ra 0.2–0.8 μm usually means a dedicated finishing toolpath or a secondary polishing step. Fine finishes on a non-functional face are wasted money.

Some features are simply hard to hold. Deep bores, thin floors, and sharp internal corners all fight the cutting tool. A corner radius smaller than the cutter can reach forces either a smaller tool, which deflects, or EDM, which adds a process. Designers who spec a generous corner radius get cheaper parts.

Tolerance stack-up is the quiet cost driver. Every setup repositions the part, and each reposition adds error. A feature that is dimensioned from a datum machined in setup one, but cut in setup three, inherits both errors. Dimension from datums that are cut in the same setup whenever possible.

Aluminum is the default for prototypes and most housings. Grades 6061 and 7075 cut fast, hold good finishes, and resist corrosion after anodizing. They are soft enough that a light clamp can leave marks, so fixture contact points matter on cosmetic faces.

Stainless steels cut slower and work-harden if the tool rubs instead of slicing. Grades 303 and 304 are common; 17-4PH holds strength after heat treatment and is used for shafts and fittings. Feeds must stay aggressive enough to stay under the hardened layer, which is why light finishing passes in stainless can ruin a part.

Titanium and nickel alloys like Inconel cut hot and wear tools fast. They need lower surface speeds, rigid setups, and plenty of coolant. These materials are chosen for temperature and strength, not for machinability, so budget more cycle time and more tool changes.

Plastics behave differently again. POM and PEEK machine cleanly but move with temperature, so holding ±0.005 mm on a long plastic part is unrealistic. ABS and PC can gum up if the cutter dwells. Sharp tools and high spindle speeds with fast feed keep the chip clear.