CNC Machining
When you need aluminum or steel parts — a few prototypes, a hundred units, or precision components — CNC is how you get them.
CNC (Computer Numerical Control) machining is a subtractive manufacturing process. A computer-controlled cutting tool removes material from a solid block — called the workpiece — to create a finished part. Unlike injection molding, which fills a cavity, CNC cuts away everything that is not the part.
For hardware founders, CNC sits at a critical intersection: it is the bridge between prototyping and production. You CNC early prototypes to test fit and function. You CNC low-volume production runs when tooling is not yet justified. And in some cases — medical devices, aerospace brackets, camera housings — CNC is the final production method.
The process begins with a digital 3D model — typically a STEP or IGES file exported from CAD. CAM (Computer-Aided Manufacturing) software translates that model into toolpaths: a series of instructions telling the machine where to move, how fast to cut, and which tool to use.
The most common machine type is 3-axis: the cutting tool moves in X, Y, and Z. This handles most parts, but features on the side of a part require flipping and re-fixturing the workpiece — which adds setup time and can reduce accuracy. 4-axis adds rotation around one axis, enabling side holes without re-fixturing. 5-axis tilts the tool or the workpiece for complex geometries in a single setup.
Material matters enormously. 6061 aluminum is the most common — machinable, lightweight, and corrosion-resistant. 7075 aluminum is stronger but more expensive and harder to machine. Stainless steel (304, 316) gives you strength and chemical resistance at the cost of slower cutting. For plastics, Delrin (acetal) and PEEK are widely used — PEEK in particular for medical and high-heat applications.
Standard CNC tolerance is ±0.005 inches (0.127 mm). A good shop can hold ±0.001 inches on critical features, but every tighter tolerance adds cost. Do not over-tolerate your drawing — most consumer product features do not need sub-thousandth precision.
What goes wrong
Over-toleranced drawings
Specifying ±0.001" everywhere makes parts 3–5x more expensive because the shop must slow down, use special tooling, and inspect far more frequently. Only dimension what matters.
Unreachable internal corners
A round cutting tool cannot make a perfectly sharp internal corner. Any inside corner will have a radius at least as large as the smallest tool. Design for the tool, not the ideal.
Deep pockets with small tools
Cutting a deep, narrow pocket requires a long, thin tool that vibrates and deflects. Keep pocket depth under 4x the tool diameter when possible.
Thin walls that chatter
Walls thinner than 0.5 mm in aluminum will vibrate during cutting, producing a rough surface and potentially breaking. Beef them up or be prepared for slower, gentler cuts and higher cost.
Material stock confusion
A "1 inch thick" plate of aluminum may actually measure 0.97 inches. Always check actual stock thickness against your design — or leave a facing allowance.
What founders should remember
CNC is for prototypes and low-to-mid volume
Under ~200 units, CNC is typically cheaper than molding because there is no tooling cost. Above that, molding amortizes and takes over. The crossover depends heavily on part complexity.
Design with tool diameters in mind
Common end mill sizes are 1/8", 1/4", 1/2". If your internal radii match a standard tool size, the shop can cut without tool changes, saving time and money.
Aluminum 6061 is the default answer
Unless you have a reason to use something else, spec 6061. It machines fast, anodizes beautifully, and every shop stocks it.