CNC tolerances explained

Tolerance is a number that gets thrown around as if it were a property of the machine. It is not. Tolerance is what the part needs to function — the machine, the material, the toolpath, and the room temperature decide whether you can hit it.

On our 3-axis router we cut wood and plywood comfortably to ±0.2 mm, which is tight enough for joinery, drawer fronts, and any kitchen cabinet you will ever build. We can hold ±0.1 mm on a single dimension if we slow the feed and finish-pass twice, but you start fighting wood movement at that point — a panel that left the machine perfect at 22 °C can drift 0.3 mm by the time it gets to the customer if humidity changes. So we quote wood at ±0.2 mm and tell people that is the honest number.

Aluminium is a different conversation. On the same 3-axis machine we hold ±0.05 mm on small brackets and pockets if the geometry allows climb-cutting and we use a single-flute carbide bit. The limit is not the spindle, it is rigidity — long thin walls chatter, and chatter shows up as 0.1 mm waves on a finished surface. If your part has features taller than three times the wall thickness, plan to either thicken the wall or accept the wave.

Plastics are the easiest material in terms of tolerance and the trickiest in terms of finish. Acetal and HDPE machine to ±0.05 mm without complaint, but heat builds up fast on enclosed pockets and the part can warp during the cut. We run plastics with a coolant mist and a slower stepdown.

When do you need 5-axis? Only when the part has a feature that cannot be reached without flipping the stock — undercuts, compound angles on a single face, organic surfaces. For 90% of furniture and B2B parts, 3-axis is what you want, and a shop that quotes 5-axis for a flat panel is selling capability you do not need.

On a drawing, specify tolerance per dimension, not as a global note. Most dimensions on a wood part can be ±0.5 mm — ±0.1 mm where it actually matters costs less if it is the only dimension marked tight.