CA CNC Machining: What the Process Can and Cannot Hold

The toolpath here comes from a CAD model rather than from a drawing and a machinist's hand. The model carries the geometry, the CAM programmer adds stock, tool and fixture decisions, and the post-processor turns that into G-code for a specific control. Every step in that chain can shift the part, so the model is a starting point, not a guarantee.

The chain is short but not free of decisions. A pocket modeled with a 3 mm internal corner cannot be cut by a 3 mm cutter at full depth without deflection, so the programmer either opens the radius or runs a smaller tool in a second pass. That choice shows up later as cost and cycle time.

Tolerance also depends on how the part is held. Thin walls move when the vise closes. A bore that is round in the model can come out lobed if the fixture pressure is uneven. Good CAM work predicts this. It does not remove it.

What the machine control receives is a set of coordinates, feed rates and tool changes. Nothing in that file knows your assembly stack-up. That is why we ask for the mating part or the fit callout before quoting, not after the first article fails.

Most prismatic parts never need simultaneous motion. A plate with holes, slots and a flat face cuts faster on a 3-axis machine, and the setup is easier to inspect. If all features are reachable from one or two directions, adding rotary axes only adds programming time.

A 4-axis mill earns its place when features repeat around a part axis. Flanges, manifolds and cylindrical housings with cross-drilled ports can be cut in one setup on a rotary table. That removes a re-fixture step, which is usually where position error creeps in.

Simultaneous 5-axis suits contoured surfaces, undercut flanges and deep pockets that a straight tool cannot reach. Impellers, turbine housings, medical instrument bodies and complex brackets are the usual candidates. The trade-off is that the tool is rarely at its stiffest angle, so feeds drop and cycle time rises.

Our shop runs 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines and 16 mill-turn centers, with 127 high-precision CNC machines in total. We route work to the smallest machine that can hold the drawing, because that is normally the cheapest route to a good part.

We hold ±0.005 mm (±0.0002 in) on features that need it, but that number should be applied only where the assembly requires it. Tightening every dimension on a drawing raises inspection time and scrap risk without improving function. Mark the critical dimensions and leave the rest at general tolerance.

Surface finish follows the same logic. As-machined surfaces land at Ra 1.6–3.2 μm, a fine finish at Ra 0.8–1.6 μm, and a polished or lapped finish at Ra 0.2–0.8 μm. Each step down costs more because it means slower passes or a second operation.

Some features simply do not hold at tight tolerance on a mill. Deep bores with a high length-to-diameter ratio deflect, sharp internal corners concentrate stress and cannot be cut to a small radius, and thin floors chatter. Where a design needs those shapes, we will say so during DFM review.

Inspection backs the claim. We check raw material on arrival, monitor the process in cut, and inspect 100% of parts before shipment, with reports available on request. If your drawing lists a datum scheme, we inspect against it rather than against the model alone.

Aluminium 6061 and 7075 cut cleanly and hold tight tolerance without much fuss. Stainless 304 and 17-4PH work-harden, so the programmer keeps the tool moving and avoids a spring pass. Titanium TC4 (Ti-6Al-4V) and Inconel need lower cutting speeds and more tool changes, which is why they cost more per part.

We machine aluminium grades 6061, 6061-T6, 2024, 5052, 5083, 6063, 6082, 7075 and ADC12; stainless 303, 304, 316, 316L, 420, 430, 431, 440C and 17-4PH; steel 1018, 1045, 4130, 4140, 4340, A36 and tool steel; copper and brass C101, C103, C110, beryllium copper, C27400, C28000 and C36000; titanium and specials TA1, TA2, TC4, Inconel and magnesium AZ31B or AZ91D; plus ABS, PC, PMMA, POM, PA, PEEK, PP, HDPE and carbon fibre.

Finish usually comes after the cut, and it changes dimensions. Anodizing builds a few micrometres on the surface. Hardcoat builds more. If a bore is at the top of its tolerance band before coating, it may finish out of spec. Tell us the coating before we set the pre-plate size.

Available finishing includes anodizing in clear, colour, hardcoat and conductive types; electroless nickel, zinc, silver and gold plating; powder coating and black oxide; bead blasting, tumbling, brushing and polishing; and laser marking with a minimum character height of 1.5 mm.