Alberta CNC machining: what engineers should know before sending a drawing

A part earns its place on a 5-axis machine when setup count, not cutting time, drives the cost. Housings with ports on four faces, impellers, brackets with angled bosses, and manifolds with intersecting bores all fall into that group. One setup gives you the datums, so hole-to-hole position stays tied to the same zero.

Simple prismatic work does not need it. A flat plate with through-holes and a pocket runs faster on a 3-axis mill with a fixture, and the tolerance outcome is the same. If a part carries a single angled face, a tilt fixture on a 3-axis machine usually beats programming a full 5-axis cycle.

The decision point is geometric. When more than two faces need machining and the angular callouts are tighter than ±0.05°, move it to 5-axis. Below that, keep the simpler machine and spend the savings on inspection.

A tolerance of ±0.005 mm is a process claim, not a machine claim. It only holds when the material is stable, the tool is fresh, and the shop controls temperature. Aluminum 6061 and 7075 behave differently after roughing; 7075 moves more as internal stress releases, so a finishing pass after a stress-relief pause is often needed.

For most parts, ±0.025 mm is the honest working number on a milled feature and ±0.01 mm on a bored or turned diameter. That is where a shop can hold a batch of 500 without sorting every piece. Pushing to ±0.005 mm on every dimension raises cost sharply and rarely improves function.

Pick tight tolerances where they matter. A bearing bore, a sealing face, or a mating spigot deserves the tight callout. A clearance hole for an M6 bolt does not. Engineers who mark only the critical dimensions get lower quotes and fewer inspection arguments.

Surface finish follows the same logic. Ra 1.6–3.2 μm is standard as-machined. Ra 0.8–1.6 μm needs a controlled finishing pass. Ra 0.2–0.8 μm usually means a secondary operation, and it should be specified only on the sealing or sliding surface.

Aluminum is the default for prototypes and housings. Grades 6061 and 6061-T6 machine cleanly and take anodizing well. Grade 7075 gives higher strength but cuts slower and distorts more, so it suits structural brackets rather than thin covers. Cast grades such as ADC12 appear when the part starts as a die casting and only the critical faces get machined.

Stainless grades 303 and 304 cover most fittings and general parts. Grade 316L is the choice for anything exposed to chlorides or sterilization, which is why medical and food equipment work lands there. Grade 17-4PH (SUS630) brings high strength after heat treatment and is common on shafts and valve components.

Steel grades 1018 and 1045 handle general shafts and plates. Alloy steels 4130, 4140 and 4340 appear on load-bearing parts where hardness and fatigue life matter. Titanium TC4 (Ti-6Al-4V) and Inconel cut slowly, wear tools fast, and should be reserved for parts where the temperature or weight requirement rules out everything else.

Plastics behave differently again. POM and PEEK hold dimensions well; ABS and PP tend to deflect under clamping, so light passes and sharp tooling matter more than spindle speed. Carbon fibre composites need diamond-coated tooling to avoid delamination.

A machined part is rarely finished when it leaves the spindle. Burrs on cross-holes, sharp edges on mating faces, and residual coolant all cause problems downstream. Integrating deburring, cleaning and surface treatment into the same production run removes a handling step and keeps the part traceable to one supplier.

Anodizing comes in clear, colour, hardcoat and conductive variants. Hardcoat adds wear resistance on sliding surfaces; conductive anodizing keeps grounding paths intact on electronic enclosures. Electroless nickel, zinc, silver and gold plating serve corrosion and conductivity needs. Powder coating and black oxide handle appearance and mild corrosion protection.

Inspection has to be planned, not added at the end. A first-article report on the critical dimensions, in-process checks on the features that drift, and a final dimensional report give the buyer something to compare against the drawing. For Alberta buyers sourcing overseas, that paperwork matters as much as the part.

Sampling every piece is not practical on a 10,000-part run. What matters is that the shop knows which dimensions move and checks those at a set frequency. On request, the full inspection record ships with the parts.