CNC Machining Vancouver: How Metal Parts Actually Get Cut

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  What CNC machining does to metal, in plain terms
• 2
  3-axis, 4-axis, and 5-axis: what each setup can reach
• 3
  Reading tolerance and surface finish before you order
• 4
  Why setup count drives both cost and accuracy
• 5
  When CNC machining is the wrong process
• 6
  Choosing the right CNC setup for your part

CNC machining is subtractive. A computer reads a toolpath file, and a spindle moves a rotating cutter through a block of material until the block becomes the part. Nothing is molded, cast, or layered. The shape comes from what you remove.

That single fact explains most of the trade-offs. You can hold tight tolerances because the machine measures its own axis positions continuously. You pay for every cubic centimeter of removed stock in spindle time. Deep pockets, thin walls, and long tools all cost more because the cutter has to reach where the metal still is.

The process covers a wide material range. Aluminum 6061, 7075, and 2024 cut fast and hold good finishes. Stainless 304 and 17-4PH cut slower and work-harden if feeds are too light. Titanium TC4 and Inconel demand rigid setups and low cutting speeds. Plastics like POM and PEEK machine cleanly when you control heat.

For engineers new to the process, the mental model is simple: geometry that a spinning tool can reach is cheap, and geometry it cannot reach is expensive or impossible.

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  Subtractive, not additiveStock is removed, so internal features need tool access.
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  Tolerance follows rigidityRigid fixtures and sharp tools hold ±0.005 mm.
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  Material drives costInconel and titanium can triple cycle time versus aluminum.

A 3-axis mill moves X, Y, and Z. The tool always points straight down. That is fine for prismatic parts with features on one or two faces, and it is the cheapest way to cut most brackets, plates, and housings. The limit shows up when a feature sits on an angled face or wraps around a boss.

A 4-axis machine adds rotation about one axis, usually the X or Y. The part turns while the tool cuts, so you can mill flats, slots, and holes around a cylindrical or square body without resetting. Shafts, manifolds, and parts with features on four sides fit this setup well.

A 5-axis machine adds a second rotary axis, so the tool can approach the part from almost any direction. We run 16 simultaneous 5-axis machining centers, with travels like 750 × 1,150 × 550 mm and a Ø400 mm rotary table. Impellers, turbine blades, deep contoured pockets, and single-setup complex parts are where 5-axis earns its cost.

The trade-off is real. Five-axis programming takes longer, and the machine hour rate is higher. If a part can be cut in two 3-axis setups with a simple fixture, that is usually the better choice.

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  3-axisBest for flat, prismatic parts and simple fixtures.
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  4-axisAdds rotation for cylindrical and four-sided work.
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  5-axisReaches contoured faces and cuts complex parts in one setup.

Tolerance and finish are two separate specifications, and mixing them up causes most back-and-forth on quotes. Tolerance is how close a dimension lands to its nominal value. Finish is how smooth the cut surface is. A part can be dimensionally perfect and still have a rough face.

We routinely hold ±0.005 mm (±0.0002 in) on critical features. That number only applies where the drawing calls for it. Blanket tolerancing a whole part to ±0.005 mm raises cost, because every feature then needs tighter process control and more inspection time. Call out critical dimensions only.

Surface finish follows the cutting parameters. As-machined surfaces land around Ra 1.6–3.2 μm. A high-quality machined finish is Ra 0.8–1.6 μm, and fine finishing reaches Ra 0.2–0.8 μm. Fine finishes usually need slower feeds, smaller stepovers, or a separate finishing pass, so they add cycle time.

One more thing: sharp internal corners cannot be machined. Every cutter has a radius, so a square corner becomes a corner with a fillet. Design in the largest corner radius you can accept and the part gets cheaper and stronger.

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  Tolerance is dimensionalApplies to size, position, and form.
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  Finish is textureMeasured in Ra; independent of tolerance.
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  No sharp internal cornersTool radius always leaves a fillet.

Every time a part comes off the table and goes back on, two things happen. You spend time re-zeroing, and you add a small positional error. A part cut in one setup has all its features located from a single datum. A part cut in four setups has four chances to drift.

This is the strongest argument for 5-axis work on complex parts. Instead of building three fixtures and moving the part between them, the machine tilts and rotates the part while the tool stays in one reference frame. Positional accuracy improves, and the fixture cost drops.

For simpler parts, setup count is a cost lever, not an accuracy problem. A well-designed soft jaw or vacuum plate can hold a part for a second operation accurately enough that the extra setup is cheap. We look at both when quoting.

Thin walls are the exception. A 0.8 mm wall will deflect under cutting force no matter how many axes you have. Light passes, sharp tooling, and support material matter more than axis count.

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  One setup, one datumFewer re-zeroing steps and less positional drift.
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  Fixtures cost moneyEach custom fixture adds tooling cost and lead time.
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  Thin walls need careBelow 1 mm, deflection is the limiting factor.

CNC machining is not always the answer. If you need 50,000 identical small parts with no tight tolerances, die casting or injection molding will beat it on unit cost after tooling is amortized. If the part is a thin sheet enclosure, sheet metal fabrication is faster and cheaper.

CNC machining wins when geometry is complex, tolerances are tight, quantities are low to medium, or the design is still changing. It also wins for functional prototypes, because you can cut the real material and test the real behavior.

A mixed approach often makes sense. Cut the prototype on a mill, validate it, then move to casting or molding for production. The prototype tells you whether the design works before you commit to tooling.

We also run 3D printing, vacuum casting, and die casting in-house, so the recommendation is not tied to one process. If a different route is cheaper for your part, that is what we will say.

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  High volume, loose toleranceCasting or molding usually wins.
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  Thin sheet partsSheet metal fabrication is faster.
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  Prototypes and low volumeCNC is the practical choice.