Seattle's Precision CNC Processing Expert: Your Quality Partner

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  How seattles precision cnc removes metal
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  What ±0.005 mm really means on a drawing
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  Surface finish callouts that match the function
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  Where seattles precision cnc fits and where it does not
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  Which process fits your part

Seattles precision cnc work is subtractive: a rotating cutter or a turning tool follows a toolpath written from your CAD model, and the material that is not part is cut away. Nothing is shaped by a mold, so geometry is defined by machine motion rather than by tooling. That is why the process handles undercuts, deep pockets and one-off shapes that casting or stamping cannot touch without a new die.

The cut itself is a controlled chip. Feed per tooth, spindle speed, axial depth and radial engagement set the chip thickness, and the chip carries the heat away. Push feed too high and the tool deflects. Push speed too low and the edge rubs, work-hardens stainless, and burns the surface.

Heat is the real constraint, not horsepower. A 12 mm carbide end mill in 6061 aluminium wants roughly 3,000–6,000 rpm and 0.05–0.15 mm per tooth. The same cutter in 316 stainless drops to 300–700 rpm and 0.03–0.08 mm per tooth. Get that ratio wrong and you scrap the part on the last pass, not the first.

Rigidity decides what you can hold. A part clamped in a vise on a 3-axis machine sees the tool cantilever out over the cut. On a 5-axis center the table tilts the work under the tool, so the same feature is cut with a short, stiff tool. That is the difference between ±0.05 mm and ±0.005 mm on a deep pocket wall.

Tolerance is a cost curve, not a checkbox. A general block tolerance of ±0.1 mm is cheap and fast. Tighten a single bore to ±0.005 mm and you add a boring cycle, a temperature-stabilized finish pass, and a CMM check. Tighten a whole part to ±0.005 mm and the quote doubles.

Seattles precision cnc shops hold ±0.005 mm on bores, spigots and mating faces. That number comes from a stable spindle, sharp tooling, a warm machine and a controlled inspection room. It does not come from the control alone. A ±0.005 mm callout on a 400 mm thin wall will move when you unclamp it, no matter how good the machine is.

Wall thickness matters more than size. A 6061 bracket with 1.5 mm walls will deflect under a finishing pass and spring back. Hold the roughing stock, take light finish passes and support the wall with soft jaws or a fixture. If the drawing shows a 0.8 mm wall at 150 mm long, expect to talk about a fixture, not a tolerance.

Datum choice drives the result. Engineers who dimension from a functional face get parts that assemble. Engineers who dimension from a rough cast surface get arguments. Pick the datum that touches the mating part, and say so on the drawing.

Ra is the average roughness of the profile, measured in micrometres. As-machined aluminium lands around Ra 1.6–3.2 μm off the cutter. A careful finish pass reaches Ra 0.8–1.6 μm. Below Ra 0.8 μm you are polishing, lapping or hard-turning, and the price climbs with every step.

Do not call out Ra 0.4 μm on a part that only needs to look clean. Sealing faces, bearing bores and sliding surfaces earn a fine finish. A cover plate does not. Marking the whole drawing with one tight Ra value forces the shop to finish every surface that way, including the ones nobody touches.

Anodizing and plating change the surface you measured. Type II clear anodize adds roughly 5–10 μm per side and can round a sharp edge. Hardcoat adds more. If a bore must stay on size after coating, mask it or machine it undersize on purpose and tell the shop which surfaces are critical.

Bead blasting hides tool marks and leaves a matte texture that paint and powder coat grip well. Brushing gives a directional grain. Polishing removes material, so never put a polish callout on a dimension you intend to hold afterwards.

Seattles precision cnc fits low-to-medium volume metal parts with real geometry: housings, manifolds, brackets, impellers, surgical instruments, fixture plates. Anywhere from one prototype to a few thousand pieces, and anywhere the shape would need an expensive die to cast or forge.

It does not fit thin sheet parts at high volume. A stamped bracket at 100,000 pieces per year will beat machining on unit cost every time. It also does not fit parts where the material must stay soft and uniform, or where a molded surface finish is the whole point.

Plastic parts are a mixed case. POM, PEEK, ABS and PC machine well for prototypes and low volume. Past a few thousand parts, injection molding wins. Machined plastic is best used to prove the design, then handed to a molder with the same CAD.

Materials we cut every week include 6061, 7075 and 6082 aluminium, 303, 304, 316L and 17-4PH stainless, 1018 and 4140 steel, C36000 brass, Ti-6Al-4V titanium, Inconel and magnesium AZ31B. Titanium and Inconel cut slowly because they hold heat at the edge, so budget more time and more tool wear.