Seattle CNC Processing Services: How to Choose Without Guesswork

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  Key takeaways
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  What to compare across Seattle CNC processing services
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  Tolerance Is a Range, Not a Number
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  Which Parts Belong on a 5-Axis Machine
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  Material Choice Drives the Quote More Than You Think
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  Lead Time Claims Need to Be Unpacked
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  Where Buyers Lose Money on CNC Parts
• 8
  Step by Step: How to Vet a Supplier in One Week

Every shop advertises tight tolerance. The useful question is where that number applies. On a 40 mm aluminum bracket with a bored bore, ±0.005 mm (±0.0002 in) is a normal working target on a 5-axis center with a Ø400 mm rotary table. On a 4,000 mm frame, the same figure describes the machine, not the part. Thermal drift, fixturing and tool wear all scale with size.

So ask for tolerance per feature class. Datum holes and bearing seats deserve the tight callout. Mounting slots, clearance holes and cosmetic edges rarely do. When a drawing puts ±0.005 mm on everything, the shop either charges for inspection it does not need or quietly ignores the callout. Both outcomes cost you.

Surface finish follows the same logic. Ra 0.2–0.8 μm comes from fine finishing passes and often a second operation. Ra 1.6–3.2 μm is as-machined and covers most brackets, housings and fixtures. Ra 0.8–1.6 μm sits in the middle and is where most functional sealing faces land.

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  Tight where it functionsReserve ±0.005 mm for datum and mating features.
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  Loosen the restGeneral tolerances of ±0.05 mm cut cost without hurting function.
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  Name the finishRa 1.6–3.2 μm as-machined is default; specify finer only on sealing faces.

5-axis machining adds two rotary axes to the three linear ones. The tool reaches the workpiece from nearly any direction in one setup. That single-setup property matters more than the axis count. Every additional setup adds a fixture, a re-zero and a stack of positional error.

Good candidates: impellers, turbine housings, medical instrument bodies, engine brackets with compound angles, and any part with undercuts a 3-axis spindle cannot reach. Parts with deep pockets and thin walls also benefit, because the tool can stay tangent to the wall and control deflection.

Poor candidates: flat plates, simple shafts, and prismatic blocks with features on one face. A 3-axis machine or a mill-turn center handles those faster and cheaper. Putting them on a 5-axis center raises the hourly rate for no functional gain. We quote both routes when the geometry allows, and the lower one usually wins on simple parts.

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  Choose 5-axis forCompound angles, contoured surfaces, deep undercuts.
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  Choose mill-turn forShafts and fittings that need turning plus cross features.
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  Choose 3-axis forPrismatic parts with features on one or two faces.

Aluminum 6061-T6 is the default for prototypes and small runs. It machines fast, holds tolerance and takes anodizing well. 7075 gives higher strength for aerospace brackets but costs more and chips less predictably. 2024 sits between the two and is common in aircraft structure.

Stainless 303 and 304 cover most general work. 316L and 17-4PH appear in medical and marine parts where corrosion resistance or strength matters. Both work-harden, so feeds and speeds need attention and cycle times run longer than aluminum.

Titanium and Inconel are where quotes diverge most. TC4 (Ti-6Al-4V) cuts at a fraction of aluminum's speed and wears tooling quickly. Inconel is worse. If a design can use 17-4PH instead of titanium, the cost difference is often larger than any weight saving on a small part. We flag that trade-off during DFM review.

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  Cheapest path6061-T6 for housings, brackets, fixtures.
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  Mid range303 or 304 stainless for general corrosion resistance.
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  Expensive pathTC4 and Inconel, justified only by temperature or weight limits.

"Fast turnaround" means different things at different shops. Ask for four separate numbers: quote turnaround, production start, ship date after start, and the historical late-delivery rate. A shop that answers all four is tracking its own schedule. A shop that answers with one vague phrase is not.

Production start within 24 hours only holds when material is in stock and the drawing is frozen. If the part needs a custom extrusion or a heat-treat batch, the clock restarts. Say so up front rather than promising a date the process cannot meet.

Documentation is the other half. A raw material certificate, an in-process inspection record and a final dimensional report are standard for regulated industries. Ask which reports ship by default and which cost extra. For medical and automotive work, the answer changes the total price.

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  Four numbers, not oneQuote, start, ship, late rate.
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  Material gates the clockStock bar starts fast; custom stock does not.
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  Ask for the report listMaterial certs, in-process records, final inspection.

Over-tolerancing is the most common leak. A ±0.005 mm callout on a non-functional face forces extra passes, extra inspection and sometimes a second operation. Multiply that across a hundred features and the part costs double for no benefit.

The second leak is finish specification. Calling out Ra 0.2–0.8 μm on an internal pocket that no one touches adds polishing time. Reserve fine finishes for sealing faces, bearing surfaces and visible exteriors.

The third is late design changes after first article. Once fixtures are cut, a geometry change means new fixturing and a new first article. Freeze the design before release, or accept the rework cost knowingly.

The fourth is unverified suppliers. A low quote from a shop with no inspection records is not a saving. It is a rework order waiting to happen, and the second shipment rarely matches the first.

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  Tolerance creepEvery unnecessary tight callout adds cost.
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  Finish creepFine finishes on hidden faces buy nothing.
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  Late changesPost-fixture edits cost more than pre-release reviews.