CNC machining San Jose: how parts actually get made

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  What CNC machining San Jose work really involves
• 2
  How tolerance and surface finish drive the process
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  When 3-axis, 4-axis, or 5-axis is the right call
• 4
  Material choice and how it changes the cut
• 5
  DFM checks that change before the first cut
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  Process route by part type

CNC machining is subtractive. A rotating cutter removes material from a solid block, and the machine follows a toolpath generated from your CAD model. Every dimension on the drawing becomes a physical relationship between tool position, spindle speed, and feed rate. Change one, and the others move.

San Jose sits inside a dense hardware corridor. Design teams there work on server racks, robotics joints, EV battery housings, and lab instruments. The parts are often low volume and geometrically awkward. A single bracket may need five setups if the geometry is handed to a 3-axis mill.

The real constraint is not the spindle. It is setup count. Each additional orientation adds a fixture, a datum shift, and a chance for stack-up error. Five-axis machines cut setups, but they do not remove the need to plan datums before the first chip flies.

So the practical question is not which shop is closest. It is which process route holds your tightest feature without multiplying cost and lead time. That decision starts with the tolerance callouts on the drawing.

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  Subtractive basicsMaterial leaves the block; the toolpath defines the final geometry.
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  Setup count drives costFewer orientations usually beat a faster spindle.
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  Datums firstPick the datum before choosing the machine.

A general tolerance of ±0.1 mm is routine on a 3-axis mill. Push to ±0.005 mm and the process changes. You need temperature control, sharp tooling, light finishing passes, and a machine that repeats itself. The tighter the number, the fewer shops can hold it across a run.

Surface finish follows a similar curve. As-machined surfaces land around Ra 1.6–3.2 μm. A high-quality finish sits at Ra 0.8–1.6 μm. Fine finishes reach Ra 0.2–0.8 μm and usually require a separate finishing pass or a secondary operation.

Here is the trap. Engineers sometimes call out a tight finish on a non-functional face. That face may be cosmetic. If it is, bead blasting or tumbling gets you there for less than a diamond-turned pass. Ask what the surface actually does before you specify it.

Tolerance and finish together set the floor for cost. A part with one tight bore and loose everything else is cheaper than a part with tight callouts on every face. Concentrate precision where the assembly needs it.

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  ±0.1 mmStandard 3-axis milling range.
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  ±0.005 mmRequires controlled conditions and tight process discipline.
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  Ra 1.6–3.2 μmTypical as-machined surface.
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  Ra 0.2–0.8 μmFine finish; adds a pass or a secondary step.

3-axis milling cuts on three linear axes. The tool stays vertical. It handles plates, pockets, slots, and prismatic parts well. If your part has features on one or two faces, 3-axis is the cheapest route. Do not over-specify the machine.

4-axis adds rotation around one axis. This suits cylindrical parts with cross-drilled holes or milled flats. A shaft with a keyway and radial ports is a classic 4-axis job. It also cuts setups on parts that would otherwise need repositioning.

5-axis moves the tool or the table on two rotary axes at once. The payoff is access to undercuts and compound angles in a single setup. Impellers, medical implants, and aerospace brackets fit this pattern. Simultaneous 5-axis also lets you keep the tool tangent to a curved surface, which improves finish and tool life.

The decision rule is simple. Count the faces that need machining. One or two faces point to 3-axis. A cylinder with radial features points to 4-axis. Freeform surfaces or five-sided access point to 5-axis. Match the machine to the geometry, not to the brochure.

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  3-axisPlates, pockets, prismatic parts. Lowest cost.
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  4-axisShafts, radial holes, milled flats on cylinders.
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  5-axisUndercuts, compound angles, freeform surfaces.

Aluminum 6061-T6 is the default for prototypes and brackets. It machines fast, holds tolerance, and takes anodizing well. 7075 gives higher strength for aerospace and racing parts, but it is less forgiving on thin walls. 2024 sits between them and is common in aircraft work.

Stainless 303 and 304 are the workhorses. 303 machines more freely, so it suits high-volume turned parts. 304 and 316L resist corrosion better and show up in medical and food-contact hardware. 17-4PH (SUS630) can be heat treated after machining for high strength.

Titanium TC4 (Ti-6Al-4V) is where cutting speed drops hard. It conducts heat poorly, so the tool edge takes the thermal load. Feed rates fall, tool life shortens, and cost climbs. Use titanium only where the strength-to-weight ratio is genuinely needed.

Plastics behave differently again. POM and PA machine cleanly. PEEK holds up at high temperature but is expensive. Carbon fiber reinforced plastics wear tools fast and can delaminate if the feed is wrong. Each material sets its own parameters, so send the material grade with the RFQ.

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  Aluminum 6061-T6Fast, stable, anodizes well. Good default.
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  Stainless 303 / 316LCorrosion resistance; 303 cuts more freely.
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  Titanium TC4High strength-to-weight; slow speeds, higher cost.
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  PEEK / carbon fiberHigh temperature or stiffness; tools wear faster.

Most costly problems are visible in the CAD model before machining starts. A deep pocket with a sharp internal corner needs a cutter with a small radius. That cutter is short and flexible. If the pocket is deeper than three times the cutter diameter, deflection rises and the corner may not clean up.

Thin walls are the second common issue. A wall under 0.8 mm in aluminum can chatter. In stainless or titanium, the limit is higher. Add a fillet at the base, or accept a slower finishing pass. Both options cost less than a scrapped batch.

Thread callouts matter too. A tapped hole needs clearance for the tap and room for chip evacuation. Blind holes should be drilled deeper than the thread depth. If the model shows a thread running to the bottom of a blind hole, the tap will likely break.

Send a STEP file plus a 2D drawing with critical dimensions. A shop that returns a DFM report within 12 hours is reading the model, not just counting holes. That feedback loop saves more time than a slightly lower hourly rate.

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  Pocket depthKeep under 3× cutter diameter where possible.
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  Thin wallsBelow 0.8 mm in aluminum invites chatter.
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  Blind tapped holesAllow drill depth beyond thread depth.