CNC machining in San Diego: how precision parts actually get made

• 1
  What "precision" actually means on a CNC machine
• 2
  How 3-axis, 4-axis, and 5-axis setups change the result
• 3
  Why the same drawing behaves differently in aluminum and titanium
• 4
  Which features are machinable and which are not
• 5
  How the part is measured decides what "in tolerance" means
• 6
  Choosing a machine and process window by part geometry

Precision is not one number. On a CNC machine it is the sum of machine rigidity, spindle thermal growth, tool wear, fixturing stiffness, and the measurement method used to check the part. Two shops can both claim ±0.005 mm and still deliver different results, because the claim only holds inside a defined envelope of size, material, and geometry.

When a San Diego hardware team sends a drawing out for machining, the tolerance callout is a contract about the final geometry. If the feature is 12 mm across and the tolerance is ±0.05 mm, almost any 3-axis mill can hold it. If the same feature is 300 mm long and the tolerance is ±0.01 mm, thermal drift during the cut becomes the dominant error source, not the machine's rated accuracy.

Material moves. Aluminum 6061 removes heat quickly but still distorts when a thin wall is cut from a solid block. Titanium Ti-6Al-4V (TC4) work-hardens at the cut edge and pulls heat into the tool. Inconel pushes tool temperature higher still. The cutting parameters that work on 6061 will destroy a cutter in Inconel within minutes.

The engineering consequence is simple: precision comes from a process window, not a specification sheet. The window is defined by what the machine can hold across the full part, not just on one clean feature.

A 3-axis machine moves the tool in X, Y, and Z while the part stays fixed. Every new face needs a new setup, and every setup adds a re-clamping error. For a bracket with four sides of features, that is four chances to drift by 0.01–0.03 mm. The part is still accurate per feature, but the relationship between features is where the error accumulates.

A 4-axis machine adds a rotary table, usually Ø400 mm. The part rotates around one axis, so features on the side of a cylindrical or prismatic part can be cut in a single setup. This removes one class of re-clamping error and often halves the cycle time on parts with radial features.

A 5-axis machine moves the tool on two additional rotary axes at the same time as the linear axes. Simultaneous 5-axis motion lets the cutter approach a contoured surface at the ideal angle, which keeps the effective cutting speed stable across a curved face. That is why impellers, turbine blades, and organic-shaped housings machine cleanly on 5-axis and poorly on 3-axis.

The trade-off is setup complexity. Five-axis work needs a verified post-processor, a collision-checked toolpath, and a fixture that does not block the rotary motion. For a simple flat plate with holes, 3-axis is faster and cheaper. The machine choice should follow the geometry, not the other way around.

GreatLight runs 16 simultaneous 5-axis machining centers, 12 four-axis mills, 27 three-axis machines, and 16 mill-turn centers. Maximum processing size is 4,000 mm, with travels of 4,000 × 400 × 150 mm for long parts and 750 × 1,150 × 550 mm for larger prismatic work.

Aluminum 6061-T6 is the default for prototypes and functional parts. It cuts fast, holds tolerance well, and anodizes cleanly. A wall 0.8 mm thick in 6061 is routinely machinable. The same wall in 316 stainless will deflect under cutting force and chatter, and the finish will show it.

Stainless 303 and 304 machine differently from each other. 303 contains sulfur, which breaks chips and improves machinability, but it is not ideal for welding or corrosion-critical service. 304 is tougher at the cut and needs lower feed per tooth and more coolant. 17-4PH (SUS630) can be machined in the solution-treated state and then aged to reach high strength, which is why it appears in aerospace and medical hardware.

Titanium TC4 (Ti-6Al-4V) has low thermal conductivity, so heat stays at the cutting edge. Tool life drops sharply if the surface speed is too high. A typical roughing pass runs at a fraction of the speed used on 6061, with high-pressure coolant to clear chips. Inconel is worse still: it work-hardens, so a cutter that rubs instead of cutting will destroy the next pass.

Plastics invert the problem. POM and PEEK machine cleanly but melt if the tool dwells. ABS and PC are soft enough that fixturing pressure can leave marks. Carbon fiber is abrasive and wears carbide quickly. None of these are reasons to avoid the material. They are reasons to pick the cutting strategy before the material is ordered.

A CNC cutter is a rotating cylinder with a limited length-to-diameter ratio. A pocket 20 mm deep and 4 mm wide needs a tool with a 5:1 ratio, which is reachable but flexible. A pocket 40 mm deep and 4 mm wide is 10:1, where tool deflection makes the wall taper and the floor chatter. The fix is often a redesigned pocket, not a different machine.

Internal sharp corners cannot be cut by a round tool. The smallest radius in a pocket is the tool radius. If the drawing calls for a true 90° internal corner, the design needs a relief or the corner will be rounded. This is the single most common DFM issue on machined parts, and it is visible before any metal is cut.

Thin floors and thin walls deflect under clamping and cutting force. A part with a 0.5 mm floor on a 50 mm span will vibrate no matter how the fixture is built. Adding a temporary support rib, changing the stock orientation, or accepting a coarser finish are the usual answers.

Deep holes are a separate case. A drilled hole beyond 5× diameter needs peck drilling or gun drilling, and the straightness tolerance widens with depth. A reamed hole holds diameter better than a drilled hole, but the position still depends on the drill that started it.

A caliper reads a diameter to about ±0.02 mm on a good day, and the reading depends on how the operator holds it. A micrometer is better for a single feature. A coordinate measuring machine (CMM) reports position and form across the part, which is what a drawing with geometric dimensioning and tolerancing actually asks for.

The measurement method has to match the tolerance. If the drawing says ±0.005 mm on a bore, a caliper cannot confirm it. The check needs a bore gauge, a micrometer with a setting ring, or a CMM with a calibrated probe. A report that does not state the instrument is not a verification.

Surface finish is measured separately. Ra 0.8–1.6 μm is a typical machined finish on aluminum and steel. Ra 0.2–0.8 μm needs a finer feed, a sharper tool, and often a finishing pass with a smaller stepover. Ra 1.6–3.2 μm is as-machined and acceptable for most non-sealing surfaces.

GreatLight inspects 100% of parts before shipment, covering raw material check, in-process monitoring, and final inspection. Inspection reports are available on request. The qualification rate across production is 99.99%.