CNC processing San Diego: how parts get made and where the limits sit

• 1
  What CNC processing San Diego shops actually remove
• 2
  Tolerances that hold, and the ones that do not
• 3
  How lead time is built, not promised
• 4
  Material choice decides the process window
• 5
  When a San Diego buyer sources offshore
• 6
  Matching the process to the part

CNC processing is subtractive. A cutter follows a toolpath generated from your CAD model and takes material away until the remaining geometry matches the nominal dimensions. On a 3-axis mill, the tool stays vertical and the table moves in X, Y and Z. That covers plates, brackets, pockets and most prismatic parts.

The limit shows up on faces that are not perpendicular to the spindle. A hole drilled at 30° to the surface, a port on a curved housing, an undercut behind a flange: a 3-axis machine has to stop, reposition, and start again, and every reposition adds a setup error. Two setups do not double the error, but they do stack it.

That is where 4-axis and 5-axis work earns its keep. A 4-axis mill adds a rotary table, usually Ø400 mm on our floor, so a part can be indexed to a new face without being unclamped. A 5-axis center tilts the spindle and the table together, so the cutter reaches the feature in one continuous pass. Fewer setups means fewer datum shifts and tighter true position on holes that reference each other.

Subtractive processing also puts heat into the part. Roughing at a heavy radial depth generates more heat than the coolant can pull out, and thin walls move as a result. The usual fix is to leave 0.3–0.5 mm of stock, let the part rest, then take a light finishing pass. It costs one extra operation and saves a scrapped batch.

A general machining tolerance of ±0.005 mm is achievable on critical features, but not on every dimension of the same print. Tolerance is a budget. Spending ±0.005 mm on a bolt clearance hole wastes inspection time and money; spending it on a bearing bore is correct. Mark the two or three features that carry the function and leave the rest at ±0.1 mm.

Feature size matters more than the number on the drawing. A 6 mm slot, 40 mm deep, deflects under cutting force no matter how rigid the machine is. A 200 mm bore in a 6061-T6 plate holds size easily because there is material around it to absorb the load. When a tolerance is tight and the wall is thin, the shop will often ask to machine it in two stages.

Surface finish is a separate budget line. As-machined surfaces land around Ra 1.6–3.2 μm. A finishing pass with a smaller stepover reaches Ra 0.8–1.6 μm. Below Ra 0.8 μm usually means a lapping or polishing step, not a different cutter path. For a sealing face, specify the finish and the flatness together, because one without the other leaks.

Thermal drift is the quiet variable. A machine that has been running since morning holds size differently than a cold one at 7 a.m. Shops that hold ±0.005 mm on production runs control the room temperature and let the spindle warm up before the first cut. Ask how the shop handles this, and the answer tells you more than a certificate.

Lead time is not one number. It is quoting, material, programming, machining, finishing and inspection stacked together. A shop that answers in 12 hours with a DFM note has already done the first two steps in parallel. A shop that answers in three days has not.

Material is the most common hidden delay. 6061 and 304 stainless sit on shelves in most contract shops. 17-4PH, Ti-6Al-4V and Inconel usually do not, and the mill lead time on those can exceed the machining time for a small batch. If your part is titanium and the schedule is three weeks, ask when the stock is ordered, not when the part ships.

Machining time scales with removal volume, not part count. One prototype with a deep pocket can take longer than ten simple brackets. This is why a shop cannot quote a price without the model. It is also why a 24-hour production start is realistic for a part that has already been quoted and programmed, and not realistic for a first-time geometry nobody has fixtured before.

Inspection sits at the end and cannot be skipped. First article on a new geometry, in-process checks on a production run, final inspection before packing. When a print calls for ±0.005 mm true position, a CMM report is the only way to prove it. Budget a day for that if the report is part of the deliverable.

Aluminium is the default for prototypes and enclosures. 6061-T6 machines cleanly, takes anodizing well, and holds thin walls better than 7075, which is stronger but more prone to stress cracking at sharp internal corners. Use 7075 where stiffness per gram matters, and radius the corners.

Stainless is where shops separate. 303 is free-machining and cheap to run. 304 galls and work-hardens if the cutter dwells, so the toolpath has to keep moving. 316L is the same story with better corrosion resistance, which is why medical and marine parts use it. 17-4PH machines well in the annealed state and then gets heat treated to its final strength.

Plastics behave differently again. POM holds tolerance and slides well; PEEK holds temperature and costs accordingly. ABS and PC are fine for fixtures and covers but move with humidity. If a plastic part has a tight tolerance across a 200 mm length, the material will fight you regardless of the machine.

Titanium and Inconel sit at the difficult end. Low thermal conductivity means heat goes into the cutter, so speeds drop and tool life shortens. These materials are machinable to the same tolerances, but cycle times are longer and the cost reflects it. Specify them only when the service condition demands it.

Local shops offer short freight and easy visits. Offshore shops offer capacity, a wider material shelf and lower hourly rates. The trade is distance against throughput, and the right answer depends on the phase of the program.

For a redesign that will change twice next week, a local shop that can turn a part in two days is usually worth the premium. You can walk the floor, hand over a fixture, and argue about a tolerance face to face. That access has value that does not show up on a quote sheet.

For a geometry that is frozen and a quantity that repeats, the calculation flips. A shop running 127 CNC machines across three plants can hold a schedule that a five-person job shop cannot, and the unit price reflects the volume. The risk you manage is communication, not capability.

That risk is manageable. A named engineer on the other side, a DFM note returned with the quote, and a first article report before the run continues cover most of it. Ask for the report on the first order. If the shop pushes back, that is your answer about how the next order will go.