CNC Machining for Automotive Parts
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Most automotive parts start as castings, forgings, stampings or moldings. CNC machining finishes the features those processes can't hold, such as bores, sealing faces and threads, and makes complete parts when volumes are too low to justify production tooling.
Where machining is used
- Engine and transmission parts. Cylinder heads, blocks, housings and valve bodies are cast, then machined at bores, faces and threads.
- Suspension and steering parts. Control arms, knuckles, uprights and brackets are machined from billet for low-volume and performance cars.
- Prototype and pre-production parts, made before casting or forging tools exist.
- Production tooling: molds, dies, fixtures and checking fixtures.
- Replacement and obsolete parts for plant equipment.
Typical tolerances
General machined features are commonly held to about ±0.1 mm. Bearing bores, dowel holes and sealing faces are often specified to around ±0.01 to ±0.025 mm, which needs extra finishing passes, temperature control and in-process measurement. Tolerances tighter than the function requires add cost without improving the part.
Machining, casting and forging
At higher volumes, a near-net casting or forging machined only at critical features usually costs less than machining from solid. At low volumes, machining from billet avoids tooling cost and lead time. Prototypes are often machined from billet in the production alloy, or a close substitute, before the casting tool is ordered.
What to define for a quote
- A 3D model (STEP) and a drawing with tolerances, datums and surface finish.
- Material and temper, for example 6061-T6 or 7075-T6 aluminum, or 4140 steel.
- Finish or coating, such as anodizing, zinc plating or e-coat.
- Order and annual quantities, and any PPAP or inspection report requirements.
See CNC machining and automotive.