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CNC Processes Behind a Custom Auto Part
You do not have to pick the process, the shop does. Knowing the main ones helps you read a quote and draw a part that is cheap to make.
The main CNC processes
Tolerances are typical ranges, not promises. The shop that quotes confirms what it holds on your geometry.
| Process | What it does | Typical auto parts | Typical tolerance |
|---|---|---|---|
| 3 axis milling | The cutter moves in X, Y and Z while the part stays clamped. Cuts faces, pockets, holes and profiles. | Brackets, covers, adapter plates, flanges | ±0.05 mm, ±0.02 mm with care |
| Multi-axis milling | The part or the spindle tilts, so the cutter reaches several faces and angled features in one setup. | Manifolds, uprights, angled ports, contoured parts | ±0.05 to ±0.02 mm, with better position between faces |
| CNC turning | The part spins and the tool cuts round features: diameters, faces, grooves and threads. | Hubs, pulleys, spacers, shafts, bushings | ±0.02 mm is routine |
| Mill-turn | A lathe with driven tools that also mills, so round parts finish in one setup. | Shafts with flats and cross holes, hubs with bolt patterns, fittings | ±0.02 mm |
| Drilling, tapping and thread milling | Makes holes and cuts threads, including AN and pipe ports. | Bolt patterns, port threads, sensor bungs | Position ±0.05 mm, thread class per spec |
| Boring and reaming | Finishes a hole to a size and position a drill cannot hold. | Bearing bores, pilot bores, hub bores | H7 fit, with roundness called out |
| Wire EDM | A thin wire cuts conductive metal by spark erosion, including hardened steel. | Hardened keyways, splines, sharp inside corners | ±0.01 mm, offered by a subset of shops |
| Grinding | An abrasive wheel finishes size and flatness after hardening. | Hardened shafts, sealing faces, wear surfaces | Tighter than milling, set per job |
Which process fits which shape
Rough rules a shop applies in the first minute of reading your drawing.
- Flat or blockyFeatures on one to three faces. 3 axis milling, the cheapest route for most brackets and covers.
- Round and concentricDiameters share one axis. Turning, which is faster and cheaper than milling a round part.
- Round with extrasFlats, cross holes or a bolt pattern on a turned part. Mill-turn, or turning then milling on a second machine.
- Angles and undercutsPorts at a compound angle, curved surfaces or features on four or more faces. Multi-axis milling, to avoid several setups.
- Hard with sharp cornersHeat treated steel with a square inside corner or keyway. Wire EDM after hardening.
- Fit tighter than a drillA bearing seat or press fit bore. Boring or reaming, sometimes grinding.
Typical numbers on a drawing
Everyday values cost the least. Tight values add machine time and inspection, so use them only where the part needs them. These are typical, not promises.
| Feature | Everyday | Tight, costs more |
|---|---|---|
| General size | ±0.1 mm | ±0.02 mm |
| Hole position | ±0.1 mm | ±0.05 mm |
| Bearing or press fit bore | H7, for example Ø16 H7 is +0.018 / 0 mm | H6, for example Ø16 H6 is +0.011 / 0 mm |
| Flatness of a sealing face | 0.1 mm | 0.02 mm |
| Surface finish | Ra 3.2 as machined | Ra 0.8 on sealing or running faces |
Work around the machine
The cutting is only part of the job. These steps are quoted with the part, and some happen at a second vendor.
Deburring and edge break
Sharp edges are removed by hand or in a process. Name any edge that has to stay sharp.
Heat treatment
Steel is usually machined soft, hardened, then finished on the critical faces. See the heat treat guide.
Anodizing and plating
A second vendor, a few extra days and a thin added layer. Say which sizes are final.
Balancing
Rotating parts are checked and corrected by removing material. Ask for it on pulleys, hubs and couplers.
Inspection
Calipers and bore gauges for simple parts, a coordinate measuring machine for position and form. Ask for a report if you need one.
Marking
Laser or engraved part numbers, if you want them. Say where, since the mark needs a flat face.
Process guides
- Process
Billet vs Cast Auto Parts: How to Pick the Right One
Machining from solid and casting solve different problems. How the two compare on strength, cost per part, lead time and design freedom, and where they cross.
- Process
Press Fits and Bearing Bores in Machined Automotive Parts
How to specify bearing bore tolerance on automotive parts: ISO fit classes, aluminum housings that loosen when hot, bore finish, chamfers and how to inspect.
- Process
AN Ports, Metric Fine and NPT: Thread Specs on Car Parts
AN, metric fine and NPT ports compared for machined car parts: dash sizes, thread callouts, sealing method and what to write on the drawing for fluid ports.
- Cost
What to Send a Machine Shop So It Can Quote Your Part
Shops price what they can see and pad what they cannot. The files, dimensions and notes that get a firm number, plus a copy and paste RFQ template to send.
Common questions
Something else? Email sales at autocncparts.com.
What is the difference between 3 axis and multi-axis milling?
A 3 axis machine cuts from one direction per setup, so a part with features on many faces needs several clampings. A multi-axis machine tilts the part or the spindle and reaches more faces in one go, which holds position between faces better and costs more per hour.
Do I need to say which process my part needs?
No. Give the geometry, material, quantity and tolerances. The shop picks the process and sets the price from it.
Can a hardened part be machined?
Yes, but slowly. Shops normally cut steel soft, heat treat it, then finish the critical faces by grinding, hard turning or wire EDM. Tell us the hardness you need so the shop plans the sequence.
Why does a sealing face need its own callout?
A standard size tolerance does not control flatness or surface finish. A leak comes from a face that is within size but not flat, so name flatness and Ra on the faces that seal.
Have a part in mind?
Send the part and we match shops with the machines it needs, or tell you if none fit. We reply within 24 business hours.