Machining and cutting
CNC milling and machining centres: what goes wrong and where to inspect
Housings, manifolds, plates and structural parts are milled, drilled and bored on vertical, horizontal and five axis machining centres.
01How the process works
A rotating cutter removes material from a fixed workpiece as the machine moves in three to five axes. Fixturing and datum strategy determine how features relate to each other; multiple set-ups introduce stacking errors.
Position tolerances of bores and holes, flatness and perpendicularity of faces and thread quality are the typical critical features. CMM reports must use the drawing datums; we often find reports that measure features against the wrong reference.
- Material. Block, plate, casting or forging verified.
- Fixturing. Datums established in fixtures.
- Roughing. Bulk material removed.
- Finishing. Critical features finished in final set-up.
- Inspection. CMM, gauges, visual and cleanliness.
02What typically goes wrong
| Defect | Typical cause | How it is found |
|---|---|---|
| Hole position error | Set-up, datum choice | CMM |
| Flatness and parallelism | Clamping distortion, residual stress | CMM, surface plate |
| Bore size and form | Tool wear, boring bar deflection | Bore gauge, CMM |
| Thread defects | Tap wear, chips | Thread gauges |
| Chatter marks | Rigidity, cutting data | Visual, roughness |
| Burrs in cross holes | No deburr step | Endoscope, visual |
03Where an inspector should look
Before production
- Datum and fixture concept
- Critical characteristics identified
- CMM program reviewed
During production
- First article layout
- In-process gauging
- Tool life control
After production
- CMM report to drawing datums
- Deburring and cleanliness
- Protection and packing
04Standards and references
ISO 5459 (datums), ISO 286 (fits), ISO 16016 (technical drawing rights), customer cleanliness specifications such as ISO 16232.
Standards are named for orientation. The applicable edition and acceptance criteria are those in your contract and specification.