Machining and cutting

CNC turning: what goes wrong and where to inspect

Rotational parts are machined on CNC lathes and turn-mill centres: diameters, faces, grooves and threads, often to tolerances of a few microns.

ISO1101 ISO2768 SPCAutomotive MSAAutomotive

01How the process works

The workpiece rotates in a chuck or collet while cutting tools move along programmed paths. Turn-mill centres add milling and drilling with live tools, finishing complex parts in one set-up. Bar feeders allow unattended production of small parts.

Typical acceptance features are diameters with ISO fits (h6, H7), runout and concentricity, surface roughness, thread gauges and chamfers. Tool wear and thermal drift move dimensions during a batch, which is why in-process measurement and SPC on key diameters are valuable.

Material. Bar or forging verified.
Set-up. Program, tools and offsets prepared.
First off. First part measured and approved.
Production. Machining with in-process checks.
Final inspection. Dimensions, runout, roughness, threads.

02What typically goes wrong

Defect	Typical cause	How it is found
Diameter out of tolerance	Tool wear, thermal drift	Micrometer, air gauge, SPC
Runout and concentricity	Clamping, second operation set-up	Dial gauge, roundness tester
Poor surface finish	Tool condition, feed, vibration	Roughness tester
Thread rejects	Insert wear, program	Go and no-go gauges
Burrs	Missing deburr operation	Visual
Wrong material	Mixed bar stock	PMI, certificate

03Where an inspector should look

Before production

Drawing review: fits, GD&T, datums
Material certificate
Gauge and measurement plan

During production

First off approval
SPC on key characteristics
Tool change records

After production

Final dimensional report
Surface finish
Cleanliness and corrosion protection

04Standards and references

ISO1101 ISO2768 SPCAutomotive MSAAutomotive

ISO 286 (limits and fits), ISO 21920 and ISO 4287 (surface texture), ISO 1502 (thread gauges).

Standards are named for orientation. The applicable edition and acceptance criteria are those in your contract and specification.

05How we support buyers of cnc turning parts

Dimensional and Geometrical InspectionDimensional, GD&T and drawing compliance verification, CMM witnessing, laser measurement coordination and first article layouts. First Article InspectionComplete verification of the first production part against every drawing and specification requirement before series release. Independent Quality ControlIncoming, in-process and final quality control performed independently at the supplier, with measured results and clear acceptance decisions.