CNC Machining — Computer Numerical Control

CNC (Computer Numerical Control) machining is a subtractive manufacturing process where a computer-controlled machine removes material from a workpiece using cutting tools to produce a finished part. It encompasses milling, turning (lathe), drilling, grinding, and EDM. CNC machining is the primary production method for precision mechanical components in high-tech engineering.

Why companies use it

·Produces highly accurate, repeatable parts directly from CAD data without tooling moulds
·Versatile — can machine a wide range of materials (aluminium, steel, titanium, PEEK) to tolerances of ±0.001 mm or better
·No setup cost for low-volume production — makes CNC ideal for prototype and NPI parts
·Modern 5-axis CNC can produce complex freeform surfaces in a single setup, reducing handling and re-fixturing errors

What hiring managers look for

·Mechanical engineers who understand CNC capabilities and limitations design better parts and communicate more effectively with machinists
·DFM for CNC (avoiding undercuts, specifying appropriate tolerances, considering tool access) is a core mechanical engineering skill
·Understanding G-code basics, fixturing concepts, and cutting parameter selection helps engineers troubleshoot machining issues
·Process engineers in semiconductor equipment, precision optics, and aerospace must work closely with CNC machine shops

Typical interview questions

What is the difference between 3-axis, 4-axis, and 5-axis CNC machining?

How does tool stick-out length affect machining accuracy and surface finish?

What design features make a part difficult or impossible to machine on a 3-axis CNC mill?

How do cutting speed, feed rate, and depth of cut interact to affect tool life and surface finish?

What is the purpose of fixturing in CNC machining and how does fixturing choice affect part accuracy?

Common mistakes

·Designing internal sharp corners — end mills have a finite radius and cannot produce a true sharp internal corner without EDM or relieving the corner
·Specifying tolerances tighter than the process capability without justification — a general machining tolerance of ±0.05 mm is appropriate; ±0.005 mm requires grinding or EDM
·Not considering tool access — long, deep features require long tool extensions that have poor rigidity and surface finish
·Mixing radically different materials in one fixture setup — different materials require different cutting parameters and cannot always be machined optimally in one setup
·Designing wall thicknesses that are too thin for the material — very thin walls deflect during machining, causing dimensional errors and chatter

Real engineering example

A semiconductor equipment engineer designed a complex aluminium vacuum chamber with 12 internal fluid channels at various depths. The original design had blind channels requiring plunge milling with L/D > 8:1. The machinist's DFM review identified that three channels would require a specialised long-reach tool with high tool deflection, leading to dimensional errors. Redesigning two channels to break through the outside (sealed with plugs) reduced the L/D to 4:1, eliminated the tolerance risk, and cut machining time by 35%.

Topics covered

CNCmachiningmillingturningprecisionmanufacturing5-axistolerances

Related interview guides

Mechanical Engineer →Manufacturing Engineer →Mechatronics Engineer →