Q02 — TECHNICAL ANSWER
CNC Turning or VMC Machining: Which Does Your Part Need?
Use CNC turning when the defining features of your part are concentric — diameters, bores, shoulders, grooves and threads arranged around one axis. Use VMC machining when they are prismatic — faces, bolt patterns and tapped holes. Parts carrying both are produced across the two processes, which is common for flanges and adaptors.
The test: what does the part have to hold?
On a turned part the critical features are almost always dimensional. A shaft lives or dies by its journal diameter, keyway and shoulder positions, because those decide how it locates, how it transmits torque, and how long its bearings last. A bush lives by its working fit, bore-to-shaft and OD-to-housing. A pin lives by its diameter and chamfer form, because the chamfer is what lets a press-fit pin start square and seat without shaving its bore.
On a milled part the critical features are positional. A mounting plate whose hole pattern is out of position does not assemble, however good the finish on any single hole. A tapped hole at the wrong depth strips on assembly. So on VMC work the relationships between features matter more than any one dimension, and inspection is referenced to the drawing datum scheme rather than to a convenient face.
When a part needs both
The clue is usually the datum. If every important dimension on the drawing is referenced to one axis, the part wants turning. If they are referenced to a face and a pair of edges, it wants milling. A part whose drawing does both — a diameter held to an axis and a hole pattern held to a face — is telling you it needs two setups whatever the supplier.
That matters commercially as well as technically, because the stack-up between those two setups is where tolerance is lost. Splitting them across two suppliers means the loss happens twice and nobody owns it.
What we need from you either way
A released drawing with the material grade and quantity. Where a drawing leaves a material, finish or thread class open we raise it before machining rather than assuming a substitute. For prismatic parts, include hole positions, thread specifications and any datum scheme the assembly depends on.
Material is verified before machining begins, the first component of each batch is checked against the critical features, in-process checks run through the batch, and finished parts are verified to the drawing before packing under batch identification.
Related questions
- What is the difference between CNC turning and VMC machining?
- CNC turning rotates the workpiece against a fixed tool, producing concentric features such as diameters, bores and threads. VMC machining holds the work still and moves a rotating cutter across it, producing prismatic features such as milled faces, drilled and tapped holes, and bolt patterns.
- Can one part be both turned and milled?
- Yes, and it is common. Flanges and adaptors have a turned face and bore with milled bolt-circle holes. Producing both in one shop avoids a supplier handover and the dimensional stack-up that can come with it.
- Does the material change which process suits the part?
- Less than the geometry does. Material affects speeds, tooling and finish rather than whether a feature is turned or milled — the datum scheme on the drawing decides that. The full grade range is listed on the turned and VMC pages.
- Do you work from a sample if there is no drawing?
- Yes. Many replacement enquiries start from a worn component rather than a drawing. Send the sample with photographs, the material if known, and the position it runs in.
Related reading
Send your drawing with the material grade, quantity and application. If you are replacing an existing part and have no drawing, a sample with dimensions and photographs is a workable starting point. An NDA can be arranged before any detailed technical exchange.