Understanding CNC Machining: Milling, Turning, Tolerances, Costs
Knowledge · Topic overview
What belongs to CNC machining?
Four processes form the core. In milling the tool rotates and creates prismatic geometries: plates, housings, mounts, frame faces. In turning the workpiece rotates, producing shafts, bushings, flanges and pins. Drilling and thread cutting usually run in the same machining centre. Surface grinding takes over where flatness and finish exceed what milling achieves economically. Many parts pass through several processes, such as a welded bracket that is milled, drilled and then painted.
Which accuracies are standard and what do they cost?
General tolerances to ISO 2768-m are the economical standard and sufficient for most faces. Fits like H7 and positional accuracies in the hundredths range are everyday work but cost extra machining and measuring steps. Every tighter tolerance should serve a function, because it raises the price directly. The key design rules are in the article on having single parts milled: tolerances only where needed, generous internal radii, standard materials.
Why do set-up and programming dominate cost?
Because they occur per order, not per piece. Drawing review, CAM programming, tooling, clamping and first-article inspection are nearly the same for one part as for fifty. A one-off carries the block alone; in a series it spreads. That is why second and third parts are disproportionately cheap and why foreseeable spare demand belongs in the first order.
When does machining become a specialist discipline?
In two directions. Upwards with large parts: from several metres and tonnes, clamping, temperature and metrology dominate the result. And with the material: aluminium demands high speeds and razor-sharp edges, stainless steel punishes timid machining with work hardening. Both specialist topics have their own articles.
All articles in this topic area
| Article | Answers |
|---|---|
| Having single parts milled | process, data package, cost logic from quantity one |
| Machining large parts | the seven challenges of XXL machining |
| Machining aluminium | cutting data, built-up edge, thin-walled parts |
| Machining stainless steel | work hardening, heat, tooling strategy |
Frequently asked questions
What is the difference between milling and turning?
In milling the tool rotates and the workpiece stands still (prismatic parts such as plates and housings); in turning the workpiece rotates (rotationally symmetric parts such as shafts and bushings). Many parts combine both.
Which tolerances are standard in CNC milling?
General tolerances to ISO 2768-m are the economical standard. Fits and tighter form and position tolerances are feasible but require extra operations and should only be demanded with a functional reason.
What does a CNC machining hour cost?
It varies with machine size and region; more decisive than the rate is the total calculation of programming, set-up, runtime and inspection. A low rate is worthless if clamping concept or experience are missing.
Can welded parts be machined?
Yes, that is the normal case in machine building: weld first, stress-relieve if needed, then mill functional faces with 3 to 5 mm allowance. That is how welded structures become accurate.