Machining removes material from solid stock. Additive adds material only where it is needed. The two behave in opposite ways as geometry gets more complex, and that single difference determines which route is appropriate.
Geometry
Machining requires tool access. Every internal cavity, undercut or enclosed channel adds setups, fixturing or becomes impossible. Additive is broadly indifferent to internal complexity, but is sensitive to overhangs, thin unsupported spans and build height.
Tolerance and surface finish
Machining wins outright. It delivers tight tolerance and fine finish directly. Additive delivers a near-net form with a rougher, more variable surface, which is why critical features on printed parts are machined afterwards.
Material efficiency and volume
- Machining generates significant swarf, which matters with titanium and nickel alloys.
- Additive uses close to the material in the part, plus supports.
- Machining scales well into higher volumes as setup cost is amortised.
- Additive is strongest from one part up to low hundreds, where tooling and setup cannot be justified.
The practical conclusion
For a simple bracket in aluminium, machine it. For a consolidated manifold with internal passages in Inconel, print it and machine the ports. Most real components sit between those extremes, and the correct answer is a route that uses both processes deliberately.