Definition
3+2 machining (positional 5-axis) uses a 5-axis machine's rotary axes to tilt the part to a fixed angle, then cuts with ordinary 3-axis linear motion while the part stays still. Simultaneous 5-axis machining moves all five axes together continuously during the cut itself, allowing the tool to maintain a specific orientation relative to a continuously changing surface.
Why the choice between them depends on surface complexity, not just machine capability
3+2 machining is generally simpler and more robust to program, and sufficient for parts needing access to multiple angled faces; simultaneous 5-axis is needed for continuously blended, complex surfaces (such as impeller blades) that a series of fixed positions cannot adequately produce.
Where the distinction shows up
Implied by the part's geometry: multiple angled but individually flat or simple faces suggest 3+2 is sufficient, while continuously blended, complex surfaces suggest simultaneous 5-axis is needed.
Common mistakes
Assuming any 5-axis machine automatically implies simultaneous 5-axis capability is being used; many jobs on a 5-axis machine actually run as 3+2, using only the positional capability.
3+2 machining is generally simpler and more robust to program, and sufficient for parts needing access to multiple angled faces; simultaneous 5-axis is needed for continuously blended, complex surfaces (such as impeller blades) that a series of fixed positions cannot adequately produce.
See how Blake reviews a partFrequently asked questions
Does 3+2 machining need a 5-axis machine?
Yes, it uses a 5-axis machine's rotary axes for positioning, but only 3-axis motion during the actual cut.
Is simultaneous 5-axis always better?
Not necessarily; it's more complex to program and not always needed. 3+2 is often sufficient and preferred when the geometry allows it.
Can the same part use both strategies?
Yes, some parts use 3+2 for most features and simultaneous 5-axis only for the specific surfaces that require it.