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2.5D vs 3.D CAM/CAM

The difference is that of full 3D contourung. A 2 1/2 D program will move in 2 axis at once, ie. linear interpolation, can cut angles and circles in a single plane at a time . It can do plunge milling and drilling once the target piont has been reached. On a Full 3D program the program makes many multiple points in 3D space that the machine moves through, the effect is cutting in 3 planes ar once if needed. To create full 3D surfaces you need this feature. To make pockets, drill holes and cut out 2D profiles you are fine with 2 1/2D program. Note : the programming time and complication is considerable between a 2D part and one that uses full 3D surfaces. Ross
 
Doug
You are pretty close to the definition , but let me add this:
The word "simultaneous" is the cause !
The first CNC's moved only 2 axis's at a time. Some machines actually had a relay to switch a driver board from one axis to another ( needed fewer drivers= $$)
The demand for 3 simultaneous axis movements brought 2 1/2 D with Z feeding while X and Y moved. Most mills fall in this I believe.
The thing they must do as pointed out earlier is move linear in one of those axis's.
When 3 axis's move non-linear (curve) simultaneously at the same time, you have true 3 D
Some of my friends from work would refer to this as "Full" 3 axis, since many would call their 2 1/2 machine a 3 axis

SUMMARY
You may look at it this way if you chuck a piece in your vise:
2D will mill a flat plate
2 1/2 D will mill a half cylinder surface ( think Quansit hut)
3D will mill a half sphere (Igloo )
 








 
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