2.5D (machining). In machining, 2.5D refers to a surface which is a projection of a plane into 3rd dimension – although the object is 3-dimensional, there are no overhanging elements possible. Objects of this type are often represented as a contour map that gives the height (i.e., thickness or depth) of the object at each point. A 2.5D image is a simplified three-dimensional ((x, y, z) Cartesian coordinates system) surface representation that contains at most one depth (z) value for every point in the (x, y) plane. All features of the part will be visible from one view meaning that it can written with simple codes and accessible technology. It also means the machining portion of the task can be completed without the need of manually removing it and re-centering the part. This leads to increased efficiency and affordability for manufacturers. 2.5 axis machining is also used in education to build an understanding of the concepts and gain experience.

Advantages 
2.5D objects are often greatly preferred for machining, as it is easy to generate G-code for them in an efficient, often close to optimal fashion, while optimal cutting tool paths for true 3-dimensional objects can be NP-complete (nondeterministic polynomial time complete), although many algorithms exist. Many milling operations can be completed using 2.5 axes. Operations that can be completed on 2.5 axes are simplistic designs containing flat bottom pockets and other terrace-like features. Drilling and tapping operations are also possible on a 2.5-axis mill. 2.5D objects can be machined on a 3-axis milling machine, and do not require any of the features of a higher-axis machine to produce. CNC machines use G-code and M-code in order to control the machine and the positioning of the spindle. Canned cycles use G-code to machine specific features such as flat-bottom pockets, drilling, or tapping cycles. These make use of 2.5 axis machines, and are used more in education then industry.

Applications 
A 2.5D machine, also called a two-and-a-half-axis mill, possesses the capability to translate in all three axes but can perform the cutting operation only in two of the three axes at a time due to hardware or software limitations, or a machine that has a solenoid instead of a true, linear Z axis. A typical example involves an XY table that positions each hole center, where the spindle (Z-axis) then completes a fixed cycle for drilling by plunging and retracting axially. The code for 2.5D machining is significantly less complex than 3D contour machining, and the software and hardware requirements are (traditionally) less expensive. Drilling and tapping centers are inexpensive, limited-duty machining centers that began as a 2.5-axis market category, although many late-model ones are 3-axis because the software and hardware costs have dropped with advancing technology. CNC(computer numerical control) routers are another example of machines that use 2.5 axes. Routers typically operate in 2 dimensions (x,y), and (z) travel is for positioning. Although routers are not capable of drilling and tapping, they can perform basic milling processes. CNC router technology is quickly becoming more advanced as companies move to produce parts for less, and some routers can operate on the (x,y,z) planes just as mills do. The key difference is the capabilities of the spindle; the spindles are often less precise and cannot produce the same torque at low RPMs compared to modern milling machines. This is why routers are rarely used in drilling and tapping operations.

References

Computer-aided engineering