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Machine World | 12-Jul-2015 | 21:10

Prismatic CNC Router Machining

Modern CNC machine tools use tool path data generated automatically from a CAM software package. This allows for highly complex parts to be machined without the extensive manual programming used in times gone past. Now, 3D models that have been designed in modelling software, as well as 2 dimensional cad drawings, can be used to generate the CNC data for use directly at the CNC machine. However, not all CNC machines have the same capabilities. What is Prismatic Machining, and what type of CNC machine do you require for it?

How Many Axes?

CNC Routers, Mills and Machining Centres are now very prevalent in manufacturing shops around the world. These come in many shapes and sizes, as well as varying levels of complexity. Often, with a little ingenuity, quite complex machining jobs can be performed with limited machine capabilities. We will briefly discuss some of the main differences between these types of machines, along with a couple of limitations.
5/6 axis machining is the optimal method for machining 3 dimensional objects. They may be a robotic arm, but more often are a vertical or horizontal mill with a bed that can rotate and tilt. This type of machine is able to machine from almost any angle to create a finished part. The down sides to this type of machine are the initial cost, as well as the extensive training required. Prismatic Machining is one of the many machining options for this type of machine.
4 axis machining is more common and usually involves a vertical spindle that can move in 3 axes, and a 4th axis to rotate the work piece so that processing can be performed on the reverse or adjacent faces. Machines such as CNC routers and Machining centres sometimes employ this method. Prismatic Machining is also suitable for this type of machine.
3 axis machining is similar but without the benefit of a rotating platform axis. This is the most common form of CNC machining centre and can perform surface machining on free-form shapes of any kind but only from a vertical perspective. Once again, Prismatic Machining is suitable for this type of machine.


2.5 axis machines are usually able to run a complex path in 2 axes simultaneously, but require a step move for the third axis. Often this is used to move to a specified Z depth into the material in one move, and then perform some form of horizontal machine routing in order to remove material. Prismatic Machining is suitable for this type of machine, although there may be some limitations on the type of processes that may be performed. 2 axis machining allows for lateral motion and usually requires a manual process to adjust the depth of cut. While Prismatic Machining is possible on a 2 axis machine, it can be quite time consuming to perform the necessary adjustments for each process.

What is Prismatic Machining?

Figure 1.This part was machined using several Prismatic Machining processes including Z level profiling, pocketing and drilling.

Prismatic Machining is a method that allows the use of a 2 or 2.5 axis machine to perform a variety of 3D machining. While the results can be very effective, the complexity of the software and the associated learning curve is often much less than 4, 5 or 6 axis machining. Although it is ideal for 2 or 2.5 axis machines, the same processes are still very applicable to more capable machines.
The main characteristic of Prismatic Machining is that the motion of the cutter is restrained to a plane that is orthogonal to the spindle axis. In other words, the machine can move up and down, or laterally, but not usually both at once. Tool path operations can be defined to perform four main types of machining operation. These are axial or drilling routines, pocketing, surfacing and contour profiling. These operations are usually performed in multi-level passes. Some machine controllers allow the work plane to be re-defined to any of the three orthogonal planes (XY, XZ, YZ). This will allow for complex paths to be performed in another plane also, but does not cater for simultaneous 3 axis movement.



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