A computer controller is used to drive an NC machine directly. (more on Numerical Control ),
The need of CNC program is be able to direct the position of the cutting tool. As the tool moves we will cut metal (or perform other processes). I we plan to indicate positions we will need to coordinate systems. The coordinates are almost exclusively cartesian and the origin is on the workpiece.
Cartesian Coordinate System
Primitive people used their 10 fingers and 10 toes to count numbers and from this
evolved our present decimal, or Arabic system where "base ten," or the power of 10, is used to
signify a numerical value. Computers and MCUs, in contrast, use the binary or base 2 system to
recognize numerical values. Knowledge of the binary system is not essential for the programmer or operator since both the computer and the MCU can recognize standard decimal system and
convert it to binary data.
Almost everything that can be produced on a conventional machine tool can be produced
on a computer numerical control machine tool, with its many advantages. The machine tool
movements used in producing a product are of two basic types: pointtopoint (straightline
movements) and continuous path (contouring movements).
The mathematician and philosopher Rene Descartes invented the Cartesian or rectangular
coordinate system. With this system, any point can be located in mathematical terms from any
other point along three perpendicular axes. CNC systems use rectangular coordinates because
the programmer can locate every point on a job precisely and independently from each other.
Eg :
1.
For a lathe, the infeed/radial axis is the xaxis, the carriage/length axis is the zaxis. There is no need for a yaxis because the tool moves in a plane through the rotational center of the work. Coordinates on the work piece shown below are relative to the work.
2.
For a tool with a vertical spindle the xaxis is the cross feed, the yaxis is the infeed, and the zaxis is parallel to the tool axis (perpendicular to the table). Coordinates on the work piece shown below relative to the work.
3.
For a tool with a horizontal spindle the xaxis is across the table, the yaxis is down, and the zaxis is out. Coordinates on the work piece shown below relative to the work.
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The advantages of CNC machines are
1. They are located very close to machine tool
2. CNC allows storage/retrieval/entry of NC programs without preprocessing of NC code
3. CNC program is only entered into memory once, so it is more reliable
4. The programs can be tested and altered at the machine
5. Increased flexibility and control options on the local computer
6. Easy to integrate into FMS systems
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