**4.4.2 Machine control**

A typical CNC control of a machine tool has two major responsibilities: i) the servo control loop and ii) the interpolator, responsible for coordinating the machine tool axis motions. The NC program, as well as others error compensation programs and routines, is located in a higher level of the control hierarchy. Supervisory tasks such as final part measurements and corrections can be accomplished by the CNC getting feedback from other controllers.

From the machine point of view, acceleration, deceleration and kinematics characteristics could be the feed limits in a machining process. According to ELBESTAWI (1997), the main issues in high speed servo control are feed rate planning and servo control loop laws. The users and/or the machine tool builders have to choose between path error and feed rate error. If the path error is allowed, the machine tries to run at the programmed feed rate; however, the trajectory is not guaranteed. If the path error has to be low, the machine keeps the path accuracy, but the feed rate can be lower than the programmed one.

In free form milling, the linear interpolation of straight line segments means a high volume of information to be processed by the CNC. Therefore, in order to manage the tasks to be carried out in real time during machining movements (i.e.: machine control loop, jerk limitation, interpolation, and so on), the CNC reduces the feed rate according to its capacity.

Advanced Free Form Manufacturing by Computer Aided Systems – Cax 565

work has implemented sophisticated numerical commands. In this case the data entered into a CNC program containing commands G01 and internally the CNC converts the straight paths into Spline equations, proportioning better performance than the traditional method of straight segments, however this conversion must be attributed to another value of tolerance. A deeper understanding can be found in SOUZA (2004) and SOUZA &

Tool Path

**N500 G6.2 P K X Y Z R** 

In the milling of complex forms, due to complexity of the paths of tool, the generation of the programs for CNC machines should be deeds with aid of software, the CAM (Computer Aided Manufacturing). Although the acronym CAM expresses a generic system for support a chain of manufacture, the term CAM is commonly employed for the generation of

The CAM calculates the paths of the tool for carry out milling, besides simulating the movement of the machine for certification, and generates the CNC programs for the

At present, the biggest application of CAM software is for the programming of machining by milling, in special 3 to 5 simultaneous axes. CAM software can also generates CNC commands for EDM machines, lathes, and laser or water pressure cutting machines, among

The CAM systems can be independent software that works in assembly with any application CAD, in general 3D, needing this, the importing of geometries in format-

A CAM system can be used to develop three main activities: i) calculate the tool paths for the machining the CNC machine; ii) carry out the simulation of the machining calculated;

COELHO (2007)ª,b.

Where:

G6.2 – syntax of NURBS

[P] – degree of the polynomial equation [K] - vector on the control point (knot) [X,Y,Z] – coordinate of the control points [R] – weight on the control points (weight)

programs for CNC machines.

manufacturing of the piece.

standard, such as, IGES, STL, VDA-FS, etc.

Fig. 9. Spline program line for the FANUC command

**5. CAM systems (Computer Aided Manufacturing)** 

other applications carried out by equipment controlled by a CNC.


When machining complex geometries, the machine often encounters curved surfaces, where short segments are required to closely follow the profile. When that occurs with the NC program set with high feed rates the machine slows down and sometimes oscillates imitating a vibration behaviour.

Feed rate oscillation happens even in the most updated CNC machines and depends on servo response, feedback devices, CNC characteristics and its implementation on the machine tool.
