The numerical control system can only generally perform linear motion interpolation and arc interpolation cutting motion. If the contour of the workpiece is a non-circular curve, the CNC system cannot directly implement interpolation, but it must pass certain mathematical processing. The method of mathematical processing is to use a straight line segment or an arc segment to approximate a non-circular curve, and the point of intersection between the line segment and the processed curve is called a node.
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For example, when the curve shown in Fig. 1.42 is approximated by a straight line, the intersection points A, B, C, D, E, F, etc. are nodes. 
Figure 1.42 Node of the part outline
In programming, we must first calculate the coordinates of the node, the calculation of the node is generally more complicated, manual calculation is difficult, and must be computer-assisted. After obtaining each node, the machining program can be programmed according to the straight line between two adjacent nodes.
This method of obtaining a node and then writing a program makes the number of nodes determine the number of program segments. As shown in Figure 1.42, there are 6 nodes. That is, the curve is approximated by five straight lines, so there are five linear interpolation blocks. The larger the number of nodes, the smaller the error δ resulting from the straight-line approximation curve and the longer the program length. It can be seen that the number of nodes determines the accuracy of processing and the length of the program. Therefore, the correct determination of the number of nodes is a key issue. Please also refer to the CAD/CAM section of this tutorial.
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