The role of tool radius compensation

In the contour processing process, because the tool always has a certain radius (such as the radius of the cutter or the molybdenum wire radius of the wire cutter), the tool center's trajectory does not equal the actual trajectory of the part to be machined, that is, the CNC machine tool. The radius of the tool must be taken into consideration when contouring. Now take a milling machine as an example (see Figure 2-41). To use a tool with a radius r to machine a workpiece with profile A, the tool center must move along a path B that deviates from the profile A by r, that is, the tool during milling. The center trajectory and the profile of the workpiece are inconsistent. We can calculate the coordinate parameters of the trajectory B based on the coordinate parameters of the profile A and the value of the tool radius r, and then compile a program for processing, but this is inconvenient. Because it is necessary to change the tool when the material, process change, or tool wear is necessary, the program must be re-created. If you do not consider the tool radius, it is more convenient to program directly according to the profile of the workpiece. However, at this time, the center of the tool is moved according to the profile of the workpiece, and the machined parts are smaller than the drawings. In order to make it easy to program, but also make the tool center along the trajectory movement, processing qualified parts, it is necessary to have tool radius compensation (or tool offset calculation, referred to as knife bias) function.

It should be pointed out that the tool radius compensation is usually not done by the programmer, and the programmer only creates the program according to the machining contour of the part. At the same time, the G41, G42, G40 commands are used to tell the CNC whether the tool moves along the inner contour or the outer contour of the part. The actual tool radius compensation is done automatically by the computer inside the CNC system. The CNC system automatically calculates the tool radius compensation based on the part contour dimensions (straight or arc and its start and end points) and tool movement direction commands (G41, G42, G40) and the tool radius used in the actual machining.

According to the ISO standard, when the tool center trajectory is on the right of the forward direction of the programmed trajectory (part contour), it is called right tool compensation, which is abbreviated as right tool compensation and denoted by G42; otherwise, it is called left tool compensation and denoted by G41; when it is not necessary G40 is used for tool radius compensation.

In the actual contour machining process, the tool radius compensation execution process can generally be divided into three steps:

(1) Tool compensation establishment.

(2) Tool compensation is performed.

(3) Undo tool compensation.