TABLE OF CONTENTS So far, we have looked at two methods of compensation for the actual position of cutting tool in relation to the machine reference point. One method was the older type, using position compensation, the other was the contemporary work coordinate system method (work offset). In both cases, the emphasis was only on the X and Y axes, not on the Z-axis. Although the Z-axis could have been included with either method, the results would not have been very practical. Main reason is the nature of the CNC work.
Normally, programmer decides on the setup of a part in the fixture and selects the appropriate location of XYZ program zero (part reference point or part zero). When using work offsets, XY axes are always measured from the machine reference point to the program zero position. By a strict definition, the same rule applies to the Z-axis as well. The major difference is that both measured XY values will remain unchanged for all tools, whether there is one tool used or one hundred tools. That is not the case with Z-axis.
The reason? Each tool has a different length.
Length of each cutting tool has to be accounted for in every program for a CNC machining center. Since the earliest applications of numerical control, various techniques of programming tool length have emerged. They all belong into one of two basic groups:
Actual tool length is known
Actual tool length is unknown
