Tool Length Offsets on the Mill - 1: What is an offset?
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In a part program written for multiple tools there must be a method of assuring that the difference in tool length of each tool used is relayed to the controller. Without this communication there would be no way to control tool-to-tool Z-depth on the part being machined. For example, each of the four tools shown in the animation above has a different length when measured from the tip of the tool to the base of the spindle. By obvious coincidence then, the distance from the tip of the tools to Z-zero on the part also varies, tool-to-tool (when the spindle is full up at the tool change position)

Assume that tool #3 was the last tool used. If a tool change was called for tool #2 and the controller was not told that tool #2 was 1.625 longer than tool #3 then when the change occurred and the program continued a bad thing would happen.

In the animation below see what would happen if the controller was aware of the diffence in length between tool #2 and tool #3?
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label: CNC mill,. Offsets, G43 H43,