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Since this field has almost no effect on the behavior of Fusion, you may find that, in your toolsettool library, it has simply been set equal to ‘Flute length'. Since CAM Assist will not use tools that go deeper than LU, this will limit the use of the endmill to a depth equal to the flute length and give you poor results in areas such as deep pockets.
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After learning about naming your End mill cutting data and checking the tool definition, the next step is use that information to decide which End mills should be in the toolsettool library?