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Tool Length Offset allows the length of tools to be measured and entered into the CNC control's tool table, so that the tool does not need to be 'touched off' on the part or on a gauge block to set the Z axis. Of course, you need a way to repeatably position the tools in the machine spindle for this to be of any use. Collets won't do for this. But ordinary end mill holders work fine.

The command is G43, and EMC will take either a positive or negative offset with that command. A positive offset means this tool is LONGER than the reference length. A negative offset would mean the tool is shorter than the reference length. (Some controls require that G44 be used for negative offsets.)

You then have to specify which tool's length is to be used for the offset, with an H word. So, you might code it as G43 H4 to use the offset for tool # 4. G49 cancels the offset, returning the the Z axis position to to what it would be without the offset added to the Z position. Note that EMC only cancels an offset when the Z axis is moved.

The way I use this is to define the top surface of a part as coordinate Z=0. I have standardized on a particular center drill (held in an end mill holder) as my reference tool, so it has an offset of zero. I measure this tool, and record its length. I then insert other tools into an end mill holder (or a Jacob's chuck on an R-8 arbor) and measure their length with a height gauge. I subtract the reference length from this tool's length, and that is the number I put in the tool table.

I have put some pictures of my fixture for measuring tool lengths and some descriptive text on my web page, A fixture for presetting tools