Where is home on a milling machine?

[Jonathan]

The more I think about it, the more I realise that it's not possible to define travel limits in software because the home position is not a constant. How is this normally overcome?

It's overcome by using two co-ordinate systems. One is referenced to the machine, and used for things like travel limits and the other is offset from this, generally relative to a convenient point on the work piece.

[birchy]

It's overcome by using two co-ordinate systems. One is referenced to the machine, and used for things like travel limits and the other is offset from this, generally relative to a convenient point on the work piece.

OK....so how would I home the tool to a work piece?

Also, can someone please clarify the directions of drives in relation to axis movement? I had assumed that on the X-axis, the table would travel from right to left when in +dir, the Y-axis would travel from the front (nearest handle) to the back (nearest column) when in +dir and the Z-axis would be travelling vertically down in +dir. Is that correct, incorrect, or does it simply depend on whatever we want our machines to do? Are there conventions, or do we just build whatever seem right?