I've used mach3 on a router and it was fine. Now I have converted a small table top milling machine.
On my router the spindle moves along the x y axis and the tool is at the intersect on the table display. But on the milling machine the spindle is stationary so the display just shows the x and y axis moving and not the position of the cutting tool. I set the home at bottom left.
My brain has switched off so Please help me out.
Merv

So you home position is -x -Y. All your movements in machine space will be in +x +y. A router and a milling machine are the same basic thing. As Mach runs it shows where the cut is in the current workspace (G54, G55,...)

Did you verify Machine Configuration? Make sure they match the mechanical setup of your milling machine.