Changing the homing direction on Centroid Acorn

Why?
As mentioned last time, I want to mount my rather oversized 4th axis unit on the LHS of The Shiz's table, as this is conventionally where you'd visualise it. Problem there is that my current home position takes the table right to its end of travel, where the 4th axis would most likely crash into the machine head. Not sure what would happen in that situation - most likely it would simply stall but nothing would be improved in the process and I might end up making mud in my underwear.

How?
I thought it would be quite simple, namely:

• Ensure the home switch trigger ramp is in the correct position (ie before the limit switch!).
• Change the .HOM file to define the "other" direction as the homing move. That's a simple matter of changing the M91/X command to M92/X. Or so I thought....

Here's the original:

;Perform Homing commands
M26/A
M92/Z L1
M26/Z
M91/Y L1
M26/Y
M91/X L1
M26/X

And what I changed it to:

;Perform Homing commands
M26/A
M92/Z L1
M26/Z
M91/Y L1
M26/Y
M92/X L1
M26/X

But there's a catch!!
Nope, that doesn't work! It changes the home direction movement but for some reason it isn't actually watching the home switch input any more. It does the slow home move - but although it activates the home switch (you can see it change state in the diagnostics screen), the home move doesn't seem to register it there. WTF??

Turns out that you have to use the wizard to make the changes, otherwise the machine doesn't know where to expect the home switch. Silly me, there was me thinking the homing move was to let the machine learn where the home switch was. Struck me as somewhat counterintuitive but what would I know?

Luckily, Marty pointed me in the right direction over on the Acorn forum.

Anything else?
Yes, the machine coordinates (MCS) are now shifted, as the home position generally denotes MCS coordinates X0, Y0, Z0. So the various soft limits (now -700mm to 0mm in X) and the G28, G30 etc return positions for the park, vise, TTS etc need to be shifted ~700mm or so negative in the X direction.

Phew. That was surprisingly painful. And yes, I thought of using the wizard originally but each time I did so, it would try to set A axis homing. As I don't have a home switch on the A axis, it would sit there spinning slowly forever. It also offers you the option of manually editing the .HOM file without pointing out that there are certain issues if you do. 

Right, now to mount the A axis unit on the table and give it a go.