I'm not an expert by any means. Someone else more familiar than me might like to comment.
I don't think G31 is a good way to home. G28.1 looks like its meant to be used for homing.
When using plasma, homing at the top of Z can be confusing, as I mentioned in above post. Sheetcam (with THC POST) compiles code that uses all positive Z moves. This means that Z should be homed at the material, best way by using a floating head, or even electrical contact method.
If you always use Sheetcam with THC POST, you may never need to home Z at top.
Please understand with conventional CNC , Z is homed at top, and alll Z moves are negative, but not when using Sheetcam with THC POST.
Sheetcam tries to make it easy for us, by putting in code ( G28.1) that homes Z at material. Then all Z moves will be positive. Then all we have to do is tell Sheetcam that pierce and cutting height are POSITIVE moves. It is easier for us to think in positive moves starting at the top of material. Its also easy because we don't even have to tell Sheetcam how thick the material is. It will always home there with G28.1 combined with floating head switch.
For now it is safer if you can to use a separate and single input for Z home on the floating switch. Tell Mach3 to home "Z negative".
You can combine all the other home and limit switches if you need more inputs available for Z Home. The floating head is not a Z limit, it is a Z home switch.
What you are doing with G31 will work, but there is a lot of room for error, and you have to measure exactly how far down the material is from your Z home at the top, and then type that figure into code.
I can understand how you are thinking, I was confused for a while too.
To answer you question about how to ignore Z homing at top of travel, is TO JUST FORGET IT FOR NOW AND HOME Z AT FLOATING HEAD ONLY AND SETUP MACH3 FOR HOME Z NEGATIVE. This only applies when using Sheetcam with a THC POST.
Ignore or disconnect switch at top of Z for now.
There are 3 things that all need to work together:-
1. Sheetcam with THC POST
2. G28.1 (included in Sheetcam THC output)
3. floating head Z Home, and set Z Home negative
When using these 3 items, try to forget "normal" Z methods.
I hope my understanding and explanation is right for you and I am not too confusing.
regards,
Shane Dwyer