z axis problems...homing issues

I have a cnc plasma using ramps1.4.I am running a heavily modified marlin firmware.I am running a9888 stepper drivers for the x and y axis and a tb6560 for the z axis which runs thru a proma sd torch height controller..There are two issues i am having..First if i run the z axis down till it hits the limit switch it will home the axis and will stop the axis and try to zero it out instead of just stopping..This causes the z axis to only go up from the postition where it hit the limit switch..If i set the firmware so it will travel to positions below the homed position it will go below it but if i crash the axis it will make the display go crazy..The bad part is i had it working great and somehow my firmware got corrupted of a setting changed..To me it seems the axis is homing anytime it hits the limit switch rather than only homing for a g28.1 z0..It was working fine before..I could zero it at the lowest position then go up and down thru several cycles then lay something in the way so it will trip the limit switch ..It would only stop the axis and not zero the position.. then it would still go to the 0 position i set when i zeroed all the axis..Now it is like it is doing a g92.1 zo anytime it hits the z axis limit switch even when manually jogging it
When Plasma cutting the z axis needs to g28.1 z0 .From there it gets a g92.1 z0 which zeros the axis then it will go to the pierce height and turn the torch on after the dwell it goes to the cut height at this time the torch height controllers takes over control of the z axis positioning.. ..So it needs to be able to home the z axis without zeroing it ..

I think the issue is from the controller auto zeroing when it hit the limit switch..It should just stop then use a g92 to set zero.When it auto zeros it cause issues with repetier as well cuz repetier won't let the axis go below 0..I am not seeing in the firmware where it is telling it to auto zero when it hits the limit switch..i looks at and tried changing the homing settings in the config h and config h adv but nothing made a difference?

Hi, I am in the process of building a cnc plasma cutting system and i was wondering if you guys could answer some of the questions i have. First off i built a 3d printer so i am pretty familiar with most things. I built a version of the prusa i3. It uses an arduino mega and am running marlin firmware. i was wondering if you can use most of the same hardware and software that is used for 3d printing. i was planning on building a x and y axis system because i figured z axis wouldn't be needed for plasma cutting because you can just set the height manually. What types of modifications need to be made to marlin software to allow plasma cutting using an arduino and stepper motors. Also what do you use to generate the g.code. for my 3d printer i currently use slic3r which converts a 3d object to g.code so it can be printed. what would i use to convert a design that needs to be cut into metal into code that the arduino can read.

You will will need a z axis and even an auto torch height controller for doing plasma.. Torch height is very critical with cnc plasma cutting..Even having a z axis that can be controlled while the program is running would be a big help .That is a hard thing to do with a ramps1.4 for your controller..I spent about 6 months 20hrs a day to rewrite the marlin firmware so i could use it effectively as a cnc plasma,cnc mill, 3d printer ,vinyl cutter or plate marker without having to do a reflash everytime i want to change process...You can use repetier as the host software but most of the other 3d printing host softwares will not work as cnc plasma or milling since they look for layers if they don't see layers they think there is nothing to print

what needs to be changed with the marlin firmware to allow cnc plasma cutting? if you create a one layer object that you want cut out of steel using CAD and generate the g.code using a 3D printing host software such as repetier and send the g.code to the arduino wouldn't it cut out the object just like it was printing it? one of the problems that i can think of is that during 3D printing the axis movements are not always the same speed, which is a problem for cutting since you need constant velocity. Is there a specific fix for that in the marlin firmware?