How do the coordinates of a drawing relate to the "home" coordinates of the LoboCNC mill?

[David Pendleton]

I have questions about where I should consider the zero points for X and Y to be on the LoboCNC mill and in my part drawings.

It seems natural for the work material to be positioned in the centers of X and Y travel on the mill, but I'm having trouble relating this method of positioning to what I'm reading about "homing" and "limits".

While reading about "homing and limit switches", the implication is that X0 should be the at the left of the X table, and Y0 should be at the front of the mill (towards the operator).  However, this calibration requires that my gcode contain only only positive coordinates.  With only positive X and Y coordinates in my gcode, if I want the work piece to be centered on the mill's X and Y travel, I then need to offset the part in the drawing by an amount relative to the size of the part in order to position the center of the part at the center of the mill's X and Y axis.  These X and Y offset amounts would be different with each part and also somewhat associates the drawing to a particular mill.

It seems a lot simpler to draw parts centered on X0 and Y0, and also zero the mill's X and Y at the centers of the X and Y travels.  The only difficulty with using all four polar quadrants in drawings and in gcode is with how to "home" and zero the mill.

I have seen many examples where the part extents are contained entirely within the +X and +Y quadrant, so I'm wondering if I'm way off-base working in the all four of the polar quadrants.

Do I have misconceptions about positioning work material at the center of the X and Y table travels?

Are the coordinate values for a part supposed to be revised at the point of gcode generation for a specific CNC target?  (Post Processors?)

Is there an automated way to "home" or zero X and Y at the centers of the X and Y travel on the LoboCNC mill?

I would like to hear about how others relate the coordinates in their drawings with the coordinates of the mill.

[Jeff]

Firstly, the homing function of the PSCNC software really only relates to the absolute machine coordinates and is used to make sure that the axes to not run into their end-stops.  In the PSCNC software, if you select the "Machine" option for the coordinates, it will show where the axes are relative to the homing position and is really only useful for seeing how far you are from the end stops.  I generally go for years without ever looking at the "machine" coordinates.

Normally, you'll be using the "Program" coordinates which you can zero out using the Set Origin buttons.  When you create your G-code, you can create an origin anywhere you want and all of the motions will be relative to that origin.  When you actually go to machine your part, typically, you will first move the table so the spindle is centered over your origin and then use the Set Origin buttons to make that position zero.  It doesn't matter where you put the origin - ie, you can have all the motions in the negative direction, all in the positive direction or you can put the origin right at the center of your part.

Lastly, if you click on a coordinate value (when displaying Program coordinates), a window will pop up and you can enter in a completely arbitrary value for the current position of that axis.  

[David Pendleton]

Jeff,

I using a TinyG controller with ChiliPeppr software with LoboCNC mill.  Even so, your explanation has answered my questions. 

I can see now, that I do not need to be concerned with "homing" the machine as part of setup for each part.  All I need to do is to position the cutter over the material where if the drawing was superimposed, the drawing's X0 and Y0 would be. Then click (ChiliPeppr's) "Zero Out" button to zero all the axis. And verify that the table will is and will remain within it's travel limits when traversing the tool path.

Concerning the "Z" axis,  I jog the Z axis down to just touching the surface of the material before the Zero Out operation.  This works for plate stock and 2D parts.  I have questions about Z axis setup for 3D parts, but I'll save them for another post.  

Thanks for clearing things up.