Home, zero, starting point...

Not sure if this is the right forum, or even if I'm using the correct terminology, but all has been going well lately with my home-taught CNC experiences. I have a Taig CNC mill, Rhino, Mach 3, and I'm about to buy SheetCam. I'll likely need a 3D CAM, but for now I don't know which to buy...
Anyway, that's just the background, the real question starts now:

Okay, between creating parts in Rhino, turning them to gcode with SheetCam and then loading that gcode into Mach 3, I am having a mental block with finding the home position on my mill or maybe I should say my metal stock?
There are so many places to describe the size of the material and where it sits on the mill that I can never get the mill to start cutting where I expect it to cut. I keep cutting objects and keep trying different positions for the start of the mill, but as soon as I start, the mill repositions and cuts somewhere else - usually a small fraction of an inch (my default unit) from where I'd positioned the cutting tip. I keep going around in circles with Mach 3 and SheetCam, but haven't had a "eureka moment" yet. To make things even more embarrassing, I've been messing around with this for so long that now I've confused myself to the point that I'm not even sure where to aim the end mill to start - just touching the outside corner of a square block? Positioned over the corner on the inside? Sometimes it even seems that the software thinks that the end mill should be positioned so that it's centered over the corner.
Anyway, I know that this is a small issue for those of you who have been doing CNC for a long time, but it's making me pull my hair. There are a bazillion places to configure this, but if you were me, where would you focus your attention? Is it mostly a Mach3 thing or is it mostly a CAM thing?

Okay, time for another coffee :-)

Thanks,
Mark

Hey Mark,

You probably realize this but just in case... there can be a difference between the box stock represented in your cam package and the outer edge of the piece you will eventually cut. You usually specify this in the cam program. I was thinking this might be why there is always a dicrepancy between where your cutter is and where it wants to start cutting. This is assuming that you are zeroing your machine before you press go.

Does this help?

Carlo

Here is one method;

Step 1. Always assume the back left edge of the raw stock is 0,0. This is so that the fixed jaw of the vice can be edge-found and will be always 0 in the Y. This means that you will be cutting in all -Y coordinates and +X. In the cartesian coordinate system this is zone 4.

Step 2. Make sure your CAD system has a "point" in the model or drawing that represents 0,0 and it is located at the back left edge.

Step 3. Export this "drawing" to your CAM system.

Step 4. The CAM system should bring in the drawing with the 0,0 at the back left.

Step 5. Export your G-code. Your controller (mach3) will need to be referenced and "zeroed" such that the 0,0 is at the back left of the raw stock you have loaded in the machine.

Now all 3 systems are coordinated. You should be able to cut material.

I must admit I know little to nothing about your software packages, so I won't attempt to offer anything direct in that regard.

What it does sound like to me is where you orient the X,Y position as your home point may get slightly altered by the cutter offset?

Some cam packages use the center of the spindle, while the offsets are controlled by the mill itself. While another cam package will offset the path as it builds it so the mill cuts to that centerline. Either way the actual path is based off of the cutter radius.

I make it a habit to use my G54 as my user home point to some feature on the part. Like a hole or corner that everything being milled MUST be in relation to. I prefer a hole rather than a part edge, but doing an MDI of G0, X0,Y0 should put the spindle over that position without offsets. Where ever the path ends does not influence those settings unless a line in your program resets them or it uses incremental values instead of absolutes.

I do know I have to be very careful on my first version EZ-Trak when restarting a program. I make it a habit to return to my user set 0,0 or it may restart the program at the point it ended at, after doing other tasks outside of the program.

Just a few hip shot guesses! :)

DC

Thanks for the input - I'll work on this some more over the next day or so and post my results ;)