Doing it wrong all this time!

[Neale]

There are two things being discussed here, and it's important not to muddle them.

First - and least important - is this matter of whether X is along the bed (long axis) or gantry, with Y being the other one. Doesn't matter a damn. Do what you like. Better, do what works for you. On my mk1 machine, there were no side rails and I stood at the machine looking at the long axis. So, made sense for X to go from left to right along the long axis and Y along the gantry. My mk2 machine has high side rails so I tend to stand at one end to load and operate the machine. It would make just as much sense to have X on the gantry so it went from left to right and have Y along the long axis. I get the impression that the US tends to do it the second way and Europe the first, but as I say, as long as you know so you put the stock on the bed the right way round, it does not matter. People argue for hours about the "logic" of one way or the other; ignore them. The only logical way is the way that works for you. As Desertboy said, though, people talk about X and Y and assume that everyone else has these the same way round. Much better to talk about long axis, gantry, etc, to avoid confusion.

The second point is very important and must not be ignored. This is to do with making sure that the positive direction of the axes is correct, and this time there is only one correct answer so that your machine is consistent with every CAD, CAM and motion control system out there. This time there are two ways to do it - the right way and the wrong way. What is described below is the right way. This is not a matter of opinion. This is the convention that everyone uses, and you ignore it at your peril. Stand looking at your machine with the X axis running side to side in front of you (long axis, gantry, doesn't matter). X goes from left to right (typically, but not necessarily, X=0 at the left end, X=max travel at the right). Whatever the numbers, as the tool moves from left to right, X is increasing. Under these circumstances, Y must increase as the tool moves away from you. Again, typically, Y=0 is nearest you, Y=max travel is furthest away. For Z, lowest coordinate is down, highest coordinate is up. People often set Z=0 at the top so as Z moves down, the Z coordinate is always negative. This will then tie in with Fusion 360, Vectric VCarve, TurboCAD, Mach3, LinuxCNC, etc. It is called a right-handed coordinate system (although the name doesn't matter) and it's what the rest of the world uses. If you set up your coordinates differently so that the numbers run the other way, you have a left-handed coordinate system and you are screwed every which way. You will never figure out quite what is going to happen when you run gcode generated by any normal software.

What I have described here works for a router, where we can all see that the work is still and the tool moves. That's easy to follow. Milling machines (table moves in X and Y) and fixed gantry machines (tool moves on one axis, table on the other) can give problems. The trick is to see that the only thing that matters is where the tool is relative to the work. Doesn't matter which of them moved. Tape a sheet of paper to the bed of your router. Draw X and Y axes on it - X from left to right at the bottom of the sheet, Y from bottom to top along the left-hand edge. Bottom left is now X=0, Y=0. Now take your piece of paper and tape it to the bed of your milling machine (or fixed gantry machine). Move the table so that the tool is over the X=0, Y=0 point. The DRO or whatever you are using should say 0,0. Now move the table so that the tool is over the max Y position. The table moved towards you. Don't worry about that - what matters is the DRO increased its Y reading. Now move the table so that the tool is at the right-hand edge of the paper. Table moved to the left. Doesn't matter - what matters is that the DRO increased its X reading. Just try to think about where the tool is on the work, and whether the DRO readings go the right way when you move the tool over the work.

So, choose for yourself which axis is X and which is Y. Make sure that when you move the tool, X and Y DRO readings move in the correct direction. Then start using the bloody machine, and the whole business will become second nature in no time. I have a moving gantry router on one side of the workshop and a vertical mill on the other with its XY moving table. There is no confusion as long as you take time to set them up right from the beginning.