Marlin for CoreXY

Good day all! I'm at the testing stage on my own-design CoreXY!!!
However this being my first CoreXY I have a few questions...
1- I want to be able to change my bed size simply by swapping beds (and some program rework) so I figure if my hot end can move in a 400*400 area and I set origin (0,0) as the centre of the bed in Marlin... then in my slicer I can just change bed size. Right? Though I'm a bit confused as to what I should then do with the "distance after homing" and "manual endstop position" settings!
2- I seem to have a scaling issue on my axis. Telling it manually to move 380mm moves it around 10(?). I'm using RAMPS so the microstep level is set by some pin jumpers. Have it set to 1/16 on XY and E and 1/1 for Z. All axis appear to be wrong. Be tried changing jumper setups but it doesn't seem to make a difference! Also telling it to move 10mm or 1mm at a time makes no difference.
3- Homing.... I have all set to lowest endstop. So it SHOULD home with carriage in front left corner and build plate as high as possible. XYZ all move in right directions in standard operation but when homing seem to move ~40mm to the positive of all axis, so carriage moves towards back right and bed lowers. It then stops.

Any ideas on what I should do to fix these? I've tried a few things to no avail!
Thanks in advance

Gee, such a helpful and inclusive community, I'm blown away with the responses! -.-
For real though, Thingiverse gave me no replies either. Is this just too challenging a topic?

I suspect that most people who post in this forum don't use Marlin and so can't help you. You could try reposting in the Firmware section.

Edited 1 time(s). Last edit at 08/29/2016 03:13AM by dc42.

---

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

1 - why swap beds? You'll need to calibrate again. Sounds like a pain to me. I can understand build surfaces, but build size?

2- steps per mm issue id guess. Or binding in mechanism so it can't travel as far as you tell it.

3- endstop wiring / configuration. Google is your friend for this.

1- currently have a standard 200*200, can easily go up to 300*300 but it's expensive. Different bed sizes heat up at different speeds too. I wouldn't change it that often if at all after swapping to 300*300. However, this is still an issue even if I don't swap beds as I have a worrying feeling that it relates to the homing error (3). Also, all the config necessary is changing bed size in the slicer.
If I swapped from a 200 to a 300 I'd have to tell Marlin that, change the offsets (if necessary, see again the issue) and several other steps. This way, all I need to do is change bed size in slicer and the printer will just work within those confines. Theoretically

2- steps per mm can't be off for all of them, especially as I used the prusa calculator to check. Would also not explain the fact different scales have the same effect. May be onto something with the binding but again, it can move the whole range, just not in the right step-scales. Gave it another run last night and seemed to act a bit more normally, so unsure what is happening here... I do have a sinusoidal resistance and I think I've narrowed it down to the motor pulley, checking that soon.

3- google is apparently not my friend as it doesn't give much help! Wiring is correct and as the standard for Marlin and for the guides I've taken advice from. Configuration I've scoured several times and also appears correct. Besides, both of these wouldn't explain why it would travel a short distance and then stop. On all axis. Unless this harks back to the offset question mentioned in point 1.