Now that I have the pitch diameters for the drive pulley and the OTS idlers, I am able to create a belt path that keeps true to the belt pitch line of all the known parts, while maintaining belt profiles parallel to the axis.

For example, here is the stepper motor drive pulley, 10.2mm on the pitch diameter, and the belt PLD is .254mm

Here is the stepper to the smooth face idler. This combination of pullys and idler determine the distance from the Y stepper stack to be 10.606mm. 

Keeping these at this distance will ensure the belt is parallel to the axis. So whatever the Y axis idler stack distance is, the rear corner pulley stack will be the same distance and the stepper motors will be that distance plus 2 x 10.606mm (21.212mm). 

Since there are 2 x 20 tooth pulleys on the idler corner to the Y axis, the distance between those pulleys must be same to keep the belt parallel. So the idler spacing A is the same for both sets of idlers. 

The last idler I need to determine is the rear large diameter idler. This one is not part of a critical parallel path, so it just needs to be large enough to position the belt just past parallel, so it doesn't hit the Y axis carriage.

I don't like the idea of using a smooth idler on the tooth side of the belts, so it's basically a GT2 sprocket with bearings to make the idler. Here a 56 tooth pulley will position the belt just past parallel by 0.25°. At this point, this is a custom made part or a modified OTS (off-the-shelf) part. TBD. 

Now that I know the belt path, next steps are to add a few more details to the halo and keep working inwards to the print head. I'm not ready to make final design decisions yet, I'll leave it rough for now until I get closer to a final design. Once I have everything rough, I'll bounce back outwards from the print head and refine everything.

The main goal is to get at least 300mm x 300mm of print travel, would like to see if I can get more,  an extra 5mm will get me 305mm x 305mm, then I can make 12" x 12" parts.