Hey ateeq,
Sorry I don't remember. I'd go measure it but I'm home now and the build is happening at work. Will try to get that for you tomorrow.
I did run across a possibly cool solution for holding the panes apart in the case of spiral bindings. Lowes has some nylon 1/4 X 20 bolts. I'm going to run a tap straight into the plywood at the front of the machine in a couple places just opposite the platen panes. Next, I'll screw two nylon bolts to contact the edge of each pane. Ought to hold them right where I want them, either together or just apart to make way for any weird bindings. Might do the same on the back side of the machine as well. Probably ought to tap the holes fairly high up where the panel is close to being supported by the crossmembers.
