Seems like that if you want to cut 20 inches - and want to figure out how long the rails ought to be you could:
Start with the 20 inches - then: Add, 1/2 the diameter of the router (assume the router is offset and that need to be included next), the offset of the router (bit) from the "back" of the y axis gantry, and then the offset from the back side to the zero point of your 20 inch reference. That ought to be close... unless I missed something (early - gotta get coffee.)
The advantage to putting the router between the Y axis rails, can be seen on my System2 in the member's gallery. The router being between the rails, just add the overall width of the Y Axis gantry to the desired cutting langth - and get that size rail!
:cheers: Jim
Experience is the BEST Teacher. Is that why it usually arrives in a shower of sparks, flash of light, loud bang, a cloud of smoke, AND -- a BILL to pay? You usually get it -- just after you need it.