So now that I’m on my third gate, and, including my arched bed my fourth or fifth or sixth plumb cut, I’ve found that the quickest and most accurate way to make the sides of the arch parallel to the sides of the bottom is to use the bottom rail as a guide. First I cut the bottom rail to width, in this case 28” (the stiles are 6” each for a total width of 40”, I hope). I then place the bottom rail on the cutoff sled and bump it up against the blade. Then I line up the middle of the arch with the middle of the bottom rail making sure the two lines are, well, plumb. I clamp the bottom rail to the sled then clamp the arch rail to the bottom rail. Now, assuming the blade is 90 degrees to the table and square to the sled everything should work out just fine.
After making the first cut I leave the bottom rail in place and clamped to the sled, then I flip the arch rail and line up the right sides so they are flush. I then reset the clamp of the arch to the bottom rail and cut the other side.
Viola! I works just fine and is very safe. Next step is to cut the curves in the arch.
-- tim hill www.newcalshop.com