Step 13: The Top, step one - determining the layout
If you don't lay it all out in advance, you could easily find that you have a bolt where you need to put a benchdog hole, or something of the sort. I sketched out ideas on graph paper, then drew the plan full-size on the top side of the bottom layer of MDF, using the actual parts as templates.
The width of the top is determined by the width of the base. The length of the top depends upon the vise or vises you uses. The end vise I had purchased was intended to be used with hardwood jaws that extend the width of the bench. I had a piece of 2x6" white oak I intended to cut down for the purpose.
The decision to be made with respect to the end vise is whether the support plate should be mounted to on the inside or on the outside of the stretcher. Mounting the plate on the inside of the stretcher reduces the reach of the vise - it can't open as far, because the support plate is back from the edge by a couple of inches. But mounting the plate on the outside of the stretcher means that we need to add some support structure for the inner jaw of the vise, which the legs would have provided if we'd mounted the plate on the inside.
I mocked up the two scenarios, and determined that with the plate inside the stretcher the vise would have a reach of 8 inches, and with it outside the stretcher it would have a reach of 9 inches. I decided that 8 inches was enough, and that the extra inch wasn't worth the extra effort. With the end vise mounted like this, the right edge of the top would have no overhang.
I wanted the left edge of the jaw of the front vise to be flush with the left edge of the top, the right edge with the left edge of the left front leg. So the amount of overhang on the left depends upon the width of the front vise jaw. The width of the jaw is, at a minimum, the width of the plate that supports it, but it's normal to make the jaw extend a bit beyond the plate. How far? The more it extends, the deeper a bite you can take with the edge of the vise, when, for example, you are clamping the side of a board being held vertically. But the more it extends, the less support it has. I decided to extend by 1-12", which gives me a 2-1/2" bite, and which should still provide solid support, given that the jaw is 1-1/2" thick. This means the top needs a left overhang of 12-1/4".
What you need to determine, by this drawing, is where you need to drill the dog holes, the mounting holes for the vises, and where you will put the drywall screws you'll be using for the lamination. As well as where the edges of the top will be cut.