View of the Underside of a Top Bar
The groove along the centre of the bar will be filled with bee wax. The bees should build their comb using this as a guide. As a secondary precaution, the chamfer on the edges of the bar will also help to keep them on the straight and narrow.
Using my trusty router, a full set of bars, plus a couple of spares, took about an hour to complete. Most of that time was used in setting and adjusting the router.