Original post on the TWW site: http://www.thewoodwhisperer.com/videos/flattening-workbenches-and-wide-boards-with-a-router/
The traditional method for flattening a workbench is to use hand planes and winding sticks. While some folks truly relish this labor of love, others prefer to delegate this grunt work to power tools. The power tool method is very similar to the action of a CNC machine. A router is placed inside a sled that rides along two parallel rails that are attached to the sides of the bench. The router sled is very easy to make from scrap 3/4” plywood. The rails can be made from 2×6 construction-grade lumber. Cheap and simple!
The only tricky part of this process, if there is one, is making sure the two guide rails are not only parallel to one another but also roughly parallel with the top. Fortunately, there is a very cool trick you can employ using string or thin cable.
Once the rails are in place, the router is dropped into the sled and the bit is plunged down. Ideally, you want to set the bit at the lowest part of the bench. One way to do this is to plunge the bit until it just touches the bench. Move the sled and router to various parts of the bench (with the power off) to see if the bit catches or slides freely. If it slides freely at any given point, you have just found a location that is lower than the original spot. Reset the router plunge depth at this new setting and continue to scan the bench. Once you have the bit set at the lowest point, bring the sled back to the end of the bench and start the routing process. Anything higher than that low point will be routed away and you’ll be left with a nice flat surface.
Perhaps one of the best parts about this jig is that you can re-use it for other things. Have an end grain cutting board that needs flattening, but you don’t want to send it through your planer? Have an oddly-shaped natural edge slab that is just too large for your tools? Both situations can be handled with a setup like this. And if you plan on using it a lot, you can get as elaborate as you want with the design of the jig. The version I show you here is stripped down to the absolute basics.
This video is an excerpt from the recent Split-Top Roubo Workbench Guild Build. Join the Guild today to see the entire set of videos and build your own lifetime workbench!
-- For free video tutorials and other cool woodworking stuff, check out http://www.TheWoodWhisperer.com