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Flattening Wide Surfaces With A Router

“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.” (Source – the video description)

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