Once the rails are in place, I have a jig that slides along them
Here is the results about halfway through the process. 
a flat benchtop
A couple years ago, I can across an article in Shop Notes about making a workbench top out of douglas fir 2x4s. I tried it and it works well. That hardest part is flattening the top. I don’t have a 24″ jointer and I’m not that good with a handplane.The I came across someting on the internet on using a router.
the trick is a sled on rails that are known to be parallel. I screwed the rails to the sides of my bench and used winding sticks to ensure they’re parallel.
Once the rails are in place, I have a jig that slides along them
Here is the results about halfway through the process.
Once the rails are in place, I have a jig that slides along them
Here is the results about halfway through the process.
