Jointing A Straight Board: The “Reverse Rainbow” Method
I use the jointer a lot. I use it to flatten the face of all the lumber I process. Then, after planing the lumber to thickness, I use it again to create at least one glue-line edge. Cleaning up hundreds of board feet adds up to more than a thousand passes on the jointer per day. I often think that I could do a class on just using the jointer because I have tricks that I want to share. Then I tell myself that using the jointer would be a boring class and even if I made it exciting no one would come because they wouldn’t think that there was much to know about the jointer. To those of you who think you know too much, I say, “Phooey!” Here is lesson #1:
The “Reverse Rainbow,” remember the term and you will remember the way to a brighter future, filled with consistently straight lumber and large pots of gold. The Reverse Rainbow is my way of reminding myself which way the bow of the board should be facing. Simple math and physics, with perhaps a little geometry thrown in, dictate that the Reverse Rainbow is achieved by placing the board on the planer with the middle on the bed and the ends in the air. This is in relation to the regular “Rainbow” that calls for the board to be placed on the planer with the ends on the table and the middle in the air.
The Reverse Rainbow seems counter intuitive to most. Everyone thinks that the jointer can’t make a straight edge when the board is sitting as unstable as a rocking chair and unable to hold its position flat on the table. Surely, flipping the board over and placing the two ends on the table will provide more stability and, in turn, more accuracy. However, that is just not the case.
The problem with putting the arch of the board up is that as the board is moved in to the jointer, it is moved upwards. This cause an arch in the cut. It will be less than before, but there will still be an arch. The next pass will be straighter, but still not entirely straight. The only way that this method can produce perfectly straight lumber is if the length of the board is always supported on the infeed and outfeed tables. This means shorter boards or a jointer with auxiliary tables for additional length.
To avoid having to make longer tables for your jointer, just flip the board over. Put the belly of the board on the jointer and start feeding it in. I do my first pass with the leading edge of the board starting on top of the outfeed table. I basically set it down just beyond the cutter head (the outfeed table stops the boards from being directly sucked in to the machine) and push it through. Keep the pressure on the outfeed table and try to maintain a straight line that sits flat on the table as long as you can. Remember, keep the pressure on the outfeed table.
This first pass will tell you all you need to know. If the bow is not too large, this first pass may run a long edge that can be cleaned up with just one more pass along the entire length. If the bow is large, the new edge may only run a third of the length. If that is the case, run the board again just like you did the first time. Start with the leading edge on the outfeed table and watch what happens. The angle of your edge will change and the belly of the board will become more centered along its length. Keep doing this until your unplaned ends are the same length, which shows that you have the angle correct and need to now just take off material until the entire edge is straight. For the finish pass, start on the infeed table, not the outfeed table, and run the entire length of the board. Slow down your feed rate to help reduce chipout, and watch your outfeed table to make sure the board sits flat through the final pass.
That’s all there is to it. Just remember to turn the rainbow over and you will get great results every time.