The author wanted to ensure the tables on the jointer were not sagging at the ends, which would have required shimming.
Rather than buy an expensive straightedge with a guaranteed degree of straightness (guaranteed, that is, until you drop it the first time
), he built what he called a "Test Bar" out of three long pieces of MDF (need only be roughly straight) and 9 dry wall screws.Since everyone isn't going to be able to read the FWW article here's the build:
Rip the MDF into three strips roughly 5 inches wide and cut to whatever length you need to check. My jointer tables beds are about 48 long total, so that's how long mine were.
Drill three holes in one long edge of each board. Put one hole near each end and one off of center. No particular measurement for placement of the holes is important; but be sure you place the holes in the same relative position on each of the three boards (the Shopsmith in horizontal boring mode works great for this). Insert dry wall screws and screw them down all relatively evenly, leaving 1-2 threads exposed.
Now, pick one board that will become the Test Board (A), and the others get labeled B and C. Put B against A with the screw heads of both boards facing each other. Bring the boards together. If all three screw heads don't touch at the same time, adjust the center screw of B only in or out until all three screws on A & B touch.
Now do the same with A & C, again, only adjusting the center screw of C.
Now do B & C, except adjust both center screws of B & C an even amount until all touch.
Now take B back to A, but this time adjust the center screw of A only.
Now take C to A but adjust C's center screw only.
Now take C and B and adjust both center screws evenly.
Take B to A again. Adjust A's center screw if necessary and if you have to make an adjustment, repeat the process. I had to adjust A's center screw a total of two times.
According to the author, now the heads of all three screws in the Test Bar (A) lie along the same exact plane.
Put the test bar on a surface. If all three screw heads touch the surface, the points where the screw heads touch are all in the same plane.
The Test Bar was great for confirming my jointer tables were co-planar and didn't droop (couldn't get a feeler gauge under the screws at either end or near the middle (one reason you don't put the third screw in the middle if you intend to use this to verify co-planar jointer tables is that a perfectly centered screw would probably be over the jointer head!).
For the Shopsmith I could see using the Test Bar to check and adjust the infeed table of the jointer (if you made the Test Bar to the proper length), or to check and adjust the the heights of the main table and any auxillilary tables and also to ensure your main and auxilliary tables were in the same plane (not tilted). Of course, if you have a 510 or 520, I supposed the table connecting tubes doe this for you. However, I suspect I will find other uses for having a long Test Bar which can always be recalibrated if it gets out of whack.
The whole procedure from ripping (I used 3/4 baltic birch), cutting to length, drilling, and adjusting the screws took less than an hour. The cost was nil because I had the drywall screws and a wide enough, long enough cut off of baltic birch to get all three pieces.
Hope this hasn't been too confusing and will be helpful to others. Seems like a much better alternative than buying an expensive precision 48 inch straight edge for this particular use.
Al
