I'm having trouble squaring up my newly constructed crosscut sled. I've got 1 1/2" thick front and back fences fastened to a 24 x 30" 1/2" MDF base. The front fence is attached with hex bolts allowing for slight adjustments. Table saw blade appears to be perfectly perpendicular to front fence. (I'm using a 12" Starett square to check alignments.) Blade is also perpendicular with the MDF base of the sled. End cuts on medium-sized boards appear perfectly square. If I take a 12" x 12" piece of 1/2" ply, however, and cut each side in sequence (I just rotate the cut around adjacent sides), side #4 is not a perfect 90 degrees with side #1. It's off by about 3/64's. I have tried adjusting the front fence slightly but nothing seems to work. Seems to me that if the blade, fence and sled base are all a perfect 90 degrees, then all cuts should be perfect 90 degree cuts. Any ideas? What could I be doing wrong?
(Solid Wood Machining Forum)
From contributor J:
One possibility is if your blade is not very sharp, it could actually move the piece slightly as it passes through. Should be easy enough to check if you can mount a toggle clamp or some sort of stop to keep the piece from being able to slide.
To test your jig you need to start with a test piece of ply that has two parallel sides. Make sure that you start with a piece that is straight and parallel. Now using your jig, cut off a piece of your wood, flip end for end and cut a 1" wide strip. Use a caliper to measure the width of the strip you just cut. Adjust your cut so that there is no more than .002" difference between the thick and thin ends of your strip. Less is better.
Let's say you have made your cuts on the right side of your test board... The slice for testing always falls to the right of the blade. Now you must test your jig with cuts that always fall to the left of the blade. If you cannot get the left and right cuts to come out the same, it's because your jig is not straight (or the jig shifts in the table slots when you're pushing from one side versus the other). This is a common problem because the jig is usually weaker where the saw kerf goes through... and jig gets bent. Or the jig is straight overall, but the wood isn't dead flat, and the squareness depends on how long a board you are cutting.
Finally, if there is any play in between your slots and the runners for the jig, you will not get consistently square cuts. Also, if your fence is not perfectly rigid, you'll have problems. Ditto if the fence is not exactly right angles to the base of the sled.
By the way, if your hex bolt is 24" from the saw blade, pivoting the crosscut fence .006" will change your cut by .003" over the 12" test piece. These are rather small adjustments, and sometimes wood gets compressed and forms grooves and doesn't want to be nudged such a tiny distance. Also, four cuts, each off by .002", adds up to .008" off, which is probably acceptable, but less is better. When you are working in thousandths of an inch, even cast iron seems like rubber putty, but just keep tweaking it and you'll get it.
And while you're at it, measure the kerf that your blade makes by cutting a groove using your jig. Compare this groove with the one you get from using the saw arm. Any difference between these measurements and the saw blade width, and you can tell if your jig is indeed running parallel to the blade, or is wobbling, etc.