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Legal,
There could be any number of things causing your readings.
1) Your square may not be square.
2) Your square may be a square square but it is sitting on the table (bed) out of parallel with the fence, and leaning ever so slightly out of square in the plane perpendicular to that of the square itself.
3) Your miter angle may not be exactly dead nuts square, and your square may not be exactly perpendicular to the bed in both planes, or exactly parallel to the fence.
Or: The bed is shaped like a "V" or an upside down "V".
The latter should be easy to check either by eyeballing or with a good straight edge. That, or bed twist, are the only things that can cause such a problem if all the above conditions are met with your measuring devices. Bed twist can be caused by screwing or bolting the saw to a twisted stand or bench.
You really need to set a stick with perfectly straight and parallel edges against the fence, butt the flat edge of the body of your square to it, and use a smaller square off the bed onto the flat of the tongue of the first square to insure it is square to the bed in that plane too.
Done that way, even if there is a slight misalignment in miter angle, you will be able to square the bevel angle accurately. That is presuming a bed that is in the same plane (ie. flat).
PS: Do not rotate the blade when checking from each side.
Cheers,
Jim
Last edited by clampman; 06-17-2009 at 08:15 PM.
Reason: Forgot " bed twist" in first edition, added PS in third edition
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