An earlier forum discussed the possibility of whether a misaligned table saw blade vs. miter slot could cause a top-to-bottom skewness in the sidewall of the workpiece , and, if so, whether the skewness could be significant (http://www.sawmillcreek.org/showthre...el-angle-error). Please permit this first-year woodworker (hobbyist) to put his $0.02 in and offer up illustrations and the math behind it.
In my case, my first end grain cutting board, made with a checkerboard-like pattern (the Wood Whisperers pattern), revealed each of the intended rectangles to have the shape of a parallelogram. The skewed sidewalls were made prior to the first glue-up (the rip cuts).
Investigating further, I confirmed with both a right angle square and a Wixley digital angle gage that the saw blade was beveled at precisely 90 degrees to the cast iron table top. But I did find that the fence was skewed to the blade, with the distance between fence and blade increasing as one travels from the front to the back of the blade. The sidewalls of the rip cuts reveal 0.015 additional material removed from the bottom of the work piece vs. the top.
The first page of the attached pdf file shows the symptom, with the aforementioned 0.015 additional material removal at the bottom of the rip vs. the top. The second page highlights my setup, with the blade skewed relative to the fence, and the blade heeled further from the fence at its back vs. its front.
Page 3 of the attached shows the length of the saw blade, front to back, at both the bottom of the work piece and its very top for my setup. Note that, for any setup, the blade length along the tabletop will, of course, be longer that the blade length along the top of the work piece due to the circular shape of the blade. This means that any skewness will cause the blade to cut further into the work piece at its bottom vs. its top (the resulting shape would appear as a straight skew due to its short length, but would in fact be a very slight cove due to the circular profile of the blade).
The final page shows the math behind the 0.015 skewness that I am experiencing. Distances x and y are the total amounts (unknown) of material removed at the bottom andtop of the wall stock, respectively. The blade lengths of 4.14 and 3 shown in this view are the distances from the center line of the blade to the end of the blade (1/2 the distances shown in page 3), measured along both the bottom and top of the work piece. Because triangles abc and ade are similar, then the ratio x/4.13 (total material removed at bottom divided by ½ the blade length along the bottom) is the same as y/3 (total material removed at top divided by ½ of blade length along top). We also know that the difference between the total material removed along the top vs. the total removed along the bottom is 0.015 (the measured skewness of the ripped sidewall). With 2 equations and 2 unknowns, we can solve for both the total material removed, and the angle of skewness. In my case, the total material removed is at bottom and top are 0.055 and 0.040, respectively (with the difference between the 2 values being the 0.015 skewness).
Knowing the total material removed, the angle of skewness is the arc tangent of either the material removed at top divided by its blade length, or the material removed at bottom over its blade length (both values are equal). My angle of skewness was 0.76 degrees (Aha!, says this newbie this would definitely account of the burning of the rip stock whenever I would set the fence to the right of the blade).
I do not have yet have a reliable method for measuring blade or fence skewness (I have ordered a Mag-DRO to use with my digital calipers), though I think that the majority of skewness is in my case is of the fence to the blade, rather than the blade to the miter (based on crude observations using my combination square). I do know that, having set a thin-stock straight-edge gently along the blade length at the table top (avoiding the teeth) and adjusting the fence square to the straight edge, my skewness problem has disappeared, along with the burning that was apparent whenever I had set the fence to the right of the blade (I know, I know, that is not the proper way to align the fence or the blade should measure to a single point on the blade at both the front and back of blade length this is why I ordered the Mag-DRO). Suffice it to say that, from the point of view of the ripped stock, it does not matter whether the blade and fence are parallel to the miter slot or skewed to the miter slot, just so long as they are parallel to one another (cross cuts, using a miter gage, are another matter entirely).
Thank you to all members of SawMillCreek for being part of and maintaining this lively and informative woodworking community. This has helped me in no small way to get underway in my woodworking endeavors. Eric S.
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Originally Posted by Eric Shapin
