William NG has a great video explaining the five cut method and how to build a cutoff sled. I highly recommend watching this!
In it he shows how to do the calculations necessary to determine exactly how far to move the sled's fence, in which direction, and from which point, and how to go about this with enough precision to get accuracy within .001" over the equivalent of a 96" cut. He does this in one try! The video is well worth watching, lots of great tips, what to watch out for, way too much information to relate here. I used it to set up the the fence on my Jessem sliding table, which is actually much more difficult to do than a sled, just because of the way it's constructed. My fifth cut was off by .001 over about 80" ( 22 x 18" rectangle). I think I can live with that
5 Cuts To A “Perfect” Cross-Cut Sled – William Ng School Of Fine Woodworking
First do a five cut, starting with a rectangle whose long dimension is at least 24", and whose width is great enough to be very stable resting against the sled fence. Measure the width of the fifth cut piece to the nearest thousandth of an inch, both top (A) and bottom (B), and the length of the cut (C) and record. Measure the distance from the pivot point of the fence to the opposite end where the adjustment will be made, and record that (D).
Now here is the formula, spelled out because I don't have a "divided by" symbol on my keyboard : A minus B; divided by 4; divided by C; times D.
Subtract the bottom measurement from the top, divide by 4 (number of cuts, which multiplies the error on the fifth piece), divide that result by the length of the last cut, multiply that by the distance from the pivot point of the fence to the point of adjustment. This tells you exactly how far to move the fence at the point of adjustment. With the pivot point on the right side of the fence, and adjusting on the left, if the result of the above calculation is a positive number, the fence needs to move down, away from the blade. If a negative number, then it needs to move up, toward the blade. In the video, he explains why each of these factors in the formula are important to the final result, and shows how to reliably move the fence by using a stop block and feeler gauges. It sounds complicated, but in reality it's very simple to do. A calculator is your friend. I was surprised to see that he had the same calculator that I bought over thirty years ago.
Dan
Eternity is an awfully long time, especially toward the end.
-Woody Allen-
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