The first time I cut a tenon I started by taking my time marking it out with my marking gauge, then with
complete confidence I made my cuts and removed the waste material and after a little clean up I test fit the tenon into the mortise only to discover the shoulders of my tenon were out of square, forcing me to fuse around with my shoulder plane until I finally achieved a good square fit.
This problem stayed with me for sometime, I would cut a tenon and then have a problem with the shoulder not being square. I tried using different marking gauges thinking maybe this was the problem, trying wheel gauges, knife gauges but nothing seemed to make a difference. Eventually I realized the problem was never in the marking gauges, it was in the material. When using a marking gauge to mark tenon shoulders the gauge references from the end of the board, and if the end of the board is not true it will cause the shoulder line to be out square, and it doesn't take much to have an issue. But if you reference from the side of a board these marks will be immensely accurate. So what I started doing is using my starrett and a marking knife to lay out my tenon shoulders referencing from the side of the board. Since I started created tenons this way I no longer have fit issues with my mortise and tenon joints.
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