Woodworker's Journal 2008-32-1, страница 35

Turning tenons, setting mortises

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back panel frame) should be only 9/16" long. Using this same tenon-forming technique, create the 7/16"-diameter x l/2"-long tenons on the ends of the backrest side posts that will make up the sides of the back panel. Turn all the rung tenons.

Finally, turn the tenons at the tops

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of the front posts. Make them long enough to accommodate the thickness of the arms, plus some extra that youH cut flush with the top of the arms.

Locating and Maiting Mortises

I use my lathe's indexing head to lay out penciled reference lines to locate the rung mortises.

An indexing head is a disk centered on the lathe's axis of rotation. The disk on my lathe has 36 equally spaced holes drilled near its circumference. A spring-loaded indexing pin allows me to lock the head at any of these 36 stops. I can, therefore, lock the head and draw a line along the length of the outside diameter of the post with my marking jig (see photo, left). That line will be exactly parallel to the lathe's axis of rotation (the post's centerline). In the case of the back posts, after drawing the first line, I release the locking pin, count 10 stops, reset the locking pin and, with my marking jig, draw a second line. These lines are then 100 degrees apart on the post's outside diameter. This 100-degree separation is the angular distance between the center-

lines of the back rungs and the centerlines of the side rungs. I repeat the process on the front posts, with one exception: Because the angular distance between the centerlines of the side rungs and the front rungs is only 80 degrees, I count off only eight stops on the indexing head.

If your lathe lacks an indexing head, here's an alternative method. First, rip a flat carpenter's pencil along its length so the lead is exposed from end to end. Then, on your bench top, clamp a pair of posts together. Lay one ripped half of the carpenter's pencil — exposed lead down — across the pair of posts. Slide the pencil along the lengths of the posts. This will leave behind a line along each post that is parallel to its centerline. Then rotate the posts (approximately 100 degrees for back posts and approximately 80 degrees for front posts), reclamp them, and create a second line by drawing the ripped pencil along the lengths of the posts.

Estimating 100- and 80-degree increments may sound foreboding, but it's not. Remember that the 100-degree distance is just a bit more

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February 2008 Woodworker's Journal