Woodworker's Journal winter-2009, страница 23
Setting the Pitch Pitch is one of the variables we can play with on a spiral. To run a single twist, well set the pitch (ratio of length to rotation) at 1:1. This means there will be one diameter's worth of length for each full rotation of the spiral. To set the pitch, divide each diameter segment you have laid out in half by running a pencil on the turning. You now have two segments for each diameter of length. Once you have the segments marked out, use your tool rest to mark out two lateral lines 180° apart (on opposite sides) along the length of what will be the spiral (see top right photo). If your lathe lacks indexing, you can use two opposite drive center spurs as indicators for where to draw your lines, or use your calipers set to just touch the O.D. (the touch points will be 180' apart). Starting the Spiral You should now have rectangular sections marked along the length of the work. To draw your With stock between centers, lay out and mark your spiral. Here the author uses a black marker to create a 1:1, single-twist ratio. The barley twist is started by using a dovetail saw with a depth stop clamped in place. Rotate the stock toward you by hand. It all starts with layout. You can't make the progressive cuts shown here without drawing the spiral first. To make a single twist you'll need a 1:1 pitch. Use the diameter of the stock as your first dividing lines (blue). Divide those segments again in half (red), then mark two horizontal (green) lines exactly 180' opposed from one another. Now you can draw the spiral (black). single spiral, simply choose one set of these boxes and run a diagonal line from corner to corner on each successive box. Note: this is also where you decide if your spiral is to be left-or right-hand. If you lay it out away from you starling at the tailstock (right) end, it will be right-hand. If you mark your layout away from you starting at the headstock (left) end, it will be left-hand. Next, use a dovetail or tenon saw with a depth stop clamped to the blade (bottom photo) to cut the line of the spiral down to depth. For a single barley twist, I recommend one third the total diameter of the turning. Cut along the line to the depth of the stop, rotating the spindle by hand as you go. Keep your saw vertical, and cut on the top of the spindle to keep your angle consistent. You will wind up with a spiral cut the width of the saw's kerf that makes one complete winter 2009 23 |
