Popular Woodworking 2009-10 № 178, страница 30
Stretchers. The stretchers will connect at the lower ring locations; turn the stretchers as needed, to fit.
Stretcher holes. I use a 9/16" brad-point bit to drill iVs"-deep holes for 1" tenons.
It is important to understand that in a measured drawing of a chair, stretcher lengths are always an approximation. These lengths vary slightly from chair to chair. Like boats and barrels, chairs tell their makers important dimensions as they are being made. For that reason, make the stretchers for your stool as you go along, not beforehand.
Remember, the four legs describe a square. Think of the two side stretchers as opposing sides of a square. The distance between them is the same as the distance between the legs. This means all three stretchers - the two sides and the medial - should all be the same length. Turn the three parts to the shapes shown in the drawing and to your lengths.
Before you begin to turn the stretchers I need to let you in on chairmaking's dirty little secret. We use socket construction -round hole and round tenon - only because it can be done so quickly. It really speeds up our work. This is, however, the second-worst j oint you can make. The worst is gluing end grain to end grain. Think about a hole drilled in wood. Its inside surface is almost completely end grain. The only edge grain is on the tangents. If you rely on glue, the glue will eventually fail. You know this from your own experiences. What is the first thing friends and neighbors ask when they find out you are a woodworker? "Can you glue my kitchen chairs?" Why? Because factory chairs always fall apart.
This is the important point. Ifwe want to take advantage of the speed and ease of socket construction, we cannot rely on glue alone. This is what factories do. They use socket construction to hold their chairs together. In other words, the joints are in tension. When the glue fails they pull apart. While we too, glue the joints, we use a completely different principal. We still use a technique that was worked out by Windsor chairmakers 250 years ago. We do not use stretchers to hold legs together. We use them to push legs apart. Ourjoints are in compression. When the glue fails, they remain tight.
To put the joints into compression, we need to make our stretchers over-long. We are going to add some extra length called preload. This is really simple. Before you turn the stretchers add 3/s" overall (not for each tenon) to the distance between the legs. We call this length shoulder-to-shoulder. Shoulder-to-shoulder is not the overall length. You still have to add a 1"" tenon on both ends. So the math in my case is 105/s" plus equals 11"". This is my shoulder-to-shoulder. When I turn the stretcher, I will add 1"-long tenons to both ends.
You are now ready to assemble the stool. It will be easy. The holes in the side stretchers are 90°, right angles. I use a 9/i6" brad-point bit. I drill the holes 11/s" deep for 1"" tenons. The deeper hole ensures that my shoulders are tight to the leg. The shoulders do the pushing apart, not the end of the tenon.
Grip the stretcher in a vise and drill. It will not rock ifyou make a simple cradle like the one in the picture below. Now, assemble the side stretchers to the medial stretcher. I glue only the inside of the hole. If you glue the tenon, the glue gets all over the place. Check to be sure the assembled "H" lies in a plane. Then, set it aside while you drill the leg holes.
You need to first determine the stretcher-hole angles. This is another case where the
Cradle. This simple cradle keeps the stretchers from rocking as you drill.
An even plane. Check to ensure your assembled "H" lies all in one plane.
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