Joining – Part 1
Use of Joints in Woodworking
Most projects require the attachment of one piece of wood to another to provide structural or functional strength. A three-legged milking stool is a great example. The stool has, as the name implies, three legs and a seat (which sometimes has a handle). The stool is intended to take a person’s sitting weight without collapsing. The joint in this case is often a dowel, which is turned on the end of a leg. This dowel leg fits into a drilled hole in the seat. In this example, when the dowel of the leg is fixed into the hole in the seat, it must result in a strong enough joint to bear the expected weight.
I am going to concentrate on three simple joints. Making these joints will allow you to gain an understanding of the amount of accuracy required to cut and fit the joints and the strengths and weaknesses of each joint. This exercise will increase your confidence in the use of hand tools and start you thinking about what is the most appropriate joint to use.
The Simple Half Lap Joint
If we were using two pieces of 1″ x 3″, you would see that the 3″ width establishes the size of both parts of the joint. Half the thickness of our 1″x 3″ gives us the depth of the joints on both pieces. The joint should be marked out using a setsquare. The cross cut line should extend across the face and down both edges. A marking gauge, set for half the thickness, can be used to mark the depth of cut on both edges. Use a dovetail or tenon saw to make two saw cuts inside the pencil lines, down to the depth marked on the edges. Hold the female piece in a vise and use a sharp chisel with the bevel down to roughly remove the wood, almost down to the gauge mark. Work from both sides. Then follow up with a hand router, set at the desired depth to clean cut the female portion of the joint.
The male part for the joint is prepared in a slightly different way. Again, use a setsquare to pencil the length line on the face of the piece and the marking gauge to scratch a line along both edges and along the end grain. As before, the cross grain cuts can be made with the tenon saw, going down to the gauge line (ie. half the thickness of our piece of wood). Now, place the wood in the vise so that the rip cut along the grain of the piece can be made comfortably with the tenon saw. Do not expect to make this cut perfectly the first time. You have now prepared the male portion of the joint. If you are very fortunate, the joint will be a snug push-fit. If the joint is too tight you can reduce the width of the male piece of wood by taking a shaving off the edge. If the male tenon is too thick, you could take a few shavings off the inside face with a rabbet plane, bull nose plane or shoulder plane.
Once you have adjusted the pieces, and they fit together so that the joint surfaces on all sides of the assembled joint fit well together, it is time to look at the joint from a structural point of view. Lay the assembled pieces flat on the bench and try to move the male portion sideways, while holding the female piece stationary. You will find that you cannot move it at all. The joint is very strong in this direction. However, if you hold the female section and slide the male section out, there is friction but it can be done. So the joint is not as strong in that direction. Lastly, if you hold the female section down and lift up on the male, the resistance is even less. Meaning that the joint is weakest in that direction. All of the comparisons are assuming no glue or fasteners of any kind have been used.
The Modified Half Lap
The modified half lap joint is to add a dovetail effect to one side of the half lap. A similar improvement to the strength of the joint would occur if dovetails were added to both sides of the half lap. The slope for the dovetail is 1:6. That means if you lay out a 1″ wide by 6″ long rectangle and draw a diagonal from corner to corner, it will give you the slope. The rest of the preparation is similar to joint #1.
The difference with the introduction of the dovetail is that the male section cannot be removed by sliding it out. The half lap joint is modified by adding a dovetail effect to one side of the half lap.
The Wedged Half Lap Joint
The construction of this joint is similar but varies in the following ways. The square edged dovetail cut of joint #2 has now been changed to an angle cut. The angle cut is a 1:6 slope but the dovetail cut is 1:12. The simplest way to cut this joint is to lay out and cut the female piece first. The narrowest end of the female dovetail gives the width of the male tenon. The half lapping is the same as before. The male tenon has 1:6 sloped edges to match the female, but the tenon is not tapered. This allows the tenon to slide into the mortise. Two additional saw cuts have been added to allow the insertion of wedges. These saw cuts are located ⅛” in from the edge of the tenon. The wedges must be an exact 1:12 slope (or taper) as the dovetail cut in the female piece of wood. When the wedges are driven into the saw cuts, they will expand the tenon to the full dovetail shape (in a final assembly of the joint, the wedges would be glued in place). The resulting joint is a double dovetail, which has considerable strength in all directions.
The use of wedges in this way is reminiscent of wedged mortise and tenon joints which are the classic high strength joints. You could, of course, add glue or fastenings to all three of these joints, which would improve their characteristics. I have deliberately discussed the joints in their dry condition because it is easier to see the differences in strength.
This exercise in cutting joints will greatly increase your skill level in the use of hand tools and your confidence level in understanding how and why these three joints perform the way they do.
In Clive’s next article, he will elaborate on simple joints and give an example of a jointed assembly.