Woodwork Joints by William Fairham
THE DOUBLE DOVETAIL PUZZLE Fig
[Illustration: Fig. 378.--Placing the Key Piece to overlap end projection of Central Bar.]
SECOND STAGE.--In the remaining part of the work the chief difficulty is to keep the puzzle from falling to pieces before the key finally locks it. Take the longer cross parts, Fig. 370, and clasp one to each arm. The six need not all be put on meanwhile, but only those which are most easily handled. The next size (Fig. 371) may then be put on.
In the ordinary course each arm could be completed with its three cross pieces till the sixth was attempted, and here the reader would find that, at the last moment, his attempt was frustrated. He could not get the last small piece in, as other bars lock the puzzle. Here it is that the "key" comes in.
THE KEY PIECE.--When the writer fits up the puzzle he finds that three of the arms may straight away be fitted complete with their three cross parts. These are the ones where the longer cross piece (Fig. 370) _lies flush with the back of the central bar_ (see Fig. 377). This is easily found out when at work on the puzzle. In the case of the other three arms there is, of course, a gap caused by the long slots of the central bars. Adjust the parts on the first-named three arms, and then deal with the fourth arm, putting in all three cross parts. For the little one
By placing the "key" so that it _overlaps the end projection_ of the arm (see Fig. 378) a space is left at the centre, and means is thus afforded for getting in the three cross parts on the remaining two arms.
This practically ends the puzzle. While the "key" is in its overlapping position the parts may be separated, but if it is _turned round on its narrow neck_, so that it is in exactly the same position as the other five small cross parts, it locks the whole thing so tightly that nothing but sheer force could loosen the twenty-one pieces.
So far as the order of putting together is concerned, there are many equally satisfactory ways, these being determined by the ease or difficulty that one experiences in holding the half-finished puzzle. It all comes to the same in the end, and the "key" must be placed on one bar before the last three arms can be completed. The "key," moreover, must be on one of the bars where a gap is left at the centre, and not on one where Fig. 370 lies flush against the central arm as in Fig. 377.
UNDOING THE PUZZLE.--To take the puzzle to pieces all that is required is to turn the "key" half round and push the other two cross bars on that arm towards the outer point. The cross bars below may then be removed, and the whole structure falls to pieces.
THE DOUBLE DOVETAIL PUZZLE (Fig. 379) consists of two pieces of wood (usually one dark and the other light) which, upon examination, appear to be dovetailed together from each face. This interlocking arrangement is obviously impossible, and the solution of the puzzle is only apparent on examining Fig. 380, where it will be seen that the joint fits together diagonally.
At Fig. 381 are given the diagrams for setting out. Draw the outline of the elevation, plan and end view. The end view in the first instance is indicated by 3, 4, 5 and 6, and it measures 1-7/8 ins. square. A 1-7/8-ins. square is simply used because 2-ins. wood generally finishes this size after it is planed up. Set out a square (A, B, C, D) which stands corner-ways in the larger square (3, 4, 5, 6). Project the lines D A and C B upwards as at 1, and on to this drawing (1), set out the dovetail according to your own idea of length, width and bevel. Project the four points of your dovetail downwards into the end view, and where these lines cut A, B, and D, C draw them downwards and rebate them into your original plan. This will give the true shape of the two dovetails and it is to this shape that you will cut your joint.