The study of construction includes many items such as strength,
proportion, joints, etc. If we look at the roof timbers shown in
outline at _a_ (Fig. 153), the interesting parts of the construction
are the three spaces enclosed by circles. The straight lines between
these circles do not interest us very much, but the parts enclosed do.
Immediately the question arises, how are the timbers fastened at these
places? In other words, what kind of joint is used? The joint then is
the critical part, or we might say the cream of the construction.
[Illustration: Fig. 153]
A very large number of joints are in use, but many of them are rare.
Our grandfathers, who built their houses and barns from oak timbers
hewn out with the axe, commonly used the mortise and tenon, fastened
with a generous hard wood pin, and many of them are still standing
after a century or two of hard usage. The fact that the beams were
rough hewn, instead of sawed, did not in any way affect their strength,
because they made good, strong joints.
Some of the more common joints are shown in the accompanying
illustrations, and may be used for reference.
No. 1. A butt joint in which the two pieces are fastened together, end
to end, by means of glue and dowels. It should be used only in cases
where there is little strain in the direction of the two pieces.
No. 2. A dowel joint joining two pieces at right angles. One form of it
is shown at No. 3 applied to the leg of a table.
No. 4. Shows two pieces fastened edge to edge by dowels. This joint
is often made without the dowels; the two strips, after jointing or
fitting, being glued and rubbed together--sometimes called a rubbed
joint.
No. 5. A butt joint fastened by nails, brads, or screws, common in box
construction.
No. 6. A butt joint where the pieces are not at right angles, owing
to the slant. This is called the hopper joint and it is fastened with
nails or brads.
No. 7. End lap. A joint much used in house framing.
[Illustration: Fig. 154. Joints used in construction]
[Illustration: Fig. 154. Joints used in construction (continued)]
No. 8. Shows the lap joint used for splicing two pieces lengthwise. It
needs to be nailed or bolted to prevent pulling apart.
No. 9. A middle lap joint.
No. 10. Dovetail lap or lap dovetail. This form resists pulling apart
and is a combination of lap and dovetail joints.
No. 11. Shows a modification of the same, only one side being
dovetailed.
No. 12. Halved joint. Both pieces are cut out to half their thickness,
and a width equal to that of the other piece. The pieces may be at
right angles or some other angle, as shown at No. 13.
No. 13. Halved joint at 45 degrees.
No. 14. Lock joint. This is a form of lap joint rarely used. It resists
pulling apart, but should be glued on account of shrinkage.
No. 15. Notched joint; used where two pieces cross, and where full
halving is not desirable, as in the sketch of pergola.
No. 16. Rabbeted or gained joint.
No. 17. Dado joint.
No. 18. Gained or housed joint.
No. 19. Through mortise and tenon, used in furniture construction and
building.
Note--16, 17, 18 are often confused, and are named differently by
mechanics. They are used in boxes, and cabinet work.
No. 20. Blind mortise and tenon, same as No. 19, except that the tenon
does not go through and is invisible. These two joints may be fastened
with glue, and are often strengthened by passing a dowel through at
right angles to the tenon. Another method is to make two or more saw
cuts in the tenon, and drive wedges into the cuts.
In door construction, where the rails meet the stiles, the tenon is
often divided, as shown by the dotted line. The two parts fitted into
separate mortises give the appearance of two distinct tenons on the
edge of the door.
No. 21. Relished mortise and tenon or door joint, a form used at the
corners of doors.
No. 22. End mortise and tenon. The tenon is seen on two sides. Used for
frames of various kinds.
No. 23. The mitre joint, used in picture frames, picture moulding,
interior finish of houses, etc.
No. 24. Lap mitre joint; a combination of end lap and mitre; rarely
used.
No. 25. Stretcher joint; a combination of end lap, mitre, and end
mortise and tenon; used by artists for frames on which their canvas is
fastened. The stretching is done by driving wedges from the inside.
No. 26. Dovetail; used as a splice.
No. 27. Single open dovetail for two pieces at right angles. When two
or more are cut in the same place, we have the open or box dovetail.
No. 28. Box dovetail; used in cabinet work and boxes.
No. 29. Half-blind dovetail. The dovetails are seen from only one side;
used in cabinet work, especially in drawer construction.
No. 30. Blind dovetail. When the two pieces are together, the dovetails
are invisible. This joint calls for very accurate work. It is used in
special cases, where strength is required, and yet it is desirable to
hide the form of construction.
No. 31. Trick dovetail; not used in construction, and only of
interest as a curiosity. The four sides of this trick combination are
apparently exactly alike. It seems impossible for them to have been
put together, and to bring out the effect it is well to have one piece
in light-coloured wood, the other dark. The method of laying out and
cutting is shown in the illustration. The dovetails that appear on the
surface are only oblique sections of dovetailed-shaped tongues and
grooves running diagonally from face to face.
No. 32. Another trick. This at first sight appears like a lap dovetail,
but the end view shows another dovetail, making it apparently
impossible to put together. The construction is shown clearly in the
drawing. It is of no value in constructive work.
No. 33. Splice or scarf joint; used in framing, occasionally; of little
value to boys.
No. 34. Tongue and groove joint; used in flooring and for sheathing.
Scores of other joints might be shown, but they are seldom used, and
are of no value to amateur mechanics.