PRINCIPLES OF JOINERY.[11]
[Footnote 11: Professor Rankine's Five Principles:
1. To cut the joints and arrange the fastenings so as to weaken the
pieces of timber they connect as little as possible.
2. To place each abutting surface in a joint as nearly as possible
perpendicular to the pressure which it has to transmit.
3. To proportion the area of each surface to the pressure which it
has to bear so that the timber may be safe against injury under the
heaviest load which occurs in practice, and to form and fit every
pair of such surfaces accurately in order to distribute the stress
uniformly.
4. To proportion the fastenings so that they may be of equal
strength with the pieces which they connect.
5. To place the fastenings in each piece of timber so that there
shall be sufficient resistance to the giving way of the joint by
the fastenings shearing or crushing their way thru the timber.]
1. _Avoid multiplication of errors by making all measurements (as far
as possible) from a common starting point, and laying off all angles
from the same line or surface._ Illustrations of this principle are
as follows: Before proceeding with other processes, a working face and
working edge and as many other surfaces as will finally appear in
the finished piece, should be trued up. At least the working face
and working edge are essential to the proper "lay-out" of the piece,
whenever measurements are made from an edge.
In laying out a series of measurements, it is important, when
possible, that the rule be laid down once for all, and the additions
be made on that, rather than that the rule should be moved along for
each new member of the series.
In scoring around a board with knife and try-square, the head of the
try-square should be held against the working face in scoring both
edges, and against the working edge in scoring both faces, and not
passed from one surface to another in succession.
In the laying out of a halved joint, Fig. 265, Nos. 15-19, p. 178, the
gaging is all done from what will be one of the flush surfaces of the
joined pieces. Then, if the gaged line should be slightly more or less
than half the thickness of the pieces the closeness of the joint would
not be affected.
2. _When possible, in laying out a joint, use the method of
superposition._ Fig. 302. By this is meant the method by which the
lay-out of one member is obtained directly from the other by laying
(superposing) the latter on the former and marking or scribing the
needed dimensions directly, instead of by measurement. It has the
advantages of simplicity, speed, and greater probability of fit.
[Illustration: Fig. 302. Marking by Superposition.]
Familiar illustrations are in the making of halved joints, Fig. 265,
Nos. 15-19, p. 178, dovetail joints, Fig. 267, Nos. 42-45, p. 180, and
scarfed or spliced joints, Fig. 264, Nos. 4-7, p. 177.
3. _Work systematically._ In case the same process is to be repeated
on a number of parts, complete this process in all before taking
up another process. This is the principle of the division of labor
applied to the individual workman.
In laying out duplicate or multiple parts, the proper cross
measurements should be carefully laid out on one piece and then
transferred with a try-square to the other parts laid accurately
beside it. So when a number of like pieces are to be gaged, all the
parts requiring the same setting should be gaged before the gage is
reset for another gaging. This is a great saving of time and insures
accuracy.
In making a number of like parts, if they are not too large much
of the work can often be done in one piece before it is cut up. For
example, to make a number of slats from a given piece of wood, the
piece may first be brought to such dimensions that the length will
be correct for the finished pieces and the thickness of the piece be
equal to the width of the slats, Fig. 303. The face may then be gaged
with a series of lines so that every other space will be equal to
the required thickness of each slat, and the alternate spaces be just
sufficient for the saw kerf and dressing. The slats may then be ripped
apart and dressed to size.
[Illustration: Fig. 303. Making a Number of Like Pieces from a Given
Piece.]
Or a long strip may be planed to thickness and width and then be
sawn up and finished to the proper lengths. For example, in a mitered
picture-frame it may be convenient to plane up two pieces, each one
long enough to make one long side and one short side.
In fitting up framed structures each part when fitted should be
distinctly marked, so that there may be no confusion in assembling.
4. _Where practicable secure the same conditions of grain in different
elements of joined structures._
Illustrations of this are as follows: The grain of the sides of a box
should run continuously around the box, or, in the case of a tall,
slim box, the grain of all the sides should run up and down. In either
case, the grain in the different sides is parallel. In a rubbed joint,
Fig. 269, No. 70, p. 182, to be planed down afterward, in case the
grain is not straight, much trouble in planing may be saved if the
different pieces are laid so that they can all be planed smooth in the
same direction. This may not be possible where the boards are joined
so as to match the grain, as in quartered oak, or where the annual
rings of slash boards are made to alternate in direction so as to
lessen warping, Fig. 280, p. 188.
5. _Where possible, allow for shrinkage without prejudice to
construction._
The most obvious illustration of this principle is panel construction.
In a panel, the frame, which is comparatively narrow, follows the
principal dimensions, and hence does not seriously shrink or swell
itself. But the panel, which is grooved into the frame can shrink or
swell without harm to the general structure.
In a gained joint, as in a case of shelves, Fig. 266, No. 29, p. 179,
the gain in the uprights does not extend quite to the front of the
shelves, and there is a corresponding slight shoulder at the front end
of the shelf, so that if the shelf and support shrink unevenly, no gap
will be apparent.
A drawing-board, Fig. 280, p. 188, is so made that it can shrink or
swell without losing its flatness. Shingles when properly laid, can
shrink or swell without the roof leaking.
6. _Where feasible, undercut joined surfaces so as to give clearance
on the inside and insure a tight appearance. But glued surfaces should
be made to meet flat._
Illustrations of this principle are as follows: The inner end of the
socket in a dovetail joint, Fig. 267, No. 48, p. 180, may be undercut
slightly so as to insure the pin's falling close into place.
The shoulder of any tenon may be undercut so as to allow the edges of
the tenoned piece to close up tight against the mortised piece.
In an end-lap halved joint, Fig. 265, No. 17, p. 178, the edges should
meet all around; if they are to be glued together, they should _not_
be undercut or they will not glue well.
In matched flooring, the underside of the boards is slightly narrower
than the upper side so that the joint may close on the upper side
without fail, Fig. 301, p. 199. The ends of flooring boards are also
slightly beveled so as to make a tight fit on the upper side.
7. _Select the simplest form of joint and use the smallest number of
abutments (bearing surfaces) possible, because the more complicated
the joint or the greater the number of bearing surfaces, the less
likelihood there is of a sound and inexpensive construction._
Illustrations of this principle are as follows: Usually a single
mortise-and-tenon joint is better than a double one because of
simplicity, strength and ease of making. Where much surface is
required for gluing, a double one may be better.
In a dovetail dado, Fig. 266, No. 28, p. 179, it is usually sufficient
to make the dovetail on one side only.
Many very elaborately spliced joints have been devised, which have no
practical advantage over the simple ones, Fig. 264, Nos. 4-7, p. 177.
A butt joint, Fig. 264, No. 11, is stronger than a mitered joint, Fig.
268, No. 52, in a box, for the latter is almost sure to shrink
apart. Where appearance is important, a ledge and miter joint has the
advantage of both, Fig. 268, No. 58.
8. _Keep a due proportion of strength between the fastenings (joints)
and the pieces fastened: i. e., the construction should neither be
frail on the one hand, because the pieces of wood are weakened by
too much cutting, nor clumsy, on the other hand, because then the
fastenings would be inordinately strong. In other words, the different
parts should be equally strong._
Illustrations of this principle are as follows: In a fished joint,
Fig. 264, No. 2, the plate should be attached so as to reinforce the
splice at the weakest point.
In a scarf joint, Fig. 264, Nos. 5 and 7, the angle should be oblique
enough to give the greatest leverage.
In a tusk tenon, Fig. 267, No. 40, the tenon is made but one-sixth the
thickness of the timber, whereas the tusk is made much larger.
Where a mortise is to be cut in a timber bearing weight, it should be
cut in the neutral axis, where the cutting of fibres will weaken it
least.
In the mortise-and-tenon of a table-rail, Fig. 267, No. 43, there
should be a wide shoulder above the tenon of the rail so that the top
of the leg above the mortise will not shear out. The mortise should be
as near the outside of the leg as possible so that the inner corner of
the leg may remain strong. The tenon should be strong enough to share
the strain with the shoulders.
A dado joint, Fig. 266, No. 25, should not be so deep as to weaken the
supporting board.
A tenon should not be so large as to weaken the mortised piece.
Pins or other fastenings, Fig. 267, Nos. 38 and 39, may weaken rather
than strengthen a joint if they are so placed or are so large as to
shear or crush their way thru the timber.
9. _Place each abutting surface in a joint as nearly as possible
perpendicular to the pressure which it has to transmit._
Illustrations of this principle are as follows: the angle in a strut
joint, Fig. 266, No. 62, should be equally divided between the two
beams.
The thrust joint, Fig. 268, No. 63, in a bridge truss, is exactly at
right angles to the pressure.
It is on account of this principle that a spliced joint for
compression, Fig. 264, No. 4, is different from a spliced joint for
tension, No. 5; and that a housed braced joint, Fig. 269, No. 66, is
better than a plain braced joint, No. 65.
A joint to resist vertical cross strain is stronger when scarfed
vertically than horizontally.
THE PRINCIPLES OF JOINERY
REFERENCES:[*]
Goss, p. 132.
Adams, p. 12.
Rivington, Vol. I, p. 57.
[Footnote *: For general bibliography see p. 4.]