It seemed to Harry that the shop was fairly well equipped, but Ralph
insisted that they must have a mitre box before making anything else
for the house.
The mitre box is, or should be, an instrument of precision, and
although simple in construction, must be perfectly accurate, or it is
useless. (Fig. 145.)
The illustration shows the common form, but elaborate affairs of iron
and wood can be bought ready made. Every boy should make his own, for
the practice, if for nothing else. The sides should be made of oak
7/8 inch thick, 18 inches long, and 3-1/2 inches high, the bottom of
7/8-inch pine or other soft wood, the same size.
When squared up, the two sides must be tested by standing them side by
side; then reverse one end for end, to see if they are alike. If not,
find where the trouble is, and correct it.
It is especially important that the edges of the bottom piece be square
and the sides perfectly parallel. This test can be made with the
marking gauge. Sides are fastened on by boring and countersinking for
three screws on each. After assembling, the whole thing must be tested
as if it were a solid block. Top edges must be true and parallel.
[Illustration: Fig. 145. The 45° mitre box and test pieces]
Near one end--about two inches in--lay out across the top with try
square a line 90 degrees with the sides. Carry the line down each side,
square with the top edges. For 45-degree angles, lay out a square by
drawing two pencil lines across the top, as far apart as the finished
mitre box is wide. Draw the two diagonals and square lines from their
ends down both sides, taking care that their position is not over the
screw in the bottom; because as the saw cuts deeper it may reach this
screw and ruin its teeth.
Make the three saw cuts directly on the lines laid out with a cross
cut or back saw, with the utmost care. If this is not done accurately,
all the labour of preparation is wasted. The blank end of the mitre
box may have an additional 90-degree cut, or be left for new cuts in
the future, as a mitre box of this description wears out and becomes
inaccurate.
Other angles may be used, as 60 degrees or 30 degrees, but it is better
to have these on another box as they are used less, and for special
purposes. (Fig. 146.)
The mitre box is not ready to use until it has been thoroughly tested.
Prepare a strip of soft wood--pine or white wood--1-1/2 inches wide and
1/2 inch thick. Cut four pieces from it on the mitre box, using the
back saw as shown at _a_, with only one of the slits. Place these four
triangular pieces together to form a square. All the four mitre joints
of this square must fit perfectly. If they do not, mark the slit "N.
G.," and test the other slit in the same way. If all right, mark "O.
K." It often happens that one may be perfect and the other inaccurate.
If they are both O. K., the box is ready for use. If one slit is
useless, lay out and cut another on the blank end of the mitre box in
the same direction, and test again.
[Illustration: Fig. 146. 30-60-90 mitre box]
In testing a 30-degree cut three pieces of the strip should be sawed
out, and when placed together they should form a perfect equilateral
triangle, while from a 60-degree cut, six pieces are needed to form a
hexagon.
These angles are valuable in inlaid work, and for getting out
geometrical designs.
The 45-degree cut is indispensable in making the mitred corners of
picture frames and in cabinet work.
In making picture frames of simple cross section, it is first necessary
to cut the rabbet (Fig. 147) with a rabbet plane. If this moulding is
made by hand, the size of the picture should be measured, the length of
all four sides added, and a liberal allowance made for waste.
[Illustration: Fig. 147. Making picture frames]
In the figure, the triangles _a a_ are waste, the rabbet being
indicated by the dotted line. After the four pieces have been sawed out
on the mitre box, they should be placed together on a flat surface,
such as the bench top or floor, to see if the mitres fit perfectly. If
they do not, one of them can be block planed to make a perfect fit, and
the other three laid close together, as shown in the illustration.
The assembling is the hardest part of the operation, and many devices
have been tried and some patented to hold the parts together while the
glue is drying.
Perhaps the surest way is to drill a hole in one piece of each joint
large enough for the passage of a wire bung-head nail.
The undrilled piece is placed vertically in the vise. The drilled
piece, after receiving a thin coat of glue, is brought into position
horizontally, and the nail driven home.
Theoretically, the nail should catch at the first blow, but the
horizontal piece will sometimes slip, even with the best of care. It is
wiser to place this piece about 1/16 inch above its final position, to
allow for this slip.
A method sometimes used is to glue near the ends of each piece a
triangular block of wood, as shown at _d_. These must be left over
night to harden.
The next day the whole four pieces can be glued and held together by
four hand screws, as shown, until the glue is thoroughly hard. This
method, of course, can only be used with plain moulding or that which
is square on the outside.
Our boys tried another way that is commonly practised. They nailed
oblong blocks to an old drawing board, as shown at _e e_, and then
placed the picture frame in the centre, after gluing the joints, and
driving wedges in between the blocks and the frame. Paper placed under
each joint prevented the frame from being stuck to the drawing board by
the glue forced out by the pressure.
This paper plan was learned by experience, as the first frame the boys
tried had to be pried up from the board, and in so doing they broke it
at two of the joints, so that it had to be made again.
It is well to remember in gluing mitre joints that end grain absorbs
more glue than a flat surface. A priming coat should be applied first,
and allowed to remain a few moments to fill up the pores. The second
coat should hold fast and make a strong joint, but an excess of glue
should always be avoided, as it must be removed after hardening, and
glue soon takes the edge from the best of tools.
Very fancy frames should be avoided. A bevel on the outside or inside,
or both, is about all the young woodworker should attempt in the way
of ornamentation. Depend on the natural beauty of the wood, as a fancy
frame draws the attention from the picture, which after all is the main
thing. We should admire the man, not his clothes, the picture not its
frame, although the latter should be neat and well made.
The finishing and polishing of frames is taken up in Chapter XLIX.