Armengaud, Jacques Eugène; Leblanc, César Nicolas   [Hrsg.]; Armengaud, Jacques Eugène   [Hrsg.]; Armengaud, Charles   [Hrsg.]
The engineer and machinist's drawing-book: a complete course of instruction for the practical engineer: comprising linear drawing - projections - eccentric curves - the various forms of gearing - reciprocating machinery - sketching and drawing from the machine - projection of shadows - tinting and colouring - and perspective. Illustrated by numerous engravings on wood and steel. Including select details, and complete machines. Forming a progressive series of lessons in drawing, and examples of approved construction — Glasgow, 1855

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CONSTRUCTION AND USE OF DRAWING- INSTRUMENTS.

15

by the square, after which the end of a paper-folder, or
other smooth article, is rubbed along the “lap,” to press out
the superfluous glue. The same operation being rapidly
applied in succession to the other edges, the sheet is left
to dry, and ultimately, by the contraction, turns out per-
fectly flat and tense. When paste or melted glue is not
to be had conveniently, a cake of glue may be dipped in
water and rubbed on the margins of the board at the
proper places.

With panelled boards, as described, the panel is taken
out, and the frame inverted; the paper being first damped
on the back with a sponge, slightly charged with water,
is applied equally over the opening to leave equal mar-
gins, and is pressed and secured into its seat by the panel
and bars. This is a ready enough way of laying a sheet,
and for damp sheets is more expeditious than the gluing
system. But for general use, plain boards are sufficient.

T Square.

The T square, Fig. 2d, is a blade or “straight edge,” a,
usually of mahogany, fitted at one end with a stock, b,
p, &9- 34.)

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Details of T.Squase.

applied transversely at right angles. The stock being so
formed as to fit and slide against one edge of the board,
the blade reaches over the surface, and presents an edge
of its own at right angles to that of the board, by which
parallel straight lines may be drawn upon the paper. To
suit a 41-inch board, the blade should measure 40 inches
long clear of the stock, or one inch shorter than the board,
to remove risk of injury by overhanging at the end; it
should be inches broad by inch thick, as this section
makes it sufficiently stiff laterally and vertically. If
thinner, the blade is too slight and too easily damaged by
falls and other accidents, and is liable to warp; if thicker,
it is too heavy and cumbersome; if broader, it is heavier
without being stiffen The tip of the blade may be secured
from splitting by binding it with a thin strip inserted in
a saw-cut as shown. The stock should be 14 inches long,
to give sufficient bearing on the edge of the board, 2 inches
broad and •§- inch thick, in two equal thicknesses glued
together. With a blade and stock of these sizes, a well
proportioned T square may be made, and the stock will
be heavy enough to act as a balance to the blade, and to
relieve the operation of handling the square. The blade
should be sunk flush into the upper half of the stock on
the inside, and very exactly fitted. It should be inserted
full breadth, as shown in the figure ; notching and dove-
tailing is a mistake, as it weakens the blade and adds
nothing to the security. The lower half of the stock should
be only If inches broad, to leave a 1-inch check or lap, by

which the upper half rests firmly on the board and secures
the blade lying flatly on the paper.

For the second size of board, 81 inches long, the blade
should be not more than 301 inches, of the same scantling
as above, or rather thinner; and the stock a little shorter.

One-half of the stock, c, (Fig. 25), is in some cases made
loose, to turn upon a brass swivel to any angle with the
blade a, and to be clenched by a screwed nut and washer,
(Fir/. 25) The loose stock is useful for drawing parallel
lines obliquely to the edges of the board, such
as the threads of screws, oblique columns, and
connecting rods of steam-engines. In most
cases, however, the sector and the other appen-
dages to be afterwards described, answer the
purpose, and do so more conveniently. A
square of this sort should be rather as an
with Swivelling' addition to the fixed square, and used only
Stock when the bevel edge is required, as it is not so
handy as the other.

The edges of the blade should be very slightly rounded,
as the pen will thereby work the more freely. It is a
mistake to chamfer the edges, that is, to plane them down
to a very thin edge, as is sometimes done with the object
of insuring the correct position of the lines; for the edge
is easily damaged, and the pen is liable to catch the edge,
and to leave ink upon it.

A small hole should be made in the blade near the end,
by which the square may be hung up out of the way,
when not in use.

No varnish of any description should be applied to the
T square, or indeed to any of the wood instruments em-
ployed in drawing. The best and brightest varnish will
soil the paper long after it has been applied and fur-
bished up. The natural surface of the wood cleaned and
polished occasionally with a dry cloth, is the best and
cleanest for working with.

Straight Edges and Triangles.

These appendages to the T square greatly facilitate the
operations of the draughtsman. They should be of close-
grained hardwood, as mahogany, well-seasoned ; straight
edges, when 5 feet long and upwards, may be of ribbon-
steel. Wood is more easily kept clean, and is less likely
to soil the paper.

Straight-edges should, like square blades, be just broad
and thick enough for the necessary stiffness, and bevelled
a little at one edge. The smallest as in (Fig. 26), may be 9
or 10 inches long, f inch broad, and inch thick; the
others from 18 inches upwards.

Triangles, or set-squares, as they are sometimes called,
should be barely inch thick, and flat on the edges, to
wear well. They should be right-angled, one of them, a,
(Fig. 26), being made with equal sides, and angles of 45
degrees each ; the other, b, with angles of 60 and 30
degrees The former, by means of its slant side, is very
l useful in laying off square figures; the vertical side, too,
saves a deal of shifting of the T square, as, when the hori-
; zontal edge is applied to that of the square, short perpendi-
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