Modern Mechanical Engineering Vol-I

38 DRAWING-OFFICE ORGANIZATION
Kinds of Prints.—The blue print is the cheapest photo print and the
one most suited to general shop purposes. It costs less than a shilling
per yard.
Blue prints for mailing purposes are taken on a very thin paper. For
machine shops, where a print will be required a great deal, and where it will
probably be pasted up in frames to keep it flat and to prevent it from going
amissing, a mounted blue print is preferable. This is a paper blue print
mounted on a tough linen backing, and it forms a very firm print indeed,
being like a thin cardboard when washed. Sometimes a linen cloth is used.
This is durable, very soft, and is suitable for folding, but the parts of the
fold are apt to rub off.
Prints which have to be coloured, say for the approval of owners, are
generally taken on black-and-white paper, i.e. black lines on a white ground,
and if occasion demands it, on black-and-white mounted paper. This
paper is much dearer. It costs probably four times as much as the blue
paper, and it takes longer to print. The white ground if slightly under-
exposed is apt to look dirty, and if slightly over-exposed the lines may, if
traced thinly, come out rather faintly.
Printing Machines.—A fairly common type of printing machine is
one formed of two semicircular cylindrical pieces of plate-glass, which together
form the curved wall of a cylinder which is open at the ends. Two prints
are usually put in at the same time, one on one semicircular portion and one
on the other portion. An arc lamp is hung from the roof over the centre of
the cylinder. The cylinder swivels, so that it may lie horizontally when
putting in the tracing, the face of which lies against the glass. Over this
is placed the photo paper, and then a felt backing is strapped on to keep
it in position. When these adjustments are satisfactorily made, the cylinder
is tilted on end so that the electric lamp may travel down its axis. The
speed of travel of this lamp is adjusted by a clockwork arrangement. As
the lamp falls gradually to the bottom the light is reflected on to the glass
and the tracing, which it penetrates. The light affects and fixes in some
degree the chemical surface of the paper, The black lines of the ink pre-
vent penetration, and the unfixed chemicals are dissolved away in developing,
leaving a white line on a blue ground, or a black line on a white ground,
and there are papers with white lines on brown grounds, Sec. The defect
of this type of frame is that the length as well as the breadth of the print
is limited, at least without folding and to some extent damaging the tracing.
•Not only so, but unless the lamp has been carefully wound clear of the
frame, it may be broken in swinging the frame to the horizontal position.
The semicircular cylindrical glass, moreover, is very costly to replace, and
awkward to handle in such a contingency, and the portion of the print at
the bottom is liable to have longer exposure than the top portion. The
latter defect betrays itself in a slight unevenness of ground-tone. Of late
years a flat plate-glass horizontal frame, which works on an endless roller
system, has been introduced. This frame takes prints of any length, say
those common in shipyards. The arc lamp travels horizontally at a fairly