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Full text of "Modern Mechanical Engineering Vol-I"

TRACING  OFFICE

35

CHAPTER   IV
Tracing Office
In the larger offices, women are now employed as tracers. Many
of the best technical men are by no means the neatest draughtsmen.
Whilst it is important to have drawings made carefully, neatly, and
to scale, it is of only secondary importance that the actual drawing
should be of a high finish. Neatness of line finish is only an incidental
accomplishment to the expert designer. But all the same, it is most desir-
able that the prints sent down to the shops should be neat and clear. For
neatness girl tracers cannot be surpassed, and it is wonderful how even the
most complicated and elaborate arrangement, say of general piping for a
large battleship or liner, can be made clear by people who do not know the
mechanical details or understand what each line signifies.
The tracing office is kept apart from the drawing office, but for obvious
reasons should be contiguous to it. It is under the charge of a head tracer,
who takes her instructions from the chief draughtsman and apportions the
work amongst her own staff.
Linen Tracings.—Most tracings nowadays are made on tough linen,
made clear with a highly starched glazed surface. The drawing is pinned
down, and a piece of tracing-cloth is stretched over it. It is necessary to
tear off a strip along the borders, as at this portion the rolls are generally
wrinkled, and if this selvedge were allowed to remain on, it would be very
difficult indeed to get the cloth properly stretched. This stretching does
prove rather troublesome, as the tracing-cloth is apt to stretch very con-
siderably. It is usual to stretch it tightly over the drawing for an hour or
two, or, in the case of a very big plan, overnight, before beginning work on
it, otherwise it would be found that if the tracing were right over the drawing
in one place it would not be so in another.
Making the Tracing.—The surface worked on is a highly glazed
surface. Water takes out the glaze by destroying the starched surface,
and makes the cloth opaque and useless for photographic purposes. It is
therefore essential to take care that no water gets on to the tracing. More-
over, a crack in the tracing will show clearly on the photograph, so the tracing
should never be folded, but should be either kept flat or carefully rolled up.
To prevent the ink running on the surface too freely, ground French
chalk is rubbed over it to enable the ink to grip.
The purpose of the tracing is to obtain the sharpest line photograph
possible. No half-tones are required. For this reason the tracing-cloth
should be as transparent as possible, and the ink as opaque as possible.
Many of the opaque papers have a strong yellow tinge, and if an ink without
much body in it be used, we get either a faint blue background, where ferro-
prussiate paper is used, or indistinct white lines, making a more difficult
photo print to read than need be.