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Full text of "Modern Mechanical Engineering Vol-I"

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THE  DETAIL OFFICE                           21
7000 i.h.p. there may be as many as 1000 pipes and possibly 300 valve
Having checked the drawing, the details and preparation of which will
be more fully dealt with in a later paragraph, it is initialled by the section
leader and given to the assistant chief. The latter, after inspecting it, will
have it sent into the tracing office, whence drawing and tracing are returned
to the section leader. It is usual then to check the tracing with the drawing,
the draughtsman's initials being put in the corner with those of the checker,
when the drawing can be photographed and sent out for circulation.
Size and Style of Drawings; Instruments; Handbooks.—The
size and style of drawings should be standardized as much as possible. It
must always be recollected that the tracing-papers and cloths, also photo-
printing papers, are made in rolls of 30 in. and 40 in. broad, and drawings
should be made accordingly.
A very good size of drawing-paper is the ordinary double elephant size,
40 in. X 27 in., although a smaller sheet, the imperial, 30 in. X 22 in. is fre-
quently adopted. The former is not only a very convenient size when on the
board, but is a satisfactory size for handling in the shops, and is economical in
tracing-cloth and photo-paper. For large arrangement drawings, paper from
the web roll is generally used. This may be the well-known sand-grained
paper, or it may be some form of mounted hand-made paper. These can
generally be procured in long rolls of 30 in., 40 in., 54 in., and 60 in. width,
and the amount required cut off. Where a drawing will be on the boards
for a long time, instead of attaching it to the board, as is usual, with small
brass-headed drawing-pins, the paper is stretched by soaking, and, whilst
wet, glued to the edges of the board. When the paper dries, it of course
contracts and gives a very tightly-stretched surface to work on, and which
will remain stretched without any ruffling up, as long as the job lasts. It
is essential to have the edges of the board planed perfectly true, and also
to have a very true T-square, also good set-squares, one of 45° and the other
a 60° one. Scales may be of paper, but are more generally of wood, and are
much more satisfactory when edged with white celluloid. Ivory scales are
frequently used, but they are very costly, and after a time the marking gets
rubbed off, and they require to be recut. Where English measures are
adopted, the usual scales are J in., J in., f in., i in., f in., f in., i J in., and 3
in. to the foot. In modern drawing-office practice, the slide rule is constantly
used for multiplying, dividing, squaring, cubing, extracting roots, &c. Each
different branch of engineering and shipbuilding possesses its own favourite
pocket and handbooks with tables, See., but these tables are frequently
standardized on sheets hung round the office.
Beginning the Drawing.—On beginning the drawing, the draughts-
man plans in his own mind how he will space it out for easy reading in the
shops, which are seldom so well lit as the office. Centre lines are used as
datum lines, and all dimensions should be calculated from them and checking
done with reference to them. Generally, two views at least are necessary,
and half a dozen may be needed, including, perhaps, an outer elevation,