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Full text of "Modern Mechanical Engineering Vol-I"



1                                     Ordinary blue-black or red writing ink is absolutely useless to get a clear

^                                line.    The main portion of the drawing, certainly, as well as the printing is

|                                done in black Indian ink.   The ink should be mixed freshly every morning,

I                                and ground down in a white-enamelled china palette to a consistency which

l                                will at once run freely and at the same time give a perfectly black line.   A

|                               little gamboge mixed with the ink helps to make it more opaque.   For

Hi                                    centre lines and dimension lines a less prominent line will do.   It used to

;i                                    be common to mix up crimson lake with water to a very thin syrup for this

I                                    purpose, but it was not generally dense enough, and has largely given place

|                                    to the use of burnt sienna.    Several firms, indeed, use nothing else but

|                                   black, chain-dotting centre lines to distinguish them from outlines.   Where

I                                        it is desired to show things faintly, such as ladders and platforms about an

II                                       engine installation, Prussian blue is employed.   As a general rule, tracings
i                                   should be made with firm, slightly heavy lines, if the most satisfactory work-
|                                    ing drawings are to be obtained.   Very thin lines do not come up well in
*                                    the photographic process.   Some of the inks can be washed out easily with
|                                   water, and it has become fairly common of late to use bottled waterproof
jl                                   inks in the tracing office.   These are much more difficult to erase. If altera-
!'                                   tions are desired in a tracing, it is better to have a small sketch of the altera-
*'                                   tion sent into the tracing office, and the erasure and alteration made there.

j'                                         When the tracing is finished it should be checked, size for size, with the

I                                   original drawing, before being allowed to leave the office.   The printing

|                                   and figuring should be as clear as possible, and, in fact, it is becoming general

practice to tolerate straight up-and-down lettering only.

Copying Order-sheets.  The copying of order-sheets is generally
done in the tracing room. The old method, was to press the sketch and
lettering through from the original sheet, by means of carbon papers, on to
the half-dozen copies required. Modern appliances have got rid of this
laborious and rather barbarous practice. The order-sheet sent in from the
drawing office is only drawn in pencil. The tracers go over it with a special
ink capable of taking a considerable number of copies. This is put on top
of special gelatine sheets and a roller run over it, so that an impression is
taken on the gelatine. This gelatine impress is now used as the original
to take the required number of copies, generally on thin tissues. Two or
more coloured inks can be used in the process, which leaves the order-
sheets very clear and satisfactory.

Several odd jobs find their way to the tracing office, although it is not
strictly tracers' work. These are the correction of a number of specifications
from an original copy, the writing up of the data-book in ink, which has
been filled in in the drawing office in pencil. In short, the tracing office
does any job arising in the drawing office that calls for neatness.