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Full text of "Modern Mechanical Engineering Vol-I"

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be no possibility of work being finished to drawings which may have been
intended for prices only, and which it may be desirable to amend as working
drawings. No sub-contract should be begun until working drawings are
received, and a note to this effect put on the acceptance.
A good estimator must watch carefully and try to arrange to do with
the minimum extras, or, where the use of these cannot be avoided, he must
allow for same. The difficulty may be exemplified in boiler plates, for
instance, which are not simply so much per ton. There is a basic price
per ton, say .£20. An extra of 2s. 6d. per ton per 3 in. over 8 ft. broad may
be payable, another extra per 5 cwt. on any plate over 4 tons weight,
another for certain surveys, one for different thicknesses of plates, and
one for tensile strengths, &c. It will be clear from this how important it
is to keep in close touch with the design office; for instance, in a large boiler
installation, the shells may be specified in one or two strakes, and the differ-
ence in prices so caused may be hundreds or even thousands of pounds.
Tables of these extras can be got from steel-makers, and up-to-date lists
should be kept in the office. A good designer, by skill and knowledge of
these extras, may save almost incredible sums to the firm.
Concurrently with the preparation of the estimate should go the drafting
of the specification and necessary schedules. This will be submitted with
tender drawings to the purchasers, and the schedules will be completed
when the contract is entered into, and the whole document will become
the guide and general working instruction for the detail office. These
specifications are usually printed, and a copy is given to each of the main
departments. Copies are carefully executed by the contracting parties
with full legal formality. The preparation of specifications is a very re-
sponsible job. Many firms have standard specifications, which are used as
a basis for the preparation of those finally approved.
Standard Specifications.—During the progress of the late War, an
increasing need was felt for the standardization, not only of parts within
a firm, but for standardization in relationship to materials, tests, &c. As
an instance, it may be stated that the manufucturers of steel plates for
boilers and ships sent out lists of standard plates they were prepared to
roll, and which, tested at the works by the surveyors of the classification
societies, could be had as if from stock, and with the minimum delay.
The previous multiplicity of requirements and tests made the work of
both designer and detailer very onerous, and not infrequently led to very
costly construction.
These differences have been realized by practical draughtsmen for years,
but it was the urgency of the War that forced on reform. Recently the
Board of Trade, Lloyds, and British Corporation, have combined to make
their rules for boiler construction identical, a vast reform when it is
remembered that previously every rule was different, and most work for
first-class jobs was made under two surveys at least, scantlings to suit the
Board of Trade often meaning an increase of weight of about 5 per cent
with steel at £20 per ton.