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178                             THE  MACHINE-SHOP
be done with a minimum of skilled attendance, and has been accom-
plished at an almost fabulous reduction in costs. The special machine
and its set-up of tools is the dominating fact. It secures the degree of
accuracy desired with continuous production, and eliminates the special
fitting of parts which used to take place, and substitutes " assembling " for
this costly operation.
An immense number of such special machine-tools has been designed.
Concurrently with this developing, new appliances have been schemed to
assist and extend the purposes to which the machines may be put, and to
economize the time of the attendants, and incidentally to relieve them of
responsibility. Always, the essential point is that accurate work is produced
in large quantities, while its cost is greatly lessened.
Machining Elementary Forms.—If the elementary geometrical forms
machined are observed, they will be found to be simple and few in number.
They comprise plane, cylindrical, and helical surfaces, though in great
variety. These are practically all; yet the types of machines built to form
these very simple shapes are numbered by the score, and the individual
designs run into hundreds. Yet the numbers constantly grow—hardly a
week passes but some new machine, possessing some special feature, is placed
on the market.
The role of producing any one of these simple geometrical forms is not
confined to one method or to one kind of machine. A plane surface can
be produced in the planer, shaper, slotter, drilling or boring machine, the
lathe, the milling machine, or the grinder. A cylindrical surface can be
machined in the lathe, the boring and turning mill, or the grinder, and, with
some limitations, in the drilling machine, the shaper, and the slotter. An
internal cylindrical surface (bore) can be made in the lathe, the drilling or
boring machine, the milling or grinding machine. A spiral or helical
surface can be produced in the lathe, the screwing machine, the milling
machine, or the grinder. Special shapes may be cut in lathes provided with
" forming" slides, and with suitable tools; in milling, grinding, and
broaching machines, and in gear-cutters. Similar forms are produced on
several kinds of machines, and this fact has had a vital bearing on the changing
practice of the present day. The machine chosen is the one which will do
the work required best and most cheaply. Three considerations arise: (i)
the selection of the best method or machine; (2) the dimensions of articles
and the relative positions of the parts to be machined; and (3) the degree of
accuracy desired.
i. TH!E SELECTION OF MACHINES.—It is not always easy to choose the
best from several possible machines. Machines naturally fall into groups,
and some machines of each group are better fitted than others for the per-
formance of certain tasks. It does not follow that because a planer will
work on short pieces it is the best tool for dealing with all short articles.
The shaper or slotter may be better. As a general principle a reciprocating
machine-tool should not be employed if a rotating one will produce satis-
factory results. Nor ought short screws to be produced on the screw-