444 NICHOLLS'S SEAMANSHIP AND NAUTICAL KNOWLEDGE

Rivets.—There are several forms of rivets named according to the
form of their heads and points. In Figure 25 A and B are called "snap"
iead rivets, A having a "snap" form of point and B a "flush" point.
Eivets C and D are "pan" heads, C having a snap point and J> a "boiler"
point, which, it will be observed, is more conical in shape than the flush
point of rivet B. The boiler point is considered to be a better holding
rivet than the others owing to the greater amount of hammering
required to get it into shape, thus filling up the rivet hole more effectively.
Snap point riveting is done with a machine, flush and boiler points are
hammered up by hand. E is a "tap" rivet used in places where a
through rivet cannot be fitted. A screw hole is tapped into the casting,
the tap rivet is then screwed into position by means of the square head
which, having served this purpose, is then cut ofi.

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Fig. 25.—Forms of Rivets
Riret Holes are punched in plates by means of a punching machine
v tich makes a hole of conical form, being smaller on the side into which
tne punching tool first enters. Bivets are, therefore, made of slightly
conical form under the head to effectively fill up the hole. Plates are
pitched from their "faying" surface, faying or facing being the sides
of the landings which bear against each other. Thus, in Figure 25, the
top plate has apparently been punched upwards and the lower plate
downwards; the two plates have then been placed together, the rivets
put in downwards as indicated by the dotted shape, and their points
hammered up by hand into flush or boiler point; or, in the case of the
snap point, with a hydraulic machine.
The Diameters of rivets vary with the thickness of the plates to be
riveted. In shell plating a f-inch rivet is generally used for J-inch
plates.
The Spacing of a row of rivets varies with different kinds of work.
It is expressed in terms of the diameter of the rivet. The space
between their centres may vary between 3J diameters in plating where
watertight and oiltight work is wanted to 7 diameters in the case of
riveting frames to shell plating.
The term "Pitch" is commonly used to indicate the spacing of the