.

To prevent inaccuracies due to this cause, reamers are often held in
what is known as a "floating" holder. This type of holder is so arranged
that the reamer, instead of being held rigidly, is allowed a slight free
or floating movement so that it can follow a hole which has been bored
true, without restraint. In this way the hole is reamed straight and to
practically the same size as the reamer.

[Illustration: Fig. 21. Two Types of Floating Reamer Holders]

There are many different designs of floating holders but the general
principle upon which they are based is illustrated by the two types
shown in Fig. 21. The reamer and holder shown to the left has a
ball-shank _A_ which bears against a backing-up screw _B_ inserted in
the end of holder _C_ through which the driving pin passes. The lower
end of the reamer shank is also spherical-shaped at _D_, and screw-pin
_E_ secures the shell reamer to this end. It will be noted that the hole
in the shank for pin _E_ is "bell-mouthed" on each side of the center
and that there is clearance at _F_ between the shank and reamer shell;
hence the reamer has a free floating action in any direction. This
holder has given very satisfactory results.

[Illustration: Fig. 22. Multiple-spindle Cylinder Boring Machine]

The holder shown to the right is attached to the face of the turret by
four fillister-head screws. Sleeve _C_ is held in plate _A_ by means of
two steel pins _B_ which are tight in plate _A_ and made to fit freely
in bayonet grooves _D_. Reamer holder _E_ floats on sleeve _C_, the
floating motion being obtained through the four steel pins _G_ extending
into driving ring _F_. Two of the pins are tight in the holder _E_ and
two in sleeve _C_. The faces of sleeve _C_, driving ring _F_, and reamer
holder _E_ are held tightly against each other by means of spring _H_
which insures the reamer being held perfectly true. Spring _H_ is
adjusted by means of nut _I_ which is turned with a spanner wrench
furnished with each holder. The reamer is so held that its axis is
always maintained parallel to the center of the hole, and, at the same
time, it has a slight self-adjusting tendency radially, so that the hole
and reamer will automatically keep in perfect alignment with each
other.

=Multiple Cylinder Boring Machine.=--In automobile and other factories
where a great many gasoline engine cylinders are required,
multiple-spindle boring machines of the vertical type are commonly us