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THE  TOOLS

189

Fig. 20.—Tapered Roughung Rtamer
Left hand not to pull in.   Serrated spirally to break up chips.

as in the twist-drills, the tool tapering towards the shank. This prevents
the rear end from rubbing in the hole. End clearance on the lips of the teeth
enables the tool to start the cut sweetly. The side or radial clearance pro-
duces a smooth and true surface. Without this the edges would rub hard
and not cut at all, and the hole would not be true. Generally the radial
clearance is a straight face, lying at an angle greater than that of the actual
cutting edge, which is very narrow, like the " land " on a drill. The edge
so formed lasts longer than it would if it were left keen.
Flutes.—The sectional forms of flutes vary. They may be straight,
concave, or convex, the first being most common as it is more readily re-
ground than the others. The flutes of tapered reamers are straight, or spiral
in the longitudinal direction. When used for roughing, the flutes are either
notched or they have a spiral groove running all round the teeth to break
up the chips (fig. 20). Some of the chucking reamers have straight flutes,
while a good many have three-grooved spirals with oil grooves for the passage
of the lubricant. All the solid reamers have shanks either parallel or tapered
to standards. Shell reamers
fit on arbors, and are only
used for the larger holes.
Floating Reamers. —
These are used in some of
the finest operations. They
accommodate themselves to
the holes which they finish. They may float perpendicularly and at an
angle. They are employed extensively in turret work, for which special
holders are provided.
Adjustable Reamers.—These are in some- degree a result of the
growth of the limit system of gauging, in which minute differences in the
diameters of holes for tight, push, and easy fits have to be made. If solid
reamers are made to deal with certain sizes of holes, they lose their dimensions
rapidly with regrinding. There are many differences in the details of
fitting and adjusting the blades in these tools. They may be classified as
follows:—
1.  Reamers having a solid body with splits, to be expanded by an
internal tapered plug (fig. 19), which is either drawn or thrust inwards with
a screw or driven with a hammer.    Only a very slight amount of expansion
is obtainable with these, but they are suitable for jobs where only fine
cuts are required with little variation in size.     They are used extensively
on turret lathes.
2.  In this group, loose blades are fitted in recesses in the body, and
expanded by the insertion of packing strips beneath them.   The one ad-
vantage of this design is that the blades bed solidly on the packing, and that
packings of increased thickness can be substituted as the blades become
worn.   They are also cheap, having few fittings, and they cannot readily be
tampered with.   Tin-foil is used for packing, the thinnest strips of which
measure 0-0005 in. thick.   These designs are used in turret lathes.