7/29/2019 spot facing cutters
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SPOT a n d
FAClNG
T0 SEAT FIRMLY, without im-
posing bending strains, a
bolt or nut requires a surfacewhich is flat and at right angles
tohe axis of the hole. At times, of
course, the essential flatness andalignment follow naturally
whenflanges or lugs are turned, milledor planed in the normal
course ofmachining. But at other timeswhen surfaces are tapered or
in the as cast condition, it is essentialo provide spot faced
seatings.
In addition, even if machining ofugs or flanges is possible,
there can
be said for spot-faced seatings that
he component is not weakened byheir presence to the same extent
ascould occur with over-all machining.
Diagram A (top) shows what wouldbe the effect of tightening a
nut on toa sloping or untrue surface-bendingof the shank of the
bolt or stud,giving rise to severe strain and leadingpossibly to
breakage. With a spot-faced seating, however-taken downust
sufficiently to bring the surfacerue-the nut seats squarely
(bottom).
Where there are several lugs andheir thickness varies
spot-facing
ensures uniformity, seatings in thick
ugs being faced down to the levelof those in thin ones.
Followingwhich, lengths of studs and projectionhrough nuts are then
everywhere theame.
The operation is performed as, B ,using a cutter with a spigot
to guidet in the hole. A hand-operated drilling
machine, a bench drilling machine, oron occasion a lathe can be
used.Speed of rotation should be slow andhe feed light, or just
sufficient pressure
applied to ensure smooth cutting.An operation of a similar kind
is to
face or square out the bottom of a
hole which has been drilled to theangle of a normal twist drill;
and its particularly important this shouldbe done if the seating is
for a ballvalve.
Depending on relative sizes of theball and the hole it covers, a
seatingmay occur partly up the conicalurface left by the twist
drill, C, in
which case the valve will not be leak-proof. If, however, the
seating isquared out, D, the ball can be lightlyapped to seal
perfectly on the squareedge.
Cutters for either operation can be
made from silver steel rod to shapes
NOVEMBER 19.56
C U T T E R S
E and F. The spot-facing cutterrequires a lathe to turn the
spigot, toenter the hole comfortably. Near thespigot the rod is
flattened each sideby filing; then the tongue so producedis
carefully backed off each side on theend to form cutting edges.For
the ball-seating tool no spigotis necessary, as it obtains its
guidefrom the drilled hole. The end of therod is flattened each
side, then ahacksaw run centrally across the
tongue and the faces backed off eachside to form cutting edges.
Both toolsmay also be backed off on the sides toclear themselves if
used at a depth.
Inserted-blade cuttersFor larger work, such as facing
bosses, cutters with mild steel shanksand inserted blades can be
used, thelatter filed to shape, then hardenedand tempered like
complete tools.
For a single-blade cutter, G? themild steel holder is turned
wtth aspigot and a reduced shank for hold-ing, leaving an enlarged
boss intowhich the cutter is inserted at an.
623 MODEL ENGINEER
angle and held by a grubscrew fromthe side.
To hold firmly, the grubscrewshould seat on a filed flat or into
acountersink. The end face of thecutter should be filed
square-check-ing with a square from the spigot-or machined in the
lathe with lightcuts. Backing off to angle X, H,should be about 8
deg.
On a double-edged cutter; as canbe used for facing piston
bosses, I,the cutter goes right through and isprovided with cutting
edges each side.Preferably, when the grubscrew hasbeen fitted, the
same as the other, theholder should be run in the latheand the tool
faced each side forthe cutting edges to spin in the same
Filing and backing off are as
Foruse, the cutter is inserted whenthe mandrel is through the
bosses.Holding the piston, which can be aproblem, can be as J,
clamping apiece of hardwood top and bottomwith bolts-but not if the
piston is asplit-skirt type. B
