Since the thermal expansion ot steel is
twice that of carbides, different rates
ot shrinkage will naturally occur as the
tool cools trom brazing temperature and
the resulting stresses are likely to produce
cracks in improperly designed tools or in
materials that are difficult to breza.
Brazing stresses depend on solidification
temperature and ductility of the solder,dimensional
relationship of carbide tip to supporting
shaft section, and area to be brazed. The
less ductile the solder and the higher its
freezing temperature the more serious will
be the stresses, which will also rise with
increasing areas to be brezad. A convenient
carbide tip-to-shank base relationship for
minimized brazing strersses reads about
1:3. With more carbide thickness, the tip
will be subjected to tensile stress; with
less, compressive stress will occur.
Crack due
to tension
Crack due
to shearing
To reduce compressive stresses, a brazing
clearance between carbide tip and steel
shank of about 0,2-0.5 mm is requried. A
wire netting, preferably ot Nicoated steel
wires or Straight Ni wires placed diagonally
to the cutting edge, may be used to ensure
a precise and uniform clearance. Stress
sensitive inserts and carbide tips overstressed
by grinding or on the job show and increased
tendency to stress cracking. Therefore,
a careful and proper brazing design is an
important and simple means to keep brazing
stresses as low as possiple. The following
illustrations are typical examples showing
the relationship between tool desing and
brazing stresses.
It carbide tips are brazed on more than
one face, e.g. slot ,brazing, they will
inevitably be exposed to higher stresses.
Therefore, whenever possible, only one a
face should be brazed. To further limit
brazing stresses; the depth of the tip seat
should not be in excess of 0.4 times the
tip thickness. For wide and comparatively
thin tools, a shank oversize is required
before brazing, to avoid cracking of the
tip due to distortion of the shank. The
shank oversize will be machined off after
brazing. Taking into account the above facst,
the seat of BÖHLER carbide tips is machined
into the shank by following as closely as
possible the tool angles of the cutting
edge.
Unnecessary grinding operations can be avoided
by grinding the shaft clearance 2 degress
larger than the tip clearance.
Whether or no brazing stresses result in
cracks will also depend on the choice of
carbide grade. Tough grades are likely to
be sate from cracking, but highly wear resistant
grades such as SB 10, HB 03, and HB 05 must
be brazed very carefully.
Brazing
Carbide Tips
