Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02214413 1997-08-29
W 096/2~287 PCT~/00139
A METHOD OF INCREASING THE S~RENG~H OF A BLADE, AND A BLADE
This invention relates to a method of increasing the
strength of the cutting point part of a blade. The invention
also relates to a cutting blade.
s The invention is primarily applicable to a sharp blade used
for chipping wood and usually made of steel for cutting wood
or other material so that the cutting force is applied to the
point of the blade and ~orms a force wearing and breaking the
point. For example, patent specification FI 79799 describes
a disc chipper provided with blades and used for chipping
wood.
A problem with present blades is that blade durability
requires a relatively big sharpening angle, and with an
impact-like cutting process the hardness of the blade must be
limited so that the blade point will not scale.
Today the durability of the blade point very o~ten limits
the sharpening angle of the blade so that it is 34 - 35~, for
example, in cellulose wood chipping. Blade hardnesses in a
range of 56 - 60 HRC can hereby be used. A too sharp or too
hard blade point will scale during use. The blade point is
nowadays made more durable by providing it with a counter-
chamfer resulting in a point angle of 38 - 40~ along a dis-
tance of about 2 mm from the point.
Blade point wear is a main problem with hard wood as blades
must be replaced when the chipper will not receive any more
wood.
According to the theory of the strength of materials it is
common knowledge that breakage will result when tensile stress
exceeds a certain limit or repeated tensile stress peaks break
the material by fatiguing, whereby an initial crack leading
to breakage will occur at a considerably lower level of ten-
sion.
A publication (Andreas Uhmeier) by Kungl. Tekniska Hogsko-
lan Stockholm 1993 deals with forces affecting the blade point
in wood chipping.
Shot-peening is a known method of cold working metal. Uti-
lization of shot-peening is taught in the Peening Reference
Manual (January 1991). Shot-peening is also the subject matter
of Konepajamies (11/1992), a publication of the VTT.
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The method according to the invention is characterized in
that by cold working the blade, compression stress is formed
in at least one blade surface forming the point. The blade
according to the invention is characterized in that conside-
rable compression stress exists on at least one blade surface
forming the point.
An advantageous application of the method according to the
invention is based on the fact that on that side of the blade
point which detaches chips, compression stress is formed when
manufacturing the blade, whereby the blade point bends towards
the sharpening surface so that compression stress also results
on the clearance side of the blade, which stress reduces
quickly as the bent area ends. In connection with the manufac-
turing method according to the invention a very short shape
also results which corresponds to the present counter-chamfer
and which strengthens the blade point and on the other side
of the point a "beak"-like blade point is formed, protruding
from the sharpening surface and adding to the chipper's
suction.
In the method according to the invention, compression
stress is rormed on the surfaces starting from the blade point
on the part actively working the wood. At the same time a
blade point results which increases chipper suction and after-
sharpening may be performed on the point.
The invention and its details are described more closely
in the following referring to the appended drawings, wherein
the blade is shown in cross-section.
Figure 1 shows the way in which a chipper works when used
for chipping pulp wood.
Figure 2 shows how a blade point penetrates into the wood.
Figure 3 shows the forces occurring in the blade point.
Figures 4 and 5 show the method of making a blade in accor-
dance with the invention.
Figures 6, 7, 8 and 9 show states of stress in the blade
point.
Figures 10 and 11 show shapes of a finished blade in accor-
dance with the invention.
Figures 12 and 13 show an after-grinding method.
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Figure 1 shows how a blade functions when used for chipping
wood. Mounting of the blade 1 on a blade disk 15 is shown in
Figure 13. Wearing plates 16 are mounted to the blade disk 15
and the blade 1 is locked in its position with a blade holder
s 17. Trees 2 to be chipped are guided by a chute 4 to slide
~ against a blade disk, and blades 1 moving with the disk in
direction s detach chip pieces 3. The blade 1 has a sharpening
angle ~ at its cutting point, and the blade 1 is furthermore
tilted so that a clearance angle ~ results between the sharp-
ening surface 6 of the blade 1 and the cut surface of the
chipped wood 2, which angle usually varies so that ~ is very
small close to the blade disk shaft and big on the outer peri-
phery of the blade disk. The thickness of the plate-like blade
1 is t and it is pressed in place from plate surfaces 10 and
21.
Figure 2 shows how wood grains 9 bend under the cutting
force of the blade point 8. How much grains 9 will bend de-
pends on the properties of wood 2 and on the sharpness of the
point 8. As the wood grains 9 bent into a curved shape are cut
off, the elastic force of the fibres will make them press
against the sharpening surface 6 of the blade 1 as regards the
surface indicated by a measure A. The surface lO of the blade
1 detaches chip pieces 3, whereby a force detaching chip
pieces results at the surface part indicated by a measure B.
Figure 3 shows force fields QA and QB occurring at the
point 8 of the blade 1. The force field QA occurring in the
sharpening surface 6 of the blade 1 prevents the tree 2 from
sliding along the feeding ramp 4. The area indicated by the
measure A becomes shorter as the angle ~ grows and hereby the
force field QA will also be reduced. When the blade point 8
becomes dull, the yielding of tree grains 9 increases, whereby
the measure A also grows adding to the force QA~ whereby
feeding of trees becomes difficult.
The width B of the force field QB affecting the surface of
the blade l varies very much during chipping.
After the chip piece 3 has come off along a line 11, the
force field QB is almost non-existent and the force pulling
the tree 2 into the chipper or the so-called "suction power"
lS zero.
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A case is then advantageous where wood is chipped simulta-
neously by two blades. This is often the case with a big tree
or when chipping takes place close to the blade disk shaft.
At times during chipping, only a force QA according to r
Figure 3 is applied to the point 8 of the blade 1, whereby
considerable tensile stress occurs in the surface 6 and there
is a great risk of breakage in the point 8 of the blade 1.
However, the area of influence of the force QA is quite nar-
row, and the measure A is 1 - 3 mm depending on the wood 2,
10 on the sharpness of the point 8 of the blade 1 and on the suc-
tion angle ~. In addition, the blade point 8 is sub~ect to a
force P which cuts off fibres 9 and gives a compression stress
to the point of the blade 1 which plays no decisive part as
regards the blade's durability.
The area B where the force is applied to the surface 10 of
the blade 1 which detaches chips and the total force QB vary
greatly and the force applied to the surface starting from the
point 8 of the blade 1 also often in respect of the surface
pressure exceeds the force applied to the surface 6 starting
from the point 8. This results in forces occurring in the
point of the blade 1 which will bend it in both directions.
It is a general notion that the blade 1 will work better
when its sharpening angle ~ can be reduced. The suction angle
~ can then be increased which reduces the force QA opposing
z5 the feeding of trees, and in the same proportion it is also
possible to reduce the suction force QB' the magnitude of
which is proportional to damages to chip pieces 3.
With this invention it is hereby possible to considerably
influence chip quality and chip production capacity. The
smaller sharpening angle ~' offers good possibilities of
increasing the suction angle ~ without turning the blade 1 in
relation to the blade disk.
With the method according to the invention, it is possible
to make a durable blade 1, where the sharpening angle ~ of the
3~ point 8 is by 5 - 6~ smaller than the one presently used.
Figure 4 shows a blade 1 from the point 8 of which extend
a sharpening surface 6 and a chip detaching surface 10, and
the sharpening angle ~' between these is considerably smaller
than with present blades. Presently the blade 1 is made more
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durable by grinding a counter-chamfer into it as indicated by
a dashed line 13.
Figure 5 shows treatment of the point of the blade 1 using
a method according to the invention which is "shot-peening"
and which is done after annealing and tempering of the blade
1. According to Figure 5, the surface lo is subjected to
peening with small grains or balls 20 having a diameter of
0.1 - 0.6 mm and hitting the surface at a speed of
50 - 150 m/sec in the way indicated by arrows V. Researches
10 show that a compression film is hereby achieved in the steel
sheet surface.
The thickness of the compression stress ~ilm is
0.1 - 0.6 mm, and the level of compression stress is 50 - 60 %
of the yield strength of the material.
After shot-peening, the point of the blade 1 has bent in
accordance with Figure 5, forming a bent point 8'. As the
point 8 bends very far, the blade 1 cannot be used without
after-grinding.
Figure 6 shows an after-ground blade 1, the surface 10 of
20 which has been shot-peened and after-ground, while the point
8" has been ground in the direction of the sharpening surface
6 so that the level of the after-ground surface 6' is higher
than the level of the sharpening surface 6 by a measure H1.
An angle ~" becomes the angle of the cutting edge 12 of the
25 point 8" of the blade 1, and it is considerably bigger than
the original sharpening angle ~'.
Figure 7 shows an alternative after-grinding method whereby
the after-ground surface 6' joins the surface 6 smoothly and
the cutting edge 12 of the point 8 of the blade exceeds the
30 level of the surface 6 by a measure H1. Also in the blade 1
according to Figure 7, the angle ~" of the cutting edge 12 is
considerably bigger than the sharpening angle ~'. With the
blades shown in Figures 6 and 7, the angle ~" of the cutting
edge changes into the angle ~' over a very small distance,
35 which determines the blade properties. Using the method in
accordance with the invention, it is hereby possible to make
an "ideal" point 8 of the blade 1 where the angle ~" of the
cutting edge 12 is usually 40 - 45~, but may even be 60~, that
is, very strong and wear-resistant, and over a distance less
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than 0.5 mm it turns into a very sharp sharpening angle ~'.
The main part of the difference between the angles ~' and ~"
is formed by the curved shape of the surface 10 in the point
8' of the blade l, whereby the curve is usually 10~ - 15~, but
may be 5 - 3C~ in other applications.
The length of the after-ground surface 6 is less than
0.5 mm and by means of it, it is achieved a very short dis-
tance of influence A for the force field QA in accordance with
Figures 2 and 3, because the point 8 of the blade 1 shown in
Figures 6 and 7 in practice gives a great effect increasing
the suction angle ~ which is due to the fact that the dis-
placement of the cutting edge 12 by the measure H1 changes
into an "additional suction angle" in its entirety.
From this follows that blades 1, where the cutting edge 12
has been displaced from the sharpening surface 6 upwards b~
the measure H1, have a smaller force QA opposing the feeding
of trees 2. The feeding of trees into the chipper is hereby
improved and there is less risk of jamming of the chipper even
with hard wood and with dull blades. The same result is achie-
ved by increasing the suction angle ~, but this will impair
the durability of the point 8 of the blade 1 more than the
very short "clearance" in the blade point given by the measure
Hl in accordance with Figures 6 and 7.
The main purpose of the invention is to increase the dura-
bility of the point 8 of the blade 1. As the blade point is
affected by forces in both lateral directions according to the
foregoing description, a thin compression stress zone must be
obtained on surfaces 10 and 6 in the blade point.
The shot-peened blade 1 whose point 8 has bent as shown in
Figures 5 and 8 achieves in its blade surfaces 6 and 10 the
compression stress level shown schematically in Figure 8. The
relative compression stress of the shot-peened surface is
shown schematically by lines 14, the length of which repre-
sents the relative magnitude of the compression stress. The
highest compression stress P1o is achieved through shot-
peening at full power. Due to the bending of the point 8, the
compression stress decreases towards the cutting edge 12 of
the blade.
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Compression stress occurs in the sharpening surface ~
because the blade point 8 bends, and partial upsetting of the
material also occurs here. The compression stress peak P6 in
~ the surface 6 is higher than the compression stress Plo in the
5 surface lO.
When after-grinding of the point 8 is performed on the
blade l, the compression stress levels will fall as shown in
Figure 9, especially close to the blade edge 12. Also the sur-
face's 6 highest compression stress P'6 < P6, and the sur-
10 face's lO highest compression stress P'lo < Plo. This partialrelease of the compression stress is advantageous because
there is hereby more loading tolerance in view of the yield
limit of the material.
Figure lO shows the end of a blade l according to the
15 invention, the point 8' of which is made in accordance with
the present invention. Figure ll shows the compression stress
level in a section of the point 8'. The surface lO has a com-
pression stress P'lo which is almost constant 0.2 - 0.4 mm
below the surface. The surface 6 has a compression stress P'6
which below the surface falls linearly, and close to the
centre of the cross-section achieves tensile stress Vmax. In
Figures 8 and 9, a dashed line l9 shows the compression-ten-
sile stress limit below the surface.
Figure 12 shows the blade point 8 in accordance with the
25 invention after use, whereby the edge 12 has vanished and been
replaced by a rounded point 8R with a rounding radius R8.
According to Figure 2, tree fibres slide over this point long
before being cut off and they wear down the point more and
more. This development can be interrupted by doing a second
30 after-grinding of the point 8 of the blade l, whereby the mea-
sure Hl is reduced or the blade surface 6 is levelled out com-
pletely. After-grinding is possible due to the "beak"-like
shape of the point 8 of the blade l, and the said after-grind-
ing comprises removal of 0.05 - O.l mm of material along a
35 band 0.2 - 0.5 mm wide. The said work can be done without
removing the blades from the chipper, and hereby the exchange
interval for blades l can be almost doubled.
Figure 13 shows how the blade l is subjected to after-
grinding while it is mounted to a blade disk 15. Using a
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manually controlled grinder 1~, a very small quantity of
material is hereby removed from the blade point and a sharp
cutting edge 12 is obtained for the blade. After-grinding is
possible only with a blade according to the invention, because
the after--grinding surface 6' protruding from the sharpening
surface 6 is very narrow. Everybody in the blade field knows
that in practice it is too difficult to grind the whole sharp-
ening surface 6 while the blade 1 is mounted to the blade disk
15.
The most usual way of making blades is by annealing and
tempering blades 1 and by grinding the sharpening surface.
However, grinding is a slow and expensive working method and
forms a great part of blade manufacturing costs. By using the
method in accordance with the invention, grinding of the
sharpening surface 6 before cold working can be eliminated
when making a new blade, because adequate quality of the sur-
face 6 is achieved by milling before annealing. After-grinding
is hereby performed only after annealing and shot-peening. The
whole sharpening surface 6 of the blade 1 is thus ground only
after use, so that the pre-grinding stage can be entirely eli-
minated for disposable blades.
Methods in accordance with the invention can be applied not
only to chippers but also when such a point 8 of a blade 1 is
needed which withstands varying loads and when a very big
clearance angle (suction angle ~) immediately after the cut-
ting edge 12 is useful when using the blade.
The method is also applicable for increasing dura~ility,
when the blade is one whose cutting edge has an angle of 90~.
Tests show that in connection with shot-peening the hardness
of the peened surface increases by 1 - 2 HRC units with pre-
sently used blade materials and considerably more with other
materials which will harden considerably in cold working. A
better wear resistance is hereby obtained also for the blade
point.
Shot-peening may also be replaced by some other cold work-
ing method creating a compression stress zone in the surface
of the material. Such a method is, for example, rolling the
surface with rollers and using a high surface pressure.
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The principal field of application of the invention is wood
chipper blades where by using the invention, blade durability
and chip quality can be improved considerably. In addition,
r the invention brings savings in blade costs, blade exchange
s time, chipper auxiliary equipment (screens, chip cutters) and
~ wood consumption.
In the above description and in the following claims the
term blade point means the part in the longitudinal direction
of the blade, located on both sides of the cutting edge. In
the applications shown in the Figures, it is formed by the
actual sharpening surface 6 and by the surface 10 detaching
chips.
