Note: Descriptions are shown in the official language in which they were submitted.
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SAFETY RAZORS
This invention is concerned with safety razors of the bind in which a blade
unit
assembly is carried by a handle and includes at least one blade with a sharp
cutting edge
which is moved across the surface of the shin being shaved by means of the
handle. A
blade unit may be mounted detachably on a razor handle to enable the blade
unit to be
replaced by a fresh blade unit when the blade sharpness has diminished to an
unsatisfactory level, or it may be fixedly attached to the handle with the
intention that the
entire razor be discarded when the blade or blades have become dulled.
Detachable and
replaceable blade units are commonly referred to as shaving cartridges. The
present
invention is particularly concerned with the blades of safety razor blade
units, and relates
specifically to elongate blades having cutting edges which extend along the
blades.
Safety razor blade units generally include, in addition to the at least one
blade, a
guard which defines a surface for contacting the skin ahead of the blades as
the blade unit
is moved across the skin in the performance of a shaving strobe, and a so-
called cap
defining a surface for contacting the shin behind the blade(s). It is well
known that to
achieve the best possible performance the or each blade must be orientated
with respect to
the cap and guard surfaces so that it is inclined to a plane which is
tangential to those
surfaces (the "tangent plane"). By the blade being inclined it is meant that
the mid-plane
of the sheet material of the blade, which plane bisects the angle defined
between the edge
facets that fonn the cutting edge at the blade tip, is at an angle, sometimes
called the
"blade tangent angle", to the tangent plane. A blade can be fixed in position
so that the
blade tangent angle does not vary to any significant extent when the razor is
being used,
but it is also lcnown to mount blades movably within the frame so that their
blade tangent
angles may change as a consequence of blade movements in the performance of a
shaving
strobe. In either case it has been the prior practice to support the or each
blade in safety
razor blade units so that a substantially planar blade element which includes
the cutting
edge and a blade section extending bacb from the faceted edge for at least
several
millimetres, lies at an appropriate angle to the tangent plane. In one well
brown
construction planar blade elements are mounted on bent blade supports which
have a
forwardly and upwardly inclined portion on which the blade elements are
carned, and
upright rear portions which are slidably guided in the frame in which the
blades are
mounted. The need to set the blade tangent angles by correct positioning of
the blades
during blade unit assembly is inconvenient and has a tendency to complicate
the
construction of the blade units as well as the assembly procedures themselves.
Further-
more, in modern safety razors it is common to have two or three blades with
parallel
edges, and having a plurality of such blades lying in respective planes
inclined to the
tangent plane can inhibit blade unit flexibility should that be desired.
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There have been proposals for safety razors with blade elements having
circular
cutting edges which extend around holes in the elements. Examples of such
safety razors
are those described in US-A-5201253 and US-A-5490329. Unlil~e a straight-edged
blade,
the blade elements with circular cutting edges can not be inclined to the
sharpened edge at
a desired angle and position with respect to the shin during shaving.
Therefore, these
blade elements need to be made to ensure the required shape and inclination of
the edges,
such as by special grinding techniques as taught by U.S. 5,490,329, and/or the
required
shape can be imparted to the circular edges by deforming the edges after
sharpeiung, as
described in US-A-5201253. These proposals liave been addressed specifically
to
overcome problems inherent with blade elements with circular cutting edges,
and they
have no obvious application to blades with straight edges which do not present
the same
difficulties.
In US-A-5010646 there is described a safety razor blade unit incorporating
blades
which are bent so that a cutting portion along which the sharpened edge
extends is at an
angle of about 112° to a base or guide portion, both portions also
being corrugated for
stress relieving purposes. Blades bent in this way have substantially reduced
flexibility
and do not offer any solution to the above mention drawbacl~s of the hitherto
l~nown
planar blades.
There is described in US-A-3938250 a composite blade assembly for a safety
razor, which composite blade assembly includes a planar blade and a
supplementary blade
member fastened to the underside of the planar blade, the supplementary blade
member
being bent to increase the span between the cutting edges respectively
provided on the
planar blade and the supplementary blade member. Also, the blade tangent angle
of the
supplementary blade member,is reduced by this blade member being bent. The
bent blade
member is specifically adapted to the production of a composite tandem blade
and it does
not offer any obvious solution to the drawbacles of the known arrangements
which limit
flexibility by requiring the blades to be set at certain inclinations to
provide the desired
blade tangent angles.
With a blade of this form a relatively small tip section can be bent to
achieve the
desired blade tangent angle with the major part of the blade being
substantially planar and
lying parallel to the tangent plane, thereby enabling enhanced flexibility,
especially in
blade units equipped with multiple blades.
The present invention has for its objective to eliminate, or at least
substantially
alleviate the limitations of the prior art blade arrangements. The invention
is directed
particularly to a blade for a safety razor blade unit, comprising an elongate
blade element
with upper and lower surfaces, a forward edge section having a sharp cutting
edge
extending therealong and defined by surface portions which taper towards the
tip of the
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cutting edge, and a substantially planar blade section extending rearwardly
from the edge
section and over which the upper and lower surfaces are parallel, wherein in a
transition
region between the planar blade section and the forward edge section the lower
surface
has a convex curvature and the upper surface is non-convex and is preferably
concave.
S According to an aspect of the invention there is provided a blade of the
form
described above wherein, along at least most of the length of the blade, the
blade cross-
section has a minimum second moment of area not greater than 1.0 X 1 O-4 mm4.
The second moment of area of the blade cross-section is dependent on the size
and
shape of the cross-section and is a measure of blade flexibility. Although
flexibility is
dependent on the Young's modulus of the blade material, razor blade materials
currently
used have Young's moduli close to one another so the second moment of area is
a good
indicator of blade flexibility. The second moment of area is measured with
respect to an
axis, and by the minimum second moment of area it is meant the moment of area
determined with respect to that axis which results in the lowest value for
that blade cross-
1S section. It is preferable that the minimum second moment of area does not
exceed O.S x
10-ø nnn4 and blades having a minimum second moment area less than 0.3 S x 10
4 mm4,
and even less 0.20 x 10 4mm4, e.g. around 0.14 x 104 mm4 or lower are
possible.
In accordance with another aspect the invention provides a blade of the form
defined above wherein a plane bisecting the tip of the blade edge is inclined
to the mid
plane of the planar blade section at an angle in the range of 10° to
35°. With the forward
edge section set at such an inclination an effective blade tangent angle can
be assured with
the planar blade section lying substantially parallel to the tangent plane.
The angle
between the tip bisector and the mid plane of the planar blade section is
preferably in the
range of 1S° to 30°, and more especially 20° to
2S°.
2S A blade in accordance with a further aspect of the invention has the
forward edge
section extending bacl~ from the tip of the cutting edge by a distance not
greater than
lmm, and preferably not greater than O.Smm.
According to a preferred embodiment the forward edge section extends bacl~
from
the tip by a distance extending not substantially beyond the tapering surface
portions, and
in particular by a distance several times less than the distance through which
the planar
section extends rearwardly from the forward section.
In accordance with the different aspects of the invention mentioned above, the
blade element conveniently comprises a metal strip having a bend at the
junction between
the forward edge section and the planar blade section.
3S The invention also resides, in accordance with a further aspect, in a
safety razor
blade unit including a guard surface, a cap surface, and at least one blade
comprising an
elongate blade element with upper and lower surfaces, a forward edge section
having a
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sharp cutting edge extending therealong and defined by surface portions which
taper
towards the tip of the cutting edge, and a substantially planar blade section
extending
rearwardly from the forward edge section and over which the upper and lower
surfaces
are parallel, wherein in a transition region between the planar blade section
and the
forward edge section the lower surface has a convex curvature and the upper
surface is
concave, whereby a plane bisecting the tip of the blade edge lies at an angle
to a tangent
plane that is tangential to the guard and cap surfaces greater than the angle
of the mid
plane of the planar blade section to the tangent plane.
The mid plane of the or each blade is preferably substantially parallel to the
tangent plane that is tangential to the guard and cap surfaces. With the upper
blade
surface concave at the transition region, the tip of the cutting edge lies
above the plane of
the upper surface at the planar blade section. A preferred blade element has
the edge tip
at a height of up to 0.25 mm, in particular in the range of 0.05 mm to 0.15
mm, above the
'plane of the upper surface.
With a blade as provided by the invention, and the blade tangent angle can be
effectively set during the production ofthe blades themselves. Furthermore, as
the major
part of each blade can be planar, two or more blades can be disposed in an
essentially
planar array enabling production of a blade assembly with substantially
greater flexibility
than hitherto attainable. The benefits of the invention are most marled when
an upwardly
inclined blade portion, at forward edge section is confined to a small region
close to the
cutting edge.
A plurality of blades, each in accordance with the invention as explained
above,
may be conveniently assembled into a blade assembly which can form a sub-unit
for
assembly with other components in the manufacture of a safety razor blade
unit, the
blades being so interconnected that their cutting edges are substantially
parallel, and the
planar sections of the blades are substantially coplanar. The blades can be
conveniently
interconnected by transverse strips attached to the undersides of the planar
sections of the
blades. These interconnecting strips may also serve to secure the blade
assembly to a
support structure of the blade unit.
Tn a safety razor blade unit incorporating the blades of the present invention
it is
advantageous if the planar sections of the blades are coplanar with, or lie in
a plane
parallel to, the tangent plane which is tangential to cap and guard surfaces
defined by the
blade unit. Such an arrangement enables maximum advantage to be talen from the
blade
flexibility that the invention males available.
The blade of the invention can consist of a blade element comprising a metal,
e.g.
steel, strip having a bend at the junction between the forward edge section
and the planar
section. The blade material can be conventional and the cutting edge can
comprise ~~
CA 02421150 2006-02-09
coatings as known in the art, including hard coatings, such as boron nitride,
amorphous
diamond or diamond-like-carbon, which coatings are preferably deposited before
the blade
element is bent near the tip although it would be possible for one or more
coatings to be
applied subsequent to bending of the blade element.
5 In accordance with yet another aspect of the invention, there is provided a
method of
making a blade according to the invention as described above, the method
comprising the
steps of:
providing a substantially planar elongate blade element blank having a sharp
cutting edge extending therealong and defined by surface portions which taper
towards the tip
of the edge; and
bending the blade element blank in a region not greater than 1 mm back from
the tip of the cutting edge so that the tip of the cutting edge is displaced
away from the initial
plane of the blank.
During the bending operation the tip of the cutting edge is preferably
displaced to a
position above an upper surface plane of the blank, e.g. to a height of up to
0.25 mm, such as
0.05 to 0.15 mm above that plane.
In addition the invention resides in tooling for making blades according to
the
invention, the tooling comprising a first member having a flat pressing
surface extended
forwardly by an inclined ramp surface, and a second member having a flat
pressing surface
facing the flat pressing surface of the first member, the members being guided
for movement
of the flat surfaces towards and away from each other for pressing a blade
element blank
therebetween.
To inhibit unwanted movements of the blade element blank during pressing, a
rear
stop can be provided for abutment with the rear edge of the blade element
blank and/or the
ramp surface can be limited by a forward stop disposed for engagement by the
front edge of
the blade element blank during the pressing thereof.
The blades of this invention are of particular benefit when used in a safety
razor blade
unit having a resiliently compliant blade support structure, such a blade unit
being described
and claimed in commonly owned Canadian Patent Application No. 2,424,739 of
even date
CA 02421150 2006-02-09
6
herewith.
A clear understanding of the invention will be gained from the following
detailed
description in which reference is made to the accompanying drawings in which:
Figure 1 shows a safety razor equipped with a blade unit having blades in
accordance
with the invention, in a front perspective view;
Figure 2 is a front elevation of the blade unit of the razor shown in Figure
1;
Figure 3 is an end elevation of the blade unit;
Figure 4 is a cross section taken along the line IV-IV in Figure 2;
Figure S is an isometric view of the blade unit;
Figure 6 is a view showing the blade unit as seen in the direction of the
arrow VI in
Figure 3;
Figure 7 is an exploded isometric view of the blade unit;
Figures 8 and 9 show the blade unit from the front and rear respectively, in a
deformed
condition as may arise during shaving;
Figures 10 and 11 show the blade unit from the front and rear, respectively,
in another
deformed condition as may arise during shaving;
Figure 12 is a cross section through a blade of the blade unit, the blade
being in
accordance with the present invention;
Figures 13A, 14A and 15A are schematic illustrations showing successive stages
in
shaping the blade of Figure 12 by a pressing operation;
Figures 13B, 14B and 1 SB are enlarged scale views of the circled areas of
Figures
13A, 14A and 15A, respectively; and
Figures 16A and 16B are cross sections through two blades indicating the axis
with
respect to which the second moment of area is at a minimum.
The razor illustrated in Figures 1 to 11 and described below is also described
and
claimed in the aforementioned Canadian Application No. 2,424,739. The razor
has a handle 1
on which a blade unit 2 is mounted. As shown the handle 1 has a fixed support
platform 3 to
which the blade unit 2 is securely fastened, but the blade unit 2 could
equally well be
releasably connected to the handle 1 to allow replacement of the blade unit 2.
The blade unit
CA 02421150 2006-02-09
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2 comprises a support structure 4 on which a blade assembly 5 is carried. The
support
structure 4 consists of a unitary moulding of rubber, or a material having
similar resiliently
flexible properties to those of rubber, and includes an upper frame 6 on the
upper face of
which the blade assembly 5 is positioned, a sub-frame 7 which has the form of
a substantially
planar sheet, and a base 8 which can also have the form of a substantially
planar sheet. The
upper frame 6 and the sub-frame 7 are connected by a living hinge 9 at their
forward edges
and are relatively positioned normally to diverge from each other rearwardly
away from the
hinge 9. The sub-frame 7 and base 8 are similarly connected by a living hinge
10 at their rear
edges and are disposed normally to diverge from each other in the direction
forwardedly away
from the hinge 10. With this configuration the upper frame 6, sub-frame 7 and
base 8, as
viewed in end elevation (Figure 3), or transverse cross-section (Figure 4)
define a Z shape,
but with the angle a subtended between the upper frame 6 and the sub-frame 7
being greater
than the angle (3 subtended between the sub-frame 7 and the base 8 so that the
upper frame 6 is
normally set at an appropriate angle with respect to the
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stem of the handle 1 and to ensure the desired deformation characteristics of
the support
structure 4 as explained below. Extending between and integrally
interconnected with the
upper frame 6 and the sub-frame 7 are several spaced flexible webs 12
uniformly
distributed along the support structure 4. As shown there are six webs 12
although more
S or less than this munber may be employed. The webs 12 normally lie in
respective
parallel planes perpendicular to the planes of the upper frame 6 and the sub-
frame 7. The
flexible webs 12 constitute respective spring elements and each is capable of
deforming
by buckling, to allow the portion of the upper frame 6 in the region of that
web to be
displaced towards the sub-frame 6 with the deformed or buckled web 12 exerting
a
substantially constant restoring force independent of the degree of buclcling
and hence the
displacement of the upper frame 6. Since the webs 12 act independently of each
other,
different portions of the upper frame 6 along the length thereof may be
readily displaced
by different amounts towards the sub-frame 7.
The sub-frame 7 is similarly supported with respect to the base 8 by a series
of six
1 S integral webs 14 uniformly spaced apart along the blade unit. Conveniently
the webs 14
are aligned and coplanar with the webs 12. The webs 14 which can also deform
by
buckling serve as independent spring elements acting between the sub-frame 7
and the
base 8, and they allow local displacement of the sub-frame 7 towards the base
8 and hence
the handle 1, while exerting a substantially constant restoring force
resisting such
displacement. The resiliently flexible nature of the support structure 4 with
the webs 12,
14 is such that localized portions of the upper frame 6 and the blade assembly
S carried
thereon can be deflected towards the razor handle 1 in order to adapt to the
skin contours
without necessarily influencing the dispositions of other portions thereof,
and the upper
frame 6 and the blade assembly S can as a consequence contort to comply with
the
2S undulations of the shin area over which they are moving. Thus, the blade
unit is
resiliently compliant to ensure close contact with the skin over the full area
spanned by
the blades.
Thus, Figures 8 and 9 illustrate the blade unit 2 with the upper frame 6 and
blade
assembly S deformed into a concave form, their medial portions M being
displaced
towards the handle 1 by a greater amount than their end portions E with the
webs 12, 14
towards the centre M of the blade unit being buckled to a greater extent than
those webs
12, 14 located nearer the ends E of the blade unit 2. Figures 10 and 11 on the
other hand
show the blade unit 2 deformed into a convex configuration, the blade assembly
S and
upper frame 6 being displaced downwardly towards the handle 1 by a greater
amount at
3 S the ends E of the blade unit than at the central portion M of the blade
unit, and in this case
the webs 12, 14 towards the ends E of the blade unit 2 being buckled more than
those
webs 12, 14 closer to the centre M of the blade unit 2. Although both sets of
spring webs
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_g_
12, 14 are shown buckled in Figures 8 to 11 this is not inevitable or
essential. It is
possible, for example as a result of downward shaving force applied towards
the rear R of
the blade unit for the webs 12 to buckle so that the upper frame 6 and blade
assembly 5
are displaced downwardly adjacent the rear edge R without the webs 14 buckling
and
without any displacement of the upper frame 6 and the blade assembly 5 at
their front
edge F. Also a force applied near the front edge can cause downward
displacement of the
upper frame 6 and blade assembly at their front edge F due to the webs 14
buclcling
without the webs 12 becoming buckled. As a consequence the upper frame 6 and
blade
assembly 5 are compliant both in the direction longitudinally of the blade
unit and in the
direction perpendicular thereto in order to adapt to conform closely to the
contours of a
slcin area being shaved. Because the angle a subtended between the upper frame
6 and
the sub-frame 7 is greater than the angle ~i subtended between the sub-frame 7
and the
base 8, the webs 12 are somewhat longer and correspondingly weaker than the
webs 14,
whereby the webs 14 exert a greater resistance to downward displacement of the
upper
frame 6 and the blade assembly 5 at their front edge F than the resistance to
downward
displacement exerted by the webs 12 at the rear edge R of the upper frame 6
and blade
assembly 5, which characteristic is considered desirable as during shaving
greater forces
are generally imparted to a blade unit in the region of the guard than those
exerted in the
region of the cap.
The blade assembly 5 comprises a guard member 15 and a plurality of elongate
blades 16, the guard member 15 and the blades 16 being formed by flexible
strips of
metal, and the blades 16 having parallel forwardly facing sharpened edges 17.
The guard
member 15 and the blades 16 are intercoimected by transverse strips 18 which
are
attached to the undersides of the blades and guard member 15. Maximum
flexibility of
the blade unit is ensured by the blades 16 and transverse connecting strips 18
being
coplanax in the normal, undeformed condition of the blade assembly 5 and the
blade unit
2. The guard member 15 is also substantially coplanar with the blades 16 and
connecting
strips 18 although as shown in Figures 6 and 7, this guard member 15 has an
upwardly
inclined rear portion, and slits 19 are spaced along the length of this
portion of the guard
member 15 to enhance the flexibility of this member. Including the guard
member 15 in
the blade assembly can be advantageous in reliably defining the shaving
geometry of the
blades, and the first blade in particular. The strips 18 have turned-down T-
shaped ends
which are engaged with notches 20, 21 moulded in the front and rear edges of
the upper
frame 6 in order to secure the blade assembly 5 to the support structure. The
upper frame
6 includes longitudinal frame members 22 at the front and rear and transverse
frame
members 23 to which the upper edges of the webs 12 are attached. In the
assembled
blade unit 2 the strips 18 of the blade assembly 5 extend above respective
frame members
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23. The cap 24 of the blade unit 2 includes a flexible lubricating strip 2S
which sits in a
groove extending along the rear longitudinal member 22 of the upper frame 6
and is held
in place by the transverse strips 18 of the blade assembly. The support
structure 4, at the
front of the upper frame 6 in the region of its hinged connection to the sub-
frame 7, forms
S a guard 26 which has longitudinal ribs 27 moulded thereon although
protrusions of other
configurations could be provided. Also, if preferred, a separate flexible
guard element
could be mounted on the support structure to provide a desired guard
configuration.
In accordance with the present invention each of the blades 16 comprises an
elongate blade element with a forward edge section SO (Figure 12) forming the
sharp
cutting edge 17, the tip of this edge being confined between opposed surface
portions
S 1, S2 referred to hereinafter as facets which taper towards the tip, and a
substantially
planar blade section S4 extending rearwardly from the forward edge section S0.
The
forward edge section 50 is canted upwardly towards the tip of the cutting edge
17 so that a
plane Pt bisecting the~~angle between the facets S 1, S2 at the tip of the
cutting edge is
1 S inclined at an angle a of around 20° to 2S° to the plane Pm
of the planar section S4. The
tip of the cutting edge 17 is can be positioned above the plane Pu of the
upper surface at
the planar section at a height of up to 0.25 mm, preferably a height in the
range of O.OS to
0.1 S mm. The inclination of the canted forward section SO is obtained by
bending a
planar blade element, blank 60 as illustrated in Figures 13A, 14A and 1 SA, by
pressing
the blade element blank 60 in a tooling assembly comprising a lower first
member 61 and
an upper second member 62. The lower tool member 61 defines a flat pressing
surface 64
which is extended forwardly by a ramp surface 6S. The upper tool member 62 has
a flat
pressing surface 66 which confronts the flat pressing surface 64 of the lower
member 61,
and the upper member 62 is movable relative to the lower member 61 for
pressing a blade
2S element blank 60 between the flat pressing surfaces. The lower member 61
has a rear
stop 67 for abutting the rear edge of a blade element blank 60 and a front
stop 68 located
on the ramp surface 65 for abutment with the tip of the blade edge. The stops
67, 68 may
be adjustable to suit the width of the blade element blanks 60 to be pressed,
and to adjust
the position of the bend to be produced with respect to the tip of the blade
edge 17, the
rear stop 67 being adjustable vertically and having an inclined front face for
this purpose.
An elongate planar blade element blank 60 with a finished cutting edge 17
along
its forward edge is introduced between the upper and lower tool members 61,
62, and is
brought to rest against the flat surface 64 of the lower member with its rear
edge in
contact with the rear stop 67 and the tip of the cutting edge 17 resting on
the ramp surface
6S and against the front stop 68 (Figures 13A and B). The blank is therefore
initially
tilted at a small angle to the flat surface 64. The upper member 62 is then
driven
downwardly bringing its pressing surface 66 into contact with the blank 60 and
causing
CA 02421150 2006-02-09
-10-
the portion of the blank 60 between the surfaces 61, 66 to be pressed flat
against the surface
64, and hence the forward edge section 50 of the blade to be pulled down
against the ramp
surface (Figs. 14A and B). In one example, the facets 51, 52 of the blade edge
extend back
from the edge tip by 0.3 mm, the blade thickness being 0.08 mm and, the front
edge of the
upper tool is aligned to contact the blade at a distance of 0.4 mm from the
tip. When the
upper member 62 is raised again, the bend produced in the blade blank 60 by
the pressing
operation recovers a little, but a permanent set in the blade blank 60 remains
so that the
forward edge section 50 is canted at the desired inclination with respect to
the planar blade
section 54 which extends rearwardly from it, and in a transition region
between the forward
section and planar section the lower surface has a convex curvature whereas
the upper surface
is non-convex, and more particularly concave. After completion of the blade
shaping
operation as described, a part of the blank including the canted forward edge
section and a
planar section several times wider, in the front to rear direction, than the
canted forwarded
edge section can be severed or snapped away from the blank to provide a
flexible blade strip
16 of the desired width. The blades produced in this manner can then be used
to produce
blade assemblies as incorporated in the safety razor of Figures 1 to 11.
The blades 16 of the invention, manufactured as described above, are
characterized by
a relatively high flexibility despite their being bent to raise the tip and
incline the cutting edge
17 to obtain an effective blade tangent angle with the planar section 54 of
the blade positioned
parallel to the tangent plane. The flexibility can be conveniently expressed
by the second
moment of area of the blade cross section since razor blade materials
currently used have
Young's moduli which differ little from one another. The second moment of area
I of a cross
sectional area with respect to an axis X lying in the plane of the cross
section is the sum of the
products obtained by multiplying each element of area dA by the square of its
distance y from
the axis X. Thus:
I= f y2dA
It will be appreciated that the second moment of area is dependent on the
shape and
size of the cross section.
CA 02421150 2006-02-09
-1 Oa-
The minimum second moment area is the second moment of area measured with
respect to that axis X which results in the smallest value for the second
moment of area Im;n
illustrated in Figures 16A and 16B are the cross sections, taken in planes
perpendicular to the
length of the cutting edges, of two blades 16 in accordance with the
invention. The blade 16
of Figure 16A is 0.08 mm thick and 0.80 mm wide with the planar section 54
being 0.40 mm
wide, and its minimum second moment of area I",~~ with respect to the axis X
is 0.322 x 10-4
mm4. The blade of Figure 16B has a thickness of 0.08 mm and a width of 0.80 mm
with the
planar section 54 being 0.10 mm wide, and in
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this case the minimum second moment of area is I",;" 0.145 X lO-4 mm4. As
described
above, the blades 16 of the invention are preferably positioned in a blade
unit 2 with the
mid planes of their planar sections 54 substantially parallel to the tangent
plane tangential
to the guard 26 and cap 24 surfaces of the blade unit, and the second moments
of area I
measured about axis parallel to these mid planes and intersecting the centres
of area for
the blades shown in Figures 16A and 16B have values of 0.443 x 10-4 mmø and
0.263 x
1 O-4 mm4, respectively. To achieve a desired level of blade flexibility in an
assembled
blade unit 2, it is also referred that the second moment of area with respect
to an axis
passing through the centre of area and parallel to the mid plane of the planar
section 54 is
not greater than 1.0 X 1 O-4 mm4, and more especially not greater than 0.5 X 1
O-4 mrn4. If
the blade 16 has a cross section which is constant along its length the I
values given above
will apply at every position along the length. However, there may be short
blade sections
where the I values are different, because the cross section is different, and
the desired
flexibility can be assured by the blade having a form such that the specified
I values apply
along at least most of the length of the blade.
Although the invention has been described above in connection with shaping a
blade having edge facets which, prior to bending of the blade, are symmetrical
with
respect to the centre plane of the blade, this is not essential and the blade
edge could have
an initial asymmetrical form including a so-called chisel edge in which the
tip lies at the
top surface plane of the blade.
Modifications to the specifically described embodiments are of course possible
without departing from the principles of the invention and will occur to those
spilled in
the art. It is to be understood, therefore, that the particular embodiments
described above
are given by way of non-limiting example only and it is the intention that the
scope of the
invention should be limited only by the claims which follow.
