Note: Descriptions are shown in the official language in which they were submitted.
I
This invention relaxes to razor blades and is
particularly concerned with the shaping of the cutting edge.
The invention and prior art will be considered in
conjunction with the accompanying drawings in which:
Figure 1 is a greatly magnified view of a blade tip
of typical, or average shape;
Figure 2 is a tip shape diagram illustrating the
principle of "tlp-width~ measurement;
Figure 3 is a highly diagrammatic representation of
lo the cutting of a facial hair;
Figures 4 to 7 are cross-sections of various
respective blades currently marketed by a variety of
manufacturers;
Figure 8 is a view, like Figures 4 to 7, of the tip
shapes described in British Patent Specification 1465697.
The invention resides broadly in a razor blade having
a cutting edge the cross-sectional shape of which within the
first 40 em measured back from the extreme edge is defined by
the formula w = awn wherein d is the distance from the tip in
em; w = the tip width (or thiclcness) in em at a given distance
d; a is a factor of proportionality of about 0.8 and n it an
exponent having a value less than 0.75, and wherein the
included angle between the tip facets in the region from 40 em
to 100 em from the extreme edge is within the range 7- 14 and
preferably 9 to 11 1/2.
In the case of a stainless steel blade n is in the
range 0.65 to 0.75 and a is in the range 0.71-0.92.
It has been found that blades having these tip
characteristics provide improved shaving on comparative shave
testing, but are sufficiently strong to give a reasonable
useful life.
In order to convey a proper understanding of the
nature of the present invention, it is convenient to describe
and illustrate the background prior art in some detail.
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Cutting edges on razor blades are sharpened by
grinding a succession of pairs of facets (usually three) of
different included angles onto a strip of steel by means of
suitably arranged abrasive wheels. The cross-section through
such an edge is illustrated in Figure 1 with typical values for
dimensions and angles shown, and its customarily described as a
"3-facet edge. While the final pair of facets is being
ground, (this stage is usually called "honing"), strip
deflection in the sharpening machine together with the
mechanical interaction between the steel and the abrasive
particles of the wheel, produces final facets which are usually
not planar but slightly convex. The curvature is a function of
the type of steel and abrasive wheel used, as well as the
sharpening machine setting parameters. Because of this
convexity of the final facets, the blade tip cross-section in
this region is customarily referred to as "Gothic arched. The
curvature prohibits precise geometrical definition of this part
of the blade tip by means of a single parameter so that it its
usual to characterize the shape by defining tip thicknesses at
various distances back from the edge. An alternative method is
to ascribe a mathematical equation
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to fit the form of each half of the facet cross-~ection.
These method Are illustrated in ~igllre 2.
During use, a nor blade it held in the razor
it angle of approximately 25, end with the edge in
contact with the Yin, it it moved over the face Jo that
when the edge encounters a beard hair, it enter and
severs it by progre~i~e penetration, tided by a wedging
action. It it believed that the cut portion of the heir
(which it on overage bout 100~um diameter remain
I pressed in contact with the blade facet remote from
the facial Ruin surface for a penetration up to only
bout half the hair diameter. Beyond this, the hair
con bend and contract nay from the blade to relive
the wedging force. The refastens to penetration
through reaction between heir and blade facet therefore
occurs only o'er about the first 50 em of the blade tip
back from the edge and the geometry of the blade tip in
this region lo regarded A being the ought important
from the cutting point of view. This it ill~tr~ted it
Figure 3.
It in slur that reduction in the included
angle of the facet killed correspondingly reduce the
resistance to continued penetration of the blade top
into the hair. However, if the included angle eureka
reduced too much, the strength of the blade tip Gould
be inadequQt~ to withstand the resultant bending force
on the edge during thy cutting prows and the tip would
deform pl~tically or fracture in a brittle phony,
depending on the mechanical properties Or the Motorola
from which it it made) and Jo sustain permanent damage,
which would impair it subsequent cutting p~rformAnceO
i.e. the edge Gould become 'blunt' or 'Doyle
In order to Dunn nuitRble ape or thy
blade tip which it Utah triune enough to prevent such
bending induced dsmag~l on Tahitian ho been nude of
the magnitude of the bending Starr imposed during
the everyone of hair. From these values and
knowledge of the yield trench of the steel from which
the blade is made, minimum dimension con be calculated
for the tip section. The trusses imposed during
cutting were amid to Roy from the ~isoo-el~stic
flow of saturated hair material past the blade tip.
lye currently produced have tip geometries
with some dominion which ore below these minimum
value and are known to become dulled by edge bendirlg
during the normal shaving life (which it on average,
approximately 10 days for a blade made from conventional
razor blade stainless steel).
We have now found that by careful control of
the tip geometry in specific regions 0 - I em from the
edge the overall cro~s-~ection con be reduced Jo that
cutting performance end shaving satisfaction are
improved, while retaining Dakota strength to resist
edge bending damage nod so maintain acceptably durability
The tip shape of awry ~anuf~cturera blades
currently on the market are one in Figures 4 to 7 7
sod Figure 8 illustrates b-lade tip forts as describe I
in British Potent 1465967.
These known blade tip shape I compared
with the preferred blade tip shape of the prevent
in mention in figures 10 and 1 ox .
In one for of the prevent invention, the blrlde
tip cro~-section it first narrowed by grinding the
three facet to smaller included ankles thin those
typified in Figure 1. This produce blade tip whose
cross-~ection it generally narrower throughout end,
importantly, in the 0 -40 em doughtiness back from the ode
which it us particular introit during her c~tti~go
Such an edge it too weak to with tend Tracy during
awing end must be further modified. This to achieved
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by adding what amount to a fourth sharpening age
It is carried out using rotting interlocking disc or
purl of leather or synthetic leather, (usually
called "strops") with abrasive material nodded to their
5 peripheries. The sharpened blued pi between the
fftropY, which polish the facet, removing a small amount
of steel from their surface, and Jo changing the
'Gothic arch" dimension. Thief stage it culled
"abrasive stropping". Buick of the flexibility of
10 the strop leather, allowing it to conform omit to
the sharpened blade tip, abrasive stropping ionizes the
curvature of the final facet, clove to the edge, while
having let effect on the facet shape further back.
It has been found tint when blade ens
15 sharpened with suitably reduced facet included angle
followed by nun appropriate abhorrer stropping treatment,
B the tip shape it changed so that the width close
to two edge become larger than those on conventionally
~hnrpened edge, utile the widths further away
20 from the edge remain smaller thin those on conventionally
sharpened edge. This royalty in the blade tip close to
the edge being stronger than normal, A tint it Can
bettor Wright the bending actresses imposed on it during
hair cumin Chile the reduced auction further back from
25 the edge, present lets resistance to penetration during
bar cutting, so facilitating the Utahan process.
The ultimate tip radius of the edge should be
conventional, with an average value of less than AYE
and preferably less than AYE as stated, for example in
Patent Specification 1,378~550 ISSUE. 3,761,374), that is,
withinthenormal range for conventionally sharpened edges.
Blades in accordance with the invention hut
been wound to haze superior shaving performance when
compared with conventional Lydia on a standard awing
text.
~3L2~
One form of blonde in accordance with the
invention and the manner in which it it formed are
described in detail below by Jay of example, with
reference to ire 9, 10 end 11, in which:
Figure 9 it diagrammatic illustration of n
blade zip stropping Operation
figures 10 and BOA are representation of blade
tip forms in accordance with the invention, compared with
the known blade tip forms teen in Figures 4 to 8. Fig. lo
is a detail from Fig. 10 on a larger scale; and
Figure 11 it a graph of width at
different distances 'd' plotted on logarithmic scale.
Stainless steel razor blade trip, of nominal
composition 13~ Or, owe C, Winnie hardened and tempered
in nccordanse with conventional practice, and sharpened
- by grinding and honing to produce edge of three facet
configuration, a illustrated in Figure I, but with
included Nile smaller than those conventionally
manu~nctured. The blued were pled between rotting
troupe of artirici~l leather, owe surface contained
fine Alumina nbra~i~e, in the anywhere of con~ntional
Brie stropping, where the angle jet on the troupe
(which it the included angle between the tangent to the
strops at their point of inter~ectiDn, shown it
25 ire 9) way in the range 30-34. The facet were
provided with metallic c08ting of an alloy of cry
end platinum (applied in accordance with Potent Us
3,829,969~ with a superimposed costing of fluorocarbon
Metro ugh go described in British Potent owe ooze j
3 The prows of grinding, honing end stropping
are well known in the art, but it Jill be understood
that lo ronYention~1 ethos could be employed for
sharpening the tip, e.g. during the strip button
nppropri~tely ehap2d die or roller, or by electrolytic
or shekel dissolution hung or by ion bombardment
` plan.
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The blade tip cros~-secti~n~ were measured
using optical interf~rom~try. A blue it planed under
thy objective lens of a m~tnllur~ic~l microscopy fitted
with n Michelson type interferometer and viewed it a
ma~nificntion of about 1000X. The interf~romcter is
adjusted to produce fringes which are oriented it right
Nile to the edge of the blade. The blade is tilted
at an appropriate angle Jo that the fringes are dip-
placed to reveal the topography of the blade facets.
The fringe spacing is adjusted so that fringe displace-
Monet can be readily measured sty various distances back
from the edge. Knowing the angle of tilt, the tip pow
is calculated from the sum of these fringe displacements,
measured it corresponding positions or. etch wide of the
I blade.
The results of these measurements ore shown in
Fire 10, in which the spread of profiles of the
preferred blaze tips over the first 40 em ore shown by
solid shaded bands, end the spread of profiles of known
20 blued is indicated by the crushed bud
B In this specific example, the widths w at
distances d from theextremeedgewereas set out below:-
d sum) w sum) 3
0.25 .20 - .30
0.5 34 - 5
-75 53 - 72
1.0 .71 .92
2.0 1.17 - 1-37
owe 1.86 - 2.16
owe 3-05 - 3.52
20.0 6.12 - 6.85
30.0 8.43 - 9.52
40.0 10.73 -12.~1
The gsometr~ of this profile was replated on
a graph using }~r~hmi~ scales fur zip thickness
I
as a function of distance from the edge and the resultant plot
is shown in Figure 11, from which it is seen that a straight
line can be fitted to the plotted points.
From the slope and intercept of the straight line,
the tip shape can be defined by the equation w = awn in which a
is a factor of proportionality of about 0.8 and n an exponent
having a value of not more than 0.75, and more specifically
within the range 0.65 - 0.75.
The known blades measured were found to have best fit
straight lines with exponents (or gradients) within the range
0.76 -- 1Ø
The smaller gradient is a primary characteristic of
the present invention and results in the fact that the blade
tip of the present invention, compared with those of the prior
art, is relatively thick and strongly arched close to the
extreme edge, but relatively thin over the remainder of the
typo
The included facet angles in the region ~10 100 Lam
from the tip are in the range 9 to 11 1/2 but making due
allowance for manufacturing tolerances could be in the range 7
to 12 or even 7 to 14.
It must be appreciated that the tip shapes described
above are for stainless steel blades and could be made
substantially thinner for harder blade materials such as
sapphire, titanium carbide or diamond.
To produce an equivalent tip shape from a material
harder than stainless steel, we reduce the corresponding tip
widths in inverse proportion to the square root of the yield
strength of the harder material in comparison with stainless
steel. In the case of diamond, for example, the tip widths
would be approximately 40% of those calculated for stainless
steel.
Furthermore, the tip region of a stainless steel
blade may be coated with a material harder than stainless steel
and having a higher yield strength. In such a case the chord
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widths given by the basic equation are reduced by adopting the
modified formula: w _ on awn in which m is the ratio of the
yield strength of the coating material to that of stainless
steel.
Furthermore, in order to ensure the integrity of the
steel substrate, the value for w must also satisfy the equation
we' (wooden, where h is the thickness of the coating.
It will be understood by those skilled in the art
that the blade tips may, in each case, be coated with materials
such as pile which further enhance the cutting action.
The thicknesses of such coatings are, of course ignored for the
purposes of calculating the tip chord widths
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