TECHNICAL FIELD
[0001] The present invention relates to a grinding wheel, in particular a depressed-centre
grinding wheel made of at least a compacted abrasive material.
PRIOR ART
[0002] As is known, grinding wheels exist of a disc-shaped type having a depressed centre,
or centres that are flat, conical, semi-flexible, dimpled or not dimpled, exhibiting
an external diameter that is substantially comprised between 20 and 230 mm, especially
used on electric portable grinding machines, or highspeed compressed air grinding
machines (60-100 m/s peripheral velocity), also known as sanders, able to perform
burring and/or cutting operations, which are essentially constituted by an abrasive
mixture reinforced with armatures constituted by one or more textile meshes, one or
more annular metal elements, commonly known as washers or sleeves, which delimit the
fixing hole of the grinding wheel to the shaft of the grinding machine, and possibly
bear a paper label, or commonly used adherent rating plate stuck on one of the two
faces of the grinding wheel (in depressed-centre grinding wheels usually the convex
face).
[0003] The abrasive mixture is generally constituted by grains of abrasive material (silicon
carbide, light green, dark green, black, corundum, modified corundums with zirconium,
semi-friable, dark brown, white, pink, ruby, with ceramic, mono-crystalline, sol-gel
abrasives or sintered ceramics, or other like materials) having a predefined granulometry
(normally measured in meshes) which are mixed with resins, for example phenolic, liquid
and/or powder and possibly modified with epoxy resins, and/or others, possibly modified
with organic compounds and/or vegetal or synthetic compounds, and other types of polyamide
resins, etc., with the use of additives and fillers. The reinforcing meshes are normally
textiles with fibreglass threads, but might also be other types of fibre such as carbon,
Kevlar or another; the meshes have a height of about 1.5 m, are first immersed in
a solution of liquid resins and solvents, squeezed between pairs of rollers and dried
in special ovens internally of which the resin dries without polymerizing (polymerization
is completed later in the firing oven together with the firing of the grinding wheel).
[0004] The meshes, thus-impregnated with resin and dried, are used for the blanking (or
other method) of the mesh discs required for reinforcing the grinding wheels.
[0005] The meshes can be pre-glued to a paper sheet or a slim polymer material sheet, or
to labels.
[0006] The annular defining elements delimiting the attaching hole of the grinding wheel
are constituted by a circular annulus plate, or another shape such as square or polygonal,
from an internal hole of which a cylindrical or non-cylindrical appendage emerges;
the plate adheres to one of the two faces of the grinding wheel, while the hollow
appendage inserts in the hole of the grinding wheel, delimiting the internal wall
thereof.
[0007] The labels are made of paper or foil or another synthetic material and normally are
circular-annulus shaped (though they could be of any other shape), and can occupy
either the whole face of the grinding wheel or a limited area of the face on which
the identifying and information data of the grinding wheel are reported.
[0008] Flat grinding wheels (in particular reference is made to a cutting disc and not a
burring disc) exhibit the whole circular annulus projecting externally of the annular
elements which delimit the attaching hole potentially active in the cutting operations,
and which can therefore exhibit a depth that is substantially equal to the radial
thickness of the circular annulus (except the part of the blocking flanges which block
the grinding wheel on the rotary shaft of the grinding machine, which normally have
an external diameter of 42 mm), and/or the grinding wheel can be used for a large
number of cutting operations.
[0009] Flat grinding wheels, however, exhibit a high flexibility and therefore low precision
in the cutting operations and a low resistance to inappropriate uses and stresses.
Further, as they are not very rigid, they can be subject to flutter during rotation,
which makes precision cuts difficult to carry out.
[0010] To obviate this drawback, grinding wheels with a central depression have been realised,
in which at the central zone proximal to the attaching hole a cupola is formed, as
shown in figure 1, which illustrates a depressed-centre cutting grinding wheel of
known type.
[0011] Depressed-centre grinding wheels have the advantage, with respect to flat grinding
wheels, of having a three-dimensional shape and therefore of providing a greater rigidity,
stability and precision.
[0012] The main disadvantage of the depressed-centre grinding wheels is that the cupola,
or cap, which generally extends over a conspicuous radial tract of the grinding wheel
(in practice, it normally exhibits an external diameter of about 65 mm, so that the
attaching flange, which has a standard external diameter of 42 mm, of the grinding
machine can be accommodated in the concave part of the cupola) and therefore does
not enable the cut to be as deep as with a flat grinding wheel and/or does not enable
making a same number of total cuts as with a flat grinding wheel.
[0013] This forces the user to replace depressed-centre grinding wheels frequently as they
are no longer fit for use, with a subsequent effect on costs for the users. Flat discs
on the other hand can be used for a longer time as they do not have the cupola interfering
with pushing the cut more deeply.
[0014] This drawback is more severely felt in cutting grinding wheels which exhibit relatively
small external diameters, as all depressed-centre grinding wheels have a cupola with
an external diameter that is greater than 60 mm and therefore this cupola is more
"unwieldy" in discs having a smaller diameter. An aim of the present invention is
to obviate the above-mentioned drawbacks in the prior art, with a solution that is
simple, rational and relatively inexpensive.
[0015] The aims are attained by the characteristics of the invention reported in the independent
claim. The dependent claims delineate preferred and/or particularly advantageous aspects
of the invention.
DESCRIPTION OF THE INVENTION
[0016] The invention relates in particular to a depressed-centre grinding wheel comprising
at least a circular cutting annulus made of a substantially planar compacted abrasive
material lying on a plane that is perpendicular to the rotation axis of the grinding
wheel and a concave central portion provided with a central through-hole for attachment.
[0017] In the invention, the internal diameter of the circular annulus coinciding with the
external diameter of the concave portion is not greater than 55 mm.
[0018] With this solution, given a same external radius of the grinding wheel, the wheel
exhibits a larger active cutting portion and therefore a greater depth of cut and/or
it can be used to perform a greater number of cuts.
[0019] In an aspect of the invention, the external diameter of the circular annulus is substantially
comprised between 100 and 230 mm, preferably between 100 and 125 mm.
[0020] In these grinding wheel formats, the increase in radial thickness of the circular
annulus, with a subsequent reduction of the external diameter of the circular cap
part, enables a significant increase in the active cutting portion of small-dimension
grinding wheels, which among other things are the most widely-used because they are
easier to handle and cost less.
[0021] The concave portion advantageously comprises a first central section that is substantially
planar and a concave cap, in which the inclination of the cap wall is substantially
comprised between 40° e 70°.
[0022] A further aspect of the invention relates to a method for realizing a depressed-centre
grinding wheel, according to any one of the preceding claims, which comprises steps
of:
- raw-moulding a substantially planar semi-finished grinding wheel;
- deforming the central portion of the semi-finished grinding wheel, so as to define
a depressed concave portion having an external diameter that is substantially identical
to an internal diameter of the external planar circular annulus of the undeformed
semi-finished grinding wheel, where the external diameter of the concave portion is
not greater than 55 mm.
[0023] Furthermore, following the deformation, the deformed semi-finished grinding wheel
is subjected to a firing step, for example at appropriate firing temperatures which
enable polymerization of the binding resin that binds the abrasive mixture together.
[0024] A further aspect of the invention relates to a grinding system which comprises a
grinding machine having a rotary shaft which can be inserted into the grinding wheel,
as described above, (i.e. into the through-hole) and a coupling flange suitable to
be inserted into the concave portion of the grinding wheel and having an external
diameter substantially equal to 42 mm.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] Further characteristics and advantages of the invention will emerge from a reading
of the description that follows, provided by way of non-limiting example, with the
aid of the figures of the accompanying tables.
Figure 1 is a section view of a depressed-centre grinding wheel of the prior art.
Figure 2 is a section view of a depressed-centre grinding wheel according to the present
invention.
BEST WAY OF CARRYING OUT THE INVENTION
[0026] With particular reference to the figures, reference numeral 10 denotes in its entirety
a depressed-centre cutting wheel.
[0027] The grinding wheel 10 comprises an abrasive disc-shaped element 11 which is attachable,
substantially coaxially, to the free end of a rotating shaft of a grinding machine.
[0028] The disc-shaped element 11 comprises at least a substantially central through-hole
12 and a first convex face 13 defining the back of the disc-shaped element.
[0029] The disc-shaped element 11 further comprises a second face 14 opposite the first
face 13.
[0030] In particular, the disc-shaped element 11 is realized by at least a layer of an abrasive
mixture that is compacted and stably bound by a binder resin.
[0031] In practice, the disc-shaped element 11 is obtained by pressing a mixture of a loose
powder of abrasive material, for example abrasive material such as natural corundum,
sand, recycled artificial corundum or the like, abrasive sol-gel or sintered ceramic,
corundum at zirconium, or other, and mixed with a suitable binder, for example resin-based,
such as phenolic, liquid and/or powder resins possibly modified with phenoxy epoxy
resins and/or others, modified with organic and/or vegetal or synthetic compounds,
and other types of poly-imide resins, etc...., and/or with additives and fillers.
[0032] The disc-shaped element 11 is constituted by an external peripheral circular annulus
15 and a disc-shaped internal cap 16 (or hub), both coaxial to the through-hole 12.
[0033] In practice, the disc-shaped cap 16, which comprises the through-hole 12, extends
radially externally from the circular annulus 15.
[0034] The circular annulus 15 lies substantially in a perpendicular plane to the axis of
the grinding wheel 10 (axis of the through-hole 12).
[0035] The circular annulus 15 of the abrasive mixture is the active portion of the grinding
wheel 10, i.e. the portion thereof that is generally used for cutting.
[0036] The circular annulus 15 has an external diameter De, variable during use, which defines
the external diameter of the grinding wheel 10 and an internal diameter Di, fixed,
which defines the external diameter of the circular cap 16. The circular cap 16 has
an external diameter substantially equal to the internal diameter Di of the circular
annulus 15 and an internal diameter substantially equal to the diameter of the through-hole
12.
[0037] In particular, for the purposes of the present invention, the external diameter of
the circular cap 16, which substantially coincides with the internal diameter Di of
the circular annulus 15, is substantially not greater than 60 mm (for example, less
than 57.5 mm).
[0038] The external diameter of the circular cap 16 is preferably substantially between
45 and 55 mm, so as to allow insertion of a coupling flange in its concavity, able
to fasten the grinding wheel 10 to the rotary shaft of the grinding machine, which
generally exhibits a diameter of about 42 mm and, at the same time, does not adversely
affect either the radial thickness of the active portion in the cutting action (circular
annulus 15) of the grinding wheel 10 or the stability and resistance of the grinding
wheel.
[0039] Advantageously, the external diameter of the circular cap 16 is substantially 55
mm.
[0040] This dimension has been found to be optimal for the aims set out above. The external
diameter (De) of the circular annulus 15 is between 100 and 230 mm.
[0041] In particular, the wheels 10 have standardized dimensions and are classified according
to the external diameter (De) of the grinding wheel 10, which exhibit the following
rated dimensions:
[0042] De = 115 mm, 125 mm, 150 mm, 180 mm, 200 mm and 230 mm.
[0043] The external diameter (De) of the circular annulus 15 is preferably between 100 and
155 mm and preferably between 115 and 150 mm.
[0044] In these formats (where the external diameter of the grinding wheel 10 is between
100-155 mm, and in particular the standard formats have external diameters of 115
mm and 125 mm) of grinding wheels 10, where the external diameter is smaller, the
peculiarity of the invention (i.e. the fact that the external diameter of the circular
cap 16 does not exceed 60 mm) is decidedly more advantageous, as it provides a percentage
active portion greater than the grinders (of the same diameter) of the traditional
type (as shown in figure 1), given an identical sturdiness of the grinding wheel 10.
[0045] In practice, even in these formats, in which the external diameter De is 115, 125
and 150 mm) the ratio between the internal diameter Di and the external diameter De
of the circular annulus 15 is substantially less than ½.
[0046] The circular cap 16 includes a first internal portion 161 that is substantially planar,
parallel and coaxial to the circular annulus 15, and a cap wall 162 (coaxial with
the circular annulus 15), connecting the first internal portion 161 and the circular
annulus 15.
[0047] Advantageously, the cap wall 162 of the concave circular cap 16 has an inclination
substantially comprised between 40 and 70°, preferably 60°.
[0048] In practice, the cap wall 162 radially occupies a radial portion substantially identical
to the distance of the rise that it defines (i.e. the axial distance between the first
internal portion 161 and the circular annulus 15, which are substantially parallel).
[0049] The abrasive mixture has a granulometry of substantially between 120 and 12 mesh
(however abrasive mixtures of a greater or smaller particle size than the above-described
range recorded can be used, according to requirements).
[0050] The layer of abrasive mixture can have a thickness of around 3 mm or less, though
this could be greater or smaller than 3 mm, depending on requirements.
[0051] The grinding wheel 10 comprises, for example, at least a reinforcement mesh 17, schematically
illustrated in figure 2, incorporated in the layer of abrasive mixture.
[0052] The reinforcing mesh 17 is for example disc-shaped and substantially coplanar to
at least the first face 14 of the disc-shaped element 11.
[0053] The grinding wheel 10 can have a plurality of reinforcing meshes 17 and a plurality
of layers of abrasive material of different nature, according to needs. By appropriately
selecting the ratio between the grain size of the abrasive mixture used, which depends
on the intended use of the grinding wheel, and the opening of the meshes of/and reinforcing
mesh/es 17, the penetration of the mixture through the mesh thereof can be advantageously
optimised.
[0054] A label 18 can be placed on the first face 14, i.e. the back of the wheel 10, illustrated
schematically in figure 2; the label can be paper or foil or other like material.
[0055] The abrasive mixtures of the layer of abrasive mixture, as well as the reinforcing
mesh 17 and the label 18, are of a type widely known in the industry.
[0056] The label 18 can otherwise be applied once the grinding wheel 10 has been fired/polymerised,
by gluing or applying self-adhesive labels, by screening, pad printing, spraying or
another form of stencil painting.
[0057] Lastly, the grinding wheel 10 includes one or more metal annular elements, commonly
known as washers or sleeves 19, which delimit the attachment hole of the grinding
wheel 10 to the pivot of the grinding machine.
[0058] The washer 19 is fixed to the first face 13 (or label) of the disc-shaped element
11 and, for example, extends radially substantially over the entire disc-shaped cap
16 exhibiting the concave shape.
[0059] The washer 19 comprises a hollow central shank 190 that inserts substantially snugly
in the through-hole 12 and which exhibits an axial thickness that is substantially
identical to (or slightly smaller than) the axial thickness of the grinding wheel
10 (the layer of abrasive mixture that constitutes the disc-shaped element 11).
[0060] The method for realising a grinding wheel 10 as described above comprises a step
of raw-moulding a semi-finished grinding wheel that is substantially planar, i.e.
so that the not-yet-concave central area that will form the disc-shaped cap 16 is
substantially coplanar to the disc-shaped circular annulus 15.
[0061] The substantially planar raw moulding grinding wheel 10 is realized using a mould
able to press the abrasive mixture together with the binder resin and give it the
desired disc-shape.
[0062] Once the semi-finished grinding wheel has been formed the forming method continues
by deforming the central portion of the grinding wheel, so as to define the disc-shaped
cap 16 having an external diameter substantially equal to the internal diameter Di
of the circular annulus 15, wherein the external diameter of the disc-shaped cap is
substantially less than 60 mm. The step of deforming the semi-finished grinding wheel
is advantageously done by inserting the grinding wheel into a mould, the bottom die
of which (and/or the punch) is shaped such as to impart the desired shape to the grinding
wheel.
[0063] Finally, the grinding wheel 10 thus-formed (i.e. deformed) is subjected to a heat
treatment of firing, for example in special polymerization oven, where the polymerization
is completed of the binder resin that solidifies and stably retains the abrasive mixture
that constitutes the disc-shaped element 11.
[0064] In practice, the grinding wheel 10 is subjected to a thermal cycle which involves
the insertion thereof in an oven at a temperature of substantially between 120° and
220° C for a time substantially comprised between 1 and 50 hours, or it can be fired
in situ in the heated mould suitable for forming of the 10 depressed-centre grinding
wheel.
[0065] The invention as it is conceived is susceptible to numerous modifications and variants,
all falling within the scope of the inventive concept.
[0066] Further, all the details can be replaced by other technically-equivalent elements.
[0067] In practice, the materials used, as well as the contingent shapes and dimensions,
can be any according to requirements, without forsaking the scope of protection of
the following claims.
1. A depressed-centre grinding wheel (10) comprising at least a circular cutting annulus
(15) made of a substantially planar compacted abrasive material lying on a plane that
is perpendicular to the rotation axis of the grinding wheel (10) and a concave central
portion (16) provided with a central through-hole (12) for attachment, characterised in that the external diameter (De) of the circular annulus (15) is comprised between 100
and 125 mm and the internal diameter (Di) of the circular annulus (15), coinciding
with the external diameter of the concave portion (16), is not greater than 55 mm.
2. The grinding wheel (10) of claim 1, wherein the external diameter of the concave portion
(16) is substantially comprised between 45 and 55 mm.
3. The grinding wheel (10) of claim 2, wherein the external diameter of the concave portion
is substantially 55 mm.
4. The grinding wheel (10) of any one of the preceding claims, characterised in that it comprises at least a reinforcing mesh sunk into the abrasive material.
5. The grinding wheel (10) of any one of the preceding claims, wherein the concave portion
(16) comprises a substantially planar first central section (161) and a concave cap
wall (162), wherein an inclination of the cap wall (162) is substantially comprised
between 40° and 70°.
6. The grinding wheel (10) of claim 5, wherein the inclination of the cap wall (162)
is substantially 60°.
7. Grinding system which comprises a grinding machine having a rotary shaft which inserted
into the grinding wheel (10), according to any of the preceding claims, and a coupling
flange suitable to be inserted into the concave portion (16) of the grinding wheel
(10) and having an external diameter substantially equal to 42 mm.
8. A method for realising a depressed-centre grinding wheel (10), according to any one
of the preceding claims from 1 to 6, which comprises steps of:
- raw-moulding a substantially planar semi-finished grinding wheel;
- deforming the central portion of the semi-finished grinding wheel, so as to define
a depressed concave portion (16) having an external diameter that is substantially
identical to an internal diameter (Di) of the external planar circular annulus (15)
of the undeformed semi-finished grinding wheel, where the external diameter of the
concave portion (16) is not greater than 55 mm.
9. The method of claim 8, characterised in that it comprises a step of firing the deformed semi-finished grinding wheel.