U.S. patent number 3,928,949 [Application Number 05/465,802] was granted by the patent office on 1975-12-30 for hollow body grinding materials.
This patent grant is currently assigned to Norddeutsche Schleifmittel-Industrie Christiansen & Co.. Invention is credited to Eckhard Wagner.
United States Patent |
3,928,949 |
Wagner |
December 30, 1975 |
**Please see images for:
( Certificate of Correction ) ** |
Hollow body grinding materials
Abstract
A grinding material comprising a multiplicity of hollow bodies
whose walls are formed of abrasive grains and a bonding means and
are arranged to be stable in resistance to grinding forces.
Inventors: |
Wagner; Eckhard (Elmshorn,
DT) |
Assignee: |
Norddeutsche
Schleifmittel-Industrie Christiansen & Co. (Hamburg,
DT)
|
Family
ID: |
25765868 |
Appl.
No.: |
05/465,802 |
Filed: |
May 1, 1974 |
Foreign Application Priority Data
|
|
|
|
|
Sep 26, 1973 [DT] |
|
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2348338 |
Oct 5, 1973 [DT] |
|
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2350139 |
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Current U.S.
Class: |
451/533;
51/296 |
Current CPC
Class: |
B24D
11/00 (20130101); C09K 3/1436 (20130101); B24D
7/00 (20130101) |
Current International
Class: |
B24D
11/00 (20060101); B24D 7/00 (20060101); C09K
3/14 (20060101); B24D 003/34 () |
Field of
Search: |
;51/164.5,358,394,395,397,398,399,401,402,204,295,296 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Smith; Al Lawrence
Assistant Examiner: Godici; Nicholas P.
Claims
What we claim is:
1. A grinding material comprising a multiplicity of hollow bodies
whose walls contain abrasive grains in an amount of more than 50%
of the wall surface and a bonding means selected from the group
consisting of a synthetic resin, a ceramic binder and a metallic
binder and are arranged to be stable in resistance to grinding
forces, the mean diameter of said hollow bodies are measured
parallel to a grinding surface lies between 0.1 and 8 mm and is not
more than 50 times the mean grain diameter and the abrasive grains
are contained substantially within the walls of the hollow
bodies.
2. A grinding material as claimed in claim 1 wherein the thickness
of the walls of the hollow bodies is on average less than 10 times
the mean abrasive grain diameter.
3. A grinding material of claim 2 wherein the abrasive grains are
disposed in the walls of the hollow bodies substantially in one
layer.
4. A grinding material as claim in claim 1 wherein the abrasive
grains are contained in the walls of the hollow bodies in a
tightly-packed arrangement.
5. The grinding material of claim 4 wherein the bonding means
content of the walls by weight is not greater than twice their
grain content.
6. A grinding material as claim in claim 1 wherein the hollow body
walls each surround a carrier particle, whose mechanical strength
of resistance to the stresses occurring during grinding is
substantially less than that of the hollow body walls.
7. A grinding material as claimed in claim 1 wherein all the
constituents of the hollow bodies not participating in the
formation of the walls are more easily removable under grinding
conditions than the walls.
8. A grinding material as claimed in claim 7 wherein the frictional
stress produced at a surface to be ground by the constituents of
the hollow bodies not participating in the formation of the walls
is less than a fifth of the frictional stress caused by the
walls.
9. A grinding material as claimed in claim 6 wherein the carrier
particle is thin-walled.
10. A grinding material as claimed in claim 7 wherein at least some
of the hollow bodies contain a substance which is fluid under
grinding conditions.
11. A grinding material as claimed in claim 7 wherein at least some
of the hollow bodies contain a substance which is in powder form
under grinding conditions.
12. A grinding material as claimed in claim 1 wherein the mean
overall diameter of the hollow bodies measured parallel to a
grinding surface is about three to 50 times as large as the mean
grain diameter.
13. A grinding tool made from the grinding material as claim in
claim 1 wherein the hollow bodies are bonded together.
14. A grinding tool as claimed in claim 13 wherein the hollow
bodies are bonded to a base material to form a grinding or abrasive
belt.
15. A grinding tool as claimed in claim 13 wherein the hollow
bodies are bonded together to form a grinding body.
16. The grinding material of claim 1 wherein said hollow bodies are
of a copolymer having abrasive grains of corundum.
17. A process for the production of a grinding material comprising
expanding polymeric material to form a multiplicity of hollow
bodies having a diameter of 0.1-8.0 mm, coating said hollow bodies
with abrasive grains so that said abrasive grains are substantially
in one layer, and depositing said coated material onto a substrate
having a layer of bonding material.
18. A process according to claim 17 wherein said substrate is a
belt.
19. A process according to claim 17 wherein said hollow bodies
contain a grinding adjuvant.
20. A process as claimed in claim 17 wherein the abrasive grains
are bonded by means of an age-hardening bonding material to the
surfaces of hollow thermoplastic beads and the hollow bodies
thereby formed are next heated to a temperature at which the hollow
bead melts but the bonding material remains sufficiently firm.
Description
This invention relates to grinding materials in which a
multiplicity of abrasive grains are held in a bonding material to
form a unit.
Such a grinding material may be formed as a tool having an abrasive
surface (that is a surface of the grinding tool for acting on a
workpiece).
Preferably, apart from the abrasive grains, the abrasive surface
has pores to receive ground-off waste material and containing if
required a coolant or grinding adjuvant. The porosity within the
body of a grinding tool can be increased by adding pore-forming
substances. For example particles can be incorporated which
volatilise during the processing of the tool U.S. Pat. No.
1,956,905, third paragraph). Since firm mutual bonding of the
abrasive grains must be maintained, such pore-forming substances
may however only be added in small quantities. The pore volume
attainable is therefore restricted and it must be anticipated that
there will be found considerable, statistically differing distances
between each abrasive grain and the nearest pore for accommodating
waste. It is also known to add hollow beads, filled with a grinding
adjuvant, to the bonding material of a grinding tool (Swiss Pat.
No. 511,678). If the mechanical strength of the grinding tool is
not to be endangered, the hollow beads must however be very small,
i.e. substantially smaller than the average dimensions of the
abrasive grains. They are not suitable therefore for forming pores
of any substantial volume. Also, it is known to produce a grinding
tool from a cellular bonding framework, to whose walls the abrasive
grains are bonded firmly (Swiss Pat. No. 257,484). Such a
construction is only useful if small quantities of an expensive
granular material, for example diamond, can be obtained in a
substantially larger quantity of the bonding framework. For normal
grinding purposes such a grinding body could not be considered.
According to this invention there is provided a grinding material
comprising a multiplicity of hollow bodies whose walls are formed
of abrasive grains and a bonding means and are arranged to be
stable in resistance to grinding forces.
Preferably all the abrasive grains forming the grinding material
are disposed in the hollow body walls.
Thus there is provided a grinding material having a large pore
volume without the mechanical strength of a grinding tool made from
the material being endangered by a diminution of the grain
coherency. The pore volume is not restricted by the size of the
abrasive grains, since even grinding materials with fine grains can
be provided with pores of large individual volume. The size of the
individual pores and their distance from the individual abrasive
grains can be made so as to vary statistically only within narrow
limits, and the individual pores can lie in a predetermined size
range, their distances from the individual grains being equal
within narrow limits. In this way, a grinding material can be
obtained whose abrasive properties, insofar as they are determined
by the grain size and the pore volume, are predetermined over wide
ranges, as is the case with known grinding materials.
The grinding material can be utilised in different forms. For
example, it may be used in the form of ballast of the hollow bodies
in polishing drums or rumbles. Here, the advantage is obtained that
a given quantity of ballast, which in general is determined by the
size and shape of the polishing drum, and also by the size and
nature of the articles to be polished, can be provided by a
comparatively small quantity of the abrasive grains. If the hollow
bodies are gas-filled, they may also have a very low specific
gravity and the ballast may be correspondingly looser, which is
favourable for a uniform abrasive action. It will be noticed that
the uniform grinding action is still retained when the hollow
bodies break up after a certain time, and act not by their outer
surfaces but by the broken surfaces of their constituent
pieces.
In using the hollow bodies for the production of grinding tools
(particularly grinding discs) it is a considerable advantage that
the desired porosity of the grinding body is attained in an
accurately predeterminable manner, and also that the pores are
directly adjacent to the abrasive grains. Each hollow body begins
its grinding action with only its outer surface. As wear of the
grinding surface (the abrasively-acting surface of the grinding
tool) develops, the hollow bodies breaks open so that their
cavities open in the manner of craters. The effective abrasive
surface of each hollow body is then formed by the crater rim which
is directly adjacent to the crater which can receive the ground-off
waste and any breaking-away grains. This advantage makes itself
felt particularly forcefully with those present-day grinding
processes in which lubricating substances are present which may
emanate from the bonding material of the grinding material or from
the workpiece, particularly when processing soft metals or using
grinding tools bonded by synthetic resin, by rubber or
metallically. However, the hollow bodies are also suitable for
ceramic bonding. In any case, the effective cutting power is
improved. A further advantage lies in that the inter-connecting of
the individual abrasive grains in the hollow body walls is not
deleteriously affected by the pores, and the hollow body walls
therefore form a framework which has a higher mechanical strength
than known grinding tools with comparable total pore volume.
Grinding discs employing the hollow bodies can therefore be
operated at higher peripheral speeds. The cutting ratio (the ratio
of the grinding material wear to the ground-off material) is
thereby improved. Furthermore the size of the volume of the
individual pores is not restricted by the grain size, as the hollow
body walls can be assembled as required from small or large grains.
The pores can be filled with an auxiliary grinding material.
It is already known to construct grinding tools from hollow balls
of grinding material (U.S. Pat. No. 2,986,455), but here hollow
balls are melted exclusively and in one piece from a hard material,
particularly corundum. Such hollow balls can be initially broken up
only under the action of powerful forces, which forces can vary
widely with different individual balls. The uncontrolled jagged
fracture rim behaves in a manner which, from the point of view of
grinding technique, is incalculable and in any case completely
different from that of the hollow ball outer surface. Furthermore
the danger exists that stray hollow ball particles could scratch
the surface to be machined. By contrast an essential advantage of
the grinding material in accordance with the invention lies in the
uniformity of the grinding action which depends exclusively on the
grain size selected.
The use of the grinding material in accordance with the invention
in the form of flat grinding tools, for example abrasive belts, is
particularly preferred.
With known flat grinding tools, in which a grain layer or a few
grain layers, is or are connected by means of a bonding material on
a backing, the grinding power varies and the surface quality
obtained varies with the degree of wear. During the life of the
tool the "peak-to-valley height" decreases. It primarily
corresponds to the mean grain diameter and therefore decreases with
increasing wear of the grinding material, until the belt has to be
replaced for lack of sufficient grinding power. If any this point
of time the surface quality aimed at, of the workpiece, has not yet
been reached, then it is not possible to attain it with a new belt
of the same grain size, since initially the roughing depth is
greater again, so that the surface quality already obtained would
be destroyed. Vice Versa, if a satisfactory surface of the
workpiece has already been obtained before the abrasive power of
the abrasive belt is exhausted, the half-used belt can no longer be
used for the same grinding operation, since the peak-to-valley
height is already too small or since the residual abrasive power is
no longer sufficient for completely processing the next
workpiece.
By contrast it has been found that abrasive belts employing hollow
bodies in accordance with the invention have a considerably longer
working life with constant abrasive properties. The hollow body
walls containing the abrasive grains continuously make fresh
abrasive grains available, whereby a large portion of the grinding
surface is continuously available for accommodating the abrasive
grains used up and the ground-off waste, and also any coolant
present. This pore proportion of the grinding surface hardly varies
in size even when the hollow bodies are worn. This characteristic
is also possessed by abrasive belts in which the hollow bodies are
initially closed, for example in the form of hollow balls. These
hollow balls engage the workpiece first of all by the abrasive
grains located on their outer surface, break up after a short time,
thus forming a crater in each hollow body, the diameter of which
crater increases as wear continues till the size is reached of the
hollow body diameter, and at its boundary a rim or crown of
abrasive grains if left free. The properties of the grinding means
only start to alter when the grinding surface, after heavy wear of
the hollow body walls, approaches the base of the cavity in the
hollow bodies and therefore sufficient accommodation volume is no
longer available.
The property, that the hollow body walls are stable and
form-resistant to the grinding forces is therefore essential for
the invention, so that as the grinding process continues they are
worn uniformly without being able to break off in large lumps and
without being compressed elastically or plastically. Filling of the
hollow bodies with grinding adjuvants is, only permissible within
the scope of the invention insofar as the self-supporting property
of the hollow body walls is not thereby affected. This
distinguishes the flat grinding tools employing hollow bodies in
accordance with the invention from those known abrasive belts
(German Pat. No. 884,004, British patent specification No.
860,920), in which the abrasive grains lie in a layer of bonding
means on a flexible backing of individual particles of cork or
expanded vermiculite.
The thickness of the abrasive-grain-holding wall of the hollow body
may lie in particular in the range up to 10, but preferably up to
3, abrasive grain diameters. As dense an abrasive grain arrangement
as possible is sought. In most cases a substantially single-layer
arrangement of the abrasive grain in the walls of the hollow body
is sufficient. It has therefore been sought to produce as stable
and dense as possible a hollow body sheath consisting of abrasive
grains and a suitable bonding material. The arrangement is all the
denser or more tightly-packed, the less is the distance apart of
the abrasive grains, and the more completely the wall surface
appears coated with the abrasive grains. A coating of more than 50%
of the surface available in each abrasive grain layer is aimed at.
The maximum, where all the grains are in contact with one another,
lies at 80%. A further measure for the density of the grain
arrangement in the hollow body walls is given by the volume ratio
of abrasive grains to the bonding means. The weight ratio, which
substantially corresponds to the volume ratio, is easier to
determine, as the specific gravity of the synthetic resin bonding
means (about 1 g/cc) and abrasive grains (about 3-4 g/cc) used in
practice, only differ negligibly. It can therefore be assumed that
the proportion of bonding means in the hollow body walls by weight
is advantageously not greater than twice, and preferably not
greater than once and preferably again not greater than 0.3 times
the grain content. A weight ratio of about 1:10 has proved very
advantageous. It is the proportion of the bonding means in the
solid condition that is meant. Synthetic resins have proved
favourable as bonding means. However, ceramic or metallic binders
can also be used. In the interests of the maximum possible
stability, the hollow bodies are made preferably substantially in
the form of a hollow sphere. They may however also be made
cylindrical or of some other shape, but preferably are closed on
all sides. A hollow cylindrical construction with both or one ends
open, is however not excluded.
A hollow-spherical construction has proved particularly favourable
with those grinding materials which have to stand up to high
grinding pressure or resist powerful centrifugal forces, and also
with those grinding materials where high requirements are set
regarding uniform particle size, such as for instance abrasive or
polishing belts.
The hollow bodies may consist exclusively of a closed wall formed
from abrasive grain and bonding means. Such hollow bodies are
obtained for example by blowing up granulates consisting of
abrasive grain and bonding material or by ensheathing carrier
particles which disappear as the bonding material binding the grain
hardens out, as they evaporate or decompose. For example
ceramically-bonded abrasive grain sheaths can be built up on
organic carrier particles which volatilise on combustion. Again
when using synthetic resin as bonding material, which is subjected
for hardening out to a high temperature, the carrier body can
disappear or lose its original form if it is fluid or gaseous at
the final hardening temperature. No harm is done however if the
finished hollow body still contains completely or partially, a
carrier particle, provided the carrier particles does not act
unfavourably in the grinding process. When using hollow bodies in
the form of a ballast the individual hollow bodies easily break up
so that in most cases there is no question of anything in the
nature of a carrier particle. It may in such a connection even be
advantageous to have a carrier particle if it is comparatively
stable, in order to support the hollow body from within against the
grinding forces. In all of the cases, however, in which during
grinding a breaking-up of the hollow body and its wear on one side
are to be anticipated, that is to say with all grinding material
present in a fixedly-bonded form, all the constituents
participating in the formation of the hollow body, including any
possible filling of the hollow body, should be more easily
removable under grinding conditions than the wall, consisting of
abrasive grain and bonding material, of the hollow body. A measure
of the innocuousness of the filling of a hollow body, if such is
present, is the difference in frictional forces which is found with
and without filling, other conditions being the same. The
frictional force produced by the filling alone should not be
greater then one-fifth of that of the wall of the hollow body and
preferably should be less than one-twentieth. If only some of the
hollow bodies are filled then the frictional proportion of the
individual filling in relation to the frictional stress of the
hollow body wall in question, may be greater. It should however not
exceed the ratio 1:1. This applied both for any carrier particles
or their remainders in the hollow body and also for other fillers,
for example grinding adjuvants.
From these points of view, a carrier particle is preferred which
takes the form of a thin-walled hollow bead. It preferably consists
of a thermoplastic material. The hollow bead of Belgian Pat. No.
702,673 has proved very suitable for this purpose. If the
mechanical strength required for the hollow body is obtained from
the envelope or sheath itself, consisting of bonding material and
abrasive grain, the carrier particle need only have a slight
mechanical strength, as it is merely a matrix for the production of
the envelope or sheath. It can in fact even volatilise during the
further processing of the grinding means, for instance during the
treatment at high temperatures, or can be disposed of in any manner
desired, in particular it may be allowed to run together in the
zone of the cavity near the base. In this way it can be removed
from the active grinding zone of the sheath, which may be
advantageous for the abrasive action, as this is not affected by
the presence of residues of the carrier particle. As the carrier
particle, taking the form of a hollow bead, may however be very
thin-walled, normally no harm is done even if it remains present in
the grinding zone, as it is made so slight by contrast with the
sheath, that it cannot take over any significant part of the
grinding pressure nor can it exert substantial frictional
forces.
This does not necessarily mean that the carrier particle has to
consist of a very soft material; it may consist of a hard, but
brittle material, provided that through its being given a thin-wall
it is ensured that its mechanical strength is substantially less
than that of the wall of the hollow body, so that with increasing
wear of the hollow body wall the carrier particle is removed by the
grinding pressure, for instance is successively broken away,
without however, because of the low forces thereby occurring and
the small quantity of material thereby involved, affecting the
grinding process. Such brittle thin carrier particles can however
be utilised for supporting the sheath.
The carrier particles could even fill the hollow bodies (or some of
them) completely, provided they are soft enough. As they are only
supposed to serve as matrices for the formation of the sheath,
their inherent strength need not be great. They may, therefore,
consist of a powdery substance, or a substance which at grinding
temperatures is soft or even fluid, or evaporates. They are of
course so selected that they do not have a deleterious effect on
the grinding process, but preferably a favourable one. For example
they could be a grinding lubricant. It is also possible for carrier
particles, made hollow, to be filled with grinding lubricant.
The size of the hollow bodies should be as uniform as possible. It
should further lie in a given proportion to the mean diameter of
the abrasive grain, that is to say it has proved advantageous if
the mean hollow body overall diameter is 3 to 50 times, and in
particular 6 to 20 times, as great as the mean abrasive grain
diameter.
The mean overall diameter of the hollow body, measured parallel to
the grinding surface, preferably lies between 0.1 to 8 mm, in
particular between 0.2 and 2 mm.
In the production of the hollow bodies the preferable procedure is
for carrier particles to be mixed in a tacky condition with the
abrasive grain. The carrier particles can be brought into the tacky
condition by themselves i.e. without the addition of an adhesive,
if for example they are heated to their softening temperature
before mixing with the abrasive grain, when for instance they
consist of a thermoplastic material. Whether the carrier particles
consist of a thermoplastic of a non-thermoplastic synthetic resin,
they can also be mixed with the abrasive grain before their
material is completely polymerized and they are therefore still
tacky. In those cases in which the carrier particles themselves are
tacky and soft, they are preferably mixed with the abrasive grain
in a fluidized bed process.
Obviously this process is also applicable when the particles have
been rendered tacky by wetting with a bonding material. However in
such a case it is usually simpler and therefore more advantageous
to effect the mixing mechanically during the filling, for example
in a moving drum.
When using the hollow balls for producing a grinding body, the said
hollow balls can be processed in the same way as has long been done
with the abrasive grains, i.e. during the production of a grinding
body they are mixed with a bonding material and brought into the
shape of the grinding body, after which the bonding material is
allowed to harden out, and during the production of an abrasive
belt the hollow bodies are bonded in the usual manner to a base
material.
The invention will be explained in more detail in the following
description of examples which is made with reference to the
accompanying drawings, in which:
FIG. 1 is a cross-section on an enlarged scale through an abrasive
belt when new,
FIG. 2 shows the abrasive belt of FIG. 1 when half worn away,
FIG. 3 is a plan view of the abrasive belt of FIG. 2,
FIGS. 4 to 6 are diagrams of comparative tests on the abrasive belt
of FIGS. 1 to 3 and
FIG. 7 is a cross-section through a grinding body on an enlarged
scale.
In FIGS. 1 to 3, a base 1 has affixed thereto by a layer 2 of
bonding material hollow bodies 3. The walls of the hollow bodies 3
consist of abrasive grains 5 held in place by bonding means 6. When
bodies 3 are ground to a half-worn condition, as is clearly
visible, in FIGS. 2 and 3 there is formed a crown or rim 4 of
grains 5. It can be seen that the coating of abrasive grains 5 is
only one layer thick, and surrounds the whole cavity 7 of each body
3. A substantial part of the bonding means 6 is located on the
inside of the hollow body wall to bond the grains 5 to hollow beads
which act as matrices for bodies 3. Of course a further part of the
bonding means 6 is located directly between the grains and some
even on their outsides.
EXAMPLE 1
Hollow beads which have been produced by expanding
vinyl-chloride/ethylene copolymers in accordance with Belgian Pat.
No. 702,673, with diameters from 0.05 to 1.00 mm, were wetted in a
mixer with a bonding material consisting of a 40% solution of an
epoxy resin of the bisphenol-A-epichlorohydrin type. The resin had
an epoxy value of 0.200; the hardener was the polyamino-amide
produced by the firm "General Mills" under the commercial name
"Versamid 115" The solvent mixture consisted of 74% xylene, 13%
butanol and 30% diacetone alcohol.
The hollow beads wetted with bonding material were mixed with
electrocorundum of grain size 400 (mean grain diameter 35 .mu.m) so
that the abrasive grains are fixed substantially in one layer and
tightly packed on the ball shell. The ratio by weight of hollow
beads to solid bonding material to corundum, is 1:10:100. The
abrasive grain balls were carefully dried and screened, only balls
with a mean diameter of 0.425 mm being used.
A cotton twill fabric conventionally provided for producing
abrasive belts was uniformly strewn, after the application of a
layer of a phenol-formaldehyde resin, with the ready-prepared
abrasive grain balls, which were given an intermediate drying in
the usual manner, and, after being coated over with a further thin
layer of the same resin, but thinned this time, hardened until
final definitive anchoring of the hollow balls was obtained. After
the completion of the hardening which (as usual) was carried out at
temperatures of more than 120.degree.C, the thin (c. 1 m) skin of
the bead was melted, leaving only the bead sheath consisting of
abrasive grain and bonding material.
An abrasive belt produced from this with the dimensions 17 .times.
2,000 mm, was tested on a belt grinding machine which was provided
with a grooved rubber contact disc with a Shore hardness of 80 with
a ratio of web width to groove width of 1:1. A surface grinding
operation was carried out on a flat steel bar of ST 37 steel and
the amount of material ground off was determined at intervals of 1
minute by weighing the workpiece. In order to compensate for
differences in the material, the same piece of flat bar steel was
used for both tests, i.e. on one narrow side for a conventional
belt and on the other narrow side for the belt of this example. For
purposes of comparison an abrasive belt produced in the usual
conventional manner with the same fabric backing and grain size was
tested. The results can be seen in FIG. 4.
It can be seen from the diagram that the belt of the conventional
type used for comparison was blunt after 3 minutes, after it had
initially achieved a very high abrasion rate. The new belt with
hollow balls attained several times the grinding capacity, the
abrasion rate remaining substantially constant with time.
After 30 minutes the test was broken off. The lower quantity of
ground material per minute during the first 2 minutes indicated
that the top of the hollow balls has to be torn off first before a
uniform attack is obtained. When the grinding operation was ended
the peak-to-valley height was measured. Here it was found that the
peak-to-valley height attained with the abrasive belt of
conventional type Rt = 5.15 m or a mean value R.sub.Z = 3.7 .mu.m,
while the belt provided with the hollow balls by contrast had 8.0
.mu.m or 5.1 as a mean value, which shows the unreduced
satisfactory cut of the latter even after a grinding time of 30
minutes, by contrast with the rapid blunting of the conventional
type of belt. The peak-to-valley height values are not indicated in
the diagram.
EXAMPLE 2
The hollow beads of Example 1 were wetted with an aqueous
dispersion of a terpolymer of butadiene-acrylonitrilestyrene with
an average nitrile content and a styrene fraction of about 5%, and
thereafter mixed with electrocorundum with a grain size of 280
(mean grain dia.52.mu.m).
The weight ratio of hollow beads to bonding means to corundum was
1:8:80. The balls were dried and screened as in Example 1. The
fabric backing and the mode of fixing the abrasive balls was again
in Example 1.
A grinding test was carried out with a belt produced therefrom. The
dimensions, the grinding machine and also the contact disc again
corresponded to the arrangements as in Example 1. Again flat steel
bar of ST 37 steel was ground.
The results are shown in FIG. 5. A conventional abrasive belt with
the same backing and grain size was taken off after 12 minutes, as
during the last four intervals no further noticeable abrasion was
attained. As a comparison the test was carried out with the belt of
this example for 30 minutes. The values of the peak-to-valley
heights measured at intervals of 4-6 minutes are shown in the graph
in FIG. 5. They only vary slightly over a wide range, which was to
be expected because of the uniform quantities of material ground
off. The total quantity of material ground off with the
conventional belt came to only about one-third of that with the new
belt.
EXAMPLE 3
An abrasive belt with dimensions 50 .times. 2,000 mm was prepared
as in Example 2 and tested for comparison wih a conventional
abrasive belt on a belt grinding machine.
The contact disc had a Shore hardness of 90. The material being
ground was a seamless drawn tube with an overall diameter of 165
mm, a wall thickness of 5.1 to 5.8 mm, a Brinell hardness of 140
kg, soft-annealed to German Standard DIN 1629. A section of the
tube was fed by the end face to the belt, rotating at 24 r.p.m.
round the cylindrical axis. Feed was automatic as soon as the
braking action, recognizable through the current consumption, left
off. The intervals were 5 minutes long.
As can be seen from FIG. 6 the belt used for cparison was worn down
after 10 minutes, having ground off 120 g of material. The belt of
this example had ground off 470 g after 35 minutes, the individual
values being uniform over a wide range.
Abrasive belts according to the invention are suitable for grinding
any desired materials, for instance metal, glass (glass-edge
grinding), synthetic plastics material, ceramics or wood. They are
particularly servicable also for wet grinding, as they have an
outstanding capacity for accommodating water in the ball craters.
Any desired underlays are also suitable, for instance woven
fabrics, paper, vulcanized fibres, fleeces or random webs, foils or
films. Belts, curved components, sheets and discs are preferred
forms of application. Non-flexible backings can also be used,
however.
For the hollow beads, silicate materials, glass, hardening
synthetic resins (phenol resin, melamine resin, urea resin, epoxy
resin, etc.), thermoplastic resins, gelatins and other processed or
unprocessed natural substances have proved particularly
suitable.
Any known bonding material can be used, both for fixing of the
abrasive grains on the hollow bead surfaces and also for fixing the
hollow bodies to the base.
Suitable abrasive grains are, for example, molten aluminium oxide,
molten zirconium oxide, mixtures of these oxides, silicon carbide,
diamond, flint, granite, emery and the like, and also polishing
agents such as pumice stone, tripolite, rouge etc.
It is evident that hollow body walls primarily comprise the
abrasive grains, the part of the bonding material located between
the abrasive grains and also the bonding material by which the
abrasive grains were fixed, during the production processes to the
hollow beads and which, if and when the bead walls are removed
(e.g. by melting), covers the inner surface of the abrasive grains
in the manner of a skin. The bonding means may be considered as
comprising the wall which provides on the outside of the hollow
body a bond between the abrasive grains associated with the same
hollow body, that is to say for example that part of the re-coating
layer which is deposited on the hollow bodies in abrasive belts.
The substances making up the hollow beads do not normally form part
of the walls. At the most a thin hollow bead layer supporting the
wall, without substantially reducing the cavity space, can be
considered as part of the bonding means as it fulfils a function of
the bonding means. In the section through a grinding body as shown
in FIG. 7 there is shown at 8 the grinding surface while 9 is a
lateral boundary surface of the grinding body not participating in
the grinding process. The other two boundary lines are shown as
thick wavy lines.
The grinding body comprises a multiplicity of hollow bodies 3, each
consisting of a bubble type carrier particle 10, of, for instance,
thermoplastic material, and a grain sheath 11 held by a bonding
material (not shown) to the carrier particle. Depending on the
nature and quantity of the bonding material used for bonding the
grains to the carrier particle this is also directly effective
between adjacent grains. Among themselves the hollow bodies are
interconnected by a bonding material 12 ensheathing the individual
hollow bodies. According to the quantity of the bonding material
used this may completely fill the gusset-like cavities between the
hollow bodies or merely cover their surfaces as in the example --
shown the gusset-like cavities remaining unfilled. If it is desired
to fill the gusset-like cavities, then the bonding material can be
enriched with a given quantity of abrasive grains 13 or filler
and/or grinding adjuvants, grains 13 being indicated by dots in the
drawing, thus being differentiated from the hollow body grains
which are indicated by peripheral lines only.
Some or all of the hollow bodies may be filled with a grinding
adjuvant 14, for example a lubricant. The grinding adjuvant may be
present in solid form, paste form, liquid form or as a gas.
It can be seen on the grinding surface 8 that some of the hollow
bodies are broken up and therefore form large pores 15, while
others 16 are still effective by their outer surfaces. It will
further be seen that the pores are large by comparison with the
grain size, while with known abrasive grains the pore size is
normally smaller than the size of the grain.
With grinding bodies it is not absolutely necessary for the
mechanical strength of the hollow body wall, substantially
consisting of abrasive grains, to be predominantly determined by
that part of the bonding means which bonds the abrasive grain to
the carrier particle. The bonding of the grain to the carrier
particle and the bonding of the abrasive grains to one another,
effected during the production of the hollow bodies, need only be
sufficiently strong for the hollow bodies to be able to withstand
the stresses occurring during the subsequent production of the
grinding body. The final mechanical strength, not only of the
grinding body, but also of the hollow bodies, is preferably
determined substantially by that part of the bonding means which
wets the hollow bodies externally during the connection to the
grinding body.
However the production of the grinding body can also be effected
predominantly or exclusively by means of that bonding means which
was used during the preceding production of the hollow bodies,
since the hollow bodies are formed before the final hardening of
this bonding means tightly packed to the grinding body.
During the production of grinding bodies the mechanical strength of
the hollow body walls need only be strong enough to avoid
deformations occuring during compression.
Grinding bodies according to the invention make a multiplicity of
fresh abrasive grains continuously available at the grinding
surface. In many cases therefore any dressing or sharpening of the
grinding body can be dispensed with, or need only be carried out at
longer intervals of time.
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