US3273984A - Grinding wheel - Google Patents

Description

United States Patent 3,273,984 GRINDENG WHEEL Roy S. Nelson, Sterling, Mass, assignor to Norton Company, Worcester, Mass, a corporation of Massachusetts No Drawing. Filed July 18, 1963, Ser. No. 296,100 2 Claims. (Cl. 51---296) This application is a continuation-in-part of Serial Number 18,255 filed March 29, 1960, now abandoned.

This invention relates to abrasive bodies and more particularly to manufactured grinding wheels having the relatively wide spacing of abrasive grains characteristic of a relatively low percent by volume of abrasive, such that many of the abrasive grains are not so closely spaced as to be mutually supporting.

In the manufacture of abrasive bodies such as grinding wheels, it has been found that as long as there is more than a certain quantity of abrasive material present in the abrasive and bond mixture, the grains of abrasive are all in contact one with another such that when the green Wheel is fired to mature the bond, the grains appear to support each other as the bond ingredients pass through a more or less plastic or liquid stage in becoming set. Due to this mutual support effect of the grains, the finished wheel retains substantially the size and shape of the green wheel which is formed under pressure prior to the firing cycle.

At a certain relative proportion by volume of grain to bond ingredients, depending upon the kind and grit size of the abrasive grain in the body, the abrasive component becomes so relatively Widely spaced within the mixture used for making the wheel that .as the bond ingredients are heated during the curing cycle and approach a liquid or flowable condition, the grains no longer can provide a mutual support for each other and before the bond can be matured the unfinished wheel shape begins to slump. A small degree of slumping can be tolerated while still attaining the desired wide spacing of the abrasive grain in the product. When it is necessary to make wheels with relatively wide spacing of abrasive grains, particularly in connection with resin bonds, it has been found that an undue degree of slumping is sometimes encountered when the abrasive grains are relatively widely dispersed so that they are not self sustaining to any appreciable extent due to the bond ingredients flowing before taking a final set.

A grinding wheel having a Wide spacing of abrasive grains is generally known in the grinding wheel art as having a high structure number. The higher the structure number, according to an arbitrary system of classification, the wider the grain spacing.

Although it is diflicult to generalize because of the many variables involved, it may be said that the area of usefulness of this invention is in wheels having an abrasive content of 54% or less by volume and a pore content of or more by volume. And when the abrasive content is less than 54% and the pore volume is to be greater than slumping becomes a serious problem in most coarse grit (840 grit) wheel specifications and it may be impossible to make such wheels without employing the teachings of this invention. Conventional techniques have been entirely unavailable, prior to this invention, to produce grinding wheels from a mix consisting of a curable resin system and abrasive grain when the finished wheel is to have a porosity of or greater and an abrasive content of not more than For example, we know of no satisfactory method other than that of the present invention to make wheels of the finished structure described in Examples I, II, III, IV, and V which follow below. Although the techniques of this invention would be applicable to wheels containing less than 30% abrasive, such wheels (except in the case where diamond abrasive is employed, to which this invention has no particular application) are not now standard items of commerce. With regard to bond content, an amount of bond equal to at least 2% of the wheel volume, in general, must "be present to produce a satisfactory wheel with presently employed materials. With a minimum abrasive content of 30 volume percent; as specified above, and a minimum bond volume of 2%, this leaves a maximum possible pore content of 68%, by volume.

The present invention makes it possible to produce a grinding wheel having a higher structure number wherein the abrasive grain in the wheel can be relatively much more widely dispersed in the bond than can be accomplished with any conventional practice known heretofore. The invention makes use of a temporary or stabilizing bonding means included in the mixture to adequately support the abrasive grain while the principal bond structure can be set up. For the purposes of this invention, a liquid resin material is provided, which hardens at room temperature and retains its integrity under the conditions of the cycle uesd for curing the principal bond, to support the abrasive grains temporarily. A green wheel structure is produced upon the curing of such a low temperature hardening resin and thereafter the green wheel can be subjected to a curing cycle to set the principal bond ingredients to provide a peramnent support for the abrasive grains.

In its preferred form, this invention may be practiced by incorporating minor quantities of a room temperature or low temperature curing liquid resin in a standard abrasive and bond mixture to control or eliminate slumping during the curing cycle for the standard bond. The use of such a stablizing bond is suggested for those mixes in which slumping during curing presents a problem, .and is especially useful for the making of wheels in which the abrasive is relatively widely spaced in the wheel.

To accomplish the best mixture, the small amount of uncured low temperature curing resin is preferably used in liquid form and is mixed with a standard mixture of uncured powdered bonding agent and abrasive material. From the resulting intimate mixture, a wheel or other shape is molded and allowed to stand at room temperature for a sufficient time to cure the low temperature setting resin comprising the stabilizing bond. The cured resin forms a supporting network throughout the green molded article to hold the abrasive grain dispersed throughout the otherwise standard bond mixture. When the low temperature curing resin has cured to a degree such that it will retain a substantially thermoset condition at the curing temperatures for the principal bond, the green bonded abrasive article may be subjected to the standard heat curing cycle.

The product resulting from following this teaching can be made in a manner to be substantially entirely free from slumping during this final curing cycle. If it is desired to minimize the quantity of stabilizing bond ingredients used, specifications can be set up such that a green wheel results in which only a slight dimensional change will be encountered during the final cure.

The low temperature curing resin may be called a stabilizer resin and is preferably added to the abrasive grain in the same manner as and in conjunction with lesser amounts of the customary liquid phenolic resins normally added to the mixture of abrasive and powdered resin used for the wheel bond. The stabilizer resin which is preferably an epoxy resin may be added to the grain and bond mixture in any form providing it, together with its curing agent, does not react with the ingredients of the principal bond under conditions such that the maturing of the stabilizer resin is accelerated in a manner which would preclude proper molding of the abrasive bond mix at the green stage. The stabilizer resin must be compatible with the principal bond in the final product such that no undesirable properties are produced either during the reaction or in the properties of the ultimate product.

It is best to add a minimum quantity of stabilizer resin which is just sufficient to hold the abrasive grain temporarily dispersed until the principal bond can be cured in place. On the other hand, the more stabilizer resin that is incorporated in the wheel mix, the less is the slumpage that is encountered. If the stabilizer resin is held to a minimum, the known properties of the final product may be attained without any significant effect due to the presence of a small amount of the stabilizing bond. I have found it desirable to substitute the stabilizer resin for a portion of the principal bond in the approximate range of from ten to twenty percent by weight of the principal bond. This controls the slumping of the product to be within tolerable limits during curing and yet apparently produces no essential change in the properties of the resulting product as compared with a similar product produced without the use of a stabilizer resin.

The stabilizer resin falling within the contemplation of this invention may be used successfully with resin bonds of either natural or synthetic resins such as shellac or phenol formaldehyde bonds. The same type of stabilizer resins can be used similarly for green bonding in connection with rubber bonds of various types for fabrication of rubber bonded abrasive containing articles by a similar process.

The stabilizer resin used in connection with any of the principal bonds mentioned above must be selected to be consistent with the nature of the cure cycle for the principal bond. The stabilizer resin should be adapted to form a rigid matrix at a temperature between room temperature and any temperature below that temperature at Which the ingredients of the principal bond ingredients begin to liquify and set up. In most cases this would be a temperature between 20 C. and 60 to 70 C. Further, the stabilizer resin must be of such a type that upon subsequent curing of the principal bond composition, the stabilizer resin retains its rigid characteristics and is not destroyed under the conditions of the curing cycle (several hours at temperatures of the order of up to 200 C.) to which the green product is subjected for maturing the principal bond components.

The important requirements for choosing the stabilizing resin may be summarized as follows:

(1) It must be a liquid prior to cure; (2) It must cure between room temperature and 70 C.;

(3) It must cure to a rigid matrix which is stable at the temperature of cure of the main resin bond.

The class of liquid epoxy resins represented by the diglycidyl ethers of polyhydric phenols are suitable for use as stabilizing resins in this invention. The principal commercial resins of this type are reacton products of epichlorhydrin and bisphenol A. Obviously other polyhydric phenols and other sources of epoxide groups however may be employed to make similar acting resins. Also, aliphatic reactants, instead of aromatic reactants, can be substituted for the bisphenol to give liquid epoxy resins satisfying the above listed requirements for the stabilizer resin of my invention. Such alternatives are known in the art. See, for example, Epoxy Resins by Lee and Neville (McGraw-Hill Book Company, 1957) and the references cited therein.

Epoxylated novolac resins having the general structure:

have also been successfully employed in the present invention.

Another important class of room temperature curing resins which are useful in the present invention as the stabilizer resin are the liquid polyester resins containin/ unsaturated groups which can be cross linked by catalysts at room temperature or slightly elevated temperatures and which satisfy the other requirements summarized above. One such polyester resin, which also contains styrene is Vibrin 157 sold by the Naugatuck Chemical Division of US. Rubber Company. This material can be cured by peroxide curing agents. A suitable catalyst is methyl ethyl ketone peroxide employed in the amount of 0.02 part per parts by weight of Vibrin 157.

The followingare examples showing the practice of my invention. The examples have been selected to show the use of different stabilizer resin compositions and the effect produced because of the use of different proportions of the stabilizer resin. In addition the examples show several compositions having different abrasive grains and different grit sizes for the abrasive grain used.

EXAMPLE I Two 6%; x 4 x 0 wheels were made up having a volume structure of 44% abrasive, 26% bond, and 30% pores, which is a structure that is known to be subject to severe slumping. The two wheels, one of which was made to standard specifications and one Was made to utilize my invention, were molded from mixes prepared in the following manner:

Standard wheel mix Into a rotary pan mixer was placed abrasive, liquid phenolic resin, powdered shellac bond, and a small amount of castor oil in that order and in the following weight percents:

Weight percent Aluminum oxide abrasive, 90 grit 85.5.

Liquid phenolic resin 2.0. Powdered shellac 12.5. Castor oil 20 cc./lb. of bond.

The materials were blended to form a uniform, freeflowing mix and a wheel shape was pressed from this mix.

Stabilizer resin mix This mix was prepared by first making two separate mixes, one standard type mix, and a second containing the stabilizer resin in the proportion by weight noted below, and finally mixing portions of these two mixes together to formulate the mix from which the wheel of my invention was to be molded.

The one standard type wheel mix was made as described above.

The second stabilizer resin containing mix was made by placing the abrasive, a liquid form of diglycidyl ether epoxy resin (and a cyanoethylated amine curing agent, 37.5 parts per hundred of the epoxy by weight, designated as ZZL-0803 by Union Carbide Corporation), and powdered shellac bond in a rotary pan mixer in that order. The Weight percents respectively of abrasive grain and total bond were the same as in the standard mix; however, the bond in this second mix was 50% epoxy resin and 5 0% shellac by weight. No castor oil was used. The epoxy resin was a liquid diglycidyl ether epoxy resin having a molecular weight of 350 to 400 purchased from Union Carbide Corporation and designated as ERL3794. It has an epoxide equivalent of 340 to 400 and is believed to be based on epichlorhydrin and bisphenol A.

Eight parts of the first standard mix were blended with two parts of the second stabilizer resin containing mix. From this combination mix, the bond of which now con tained 10% by weight of stabilizer resin a 6 x4x 0 wheel shape was molded from this mix.

Both the molded standard wheel shape and. Stabilizer resin containing wheel shape were allowed to stand at room temperature of about 25 to 28 C. for approximately 24 hours. The dimensions of the green wheels were then recorded and the wheels were cured in a standard shellac bond curing cycle in which the temperature of the wheels was raised gradually from room temperature to 160 C. over a period of 64 hours, the wheels were held soaking at 160 C. for 8 hours and then the oven heat supply was turned oil? and the wheels allowed to cool to near room temperature in the oven.

After completion of the above described curing, the standard wheel had slumped 163 mils, while the epoxy resin stabilized wheel had slumped only 3 mils.

EXAMPLE II Two 8X3 x2" wheels, one a standard mix and the other a mix containing a stabilizer resin, were molded from mixes having a volume structure of 44% abrasive, 26% bond, and 30% pores, the mixes having been prepared in the following manner:

Standard wheel mix Liquid phenolic resin 3.1. Powdered shellac 14.2. Castor oil 20 cc./ lb. of bond.

The materials were blended in the mixer to form a uniform, free-flowing mix.

Stabilizer resin mix This mix was made in an identical manner to the standard wheel mix, and using the same quantities of material. Instead of wetting the abrasive with liquid phenolic resin, however, 3.1% by weight of a liquid diglycidyl ether epoxy as in Example I (and amino polyamide curing agent) was employed in its place. This gave a mix, the total bond which contained 18% by weight of the stabilizer resin. The curing agent was Genamid 250 from General Mills Incorporated and 30 parts of curing agent were employed for each 70 parts by weight of resin.

Green wheels were molded from both the standard wheel mix and the stabilizer resin contining wheel mix, the green wheels were allowed to stand 24 hours at room temperature. The dimensions of the two wheels were noted and the wheels were finally cured under the conditions of a conventional shellac curing cycle as described above. After this cure, the standard wheel had slumped 200 mils, while the stabilized wheel had deformed only 36 mils.

EXAMPLE III Two 8 x 3 x 2" wheels, one a standard wheel and one a stabilizer resin containing wheel, were formed with a volume structure of 44% abrasive, 26% bond, and 30% pores, from mixes prepared in the following manner:

BRL-2332 1.4. Powdered shellac 14.2. Castor oil 20 cc./lb. of bond.

EXAMPLE IV Two 8 x 3 x 2" wheels, a standard and stabilized wheel, with a volume structure of 44% abrasive, 26% bond, and 30% pores, were molded from mixes prepared in the following manner:

Standard wheel mix Prepared as described in Example II.

Stabilizer resin mix Into a rotary pan mixer was placed abrasive, liquid epoxy-novolac resin (and cyanoethylated amine curing agent), liquid phenolic resin, shellac, and castor oil, in that order and in the following weight percents:

Weight percent Silicon car-bide abrasive, 30 grit 82.7.

Liquid epoxy-novolac resin 1.7.

Liquid phenolic resin 1.4.

Powdered shellac 14.2.

Caster oil 20 cc./lb. of bond.

The ingredients were blended to form a uniform, freeflowing miX.

The liquid epoxy novolac resin was Dow Chemical Company D.E.N. 438, having a molecular weight of about 600. The catalyst was the same as that in Example I and was employed the same amount based on the weight of the resin.

After molding, the wheels were allowed to stand at room temperature for 24 hours. The dimensions of both wheels were noted and the wheels cured as above described. After curing, the standard wheel had slumped 2 00 mils and the stabilized wheel only 52 mils.

EXAMPLE V A 6 7 x 4 x 0" phenol-formaldehyde bonded wheel was made according to the teachings of the instant invention with a volume structure of 46% abrasive, 20% bond, and 34% pores; a specification which is well known to slump badly without the benefit of the teachings of the instant invention.

This structurally stabilized wheel was made by first making two separate mixes, one standard type mix and a second containing the stabilizer resin, then finally mixing portions of these two mixes together to give the mix from which the wheel was to be molded.

Standard type mix Into a rotary pan mixer was placed abrasive, liquid phenolic, and neutral anthracene oil Wetted powdered phenolic resin, and in the following quantities:

Weight percent Aluminum oxide abrasive, 36 grit 89.9. Liquid phenolic resin (BRL-2332) 2.02. Powdered phenolic resin (plus 9% hexa by wt.) 8.08. Neutral anthracene oil 30 cc./lb. of bond.

The materials were blended to a uniform, free-flowing mix.

The powdered phenolic was a two stage phenol formaldehyde resin (a novolac), molecular weight 590, designated as BRP-54l7 by Union Carbide Corporation.

Stabilizer resin mix Into a rotary pan mixer was placed abrasive and liquid diglycidyl ether epoxy resin (and cyanoethylated amine curing agent) in the following quantities:

Weight percent Aluminum oxide abrasive, 36 grit 89.9 Liquid epoxy resin 10.1

Eight parts of the standard type mix were blended with two parts of the stabilizer resin mix. The bond of this final mix then contained 20% by weight of the epoxy stabilizer resin. From this mix a 6 7 x 4 x wheel was molded and allowed to stand at room temperature for approximately 24 hours. The dimensions of the green wheel were recorded and the wheel cured in a bake cycle which consisted of subjecting the wheel to a heat treatment such that the temperature in the center of the mass of said wheel progressed from a room temperature of 25- 28 C. to an ultimate temperature of approximately 185 C. in four hours.

The dimensions of the cured product were identical to those of the green wheel.

The above examples give typical examples of the way in which my invention may be performed. The wheels of my invention characterized by high structure numbers have especially good grinding characteristics for finishing the metal rolls used in certain operations in paper mills.

The grinding of paper mill rolls is one of the more diflicult grinding operations known which entails roughing (or reshaping) and polishing of the rolls. The difficulty arises as a result of the stringent demands made on the grinding wheels employed. For the ultimate in grinding performance a wheel should exhibit the following characteristics (1) The wheel must be of such a nature as to allow the bond to break down, exposing new cutting grains, under the extremely light infeed pressure necessary to avoid bending or distorting the long rolls during the roughing operation. At the same time the wheel must possess sufficient durability to maintain cutting action for a continuous pass on the entire length of the roll, in order to impart true dimensions.

(2) The wheel must possess resiliency in order to perform the necessary polishing operation. The polishing capability of a wheel is dependent on the wheels ability to hug the roll with little or no infeed for a pass on the entire length of the roll with no chattering or bumping. The ability of the wheel to hug the roll is in turn dependent on the resiliency of the wheel bond. So a strong but yielding bond is required. It was believed that a wheel with a very high structure number in a medium grade would possess these desired properties. However, medium grade wheels with a very high structure number in the coarse grit sizes required for this type of operation have not been manufacturable heretofore; because when an attempt is made to manufacture such a wheel, slumping occurs during the cure, resulting in a low structure number and/or harder grade wheel than is desirable.

I have found that such a medium grade wheel with a high structure number such as I have described above, can be manufactured by incorporating the stabilizer resin of my invention. The following are grinding tests comparing the grinding quality of shellac bonded wheels with a high structure number manufactured in accordance with my invention, as compared to the Nortion wheel of the highest structure number manufacturable without employing this invention, and the best competitive wheel in the field.

Grinding test of standard vs. new wheel with high structure number WHEEL DATA The standard wheel was 20 x 3 x 12'', contained 36 grit silicon carbide abrasive and the resins used were liquid 8 phenolic andshellace. The volume structure of the wheel was 450% abrasive, 20% bond and 30% pores.

The new wheel with a relatively higher structure number made according to this invention was also 20 x 3 x 12", contained 36 grit silicon carbide abrasive and the resins used were liquid phenolic, liquid epoxy, and shellac. The volume structure of the wheel was 44% abrasive, 24% bond, and 32% pores.

TEST METHOD resulting in only fair 1, 600 None or only very light chatter; grinding action and linish excellent.

High Structure Number.

The standard wheel exhibited poor grinding action characterized by a sudden breakdown of a glazed wheel face resulting in chatter and the usually accompanying chatter marks on the iron roll.

The high structure wheel, on the other hand, exhibited excellent grinding action breaking down evenly as indicated by the low power consumption and the superior finish applied to the iron roll.

Grinding test of highest quality competitors wheel vs. new wheel with high structure number WHEEL DATA The competitors wheel was 20 x 3 x 12", contained 36 grit silicon carbide, and the resin appeared to be shellac. The volume structure, after examination, appeared to be 52% abrasive, 28% bond, and 20% pores.

The new wheel of high structure number was the same as described above.

TEST METHOD The test method was the same as described above.

RESULTS Average Wheel Average Wheel Wear on Net Power Remarks Diameter Ier Run, Per Run, watts mils Competitors 2.0 3, 300 Vefry light chatter; good iiiish. High Structure 2.8 1, 600 None or very light chatter;

Number. excellent finish.

The test results indicate that the new high structure wheel is superior to the best competitive wheel in the field. Although the new high structure wheel showed more wheel wear than did the competitive product, it consumed only half as much power and imparted a superior finish on the work.

9 EXAMPLE VI A suitable formulation for employing a room temperature curing polyester resin is as follows:

Weight percent SIC abrasive, 30 grit size 82.7 Liquid polyester resin (Vibrin 157, with 0.02 part The above ingredients were blended to a uniform free flowing mix, molded, and allowed to stand 24 hours, and then cured as in the previous examples. The use of the stabilizer resin effectively controlled slump as compared to a standard wheel of the same mix but without the stabilizer resin. Instead of the particular commercial polyester disclosed, polyesters such as disclosed in U.S. Patent 2,493,948, including a room temperature curing agent, can be employed.

The above description covers typical examples of my invention. Many modifications thereof will occur to those skilled in the art, which fall within the scope of the following claims.

I claim:

1. A grinding wheel of organic resin bonded abrasive grains consisting of between 30 and 50% by volume of abrasive, more than 20% but not more than 68% pores and at least 2% bond, said bond consisting of a principal bond which is essentially a'shellac bond containing a minor amount of phenol formaldehyde resin, and a stabilizer bond in the amount of 10 to by weight of said principal bond selected from the group consisting of diglycidyl ether epoxy resins, epoxylated novolac resins, and unsaturated polyester resins.

2. A method of making grinding wheels, having an abrasive content of from 30 to 54% by volume and a pore content of from 20 to 68% by volume, in which a principal relatively high temperature curing resin bond is employed which goes through a plastic state during cure of the wheels and in which the abrasive grains are so widely spaced that slumping of the wheel during cure is ordinarily encountered, comprising including in the wheel mix abrasive grain and a bond, said bond consisting of a liquid relatively low temperature curing resin and a principal relatively high temperature curing resin which passes through flowable liquid state below its curing temperature, curing said liquid resin to a solid state at a temperature below that temperature at which said principal bond becomes a flowable liquid, and finally curing said principal resin bond, said principal bond being selected from the group consisting of shellac, phenol formaldehyde resins, rubber, and mixture thereof, said liquid low temperature curing resin system being persent in the amount of between 10 and 20% by weight of the principal bond and being selected from the group consisting of diglycidyl ether epoxy resins, epoxylated novolac resins, and unsaturated polyester resins.

References Cited by the Examiner UNITED STATES PATENTS 2,521,911 9/1950 Greenlee 51298 X 2,703,765 3/1955 Osdal 51298 2,709,647 5 1955 Goepfert et a1 51-298 2,779,668 1/1957 Daniels et al. 51-298 2,793,105 5/1957 Lection 51-298 2,811,430 10/1957 Gregor et al 51-298 2,825,638 3/1958 Booth 51298 2,907,740 10/1959 Greenlee 260-831 2,939,777 6/1960 Gregor et al 51298 3,102,010 8/1963 Lang 51298 FOREIGN PATENTS 787,022 11/1957 Great Britain.

ALEXANDER H. BR-ODMERKEL, Primary Examiner.

D. J. ARNOLD, Assistant Examiner.

Claims (1)

1. A GRINDING WHEEL OF ORGANIC RESIN BONDED ABRASIVE GRAINS CONSISTING OF BETWEEN 30 AND 50% BY VOLUME OF ABRASIVE, MORE THAN 20% BUT NOT MORE THAN 68% PORES AND AT LEAST 2% BOND, SAID BOND CONSISTING OF A PRINCIPAL BOND WHICH IS ESSENTIALLY A SHELLAC BOND CONTAINING A MINOR AMOUNT OF PHENOL FORMALDEHYDE RESIN, AND A STABILIZER BOND IN THE AMOUNT OF 10 TO 20% BY WEIGHT OF SAID PRINCIPAL BOND SELECTED FROM THE GROUP CONSISTING OF DIGLYCIDYL ETHER EPOXY, RESINS, EXPOXYLATED NOVOLAC RESINS, AND UNSATURATED POLYESTER RESINS.