"R API D Curing Of Resin Bonded Grinding Wheels "

Abstract

Rapid microwave curing of resin bonded grinding wheels using multifunctional sample holders made from microwave susceptor materials involving accelerated and volumetric controlled heating for curing of resin bonded grinding wheels. Rapid curing of grinding wheels by exposing them to the electromagnetic radiations (EMR) in microwaves frequency in the range of 900 to 3000 MHz, more particularly in the range2450 ±50 MHz. Susceptors are not only used as sample separators but also absorb microwaves effectively and efficiently at room temperature, which in turn heat the grinding wheels initially and then microwaves heat them volumetrically and rapidly. They also act as load that maintain the final geometry of the grinding wheel after curing, and absorb reflected microwaves from metallic constituents present in the grinding wheel. The microwave susceptor material used in the present invention is carbon bearing material e.g. graphite and/or silicon carbide. The microwave process provides an attractive alternative route to the curing of grinding wheels rapidly and economically.

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FORM 2
THE PATENT ACT, 1970
(39 of 1970)
&
The Patent Rules, 2003
COMPLETE SPECIFICATION
(See Section 10 and Rule 13)
RAPID CURING OF RESIN BONDED GRINDING WHEELS
PRADEEP METALS LTD.
R-205,M1DC, Rabale,
Navi Mumabi-400 701,
India
THE FOLLOWING SPECIFICATION PARTICULARLY DESCRIBES THE INVENTION AND THE MANNER IN WHICH IT IS TO BE PERFORMED:

FIELD OF THE INVENTION:
The present invention relates to Rapid Curing of Resin Bonded Grinding Wheels.
The Invention relates to accelerated heating (i.e. curing) of resin bonded grinding wheels embedded with fiber reinforcement, with the aid of electromagnetic radiations, such as microwave.
The invention more particularly relates to Rapid Curing of Resin Bonded Grinding Wheels with the aid of electromagnetic radiations such as microwave, specifically in the range of 900 to 3000 MHz, more specifically at 2450±50MHz and the use of pre-designed, customized sample holders made from microwave susceptor materials in the curing process.
BACKGROUND:
Grinding wheel is a widely used cutting tool to remove undesired material from work piece by abrasive action. Industrial applications of grinding wheels are: cylindrical grinding, profile grinding, internal grinding, honing and super-finishing, spring-end grinding, centreless grinding, surface grinding etc. The grinding wheels are typically used in various industries including bearing industries, automobile, defense, foundries, steel plants, and machine/cutting tool manufacturing etc. Generally, the grinding wheels are used in the entire engineering industry. Efficient grinding wheels should have high and constant cutting capacity and excellent profile durability. It is desirable that it should make optimum utilization of the abrasive grain and have an extended service life.
In the manufacturing of grinding products such as resinoid grinding wheels, which are designed to perform heavy duty tasks such as metal cutting and rail track grinding, an abrasive material is intimately mixed with the bonding ingredients and temporary binder. The bonding ingredient consists of such compounds as are necessary to combine to form the required resinoid bond during curing. The ingredients are mixed and pressed into the required shape. The green product thus obtained, is then placed in the oven for curing for several hours in order to achieve a slow heating to avoid any damage to the product. Typically in the conventional process, green wheels are cured for several hours ranging from 15-30 hours in an electrical oven at about 180-22O°C with several intermediate holds at different temperatures by using radiant heating. During curing, pressure is also employed by inserting metallic plates between the samples.

The conventional process of curing green wheels requires longer duration heating for achieving desired bond strength between ceramic grains and phenolic resins, as both are bad conductors of heat. This results in spending of considerable time and energy to achieve target properties.
Thus, there Is a need in the prior art to lower such time and energy required in conventional process of curing.
Prior art teaches various techniques used to lower such time and energy in the process of manufacture of grinding wheels, with the help of use of electromagnetic radiations such as microwave. Examples can be seen as:
1) US Patent 5072087 titled* claims a process of producing a heat-treated body of a
refractory material from a substance that does not couple well with microwaves. The said
patent provides a process that enables non-susceptors to be heated by microwaves without
contaminating the final product with an undesired material.
2) US Patent 4305898 discloses a method for the manufacture of a bonded abrasive grinding product. According to this patent, the green sample is placed in the microwave oven for curing in the manner of batch type, semi-continuous and continuous furnace. The said patent also includes a steel or other metallic reinforcing ring in a grinding wheel without it being damaged or destroyed during firing. The invented process is used only for the manufacturing of swing frame & pedestal grinding wheels where maintaining of profile is not required. Hence using this process a flat and thick swing frame & pedestal wheel can be processed in a single layer only. The limitation of this process is it cannot maintain the critical and complex profile of the wheel after processing and cannot cure grinding wheels in multiple stacking for better economics.
3) US Patents 4150514 and 4404003 disclosed the process in which the mixture was prepared by blending of refractory particles, binder and filler, then transfer the mixture into the grinding wheel molds at room temperature. After molds were filled with mixture they are subjected to microwave energy at about 2.45GHz. This heated the mold and their charges to a temperature within the range of about 35-120°C. This is called the preheating process of the grinding wheel. Then mold was transferred between the platens of a hot mold press and mold was subjected to post cure heating step in accordance with conventional procedures. The said patent used the microwave only for the preheating of the mixture which provides fluidization and minimizes the degree of pressure required for the production of any given density of resin-abrasive mixture; final curing of the grinding wheel was followed by conventional route.
However, as seen above, these techniques have some limitations and there is further scope of conserving time and energy in spite of using Microwave heating.

It is an object of the invention.to carry out the process of curing using microwave energy, in further shortened time period and with further less energy.
In the conventional process, a steel plate weighing nearly about lOOOgm is used for the curing of single sample weighing about 90g to retain the geometry of the sample. This creates nothing but excessive dead load. During the conventional heating process, unnecessary heating of the steel plates and side walls of the furnace/oven also consumes disproportionate energy.
Besides, the use of such metallic plates in microwave heating creates reflections of microwaves from the metal plates which may damage the magnetron and there is fear of forming hot spots in the microwave cavity. Therefore, there is an increased risk of damage to be caused to the magnetron and microwave chamber internally, as such. It also results in uneven heating of the sample and waste of energy due to reflections.
Thus it is seen that there is a need in the prior art to lower unnecessary and wasteful dead load created in conventional process of curing and also save the energy wasted in heating the same.
It is another object of the invention to use precisely quantified material in the process, so as to avoid unnecessary and wasteful heating, thereby saving further considerable energy and also achieving uniform heating of the sample.
It is clear that plain substitution of microwave process to the conventional heating process is not the solution to the limitations posed by the prior art.
Further, the sample to be cured needs to be evenly heated with a uniform temperature all over its body shape.
The shape of the sample to be cured should not be affected while curing, due to uneven weight load or uneven heating.
Thus, there is need in the prior art to devise a system of rapid and uniform curing of grinding wheels using electromagnetic radiations such as microwave; which steers away from the limitations noted above, and achieves the intended objects.

OBJECT OF THE INVENTION:
The main object of the present invention is to provide Rapid Curing of Resin Bonded Grinding Wheels using Microwave which obviates the drawbacks of the hitherto known prior art as detailed above.
Another objective of the invention is to cure the wheels rapidly and produce the finished product with acceptable and desired physical properties, in an economical and safe way.
SUMMARY OF THE INVENTION:
Accordingly, the present invention provides a system of rapid curing of resin bonded grinding wheels using microwave energy, comprising of pre-designed customized sample holders made from microwave susceptor materials during processing.
The use of pre-designed, customized sample holders made from microwave susceptor materials in the curing process of grinding wheels with microwave energy, provides the necessary objectives viz.
a) while they function as separators of green grinding wheel samples to be cured; they absorb microwaves effectively and efficiently and in turn heat the grinding wheels volumetrically and rapidly during the curing process;
b) they act as a load to press the green samples during curing; and also enable maintain the shape and profile of the grinding wheel during the curing process.
Thereby the time taken for curing the sample is reduced drastically and uniform curing of the sample is also achieved.
The crux of the invention is the pre-designed customized sample holder made of microwave susceptor material such as graphite, which is predesigned and customized according to the desired shape of the finished product, whereby rapid and uniform curing of resin bonded grinding wheels is achieved using microwave energy, time and resources are further saved.
DETAILED DESCRIPTION OF THE INVENTION:
Microwave technique is an internal heating process where the heat is generated by interaction of electromagnetic waves with the material at the atomic level. The microwave heating process is also known as dielectric heating. As the microwaves interact with the sample they cause rapid oscillation of the dipoles of the molecules of the constituents such

as ceramic grains and organic binders, causing inter-molecular friction. Due to this heat is generated very rapidly, resulting in heating the sample volumetrically and uniformly. The volumetric heating equilibrates the reaction kinetics and forms bonding rapidly and as a result the intermediate soaking steps of the conventional process are minimized or completely eliminated. In our invention, this microwave heating system installed with infrared temperature sensor and the temperature controller is of prime importance. In the present invention, green resinoid grinding wheels are stacked suitably in the predesigned customized sample holders made from machinable susceptor material such as graphite, which is a good absorber of microwave radiations. These are placed in the microwave cavity in such a way to get uniform exposure in the microwave field at 2.45GHz. Another component of our system; an infrared temperature sensor is focused on the sample for monitoring and controlling the temperature, and maintaining a typical heating profile of the grinding wheels during curing process. The infrared sensors can be replaced by thermocouples with proper design and arrangement. Samples are heated as per optimized heating profile in a microwave cavity. The total time taken for the curing is significantly lower compared to the conventional method.
The sample holders in one embodiment of our invention for curing green resinoid grinding wheels are customized plates, made of graphite. They do not only aid as the lender of support to the samples, but also play an important role in maintaining the shape and size of the grinding wheel too. Without the use of such pre-designed customized sample holders, the entire system would be rendered similar to that of the normal microwave heating system with or without microwave susceptible sample holders known in the prior art and with all its inherent limitations.
BRIEF DESCRIPTION OF THE DRAWINGS:
Figure 1 represents Schematic diagram of the microwave system.
Figure 2 represents schematically the sample and sample holder arrangement.
Figure 3 represents Time- Temperature profile of curing of grinding wheels in a Microwave system.
Figure 4 represents Comparison of Time-temperature profile of curing Grinding Wheels: Microwave Vs Conventional oven.

DETAILED DESCRIPTION OF THE DRAWINGS:
As required, details of one embodiment of the present invention are disclosed herein. However, it is to be understood that disclosed are merely exemplary of the invention which may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention in virtually any appropriately detailed structure.
In the present invention, green resinoid grinding wheels are stacked suitably in the predesigned sample holders made from susceptor 1 material such as graphite, which is a good absorber of microwave radiation at room temperature. The sample holder 1 and green grinding wheel 2 are stacked alternately one above the other as shown in figure. 2. The sample 2 is always pressed between sample holders 1 which apply the optimum load in the gravitational direction required for maintaining the desired profile of the grinding wheel 2 during curing. These are placed in the microwave cavity 5 as shown in figure 1, in such a way to get uniform exposure in the microwave field at 2.45GHz. A microwave inlet 4 is provided at the top of the microwave cavity. An infrared temperature sensor 3 is focused on the sample for monitoring and controlling the temperature, and a typical heating profile is shown in figure 3. Samples 2.are heated as per optimized heating profile in a microwave cavity. The total time taken for the curing is significantly lower compared to the conventional method as shown in figure 3.
The invention describes the heating i.e. curing of resin bonded fiber reinforced grinding wheels by electromagnetic radiation (EMR), such as microwave, by using a special susceptor 1 which is a good absorber of microwaves at room temperature. The susceptor material selected in the invention is graphite, which can be machined in predesigned shapes of the grinding wheel. There are alternatives of graphite materials like silicon carbide, zirconia based materials, and ferrites etc. But, these alternatives have inherent problem as the machining of these ceramic susceptor materials is difficult. They need to be either pressed or casted in the desired shape and then sintered to high temperature to attain the strength and geometry. Therefore, the material used in this invention is graphite which is easily machinable. The multiple roles of graphite in this invention are a) susceptor 1 for initial heating of grinding wheel, b) sample holder and separator, c) load and pressure provider to maintain the final geometry of the grinding wheel and d) absorber of the reflected microwaves form the metallic ingredients present in the grinding wheel.

Though it is a prior art to have microwave susceptible sample holders in microwave systems for heating purposes, it is not at ail obvious to use graphite in curing grinding wheels because, graphite is very fragile and hence vulnerable to breakage in sturdy process. Besides, it may get oxidized if it is subjected to heat beyond a permissible limit, whereby the intended objective will not be obtained. Moreover, it should also be sturdy enough to maintain the physical shape of the grinding wheel, as it is subjected through the curing process, without itself getting changed or damaged. And it is also not the case that the sample holders would be such as used and thrown. It should be durable enough to not only deliver the benefits but also .be fit for repeated use in the system, to make the system economical. Hence, the right kind of susceptible sample holder with proper design and customized properties, based on extensive research on trial and error'basis has been the foundation of this seemingly easy, but considerably intricate invention.
As mentioned earlier, the preferred form of this invention makes use of optimum size sample holders made from machinable susceptor 1 material such as graphite, arriving at the appropriate dimension of the sample holder is a part of the ingenuity in this invention. The thickness of the susceptor plate for the curing of depressed and cut-off grinding wheel of 100 mm and 180mm respectively can vary from minimum 10 mm thickness and weight about 100 g to a maximum of 15 mm thickness and weight about 250 g; for 100mm depressed wheels; and minimum 5 mm and weight 300g to maximum 10 mm thickness and weight 400 g for 180 mm cut-off wheels. This range of thickness and weight holds true to exert the desired optimum load in the gravitational direction as shown in figure 2 for maintaining the profile of the grinding wheel 2 after curing. This optimum load allows plastic fiber to enter uniformly in the matrix and strengthening or fiber reinforcement of the grinding wheel. The susceptor plate made of graphite used in this invention exhibits good absorbing characteristics for microwave radiation at room temperature, thereby absorbing part of the microwave energy to increase its temperature, when placed in the microwave cavity. By increasing the thickness or weight beyond the optimum values invented, more energy is consumed and reduced economic advantage of the so invented process. By decreasing the thickness and weight beyond the optimum values invented, the susceptor plate shall exert less load than the desired optimum load for maintaining the profile of the grinding wheel after curing and under low load the cured wheel may get distorted. By reducing thickness below invented optimum value, the fragile property of the susceptor plate such as graphite is increased and it becomes soft, brittle and need frequent replacement after few cycles, this makes the invented system and process uneconomical.
In the conventional process a steel plate weighing about lOOOgm is used for the curing of single sample weighing about 90g to retain the geometry of the sample 2. But, in the

present invention, susceptor plate made of graphite, weighing about 400gm was employed thereby reducing the dead load by about 60%. During the conventional heating process, unnecessary heating of the steel plates consumes disproportionate energy which is reduced drastically by the employment of light weight susceptor which works as an active heater under electromagnetic field. The function of graphite in this invention Is also to absorb any reflected microwaves from the metallic components present in the green shape to avoid reflection of microwaves going to the magnetron for its protection. It also exerts the desired pressure on the wheel that enables the fiber reinforcement to penetrate in the matrix to bond the matrix, thereby achieving the desired strength and geometry. The active participation of the sample holders during microwave curing helps in drastic reduction of total time required for curing of resinoid grinding wheels compared to the conventional processing in an electric heating system with metal separators as shown in figure.4. In the present invention, drying of green samples 2 before curing is not required thereby making the entire process much simpler and faster.
Processing conditions of depressed and cut-off wheels are given below in Table I: Table l: Typical processing conditions of depressed and cut-off wheels

Type of sample Curing temperature °C Total time (mins.) Power consumed (KWh)
Depressed
Resinoid
Wheel 220 90 1.12
Cut-Off
Resinoid
Wheel 220 100 1.31
Many modifications in addition to those describes above may be made to the technique described herein without departing from the spirit and scope of the invention. Accordingly, following are examples only and or not limiting of the scope of the invention.
Example 1
The green compact sample of cut-off resinoid grinding wheel, 180 mm diameter, 4.5mm thick and 22 mm hole diameter weighing 180gm was placed between 12 mm thick graphite susceptors weighing 300gm each. The grinding wheels consisting of aluminum oxide with mixture of phenolic resin and fillers are cured at 220T in 700W microwave system within 100 minutes. The power consumed was approximately 1.31KWh. A typical time temperature profile is given in fig. 4.

Example 2
The green compact sample of depressed resinoid grinding wheel, 100 mm diameter, 5mm thick and 15mm hole diameter, weighing 90gm was placed between 12 mm thick graphite susceptors weighing 200gm each. The grinding wheels consisting of aluminum oxide with mixture of phenolic resin and fillers were cured at 220°C in 700W microwave system within 90 minutes. The power consumed was approximately 1.12KWh.
Example 3
The wheel produced above was tested for metal removal rate estimation. For this purpose cured grinding wheels are mounted on a lathe machine installed with electrical motor that delivers 6200 rpm to the 5 mm thick depressed resinoid grinding wheel of 100 mm diameter. Trial was conducted on 2.8 mm dia., 338 mm long C22.8 grade carbon steel rod weighing 1.6 kg. The carbon steel rod was mounted on the lathe and by adjusting auto motor travel settings maintained uniform travel speed of the wheel with a constant rate in a forward direction giving a cut of about 1 mm on the rotating carbon steel rod. The duration of cutting was 30 min. for both microwave cured wheel and commercial grinding wheel. After completion of 30 min., the rod was removed from the lathe and its final dimensions, and weight were noted. Similarly, the change in the diameter and weight of grinding wheel were noted. Results are listed below in tabular form in the following Table II:
Table II: Material removal data collected during trial of the depressed resinoid grinding wheel, 100 mm diameter, 5mm thick on C22.8 carbon steel bar

MW cured Wheel Commercial
Initial dia. of steel rod [dir ,mm) 28.4 28.4
final dia. of steel rod (dfr, mm) 27.3 27.6
Initial dia. of wheel (diw mm) 100.5 99.96
final dia. of wheel ( dfw mm) 100.34 99
Initial wt. of steel rod (Wir gm) 1634.3 1608.1
final wt. of steel rod (Wfr gm) 1608.1 1590
Initial wt. of wheel {Wiw gm) 98.62 97.17
final wt. of wheel (Wfw gm) 98.3 96.69
Initial vol. of steel rod (V, mm3) 214004.56 214004.6
final vol. of steel rod (Vf mm3) 209772.14 210379.5
Initial vol. of wheel ( vt mm3) 39643.5 41963.9
final vol. of wheel ( vf mm3) 39517.4 41161.8

Using the standard formula "i & jj mentioned below, the Metal Removal Rate (MRR, mm /min.) and G-Ratio i.e. the ratio of metal volume removed to volume of wheel consumed was estimated.
Metal Removal Rate (MRR)= (V; - Vf)/T -— (i)
Where, Vj - Initial volume of the rod, Vf- Final volume of the rod, T - Time (min.)
G-Ratio = (Vi-Vf)/(Vi-Vf) —(ii)
Where, v- Initial volume of the grinding wheel, vf- Final volume of the grinding wheel Table III: Performance comparison @6200 rpm

Wheel MRR mrnVmin. G-Ratio
MW-1 141.1 33.6
Commercial-1 120.8 4.5
The results as shown above in Table III demonstrate that MRR and G-Ratio values of MW cured grinding wheel are better than the commercial wheel (commercial-1) when they are compared at 6200 RPM.
Example 4
The wheel produced by above method was used for cutting metal and for this purpose was mounted on a lathe machine installed with electrical motor that delivers 11500 rpm to the depressed resinoid grinding wheel of 100 mm diameter. Trial was conducted on 2.8 mm dia., 338 mm long C22.8 grade carbon steel rod weighing 1.6 kg. The carbon steel rod was mounted on the lathe and by adjusting auto motor travel settings maintained uniform travel speed of the wheel with a constant rate in a forward direction giving a cut of about 1 mm on the rotating carbon steel rod. The duration of cutting was 30 min. for both microwave cured wheel and commercial grinding wheel. After completion of 30 min., the rod was removed from lathe and its final dimensions, and weight were noted. Similarly the change in the diameter and weight of grinding wheel were noted. From the data and using the standard formula i & ii mentioned above in example 3, the Metal Removal Rate (MRR, mm3/min.) and G-Ratio i.e. the ratio of metal volume removed to volume of wheel consumed was estimated.

Table IV: Performance comparison @11500 rpm

Wheel MRR mmVmin. G-Ratio
Mw 169.2 72.5
Commercial-2 164.7 11.8
MW 177.8 42.7
Commercial-3 100.0 20.6
The results as shown above in Table IV show that MW cured wheels of this invention perform with better durability (G-ratio) at both high and low cut rates compared with the conventionally cured grinding wheel with the same composition (commercial) as well as with the commercial grinding wheels of the same type (specifications) (commercial-3) made by different manufacturers using their own formulation and process. The microwave cured wheels show significantly higher wheel life and also lower level of burn.
Advantages of the invented process:
1) This invented process is simple, affordable and economical.
2) In the invented process grinding wheel is rapidly, volumetrically and selectively heated by microwave, thereby requiring much less time for curing.
3) The present process maximizes the utilization of electromagnetic energy such as microwave by using the pre-designed microwave susceptor.
4) The present process provides better properties of the final product such as Metal Removal Rate (MRR) and G- ratio.
5) In conventional curing metallic plates are used as separators and load for retaining the shape of the wheel. In the invented process the input energy is utilized only for heating the desired material due to drastic reduction in the unnecessary dead load of the metallic plate placed as separators and replaced by thin lightweight materials.
6) In the invented process, by using sample holders made from microwave susceptor materials, the reflected microwaves from the metallic ingredients present in the grinding wheel are absorbed by the sample holders, thereby enhancing the energy efficiency.
7) The present novel invented process uses a simple design with huge economic benefits.

While the invention has been described, disclosed, illustrated and shown in certain terms or embodiments or modifications which has been undertaken practically, the scope of the invention is not intended to be nor should it be deemed to be limited thereby and such other modifications or embodiments as may be suggested and teachings herein are particularly reserved especially as they fall within the breadth and scope of the claims here appended. The preferred form of this invention has been described above. It is possible that modifications thereof may occur to those skilled in the art which will fall within the scope of the following claims.

We Claim,
1) A system of rapid curing of resin bonded grinding wheels; comprising
a) Microwave heating cavity,
b) Sample holders made from microwave susceptor materials suitable for such cavity and samples,
c) Green Samples of resinoid grinding wheels,
d) Infrared temperature sensor,
e) Mechanism to control the temperature within the microwave cavity,
wherein the sample holders are pre-designed and customized with respect to the
desired geometry and properties of the grinding wheels.
2) A system of rapid curing of resin bonded grinding wheels as described in claim 1; wherein the sample holders are made of carbon bearing material of non ceramic nature, preferably graphite
3) A system of rapid curing of resin bonded grinding wheels as claimed in claim 1; wherein both batch wise or continuous process of curing of the grinding wheels can be deployed.
4) A system of rapid curing of resin bonded grinding wheels as claimed in claim 1; wherein the microwave radiations used could be continuous or pulsed.
5) A system of rapid curing of resin bonded grinding wheels as described in claim 1; wherein at least the green sample of resinoid grinding wheels to be cured is held in the pre-designed and customized sample holders and subjected to microwave heating in the microwave cavity
6) A system of rapid curing of resin bonded grinding wheels as claimed in claim 1; wherein the microwave energy is used in the frequency range of 800 to 5000MHz, preferably between 890 to 2450MHz and more preferably 2450±50 MHz.
7) A method of rapid curing of resin bonded grinding wheels as described in claim 5; wherein the samples are heated in a temperature range of 180 degrees to 220 degrees; preferably at 220 degrees.
8) A system and process of rapid curing of resin bonded grinding wheels using sample holders made from microwave susceptor materials as described in the text and examples.

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