Title of Invention	A METHOD OF MAKING A MONOLAYER ABRASIVE TOOL CONTAINING A MONOLAYER, CUBIC BORON NITRIDE (CBN) AND A CBN ABRASIVE TOOL THEREOF
Abstract	The invention relates to a method of making a monolayer abrasive tool containing a monolayer cubic boron nitride (cBN) abrasive grit bonded to a steel substrate. Method comprises applying uniformly on the surface of a plain carbon steel substrate, a brazing paste made of Ag-Cu eutectic alloy (72% Ag, 28% Cu) mixed with 1% to 3% of titanium or titaniumhydride powder, placing cBN grits (nono crystalline/microcrystalline type) on said brazing paste, uniformly placing said grit on the surface of said steel substrate, drying the tool substrate alongwith the brazing paste and cBN grit at a temperature varying from 75°C - 125°C for 5 to 15 minutes, placing the abrasive

Title of Invention

A METHOD OF MAKING A MONOLAYER ABRASIVE TOOL CONTAINING A MONOLAYER, CUBIC BORON NITRIDE (CBN) AND A CBN ABRASIVE TOOL THEREOF

Abstract

The invention relates to a method of making a monolayer abrasive tool containing a monolayer cubic boron nitride (cBN) abrasive grit bonded to a steel substrate. Method comprises applying uniformly on the surface of a plain carbon steel substrate, a brazing paste made of Ag-Cu eutectic alloy (72% Ag, 28% Cu) mixed with 1% to 3% of titanium or titaniumhydride powder, placing cBN grits (nono crystalline/microcrystalline type) on said brazing paste, uniformly placing said grit on the surface of said steel substrate, drying the tool substrate alongwith the brazing paste and cBN grit at a temperature varying from 75°C - 125°C for 5 to 15 minutes, placing the abrasive

Full Text	TECHNICAL FIELD The present invention relates to a method of making a monolayer abrasive tool used in surface grinding and said tool. The present invention particularly relates to such tools employing a brazing process for bonding a monolayer of cBN cubic boron nitride grits to a substrate with uniform grit spacing and bond uniformity. BACKGROUND OF THE INVENTION In recent years, brazing of abrasive particles to a suitable substrate has gained importance in the material processing industry with a view to manufacturing tools which can out perform conventional galvanically bonded tool. In principle the advantage of brazing lies in the fact that the chemical nature of bonding as obtained during brazing allows low bond coverage to hold the grits than that required in mechanical type bonding in galvanic process. This feature results in large grit protrusion and hence large chip clearance space in the brazed tool in comparison to that in galvanically bonded wheel for a given grit density on both the wheels. According to US Patents 3894673 and 4018576 first monolayer diamond brazed tool which could outperform galvanic type diamond tools uses hard durable Ni-Cr based alloys. It is further claimed that soft low melting point braze alloy such as silver/copper eutectic and near eutectic composition containing a relatively........... small amount of titanium which could readily wet diamond foils to form a bond exhibiting adequate mechanical properties which would retain the diamond particles under rigorous grinding and cutting application of commercial acceptance. Such brazed diamond tools could not compete with the electroplated tools. It is also claimed that Ni-Cr alloys are superior to Ag-Cu base soft alloys in terms of higher mechanical strength and resistance to swarf abrasion and diamond grit retention. However, the said patent did not teach about any merits or demerits of Ag—Cu base alloys and Ni--Cr alloys in making monolayer cBN grinding wheel and its performance with respect to galvanic type cBN grinding wheel. It has been first revealed (US Patent 5,129,918) that Ni-Cr base alloys cannot wet or bond cBN grits effectively. Good wetting is possible when cBN grits coated with a film of TiC, CrC (US Patent 5,129,918) or Sic (US Patent 5,389t118). Even a Cr free Ni-base alloy can wet and bond TiC coated or CrC coated cBN particles. It is further claimed (US Patent 5,492,771) that single layer cBN brazed wheel fabricated with Ag-Cu eutectic alloy containing small amount of Ti is less performing than a corresponding galvanically bonded cBN wheel. It is mentioned in the said patent that Ag—Cu—In alloy (combined amount of Ag and Cu at least 70%, Indium 8-15% by weight) containing small amount of Ti sufficient to wet and bond cBN grits could only out perform the galvanically bonded cBN wheel. It is claimed that the primary difference between the exceptional performance of the braze alloy as mentioned in the said patent for bonding a monolayer of cBN particles on the tool and failure of the less strong eutectic and near eutectic silver/copper alloys to make commercially acceptable abrasive tools appears to be relatively small increase in yield strength. In the said patent it has also been mentioned by way of example that performance of cBN wheel fabricated with Ag-Cu eutectic alloy containing small amount of Ti is 10% inferior to a galvanically bonded monolayer cBN interms of number of parts produced whereas performance of monolayer cBN wheel brazed with Ag-Cu-In alloy (Ag 61.5%, Cu 24%, In 14.5) containing small amount of Ti was improved by 60% with respect to the galvanically bonded monolayer cBN wheel. However, it has been found by the applicant that the brazed cBN wheel with high grit protrusion than that of galvanically bonded cBN wheel and having similar grit concentration as that of galvanic type shows no advantage in grinding steel with high material removal rate. This is observed for both unhardened and hardened bearing steels. Three brazing alloys viz. Ag—Cu eutectic with small amount of Ti, Ag 61.5% Cu 25%, In 14.5% containing small amount of Ti and a Ni-Cr base braze alloy containing 76% Ni , 14% Cr, 10%P were used to fabricate three brazed monolayer cBN wheel. For Ni-Cr braze alloy TiC Coated cBN grits were used. The brazed cBN wheels did not have any problem of bond erosion by abrasion of grinding swarf and its effect on retention of cBN particle in the bond. The grinding wheels experienced wheel loading leading to premature failure of the wheel. This observation points to the fact that just by increasing the grit protrusion, life of monolayer cBN grinding wheel cannot be increased. Similarly increase of yield strength of the braze material did not necessarily mean an improved performance of the same. It can be very well realized that for monolayer diamond abrasive tool which machines abrasive materials producing frangmented or gritty chips, wheel loading is not the main mode of failure. It has been found that improved abrasion reaistance of Ni-Cr alloy compared to galvanic Ni-bond is advantageous in holding the diamond crystals on the tool substrate. Additionally large protrusion of diamond grit in brazed wheel leads to extended tool life for brazing wheel in comparison to galvanically bonded wheel. It is further noted that one of the essential requirement of brazed cBN tool for free and fast cutting is the uniformity of the bonding layer over the entire working surface of the abrasing tool. It has been found that AG-Cu eutectic or near eutectic tool............... containing small amount of Ti when applied in the form of a paste layer, during brazing, not only wets cBN grits but flows well on the plain carbon steel substrate making uniform bond level on the substrate. Similar result was also obtained when Ti coated cBN grits are used with Ag-Cu eutectic near eutectic alloy containing no Ti. Also, during brazing with the Ag-Cu-In braze alloy (contained amount of Ag and Cu at least 70%, Indium 8-15% by weight) containing small amount of Ti (USA Patent 5,492,771) uniform bond level could be obtained on the plain carbon steel substrate. However, it is also known that for brazing purpose, stainless steel, tool steel are particularly suitable because of their general resistance to wrapage or dimension instability during high temperature processing. Similarly tungsten carbide would be a better candidate as the substrate instead of steel because it is not only stable at brazing temperature but also having higher rigidity than steel. The applicants have found that Ag-Cu base alloys with or without any addition of Ti, when applied on high speed steel, stainless steel or (Tungstel Carbide) WC substrate could not form uniform bond level during brazing of cBN grit. During brazing of cBN grits with Ag-Cu-In ................................................ alloy (contained amount of Ag and Cu at; least 70%, Indium 8-15% by weight) containing small amount of Ti (Re: USA Patent 5,498,771) uniform bond level could not form on high speed steel, stainless steel and tungsten carbide substrate. Two step brazing wherein the filler material was applied to the substrate in the form of paste and then melted followed by deposition of cBN particles on the solidified layer and subsequent brazing, however, did not bring about any improvement in the uniformity of the bonded layer. Thus, there remains a demand for a low cost practical method of brazing of a monolayer of cBN grits to substrate like carbon steel, stainless steel, alloy steel like high speed steel, tungsten carbide-colbalt composite. OBJECT OF THE INVENTION One of the objects of the present invention is to provide a cBN abrasive wheel. Another object of the present invention is a method of manufacturing a cBN abrasive wheel which can satisfy the requirement of load free grinding. With this object in view, the main aspect of the present invention is to provide a cBN abrasive tool comprising a steel or cemented carbide substrate and a single layer of cBN grit bonded to thesaid substrate by a brazing alloy characterized in that the said cBN abrasive tool satisfies the requirement of load free grinding. The present invention provides a method of making a monolayer abrasive tool containing a monolayer cubic boron nitride (cBN) abrasive grit bonded to a steel substrate and which method comprises of the following steps — a) applying uniformly on the surface of a plain carbon steel substrate, a brazing paste made of Ag-Cu eutectic alloy (72% Ag.28% Cu) mixed with 1% to 3% of titanium or titaniumhydride powder; b) placing cBN grits (mono crystalline/microcrystalline type )said brazing paste in a manner that a monolayer configuration of the grits with inter-grit spacing of 50 µm to 250 µm is obtained; c) uniformly placing said grit on the surface of said steel substrate with the help of a sieve or mesh of specific size so that only one grit of a particular grit size may pass through the mesh; d) drying the tool substrate along with the brazing paste and a cBN grit placed on it at a temperature varying from 75 oC - 125 oC for 5 to 15 minutes to remove volatile materials from the brazing paste; e) placing the abrasive tool specimen in a vacuum brazing furnace, where vacuum in the order of 2x10-5 Torr to 8x10-6 Torr is obtained in 10 to 12 min. with help of a rotary pump and a diffusion pump. f) raising the temperature of the tool, specimen to the brazing temperature varying in the range from 800o C to 900o C in a manner that the pressure inside the vacuum chamber never rises above 5x10-5 Torr; g) maintaining the temperature of the tool specimen for 0.5 mimutes to 5 minutes; and h) cooling said tool specimen in vacuum (1x10-5 Torr — 8x10-6 Torr before removing the same from the furnace. The invention further provides a cBN abrasive tool comprising metal or cemented carbide substrate and a single layer of cBN grits bonded to the said substrate by a braze alloy which offers load free grinding wherein the abrasive tool is provided with high grit protrusion, regular grit distribution and adequate grit spacing to overcome failure by wheel loading. In said cBN abrasive tool, cBN grits are disbributed in a predetermined pattern on the braze layer applied on the substrate and are not displaced during brazing, when the braze alloy becomes totally liquid and remain in the same position and no clamping device is required to hold grits in respective position. The braze alloys comprise of passive Ag-Cu base alloys capable of wetting and bonding Ti coated cBN or Ag-Cu base active alloys capable of wetting and bonding uncoated cBN grits at temperatures below 950oC where during brazing, said braze alloy hold the cBN grits intact in their position without causing any displacement of grits and capable of forming uniform bond layer on the plain carbon steel substrate. In the cBN abrasive tool of the present invention Ag-Cu passive or Ag-Cu active alloys are incapable of providing uniform bond layer during brazing of cBN grits on stainless steel, high speed steel and tungsten carbide substrate but are capable of providing uniform bond layer when the surface of the said substrate materials are modified. The said substrate in the abrasive tool are coated with a film of titanium nickel. Said substrate may also be coated with a primary coating of titanium and a secondary coating of nickel. DESCRIPTION OF THE DRAWINGS The present invention will now described in detail with reference to the accompanying drawings wherein Figure 1 shows the cross sectional view of an monolayer abrasive disc or tool embodying the present invention, and Figure 2 shows another example of cBN abrasive tool as illustrated in Figure 1. In the accompanying drawings Figure 1 shows the cross-sectional view of an illustrative disc embodying the invention wherein cubic boron nitride abrasive particles 'D' are bonded to the circular disc substrate 'A' in a monolayer configuration by the layer of a brazing alloy 'B' having a thickness C. The principal characteristic features of the cBN abrasive wheel are grit protrusion 'E' above the braze layer and the inter grit spacing 'F' along the periphery of the disc . Cross-sectional view in Figure 2 illustrates another example of the cBN abrasive tool described in Fig.1. In this instance the cBN grits 'D' are provided with a surface coating 'E'. The circular disc substrate 'A' is also provided with a coating 'B' on its peripheral surface. DETAILED DESCRIPTION OF THE INVENTION During the course of invention it has been observed by the applicant that the problem of wheel loading which occurs in the grinding wheels offering large chip clearance space through high crystal protrusion can be solved if all the grits on the working surface are bonded to the tool body keeping uniform space between The adjacent grits unlike that of a galvanic type wheel where grit density is high and spacing is also not uniform. However, according to the principle of grinding, with increase of grit spacing, ie. low grit density, chip volume to be removed by each grit also increases and apparently there is no advantage in increasing the light-grit spacing. During the course of invention it has been also found by the applicant that Ti or TiC coated cBN grits which are wetted by commercially available Ni-Cr based braze alloy can be precisely placed in a regular pattern on the braze paste layer of the said braze alloy applied on the carbon steel substrate. But the grits shifted from their location during brazing when the alloy becomes totally molten and present grit spacing on the wheel surface cannot be maintained. This behaviour of the Ni-Cr based brazed alloy can be regarded as the major incapability of the braze alloy in manufacturing abrasive tool with predetermined grit spacing and got unnoticed by those skilled in the art and this was never reported. Surprisingly it has been further found by the applicant that when Ag-Cu base eutectic alloy used for brazing; same Ti coated cBN grits placed precisely in a regular pattern keeping a well defined spacing between the adjacent grits on the braze paste layer applied to the carbon steel substrate, did not shift from the adjacent grits unlike that of a galvanic type wheel where grit density is high and spacing is also not uniform. However, according to the principle of grinding, with increase of grit- spacing i.e. low grit density, chip volume to be removed by each grit also increases and apparently there is no advantage in increasing the inter-grit spacing. During the course of invention it has been also found by the applicant that Ti or TiC coated/cBN grits which are wetted by commercially available Ni-Cr based braze alloy can be precisely placed in a regular pattern on the braze paste layer of the said braze alloy applied on the carbon steel substrate. But the grit© shifted from their location during brazing when the alloy becomes totally molten and preset grit spacing on the wheel surface cannot be maintained. This behaviour of the Ni-Cr based brazed alloy can be regarded as the major incapability of the braze alloy in manufacturing abrasive tool with predetermined grit spacing and got unnoticed by those skilled in the and this was never reported. Surprisingly it has been further found by the applicant that when Ag-Cu base eutectic alloy used for brazing; same Ti coated cBN grits placed precisely in a regular pattern keeping a well defined spacing between the adjacent grits on the braze paste layer applied to the carbon steel substrate, did not shift from their initial position during brazing. Preset position of cBN grits could be precisely maintained even when the alloy became liquid totally. Similar behaviour was also discovered with Ag-Cu eutectic or near eutectic alloy containing small amount of Ti during brazing of uncoated cBN grits on plain carbon steel substrate. This behaviour of the braze alloy can be regarded as its remarkable capability in the manufacture of a abrasive tool with predetermined grit setting without the requirment of any fixture for holding the grits in position when braze alloy became completely liquid and it is quite surprising that such behaviour of the Ag-Cu base alloy has gone completely unnoticed by those skilled in the art. This characteristics of the Ag-Cu alloy is truly unexpected, when one consider the facts that similar characteristics has not been demonstrated by Ni or Ni-Cr base brazing alloys. The cBN wheel with uniform grit spacing comprising of a substrate, Ti coated cBN grit and Ag-Cu alloy or uncoated cBN grit and Ag-Cu-Ti active alloy has revealed the fact that grinding performance of this wheel could surpass that of galvanically bonded wheel and this has escaped the notice of those skilled in the art. It was only revealed that to make high performance wheel use of Ni-Cr alloy or Ag-Cu-In-Ti alloy is not sufficient. On the other hand, by ascertaining the characteristics of Ag-Cu alloy it was possible to overcome the limitations of the brazed cBN in a very simple way. For cBN wheel which is prone to failure like wheel loading in dry grinding of hardened and unhardened material at high material removal rite, the problem can be solved by using the new method as disclosed in the invention and this has gone completely unnoticed by those skilled in the arts. The available yield strength of Ag-Cu or Ag- Cu-Ti alloy was found to be adequate to prevent failure of the cBN wheel by mechanical failure of the braze alloy. However, there is no limitation in using Ag-Cu base alloys with higher yield strength provided they show the same characteristics that is in the liquid state, the braze alloy will not casue any movement of the grit from its initial set position and grit spacing can be maintained during brazing. Braze filler metals are available commercially or can be prepared based on system other than Cu-Ag provided the braze alloy can wet Ti coated cBN or when activated can also wet uncoated cBN grit and also show the same characteristics as found with Ag—Cu alloy in the present invention. It is obvious that exact grit spacing will depend on the material removal rate, ductility of the work material. It will be appreciated that grit spacing should be chosen so that chip clogging and wheel loading is avoided. Unnecessary increase in grit spacing will increase specific grinding force on each grit and may cause rapid grit wear by fracture. However, according to the specific grit force, the friability of the grit can be changed. Similarly size of the grit can also be changed. During the course of invention it was also found that the Ag-Cu base alloys applied to the tool steel, stainless steel or tungsten carbide substrate in the form of a paste can easily form a uniform bond level during brazing of cBN grits when the surface chemistry of the substrate is modified. The choice of the coating may depend on the type braze material, ie. whether the braze material is a passive not containing any active element like Ti or an active braze alloy containing transition elements like Ti. Similarly the coating deposition technique may be CVD, PVD which depends on the material of substrate, geometry of the substrate and the film desired to be deposited. For economic reason it is obvious that conventional or well established techniques may be used but in principle there is no limitation imposed on use of non-conventional deposition technique. The thickness of the coating should be as minimum as possible but sufficient to cover the entire surface of the substrate. The purpose of the coating is only to change the surface chemistry of the substrate. Generally speaking 1~2 µm thick coating would be sufficient for the said purpose. The................................................................................................ deposition may be high temperature or low temperature process. It is also evident that coating material improves its adhesion with the substrate during brazing operation when some thermochemical interaction takes place between the film and the substrate. The present invention has been described in more detail and can be appreciated from the following examples, which are in no way intended to limit the scope of the present invention. Example I - Small cylindrical grinding wheels were fabricated using a plain carbon steel substrate and monolayer of cBN grits and silter-copper eutectic alloy. The silver-copper brazing alloy "Cusil" from WESGO Inc, USA in the form of paste was applied uniformly on the round surface of the plain carbon steel substrate. This was followed by placement of cBN grits in monolayer configuration with predetermined inter grit spacing. The cBN grits were of size B151 (150 µm/125 um mesh width) of grade Borazon 510 (Ti coated cBN) from General Electric Company. Uniform placement of grit could be done with the help of a sieve or mesh, the holes in which could accept just one grit at a time. The size of the hole would be such just to allow the largest grit to pass through. The pitch of the hole would be determined by the requirement of grit spacing. There could be other methlod of placement of grit in a regular pattern. The braze layer in the form of paste could hold the grits at their deposited position temporarily without any difficulty. The .................................................................................. tool specimen was then dried at 100oC for 15 min to remove excessive volatile materials. The tool specimen was then placed inside a vacuum brazing furnace. Rough vacuum in the order of 10-2 Torr was obtained in 8 min by the rotary pump and a vacuum better than 1 x 10-5 Torr could be obtained in 15 min by connecting the diffusion pump, Temperature was then raised to 450o C and kept at that point for about 1 min followed by further increase to 850oC in 4 minute™ The specimen was held at temperature for just 1 min. This was followed by cooling under same atmosphere i.e., in vacuum not less than 1 x 10-5 Torr. The brazed tool specimen was then removed from the furnace for physical examination. The Ti coated cBN grits were effectively wetted by the "Passive" Ag-Cu eutectic alloy. The braze layer also maintained its uniformity throughout, the working surface. The cBN grits were found to set to the substrate in the same location where those were placed before brazing and on displacement of grit took place. Example II The brazing process of Example I was define carried out except that grits were uncoated cBN of grade Borazon 500 and the braze alloy was an active braze alloy prepared by adding 2% by wet of titanium hydride with commercially available Ag-Cu eutectic braze alloy powder "Cusil" from WESGO. It is pbvious that titanium hydride powder was thoroughly mixed with the eutectic powder to obtain a good homogenity of the active metal in order that upon melting necessary wetting properties of the liquid could be obtained through out the entire mass. After brazing the cBN grits were seen not only to have good bond with the braze alloy but also to maintain the same location where those were placed before brazing and no displacement of grit took place. Example III The brazing process of example II was carried out except that the grits were uncoated microcrystalline cBN grit of grade Borszon 570 from Beneral Electric with grit size B 151. After brazing, microcrystalline grits were found to be effectively wetted by the brazing alloy. The braze also maintained a low but uniform level throughout the working surface of the tool resulting in large Crystal protrusion. The microcrystal1ine cBN grits were found to S~et to the substrate in the same location, where those were placed before brazing and no displacement of grit took place. Example IV The brazing process of example III was carried out except that substrate material was a sustentic stainless steel and the substrate was coated 1~2 um thick PV (Physical Vapour Deposition) Ti. After brazing a uniform braze layer was obtained on the working surface of the grinding wheel resulting in large grit protrusion. The microcrystal1ine cBN grits were found to set to the substrate in the same position where those were placed before brazing and no displacement of grit took place. Example V The brazing process of example IV was carried out except that the substrate material was martensitic stainless steel and the s substrate was coated with 1~2 µm thick PVD Ti Coating. After brazing a uniform braze layer was obtained on the working surface of the grinding wheel resulting in large crystal protrusion. The microcrystal1ine cBN grits were found to have bonded to the substrate and remain in the same position where such grits were placed before brazing and no displacement of cBN grits occurred during brazing. Example VI The brazing process of example I was carried except that stainless substrate was austentic/steel and the substrate was coated with a primary coating of PVD Ti and secondary coating of galvanic Ni. After brazing a containuous and uniform layer of braze alloy was obtained which wetted and bonded the cBN crystals resulting in large crystal protrusion. The cBN grits were found to set to the substrate in the same location where those were fixed before brazing and no displacement of grit took place during brazing. Example VII The brazing process of example VI was carried except that substrate was hardmetal having 94% WC and 6% Co. The substrate was coated with a primary coating Ti and a secondary coating of Ni. After the 'one step' brazing a continuous and uniform layer of braze alloy was obtained. The cBN grits were found to set to the substrate in the same location where those were placed before brazing and no displacement of grit occurred during brazing. The product resulting from the practice of the invention represents a significant advancement in the construction of monolayer cBN abrasive tool. This advancement can be easily realized when one compares the performance of brazed tool with galvanic type cBN tool. Appl :i.cat ion I Operation : Surface grinding Work material : 103 Cr1 (IS) hardened bearing steel (HRC62) Abrasive Tool : i) Salvanic type monolayer cBN wheel (grit size : B 151) ii) Braze type monolayer cBN wheel (grit size : B 151) Process parameters : Wheel speed : 16 m/s. 27.5 m/s. 41 m/s Table speed : 4 m/min Down feed : 10 µm, 20 µm, 30 µm and 40 µm Environment : dry. Sign used in the Table 1 a) 41 : 10 = wheel speed of 41 m/s and downfeed of 10 µm b)v = the wheel behaviour became steady after few passes of spark-in grinding c) X = the wheel loading occurred just after few spark—in grinding passes and the wheel had to be withdrawn. Long duration grinding test 1) With 27.5 m/s wheel speed, 30 µm down feed and 4 m/min table speed, galvanic wheel could remove a specific volume 40 mm3 /mm2 working surface, when the forces rose to a high value indicating wheel loading. On the other hand brazed type tool could remove as high as 100 mm3 /mm2 specific volume of work material without any sign of rise of grinding force and use the brazed cBN tool could be extended further. ii) At higher material removed rate i.e., with 41 m/s wheel speed, 40 µm down feed and 4 m/min table speed, galvanic wheel could remove a specific volume of 20 mm /mm before wheel was withdrawn because of wheel wear and loading leading to high grinding force. The brazed type wheel could remove a specific volume of 60 mm3 /mm2 before it was withdrawn. Application II Operation ; Surface grinding. Working material : Titanium alloy Ti - 6A1 - 4v (32 HRC) Abrasive Tool : i) Galvanic type monolayer cBN wheel (grit size : B151 ) . ii) Braze type monolayer cBN wheel (grit size : B.151) Process parameters : Wheel Speed : 16 m/s, 20 m/s. 27.5 m/s. Table speed : 2 m/min. and 3 m/min Downfeed : 10 µm, 20 µm, 30 µm and 40 µm. Environment : dry. WE CLAIM: 1. A method of making a monolayer abrasive tool containing a monolayer cubic boron nitride abrasive grit bonded to a steel substrate and which method comprises of the following steps- a) applying uniformly on the surface of a plain carbon steel substrate, a brazing paste made of Ag—Cu eutectic alloy (72% Ag, 28% Cu) mixed with 1% to 354 of titanium or ti taniumhyclride powder; b) placing cBN grits (mono crystalline/microcrystalline type) on said brazing paste in a manner that a monolayer configuration of the grits with inter—grit spacing of 50 µm to 250 µm is obtained c) uniformly placing said grit on the surface of said steel substrate with the help of a sieve or mesh of specific size so that only one grit of a particular grit may pass through the mesh; d) drying the tool substrate along with the brazing paste and cBN grit placed on it at a temperature varying from 75 oC - 125 oC for 5 to 15 minutes to remove volatile materials from the brazing paste; e) placing the abrasive tool specimen in a vacuum brazing furnace, where vacuum in the order of 2x10-5 Torr to 8x10-6 Torr is obtained in 10 to 12 min, with help of a rotary pump and a diffusion pump; f) raising the temperature of the tool specimen to the brazing temperature varying in the range from 800°C to 900°C in a manner that the pressure inside the vacuum chamber never rises above 5x10-5 Torr; g) maintaining the temperature of the tool specimen for 0.5 minute to 5 minutes; and h) cooling said tool specimen in vacuum (1x10-5Torr- 8x10-6 Torr) before removing the same from the furnace. 2. A cBN abrasive tool comprising metal or cemented carbide substrate and single layer of cBN grits bonded to the said substrate by a braze alloy as herein described which offers load free grinding wherein the abrasive tool is provided with high grit protrusion, regular grit distribution and adequate grit spacing to overcome failure by wheel loading. 3. A cBN abrasive tool according to Claim 2 wherein said cBN grits which are distributed in a predetermined pattern on the braze layer applied on the substrate are not displaced during brazing, when the braze alloy becomes totally liquid and remain in the same position and no clamping device is required to hold grits in respective position. The invention relates to a method of making a monolayer abrasive tool containing a monolayer cubic boron nitride (cBN) abrasive grit bonded to a steel substrate. Method comprises applying uniformly on the surface of a plain carbon steel substrate, a brazing paste made of Ag-Cu eutectic alloy (72% Ag, 28% Cu) mixed with 1% to 3% of titanium or titaniumhydride powder, placing cBN grits (nono crystalline/microcrystalline type) on said brazing paste, uniformly placing said grit on the surface of said steel substrate, drying the tool substrate alongwith the brazing paste and cBN grit at a temperature varying from 75°C - 125°C for 5 to 15 minutes, placing the abrasive

Full Text

TECHNICAL FIELD
The present invention relates to a method of making a monolayer abrasive tool used
in surface grinding and said tool. The present invention particularly relates to such
tools employing a brazing process for bonding a monolayer of cBN cubic boron
nitride grits to a substrate with uniform grit spacing and bond uniformity.
BACKGROUND OF THE INVENTION
In recent years, brazing of abrasive particles to a suitable substrate has gained
importance in the material processing industry with a view to manufacturing tools
which can out perform conventional galvanically bonded tool. In principle the
advantage of brazing lies in the fact that the chemical nature of bonding as obtained
during brazing allows low bond coverage to hold the grits than that required in
mechanical type bonding in galvanic process. This feature results in large grit
protrusion and hence large chip clearance space in the brazed tool in comparison to
that in galvanically bonded wheel for a given grit density on both the wheels.
According to US Patents 3894673 and 4018576 first monolayer diamond brazed
tool which could outperform galvanic type diamond tools uses hard durable Ni-Cr
based alloys. It is further claimed that soft low melting point braze alloy such as
silver/copper eutectic and near eutectic composition containing a relatively...........
small amount of titanium which could readily wet diamond foils to
form a bond exhibiting adequate mechanical properties which would
retain the diamond particles under rigorous grinding and cutting
application of commercial acceptance. Such brazed diamond tools
could not compete with the electroplated tools. It is also
claimed that Ni-Cr alloys are superior to Ag-Cu base soft alloys
in terms of higher mechanical strength and resistance to swarf
abrasion and diamond grit retention. However, the said patent did
not teach about any merits or demerits of Ag—Cu base alloys and
Ni--Cr alloys in making monolayer cBN grinding wheel and its
performance with respect to galvanic type cBN grinding wheel.
It has been first revealed (US Patent 5,129,918) that Ni-Cr base
alloys cannot wet or bond cBN grits effectively. Good wetting is
possible when cBN grits coated with a film of TiC, CrC (US Patent
5,129,918) or Sic (US Patent 5,389t118). Even a Cr free Ni-base
alloy can wet and bond TiC coated or CrC coated cBN particles. It
is further claimed (US Patent 5,492,771) that single layer cBN
brazed wheel fabricated with Ag-Cu eutectic alloy containing
small amount of Ti is less performing than a corresponding
galvanically bonded cBN wheel. It is mentioned in the said patent
that Ag—Cu—In alloy (combined amount of Ag and Cu at least 70%,
Indium 8-15% by weight) containing small amount of Ti sufficient
to wet and bond cBN grits could only out perform the galvanically
bonded cBN wheel. It is claimed that the primary difference
between the exceptional performance of the braze alloy as
mentioned in the said patent for bonding a monolayer of cBN
particles on the tool and failure of the less strong eutectic and
near eutectic silver/copper alloys to make commercially
acceptable abrasive tools appears to be relatively small increase
in yield strength. In the said patent it has also been mentioned
by way of example that performance of cBN wheel fabricated with
Ag-Cu eutectic alloy containing small amount of Ti is 10%
inferior to a galvanically bonded monolayer cBN interms of number
of parts produced whereas performance of monolayer cBN wheel
brazed with Ag-Cu-In alloy (Ag 61.5%, Cu 24%, In 14.5) containing
small amount of Ti was improved by 60% with respect to the
galvanically bonded monolayer cBN wheel.
However, it has been found by the applicant that the brazed cBN
wheel with high grit protrusion than that of galvanically bonded
cBN wheel and having similar grit concentration as that of
galvanic type shows no advantage in grinding steel with high
material removal rate. This is observed for both unhardened and
hardened bearing steels. Three brazing alloys viz. Ag—Cu
eutectic with small amount of Ti, Ag 61.5% Cu 25%, In 14.5%
containing small amount of Ti and a Ni-Cr base braze alloy
containing 76% Ni , 14% Cr, 10%P were used to fabricate three
brazed monolayer cBN wheel. For Ni-Cr braze alloy TiC
Coated cBN grits were used. The brazed cBN wheels did not have any problem
of bond erosion by abrasion of grinding swarf and its effect on retention of cBN
particle in the bond. The grinding wheels experienced wheel loading leading to
premature failure of the wheel. This observation points to the fact that just by
increasing the grit protrusion, life of monolayer cBN grinding wheel cannot be
increased. Similarly increase of yield strength of the braze material did not
necessarily mean an improved performance of the same. It can be very well realized
that for monolayer diamond abrasive tool which machines abrasive materials
producing frangmented or gritty chips, wheel loading is not the main mode of
failure. It has been found that improved abrasion reaistance of Ni-Cr alloy
compared to galvanic Ni-bond is advantageous in holding the diamond crystals on
the tool substrate. Additionally large protrusion of diamond grit in brazed wheel
leads to extended tool life for brazing wheel in comparison to galvanically bonded
wheel.
It is further noted that one of the essential requirement of brazed cBN tool for free
and fast cutting is the uniformity of the bonding layer over the entire working
surface of the abrasing tool.
It has been found that AG-Cu eutectic or near eutectic tool...............
containing small amount of Ti when applied in the form of a paste layer, during
brazing, not only wets cBN grits but flows well on the plain carbon steel substrate
making uniform bond level on the substrate. Similar result was also obtained when
Ti coated cBN grits are used with Ag-Cu eutectic near eutectic alloy containing
no Ti.
Also, during brazing with the Ag-Cu-In braze alloy (contained amount of Ag and
Cu at least 70%, Indium 8-15% by weight) containing small amount of Ti (USA
Patent 5,492,771) uniform bond level could be obtained on the plain carbon steel
substrate.
However, it is also known that for brazing purpose, stainless steel, tool steel are
particularly suitable because of their general resistance to wrapage or dimension
instability during high temperature processing. Similarly tungsten carbide would be
a better candidate as the substrate instead of steel because it is not only stable at
brazing temperature but also having higher rigidity than steel.
The applicants have found that Ag-Cu base alloys with or without any addition of
Ti, when applied on high speed steel, stainless steel or (Tungstel Carbide) WC
substrate could not form uniform bond level during brazing of cBN grit. During
brazing of cBN grits with Ag-Cu-In ................................................
alloy (contained amount of Ag and Cu at; least 70%, Indium 8-15% by
weight) containing small amount of Ti (Re: USA Patent 5,498,771)
uniform bond level could not form on high speed steel, stainless
steel and tungsten carbide substrate.
Two step brazing wherein the filler material was applied to the
substrate in the form of paste and then melted followed by
deposition of cBN particles on the solidified layer and
subsequent brazing, however, did not bring about any improvement
in the uniformity of the bonded layer.
Thus, there remains a demand for a low cost practical method of
brazing of a monolayer of cBN grits to substrate like carbon
steel, stainless steel, alloy steel like high speed steel,
tungsten carbide-colbalt composite.
OBJECT OF THE INVENTION
One of the objects of the present invention is to provide a cBN
abrasive wheel.
Another object of the present invention is a method of
manufacturing a cBN abrasive wheel which can satisfy the
requirement of load free grinding.
With this object in view, the main aspect of the present
invention is to provide a cBN abrasive tool comprising a steel or
cemented carbide substrate and a single layer of cBN grit bonded
to thesaid substrate by a brazing alloy characterized in that the
said cBN abrasive tool satisfies the requirement of load free
grinding.
The present invention provides a method of making a monolayer
abrasive tool containing a monolayer cubic boron nitride (cBN)
abrasive grit bonded to a steel substrate and which method
comprises of the following steps —
a) applying uniformly on the surface of a plain carbon
steel substrate, a brazing paste made of Ag-Cu eutectic
alloy (72% Ag.28% Cu) mixed with 1% to 3% of titanium
or titaniumhydride powder;
b) placing cBN grits (mono crystalline/microcrystalline
type )said brazing paste in a manner that a monolayer
configuration of the grits with inter-grit spacing of
50 µm to 250 µm is obtained;
c) uniformly placing said grit on the surface of said
steel substrate with the help of a sieve or mesh of
specific size so that only one grit of a particular
grit size may pass through the mesh;
d) drying the tool substrate along with the brazing paste
and a cBN grit placed on it at a temperature varying
from 75 oC - 125 oC for 5 to 15 minutes to remove
volatile materials from the brazing paste;
e) placing the abrasive tool specimen in a vacuum brazing
furnace, where vacuum in the order of 2x10-5 Torr to
8x10-6 Torr is obtained in 10 to 12 min. with help of a
rotary pump and a diffusion pump.
f) raising the temperature of the tool, specimen to the
brazing temperature varying in the range from 800o C to
900o C in a manner that the pressure inside the vacuum
chamber never rises above 5x10-5 Torr;
g) maintaining the temperature of the tool specimen for
0.5 mimutes to 5 minutes; and
h) cooling said tool specimen in vacuum (1x10-5 Torr —
8x10-6 Torr before removing the same from the furnace.
The invention further provides a cBN abrasive tool comprising
metal or cemented carbide substrate and a single layer of cBN
grits bonded to the said substrate by a braze alloy which offers
load free grinding wherein the abrasive tool is provided with
high grit protrusion, regular grit distribution and adequate grit
spacing to overcome failure by wheel loading.
In said cBN abrasive tool, cBN grits are disbributed in a
predetermined pattern on the braze layer applied on the substrate
and are not displaced during brazing, when the braze alloy
becomes totally liquid and remain in the same position and no
clamping device is required to hold grits in respective position.
The braze alloys comprise of passive Ag-Cu base alloys capable of
wetting and bonding Ti coated cBN or Ag-Cu base active alloys
capable of wetting and bonding uncoated cBN grits at temperatures
below 950oC where during brazing, said braze alloy hold the cBN
grits intact in their position without causing any displacement
of grits and capable of forming uniform bond layer on the plain
carbon steel substrate.
In the cBN abrasive tool of the present invention Ag-Cu passive
or Ag-Cu active alloys are incapable of providing uniform bond
layer during brazing of cBN grits on stainless steel, high speed
steel and tungsten carbide substrate but are capable of providing
uniform bond layer when the surface of the said substrate
materials are modified.
The said substrate in the abrasive tool are coated with a film
of titanium nickel. Said substrate may also be coated with a
primary coating of titanium and a secondary coating of nickel.
DESCRIPTION OF THE DRAWINGS
The present invention will now described in detail with reference
to the accompanying drawings wherein
Figure 1 shows the cross sectional view of an monolayer abrasive
disc or tool embodying the present invention, and
Figure 2 shows another example of cBN abrasive tool as
illustrated in Figure 1.
In the accompanying drawings
Figure 1 shows the cross-sectional view of an illustrative disc
embodying the invention wherein cubic boron nitride abrasive particles 'D' are bonded to the circular disc substrate
'A' in a monolayer configuration by the layer of a brazing alloy
'B' having a thickness C. The principal characteristic features
of the cBN abrasive wheel are grit protrusion 'E' above the braze
layer and the inter grit spacing 'F' along the periphery of the
disc .
Cross-sectional view in Figure 2 illustrates another example of
the cBN abrasive tool described in Fig.1. In this instance the
cBN grits 'D' are provided with a surface coating 'E'. The
circular disc substrate 'A' is also provided with a coating 'B'
on its peripheral surface.
DETAILED DESCRIPTION OF THE INVENTION
During the course of invention it has been observed by the
applicant that the problem of wheel loading which occurs in the
grinding wheels offering large chip clearance space through high
crystal protrusion can be solved if all the grits on the working
surface are bonded to the tool body keeping uniform space between
The adjacent grits unlike that of a galvanic type wheel where grit density is high and
spacing is also not uniform. However, according to the principle of grinding, with
increase of grit spacing, ie. low grit density, chip volume to be removed by each
grit also increases and apparently there is no advantage in increasing the light-grit
spacing.
During the course of invention it has been also found by the applicant that Ti or
TiC coated cBN grits which are wetted by commercially available Ni-Cr based
braze alloy can be precisely placed in a regular pattern on the braze paste layer of
the said braze alloy applied on the carbon steel substrate. But the grits shifted from
their location during brazing when the alloy becomes totally molten and present grit
spacing on the wheel surface cannot be maintained.
This behaviour of the Ni-Cr based brazed alloy can be regarded as the major
incapability of the braze alloy in manufacturing abrasive tool with predetermined
grit spacing and got unnoticed by those skilled in the art and this was never reported.
Surprisingly it has been further found by the applicant that when Ag-Cu base
eutectic alloy used for brazing; same Ti coated cBN grits placed precisely in a
regular pattern keeping a well defined spacing between the adjacent grits on the
braze paste layer applied to the carbon steel substrate, did not shift from
the adjacent grits unlike that of a galvanic type wheel where
grit density is high and spacing is also not uniform. However,
according to the principle of grinding, with increase of grit-
spacing i.e. low grit density, chip volume to be removed by each
grit also increases and apparently there is no advantage in
increasing the inter-grit spacing.
During the course of invention it has been also found by the
applicant that Ti or TiC coated/cBN grits which are wetted by
commercially available Ni-Cr based braze alloy can be precisely
placed in a regular pattern on the braze paste layer of the said
braze alloy applied on the carbon steel substrate. But the grit©
shifted from their location during brazing when the alloy becomes
totally molten and preset grit spacing on the wheel surface
cannot be maintained.
This behaviour of the Ni-Cr based brazed alloy can be regarded as
the major incapability of the braze alloy in manufacturing
abrasive tool with predetermined grit spacing and got unnoticed
by those skilled in the and this was never reported.
Surprisingly it has been further found by the applicant that when
Ag-Cu base eutectic alloy used for brazing; same Ti coated cBN
grits placed precisely in a regular pattern keeping a well
defined spacing between the adjacent grits on the braze paste
layer applied to the carbon steel substrate, did not shift from
their initial position during brazing. Preset position of cBN
grits could be precisely maintained even when the alloy became
liquid totally. Similar behaviour was also discovered with Ag-Cu
eutectic or near eutectic alloy containing small amount of Ti
during brazing of uncoated cBN grits on plain carbon steel
substrate.
This behaviour of the braze alloy can be regarded as its
remarkable capability in the manufacture of a abrasive tool with
predetermined grit setting without the requirment of any fixture
for holding the grits in position when braze alloy became
completely liquid and it is quite surprising that such behaviour
of the Ag-Cu base alloy has gone completely unnoticed by those
skilled in the art. This characteristics of the Ag-Cu alloy is
truly unexpected, when one consider the facts that similar
characteristics has not been demonstrated by Ni or Ni-Cr base
brazing alloys.
The cBN wheel with uniform grit spacing comprising of a
substrate, Ti coated cBN grit and Ag-Cu alloy or uncoated cBN
grit and Ag-Cu-Ti active alloy has revealed the fact that
grinding performance of this wheel could surpass that of
galvanically bonded wheel and this has escaped the notice of
those skilled in the art. It was only revealed that to make high
performance wheel use of Ni-Cr alloy or Ag-Cu-In-Ti alloy is not
sufficient. On the other hand, by ascertaining the
characteristics of Ag-Cu alloy it was possible to overcome the
limitations of the brazed cBN in a very simple way. For cBN wheel
which is prone to failure like wheel loading in dry grinding of
hardened and unhardened material at high material removal rite,
the problem can be solved by using the new method as disclosed in
the invention and this has gone completely unnoticed by those
skilled in the arts. The available yield strength of Ag-Cu or Ag-
Cu-Ti alloy was found to be adequate to prevent failure of the
cBN wheel by mechanical failure of the braze alloy. However,
there is no limitation in using Ag-Cu base alloys with higher
yield strength provided they show the same characteristics that
is in the liquid state, the braze alloy will not casue any
movement of the grit from its initial set position and grit
spacing can be maintained during brazing.
Braze filler metals are available commercially or can be prepared
based on system other than Cu-Ag provided the braze alloy can wet
Ti coated cBN or when activated can also wet uncoated cBN grit
and also show the same characteristics as found with Ag—Cu alloy
in the present invention.
It is obvious that exact grit spacing will depend on the material
removal rate, ductility of the work material. It will be
appreciated that grit spacing should be chosen so that chip
clogging and wheel loading is avoided. Unnecessary increase in grit spacing will
increase specific grinding force on each grit and may cause rapid grit wear by
fracture. However, according to the specific grit force, the friability of the grit can
be changed. Similarly size of the grit can also be changed.
During the course of invention it was also found that the Ag-Cu base alloys applied
to the tool steel, stainless steel or tungsten carbide substrate in the form of a paste
can easily form a uniform bond level during brazing of cBN grits when the surface
chemistry of the substrate is modified. The choice of the coating may depend on the
type braze material, ie. whether the braze material is a passive not containing any
active element like Ti or an active braze alloy containing transition elements like Ti.
Similarly the coating deposition technique may be CVD, PVD which depends on
the material of substrate, geometry of the substrate and the film desired to be
deposited. For economic reason it is obvious that conventional or well established
techniques may be used but in principle there is no limitation imposed on use of
non-conventional deposition technique. The thickness of the coating should be as
minimum as possible but sufficient to cover the entire surface of the substrate. The
purpose of the coating is only to change the surface chemistry of the substrate.
Generally speaking 1~2 µm thick coating would be sufficient for the said purpose.
The................................................................................................
deposition may be high temperature or low temperature process. It is also evident
that coating material improves its adhesion with the substrate during brazing
operation when some thermochemical interaction takes place between the film and
the substrate.
The present invention has been described in more detail and can be appreciated from
the following examples, which are in no way intended to limit the scope of the
present invention.
Example I - Small cylindrical grinding wheels were fabricated using a plain carbon
steel substrate and monolayer of cBN grits and silter-copper eutectic alloy. The
silver-copper brazing alloy "Cusil" from WESGO Inc, USA in the form of paste
was applied uniformly on the round surface of the plain carbon steel substrate. This
was followed by placement of cBN grits in monolayer configuration with
predetermined inter grit spacing. The cBN grits were of size B151 (150 µm/125
um mesh width) of grade Borazon 510 (Ti coated cBN) from General Electric
Company. Uniform placement of grit could be done with the help of a sieve or
mesh, the holes in which could accept just one grit at a time. The size of the hole
would be such just to allow the largest grit to pass through. The pitch of the hole
would be determined by the requirement of grit spacing. There could be other
methlod of placement of grit in a regular pattern. The braze layer in the form of
paste could hold the grits at their deposited position temporarily without any
difficulty. The ..................................................................................
tool specimen was then dried at 100oC for 15 min to remove
excessive volatile materials. The tool specimen was then placed
inside a vacuum brazing furnace. Rough vacuum in the order of
10-2 Torr was obtained in 8 min by the rotary pump and a vacuum
better than 1 x 10-5 Torr could be obtained in 15 min by
connecting the diffusion pump, Temperature was then raised to
450o C and kept at that point for about 1 min followed by further
increase to 850oC in 4 minute™ The specimen was held at
temperature for just 1 min. This was followed by cooling under
same atmosphere i.e., in vacuum not less than 1 x 10-5 Torr. The
brazed tool specimen was then removed from the furnace for
physical examination. The Ti coated cBN grits were effectively
wetted by the "Passive" Ag-Cu eutectic alloy. The braze layer
also maintained its uniformity throughout, the working surface.
The cBN grits were found to set to the substrate in the same
location where those were placed before brazing and on
displacement of grit took place.
Example II
The brazing process of Example I was define carried out except that
grits were uncoated cBN of grade Borazon 500 and the braze alloy
was an active braze alloy prepared by adding 2% by wet of
titanium hydride with commercially available Ag-Cu eutectic braze
alloy powder "Cusil" from WESGO. It is pbvious that titanium
hydride powder was thoroughly mixed with the eutectic powder to
obtain a good homogenity of the active metal in order that upon
melting necessary wetting properties of the liquid could be
obtained through out the entire mass. After brazing the cBN
grits were seen not only to have good bond with the braze alloy
but also to maintain the same location where those were placed
before brazing and no displacement of grit took place.
Example III
The brazing process of example II was carried out except that the
grits were uncoated microcrystalline cBN grit of grade Borszon
570 from Beneral Electric with grit size B 151. After brazing,
microcrystalline grits were found to be effectively wetted by the
brazing alloy. The braze also maintained a low but uniform level
throughout the working surface of the tool resulting in large
Crystal protrusion. The microcrystal1ine cBN grits were found to
S~et to the substrate in the same location, where those were
placed before brazing and no displacement of grit took place.
Example IV
The brazing process of example III was carried out except that
substrate material was a sustentic stainless steel and the
substrate was coated 1~2 um thick PV (Physical Vapour
Deposition) Ti. After brazing a uniform braze layer was obtained
on the working surface of the grinding wheel resulting in large
grit protrusion. The microcrystal1ine cBN grits were found to
set to the substrate in the same position where those were placed
before brazing and no displacement of grit took place.
Example V
The brazing process of example IV was carried out except that the
substrate material was martensitic stainless steel and the
s
substrate was coated with 1~2 µm thick PVD Ti Coating. After
brazing a uniform braze layer was obtained on the working surface
of the grinding wheel resulting in large crystal protrusion. The
microcrystal1ine cBN grits were found to have bonded to the
substrate and remain in the same position where such grits were
placed before brazing and no displacement of cBN grits occurred
during brazing.
Example VI
The brazing process of example I was carried except that
stainless
substrate was austentic/steel and the substrate was coated with
a primary coating of PVD Ti and secondary coating of galvanic
Ni. After brazing a containuous and uniform layer of braze alloy
was obtained which wetted and bonded the cBN crystals resulting
in large crystal protrusion. The cBN grits were found to set to
the substrate in the same location where those were fixed before
brazing and no displacement of grit took place during brazing.
Example VII
The brazing process of example VI was carried except that
substrate was hardmetal having 94% WC and 6% Co. The substrate
was coated with a primary coating Ti and a secondary coating of
Ni. After the 'one step' brazing a continuous and uniform layer
of braze alloy was obtained. The cBN grits were found to set to
the substrate in the same location where those were placed before
brazing and no displacement of grit occurred during brazing.
The product resulting from the practice of the invention
represents a significant advancement in the construction of
monolayer cBN abrasive tool. This advancement can be easily
realized when one compares the performance of brazed tool with
galvanic type cBN tool.
Appl :i.cat ion I
Operation : Surface grinding
Work material : 103 Cr1 (IS) hardened bearing steel (HRC62)
Abrasive Tool :
i) Salvanic type monolayer cBN wheel (grit size : B 151)
ii) Braze type monolayer cBN wheel (grit size : B 151)
Process parameters :
Wheel speed : 16 m/s. 27.5 m/s. 41 m/s
Table speed : 4 m/min
Down feed : 10 µm, 20 µm, 30 µm and 40 µm
Environment : dry.

Sign used in the Table 1
a) 41 : 10 = wheel speed of 41 m/s and downfeed of 10 µm
b)v = the wheel behaviour became steady after few passes
of spark-in grinding
c) X = the wheel loading occurred just after few spark—in
grinding passes and the wheel had to be withdrawn.
Long duration grinding test
1) With 27.5 m/s wheel speed, 30 µm down feed and 4 m/min table
speed, galvanic wheel could remove a specific volume 40
mm3 /mm2 working surface, when the forces rose to a high
value indicating wheel loading. On the other hand brazed
type tool could remove as high as 100 mm3 /mm2 specific
volume of work material without any sign of rise of grinding
force and use the brazed cBN tool could be extended further.
ii) At higher material removed rate i.e., with 41 m/s wheel
speed, 40 µm down feed and 4 m/min table speed, galvanic
wheel could remove a specific volume of 20 mm /mm before
wheel was withdrawn because of wheel wear and loading leading
to high grinding force. The brazed type wheel could remove a
specific volume of 60 mm3 /mm2 before it was withdrawn.
Application II
Operation ; Surface grinding.
Working material : Titanium alloy Ti - 6A1 - 4v (32 HRC)
Abrasive Tool :
i) Galvanic type monolayer cBN wheel (grit size : B151 ) .
ii) Braze type monolayer cBN wheel (grit size : B.151)
Process parameters : Wheel Speed : 16 m/s, 20 m/s. 27.5 m/s.
Table speed : 2 m/min. and 3 m/min
Downfeed : 10 µm, 20 µm, 30 µm and 40 µm.
Environment : dry.
WE CLAIM:
1. A method of making a monolayer abrasive tool containing a
monolayer cubic boron nitride abrasive grit bonded to a
steel substrate and which method comprises of the following
steps-
a) applying uniformly on the surface of a plain carbon
steel substrate, a brazing paste made of Ag—Cu
eutectic alloy (72% Ag, 28% Cu) mixed with 1% to 354 of
titanium or ti taniumhyclride powder;
b) placing cBN grits (mono crystalline/microcrystalline
type) on said brazing paste in a manner that a
monolayer configuration of the grits with inter—grit
spacing of 50 µm to 250 µm is obtained
c) uniformly placing said grit on the surface of said
steel substrate with the help of a sieve or mesh of
specific size so that only one grit of a particular
grit may pass through the mesh;
d) drying the tool substrate along with the brazing paste
and cBN grit placed on it at a temperature varying from
75 oC - 125 oC for 5 to 15 minutes to remove volatile
materials from the brazing paste;
e) placing the abrasive tool specimen in a vacuum brazing
furnace, where vacuum in the order of 2x10-5 Torr to
8x10-6 Torr is obtained in 10 to 12 min, with help of a
rotary pump and a diffusion pump;
f) raising the temperature of the tool specimen to the brazing temperature
varying in the range from 800°C to 900°C in a manner that the pressure inside the
vacuum chamber never rises above 5x10-5 Torr;
g) maintaining the temperature of the tool specimen for 0.5 minute to 5
minutes; and
h) cooling said tool specimen in vacuum (1x10-5Torr- 8x10-6 Torr) before
removing the same from the furnace.
2. A cBN abrasive tool comprising metal or cemented carbide substrate and
single layer of cBN grits bonded to the said substrate by a braze alloy as herein
described which offers load free grinding wherein the abrasive tool is provided with
high grit protrusion, regular grit distribution and adequate grit spacing to overcome
failure by wheel loading.
3. A cBN abrasive tool according to Claim 2 wherein said cBN grits which
are distributed in a predetermined pattern on the braze layer applied on the substrate
are not displaced during brazing, when the braze alloy becomes totally liquid and
remain in the same position and no clamping device is required to hold grits in
respective position.

The invention relates to a method of making a monolayer
abrasive tool containing a monolayer cubic boron nitride (cBN) abrasive
grit bonded to a steel substrate. Method comprises applying uniformly
on the surface of a plain carbon steel substrate, a brazing paste made of
Ag-Cu eutectic alloy (72% Ag, 28% Cu) mixed with 1% to 3% of
titanium or titaniumhydride powder, placing cBN grits (nono
crystalline/microcrystalline type) on said brazing paste, uniformly
placing said grit on the surface of said steel substrate, drying the tool
substrate alongwith the brazing paste and cBN grit at a temperature
varying from 75°C - 125°C for 5 to 15 minutes, placing the abrasive

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Patent Number

270425

Indian Patent Application Number

346/CAL/2001

PG Journal Number

52/2015

Publication Date

25-Dec-2015

Grant Date

21-Dec-2015

Date of Filing

22-Jun-2001

Name of Patentee

DEPARTMENT OF SCIENCE & TECHNOLOGY

Applicant Address

NEW MEHRAULI ROAD, NEW DELHI 110016

Inventors:

#	Inventor's Name	Inventor's Address
1	ASIT BARAN CHATTOPADHYAY	INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR - 721 302
2	AJAY KUMAR CHATTOPADHYAY	INDIAN INSTITUTE OF TECHNOLOGY, KHARAGPUR - 721 302

PCT International Classification Number

B05D5/02

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA