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Abstract
Cubic boron nitride (cBN) is a unique synthetic material on account of its high hardness, high wear resistance, excellent cutting edge stability and relative chemical inertness compared to diamond. The introduction of monolayer electroplated cBN wheels replaced the complex pre-grinding wheel preparation work (truing and dressing) of composite cBN wheels and thereby extensively facilitating the application in high-efficiency deep grinding, creep feed grinding, etc. The present work has aimed at developing a precisely controlled brazing technique suitable for bonding the cBN grits to a steel substrate in mono layer form with higher bond strength, larger grit protrusion and more uniform grit distribution compared to that in the currently used galvanically bonded wheels. Experimental investigation have clearly demonstrated the potential of the newly developed brazed wheels under varying grinding conditions for processing materials like bearing steel. Improved capability of these wheels over galvanically bonded wheels could be better recognised during dry grinding at high material removal rate and for large stock removal when galvanically bonded wheels were found to suffer from severe wheel loading in grinding bearing steel and from unusual increase in grinding forces due to grit pullout. Creation of wider inter-grit spaces with strong bonding and uniform grit spacing happened to be the essence of the present brazed cBN wheel.
1 Introduction
Super abrasive wheels are widely used essentially for grinding strong, hard, heat-resistive and abrasive-type materials like exotic steels, glass, ceramics, stones, plastics and fibrere in forced plastics with high stock removal rate as well as high precision. Mono layer cubic boron nitride (cBN) super abrasive wheels are especially suitable for constricted applications like internal grinding, form grinding and stress-free precision grinding. The mono layer cBN super abrasive wheels presently manufactured by galvanic bonding suffer from inadequate bond (mechanical) strength, no control over grit distribution and lesser protrusion of the grits. Keeping all such points in view in the present work, an attempt has been made to develop small monolayer brazed cBN wheels where cBN grits are uniformly spaced and strongly bonded (chemical and mechanical) on the peripheral steel shank by special brazing technique. Brazing of ceramic particles like cBN calls for flux-less brazing because of disintegration of the flux under the high brazing temperature. Wetting of cBN by braze alloy is greatly affected even in the inert atmosphere provided by technical quality argon because of the presence of the oxygen.Presence of even a few partsper million of oxygen can prevent proper wetting. High vacuum environment beyond 10?5torr can offer a solution to the problem of brazing cBN . A lot of work has been carried out to braze cBN on steel shank, but the potential of cBN grits were not been fully utilised due to non-uniform distribution . The cBN grits of size B151 (150/125-μm mesh width) were used and micro crystalline in nature which,unlike mono crystalline grits, are free from well-defined cleavage plane.
Their cutting tips under grinding forces and offer better auto sharpening character in comparison to mono crystalline cBN. This self-sharpening property of cBN is very much desirable in a single-layer wheel. Micro crystalline cBN grits also possess higher toughness and high-temperature stability than the mono crystalline counterpart. Evaluation of the performance of the new brazed cBN wheel compared to that of galvanically bonded cBN wheel has been carried out in a surface grinding machine fitted with the high-speed spindle . A recent investigation has shown that the grit can be distributed uniformly on the flat surface. This paper evaluates the comprehensive performance of the newly developed peripheral tools in the comparative grinding tests with the electroplated counterparts on bearing steel with high speed, high feed and moderate depth of cut for longer duration to use the maximum potential of cBN grit before dislodging. It is also proven that newly developed cBN brazed wheel has performed much better than electroplated wheel in ductile mode dry grinding.
2 Experimental procedure
2.1 Brazing
In the present study, a Ag–Cu base alloy of eutectic composition (72% Ag and 28% Cu) and melting point 780°C was made active by adding TiH2 instead of a prealloyed braze material. The mixture in the form of paste was uniformly applied on the peripheral surface of the wheel blank. The cBN grits were laid uniformly in desired spacing on the periphery of the wheel. It was possible to distribute the grits in a monolayer configuration without undesirable clustering. The laying of the grits has been done essentially in such a way that adequate spaces are provided for free accommodation of the grinding chips without wheel loading. Figure 1 shows the preparation of
distribution of grits on the peripheral surface of the wheel. During brazing, it was observed through the glass windows repeatedly that wetting of the cBN grits by the active alloy occurred almost instantaneously when the temperature was raised to 850°C. The tool specimen was kept at that temperature just for 1 min to achieve good bond strength. The surface of both the brazed and the standard galvanic wheels were visually examined under SEM. The surface topography of SEM micrographs of the brazed type wheel (BT) and the galvanically bonded (GB) wheel are shown in Fig. 2. The first impression gathered through visual inspection was that the cBN grits in the galvanically bonded wheel were seen to lie embedded in the Ni matrix in a closely packed and non-uniform manner. In contrast, the cBN grits in the brazed wheel were found to be fairly uniformly distributedbutwellprojectedabovethebondinglayer.Themean grit protrusion was found to be 35–55 μm for GB wheel. The same was found to be in the range 85–100 μm for BT wheel.
2.2 Grinding setup
The grinding experiments for evaluating the performance of the new brazed cBN wheel compared to that of galvanically Fig. 1 Uniform placing of cBN grits in mono layer configuration bonded cBN wheel have been carried out in a surface grinding machine fitted with the high-speed spindle (Fig. 3). The maximum run out on the wheel was checked and found to be less than 5 μm. The job specimen, 80 mm long, 20 mm deep and 3 mm wide, was held in a specially designed fixture which was mounted on a 3D Kistler dy namometer (KISTLER 9257B, Switzerland). Experimental conditions have been given in Table 1. The performance of the present BT cBN wheel in comparison to that of the galvanically GB standard cBN wheel has been assessed for the present grinding tests primarily on the basis of grinding forces, specific energy consumption, wheel condition (wheel loading and fracture, wear and pullout of the grits) during grinding operation and effective service in longer duration grinding.
3 Result and discussion
3.1 Grinding forces, specific energy, metal removal rate
The roles of variation in the major grinding process parameters, viz. wheel speed, Vc, table speed, vw, and downfeed,
d, on the magnitude of the tangential component, Ft, and the normal component, Fn, of the grinding force recorded during grinding the bearing steel by BT wheel and GB wheel have been shown in Fig. 4. It can be seen from those figures that the magnitude of both Ft and Fn decreased or tended to decrease with the increase in Vc and increased with the increase in vw and d for all the present work–wheel combinations. In grinding, the magnitude of the cutting forces depend primarily on the property of the work material, crosssectional area of the layer to be removed from the work surface in each pass, i.e. total chip load and the form factors which again are governed by the cutting tip geometry (sharpness), and interaction at the chip–tool interfaces. Specific forces and hence specific energy requirement are always larger in grinding where the rake angle is adverse in the order of ?35° to ?80°. In addition, the rubbing or interactions at the chip–bond, chip–workpiece or bond– workpiece interfaces in different degrees can also be attributed to high force and specific energy of grinding. All along in the present grinding experiments, Fn has been much larger (almost double) than Ft under all the conditions undertaken, as can be seen in the concerned figures. The ratio between the tangential force and the normal force in any machining process is governed mainly by the tool rake angle and friction at the chip–tool interface. Large negative rake, lack of sharpness of the grits due to rounding and flattening (by micro fracturing and wear) of the grit tips and relatively much smaller penetration of the cutting points in the work surface are the main causes behind the unusually very large value of Fn compared to Ft in grinding. This unique feature of grinding process as such has also been noted in the present grinding experiments with the monolayer cBN wheels. Figure 4 shows that for both the wheels, Ft and Fn gradually decreased with the ncrease in Vc. In any machining of ductile metals, the cutting forces decrease sizeably with the increase in cutting velocity due to plasticisation and shrinkage of the shear zone ahead the cutting edge.
摘要
立方氮化硼（CBN）是一種獨特的合成材料，由于其高硬度、高耐磨、抗疲勞性、優(yōu)良的切削刃穩(wěn)定性和相對化學(xué)惰性能與鉆石相差無幾。為了復(fù)合CBN砂輪的廣泛應(yīng)用,所以引入單層電鍍CBN砂輪來取代復(fù)雜的立輪做準(zhǔn)備工作(校準(zhǔn)和整合)。目前的工作是為了發(fā)展精確控制的釬焊技術(shù)，由于CBN磨粒在單層鋼底形成的粘結(jié)強度較高,較大磨粒突出和更均勻分布,所以使用電流的保稅輪子。實驗研究清楚地表明在不同的加工材料，如軸承鋼的磨削條件下更能開發(fā)釬焊車輪的潛力。改善這些輪子的功能在通電的保稅輪子下可以更好的去磨削并且能使材料去除率高，但對于大型切削時，電流的保稅輪子在磨軸承鋼輪中很難加載。為了建立更廣泛的加工空間，能到達(dá)較強的結(jié)合和統(tǒng)一性，這是目前焊接CBN砂輪的本質(zhì)。
1?介紹
超硬磨料砂輪廣泛主要用于磨削強、硬、熱電阻和耐磨類型的材料。如玻璃，陶瓷，鋼材，石材，塑料和纖維增強塑料，能達(dá)到高材料去除率以及高精度。單層立方氮化硼（CBN）超硬磨料砂輪特別適用于狹窄的應(yīng)用如內(nèi)圓磨削，成型磨削和無壓力的精密磨削。目前通過電偶結(jié)合制造的單層CBN超硬磨料砂輪機械強度不足，無法控制粒度分布和較小的顆粒突起。在目前為了解決這些問題，已嘗試開發(fā)小型單層釬焊CBN砂輪，通過特殊的釬焊技術(shù)（化學(xué)和機械）使CBN磨粒在外圍的鋼圈分布均勻間隔緊密。陶瓷釬焊就像量少的CBN釬焊，由于釬焊溫度較高而造成解體。因為氧氣的存在，所以在釬焊時需要提供稀有氣體氬做保護氣，防止CBN釬焊時對合金的影響。這樣即使外圍有數(shù)以百萬計的氧也能防止被濕潤。超過10ˉ5托的真空環(huán)境給CBN釬焊提供了一個解決辦法。大量的工作已經(jīng)在釬焊CBN鋼柄中進行，但由于CBN磨粒沒有分布均勻所以使它的潛力沒有得到充分的利用。
CBN磨粒在B151尺寸時（150/125μm網(wǎng)格的寬帶）跟自然界的微晶差不多，不像單結(jié)晶粉末，我明確的理解面。這種通過高溫高壓技術(shù)使砂結(jié)構(gòu)由微米級單晶粘貼在一起的技術(shù)，與單晶CBN比較能造成磨粒進行微壓使能在刀片的磨削力和提供更好的自動銳化特性。這種自銳性CBN為單層盤是非?？扇〉?。微晶CBN磨粒具有比單晶硅對應(yīng)更高的韌性和高溫穩(wěn)定性。對新的釬焊CBN砂輪的性能評價相比，電鍍CBN砂輪進行表面研磨機配有高速電主軸。最近的一項調(diào)查表明，砂可以在平坦的表面均勻分布。本文對新開發(fā)的外圍工具綜合性能比較磨削試驗與電鍍同行對高速軸承鋼，持續(xù)時間較長，在去除使用CBN磨粒的最大潛力降低深度適中。這也證明，新開發(fā)的CBN砂輪比電鍍輪更能抗干擾。
2 實驗步驟?
2.1釬焊
在目前的研究中，銀銅合金的共晶組成（72%銀和28%銅）合金熔點為780℃有由于通過添加TiH代替了原來的纖材料，在表面形成混合武均勻地施加在輪坯外表面。CBN粒子被放置在間距均勻的輪坯外圓上。這些均勻分配的粒子在單層結(jié)構(gòu)表面沒有聚集。粒子的鋪設(shè)已基本存在這樣一種形式，有足夠的空間用于粒子屑的存在。
釬焊過程中，觀察到的玻璃窗，通過反復(fù)的潤濕CBN磨粒的活性合金幾乎在瞬間升溫到850°C.刀具試樣保持在該溫度下才1分鐘，就達(dá)到了良好的粘結(jié)強度。釬焊和標(biāo)準(zhǔn)電偶的表面在視覺掃描電子顯微鏡下檢查。發(fā)現(xiàn)釬焊式車輪SEM表面形貌（BT）和電連接（GB），通過視覺的第一印象是電鍍砂輪CBN粒子被發(fā)現(xiàn)通過一個緊密的和非均勻的方式嵌入鎳基，相比之下，在釬焊砂輪CBN粒子被發(fā)現(xiàn)是相當(dāng)均勻的。平均磨粒出刃是35–55μM?GB輪。同樣是在范圍85–100μm之內(nèi)。
2.2 磨削裝置
通過磨削實驗，新的釬焊CBN砂輪相比電鍍CBN砂輪在性能上表面研磨機配有高速主軸。輪的最大消耗檢查發(fā)現(xiàn)是小于5μM。這個樣本，長80毫米，深20毫米，寬3毫米，在一個被安裝在一個三維的測力計的專門設(shè)計的夾具中進行（Kistler 9257B，瑞士）。得到的實驗條件在表一中給出。目前磨削試驗主要是對BT CBN砂輪與電鍍CBN砂輪的性能比較磨削力為主，在進行長時間的磨削中具體的能源消耗情況，輪的狀態(tài)（輪的載荷和斷裂，粒子的磨損和抗壓）。
3 結(jié)果與討論
3.1磨削力，具體的能耗，金屬去除率
在主要的磨削工藝參數(shù)變化的作用下，即輪速，線速度，進給速度，角速度，供水和直徑，對于切向分量的大小，正常組成部分的Ft和在磨削過程中對于軸承鋼輪和BT?GB輪的磨削力Fn已在圖4所示。從這些數(shù)據(jù)中可以發(fā)現(xiàn)，F(xiàn)t和Fn的下降幅度或趨于在線速度與加速度的增加，并且直徑對于目前所有的工作輪組合都有影響。在磨削加工中，切削力的大小主要取決于工件材料的性質(zhì)，是在每一個經(jīng)過的工作表面去除的橫截面積，即總芯片負(fù)載，又是由刀片幾何形式的因素（鋒利度），并在芯片工具界面相互作用。因此具體的能源需求總是大于的磨削力的需求，并且刀具傾斜角度在?35°到?80°是不利的。
在目前的磨削實驗中，通過有關(guān)數(shù)據(jù)可以看到，在所有條件下進行的，F(xiàn)N都比FT要大的多（幾乎一倍）。切向力和法向力是任何加工過程主要受刀具前角和摩擦在芯片–工具界面之間的比率。大的負(fù)前角，由于舍入和壓扁的磨粒鋒利的缺乏（由微破裂和磨損），砂的技巧和在工作面切削點的滲透相對較小的背后是非常非常大的價值比較磨FN FT的主要原因。這種獨特的功能磨削工藝等也已經(jīng)在在目前與單層CBN砂輪磨削實驗中引起了了注意。顯示了兩個輪子，在VC的增加FT和FN逐漸降低。在任何加工韌性金屬時，由于剪切帶的前面的切削刃的增塑和收縮切削速度增加會使切削力降低。