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磨床的類型 磨床的類型 從最簡(jiǎn)單的磨床對(duì)最復(fù)雜, 磨床可能被分類作為公共磨床、圓柱形磨床和表面磨床。一般的機(jī)械師將牽涉主要到地板鑲嵌和長(zhǎng)凳登上的公共磨床和交換表面磨床。 公共磨床 公共磨床意欲為臨時(shí)研制件支持在手和被利用反對(duì)轉(zhuǎn)動(dòng)的研的磨蝕輪子的地方。這類型磨床取決于準(zhǔn)確性取決于操作機(jī)器的智慧. 技能、知識(shí)和機(jī)械的性能和工作性質(zhì).公共磨床包括一個(gè)水平地登上的馬達(dá)與一個(gè)研的磨蝕輪子附有各個(gè)馬達(dá)軸的末端。電動(dòng)發(fā)動(dòng)機(jī)驅(qū)動(dòng)器是簡(jiǎn)單而常見(jiàn)。它也許長(zhǎng)凳登上或地板鑲嵌。通常, 軸軸承的情況和設(shè)計(jì)并且馬達(dá)規(guī)定值確定機(jī)器的輪子大小容量。適當(dāng)?shù)妮喿邮匦l(wèi)并且工具休息為安全和操作方便被提供。磨床進(jìn)來(lái)各種各樣的大小和形狀依照下面被列出。 地板鑲嵌公共磨床 典型的地板鑲嵌公共磨床站立齊腰高和綁住到地板螺栓。地板鑲嵌公共磨床請(qǐng)見(jiàn)圖5-1座212英寸口徑的2英寸全磨料磨輪子。安排兩個(gè)輪子安裝粗面轉(zhuǎn)動(dòng)為粗磨目的在軸的一個(gè)末端并且一個(gè)美好的五谷輪子為精整目的在另一邊末端這節(jié)省否則會(huì)被消耗在改變輪子里的時(shí)間。 各個(gè)砂輪由都有砂輪護(hù)罩保證機(jī)器的安全。每個(gè)砂輪磨床都有透明眼罩，火花逮捕和可調(diào)整的工具臺(tái)。工具臺(tái)和冷卻臺(tái)在基地或墊座的邊。在磨削碳鋼切口事采用冷卻液冷卻。使用12 英寸輪子, 機(jī)器提供大約5.500 SFPM 的最大切口速度。2-HP 電動(dòng)機(jī)這個(gè)機(jī)器的最大速度有1.750每分鐘/轉(zhuǎn)。 例1地板鑲嵌公共磨床 臥式實(shí)用磨床 象臥式實(shí)用磨床, 一個(gè)劣質(zhì)砂輪和一個(gè)優(yōu)質(zhì)的砂輪通常裝在在機(jī)器為操作便利。各個(gè)輪子帶有可調(diào)整的工具臺(tái)和眼罩為保護(hù)。這個(gè)機(jī)器,裝有發(fā)動(dòng)機(jī)熱過(guò)度負(fù)荷開(kāi)關(guān)停止發(fā)動(dòng)機(jī)如果采用過(guò)高的壓力使砂輪焚燒了馬達(dá). 馬達(dá)旋轉(zhuǎn)到3.450轉(zhuǎn)最大切割速度最高為七英寸的機(jī)輪每分鐘約6300英尺表面(SFPM). 例2臥式實(shí)用磨床 臥式實(shí)用鉆磨床 臥式實(shí)用鉆磨床是為了磨削鉆子準(zhǔn)備的。這類型準(zhǔn)確性研磨機(jī)不依靠操作員的手巧和技巧因?yàn)殂@子被安置在保護(hù)裝置。保護(hù)裝置安置鉆子在正確位置為清除和包括的角度。對(duì)于更多信息關(guān)于這個(gè)機(jī)器參見(jiàn)章節(jié)4 。 臥式實(shí)用磨床與拋光機(jī) 臥式實(shí)用磨床和拋光的機(jī)器是混合磨削和清洗, 并且拋光它不被推薦為工具研因?yàn)樗话ぞ咝菹⒒蜍囕啌醢?。這個(gè)機(jī)器通常登上一個(gè)4 條英寸直徑導(dǎo)線輪子在一個(gè)末端。鋼絲輪被使用為清潔并且磨蝕輪子被使用為一般研。二個(gè)輪子的當(dāng)中一個(gè)可能被去除并且一個(gè)拋光的輪子登上在它的地方為拋光和擦亮。1/4-HP 電動(dòng)機(jī)旋轉(zhuǎn)在3,450 轉(zhuǎn)每分鐘最大值。4 英寸直徑輪子的最大切口速度是大約3,600 SFPM。 例3臥式實(shí)用磨床與拋光機(jī) 臥式工具和切削刀研磨機(jī) 臥式工具和切削刀研磨機(jī)，見(jiàn)圖5-4。主要被設(shè)計(jì)研末端磨房。它以及切開(kāi)可能并且研小木和鋼切削刀大品種鋸切削刀直徑12 英寸使用鋸研的附件。典型的臥式工具和切削刀研磨機(jī)的容量是如下: 砂輪移動(dòng)7 l/2垂直英寸 砂輪移動(dòng)5 l/2垂直英寸 表面移動(dòng)6英寸 距離中心-14英寸. 回轉(zhuǎn)中心(直徑)直徑l/2-英寸直徑 回轉(zhuǎn)工作臺(tái)(直徑)l/2-英寸直徑 非常規(guī)圓柱磨床在大規(guī)模維護(hù)系統(tǒng)包括加速磨削工具 工具庫(kù)磨床 工具庫(kù)磨床, 看圖5-5, 是機(jī)械工具附件被設(shè)計(jì)登上對(duì)引擎車床工具崗位。它被使用為內(nèi)部和外在研圓柱形工作場(chǎng)所。參見(jiàn)章節(jié)7 為這個(gè)機(jī)器的描述。 碾碎和研磨車床裝置 并且叫這個(gè)附件是一個(gè)多功能的機(jī)械工具附件，它執(zhí)行內(nèi)部和外在圓柱形研在它的其它作用之中。參見(jiàn)章節(jié)9 為這個(gè)機(jī)器的描述。 表面磨床 表面磨床被使用為磨平面。工件往復(fù)運(yùn)動(dòng)和交換在砂輪之下的一張長(zhǎng)方形工作臺(tái)。交換表面磨床有水平的輪子紡錘和一般登上直接或圓筒類型研磨蝕輪子。 表面交換磨床 交換的表面磨床是水平的型表面磨床。工件被緊固對(duì)桌,砂輪用手或自動(dòng)進(jìn)給移動(dòng)。磁性卡盤(pán)用來(lái)把工件緊固在工作臺(tái)上。這臺(tái)磨床有一個(gè)內(nèi)部泵浦和管道系統(tǒng)為自動(dòng)制動(dòng)時(shí)的冷卻和工件以及砂輪再次工作事的冷卻。砂輪裝配對(duì)水平的軸是平直的和只切開(kāi)它的圓周表面。砂輪速度是可調(diào)整的砂輪。 標(biāo)準(zhǔn)砂輪的類型 砂輪有著許多不同的大小、形狀和研磨劑(圖5-7) 。一些各種各樣的型下面是列出的： 盤(pán)形 盤(pán)形砂輪 第1, 5, 和7型號(hào), 向內(nèi)部, 圓柱形, 水平的紡錘、表面, 工具和即時(shí)磨和粗磨共同地被應(yīng)用。凹進(jìn)處在第5 和7 型號(hào)容納架置耳輪緣。第1型號(hào)輪子從0.006 英寸到l/8 英寸厚度被使用為切除棱角和開(kāi)槽。 圓筒形 圓筒形砂輪,第2型號(hào)，被安排研在或輪子的周圍或邊。 錐形 錐形砂輪,第4型號(hào), 當(dāng)粗磨可以保證安全的擋住砂輪破損時(shí)的飛屑。 直杯形 直杯形砂輪,第6型號(hào),主要用在表面磨削, 但可能并且被使用為臨時(shí)磨平面。簡(jiǎn)單或斜面孔是可利用的。 杯形砂輪 杯形砂輪，第11型號(hào), 是常用在工具磨削。以樹(shù)脂狀粘結(jié), 它在磨蝕時(shí)很有用，它的表面也許是平面或成斜面。 蝶形 蝶形砂輪，第12型號(hào)的首要用途,是在工具修整。它稀薄的邊緣可能被插入入狹窄的地方, 并且它是方便為研磨被解除的銑刀和鉆孔的表面。 茶碟形 茶碟砂輪,第13型號(hào), 是亦稱除毛刺機(jī)因?yàn)樗皇褂脼榍宄獭? 例4砂輪的各種標(biāo)準(zhǔn)類型 研磨材料 磨粒是從砂輪切割式脫落的。他們實(shí)際上切開(kāi)小片斷或芯片工作當(dāng)輪子轉(zhuǎn)動(dòng)。磨粒形狀是不規(guī)則以幾鋒利的先鋒。當(dāng)這些邊緣增長(zhǎng)愚鈍, 力量行動(dòng)在輪子傾向于碾碎磨粒和產(chǎn)生新刃口。 研磨工具 多數(shù)研的輪子由碳化硅或氧化鋁制成, 其中之二是人為(制作的) 研磨劑。碳化硅是極端堅(jiān)硬但易碎的。氧化鋁輕微地更軟但比碳化硅堅(jiān)韌的。它默鈍化迅速, 而是它容易地不破碎因此它更好適用與相對(duì)地高抗拉強(qiáng)度研的材料 磨粒的大小 磨粒被選擇是根據(jù)他們穿過(guò)濾網(wǎng)相嚙合。例如,第40號(hào)磨粒表明, 磨粒通過(guò)篩子有大約40 個(gè)濾網(wǎng)對(duì)線性英寸。一個(gè)磨料被選定粗糙, 中等, 或良好根據(jù)各自的磨粒的大小做砂輪。 粘結(jié)材料 粘結(jié)劑 磨粒在一個(gè)砂輪由粘合劑粘結(jié)到位。粘結(jié)的百分比在輪子確定, 到大規(guī)模范圍, "堅(jiān)硬" 或"等級(jí)" 輪子。砂輪將是更加高的百分比和力量, 更加堅(jiān)硬的。堅(jiān)固的砂輪將保留磨粒更久, 當(dāng)"軟性" 輪子迅速脫落磨粒。如果一個(gè)磨粒是"太堅(jiān)硬的" 為工作, 它將給上釉因?yàn)檎辰Y(jié)防止鈍化 磨蝕微粒從被發(fā)布因此新磨?？赡鼙槐┞稙榍锌凇３刂茍?jiān)硬和拿著研磨劑以外,粘結(jié)并且提供適當(dāng)?shù)陌踩蛩卦诟咚贂r(shí)。它使輪子結(jié)合在一起當(dāng)離心力設(shè)法分開(kāi)撕毀它。最共普通粘結(jié)被使用在砂輪里玻璃化, 硅酸鹽, 紫膠, 樹(shù)脂狀, 和橡膠。 陶瓷 大多數(shù)研的輪子有玻璃化的粘結(jié)。玻璃化的粘結(jié)的輪子比其他粘結(jié)是熱或寒冷影響的和被做成比其他的粘結(jié)更加堅(jiān)固。他們適應(yīng)實(shí)際所有類型研磨。有一著名的例子: 如果輪子不是足夠濃厚, 它不承受旁邊壓力和在稀薄的情況下切除輪子。 硅酸鹽 硅酸鹽粘結(jié)研磨顆粒比燒結(jié)粘合劑更容易脫落。硅酸鹽粘結(jié)得砂輪很好適用與磨削溫度必須保持在較低的領(lǐng)域, 譬如磨削鋒利的切割工具。它不適用重型磨削。薄片切斷砂輪用有時(shí)以蟲(chóng)膠結(jié)合因?yàn)樗挥迷诳焖倮淝懈睢? 樹(shù)脂 樹(shù)脂狀粘結(jié)是牢固的和柔軟的。它廣泛被應(yīng)用在粗磨砂輪里(為磨削不規(guī)則性的粗糙鑄件), 操作在9,500 SFPM 。它并且被使用在切斷砂輪。 橡膠 在橡膠粘結(jié)的砂輪里, 純凈的橡膠里混入了硫磺。它有非常靈活得切割速度和它被準(zhǔn)許制造稱0.006英寸薄的砂輪用以在大家伙上切割裂縫。多數(shù)切斷機(jī)的砂輪都是由有橡膠粘結(jié)的。 硬度的等級(jí) 一個(gè)砂輪的等級(jí)由粘結(jié)材料的硬度來(lái)決定。下面列出一些砂輪等級(jí)的例子： 一個(gè)較軟的砂輪在切削時(shí)切屑很容易飛濺出來(lái)但是一個(gè)較硬的砂輪切削時(shí)則能成功的阻止切屑飛濺出來(lái)。 多數(shù)砂輪被硬度等級(jí)辨別根據(jù)字母標(biāo)明。多數(shù)砂輪制造商使用一個(gè)信件代碼范圍從A (非常軟) 對(duì)Z (非常硬) 。陶瓷和硅酸鹽通常歸類從非常軟到非常硬, 蟲(chóng)膠和樹(shù)脂通常歸類從非常軟到非常硬, 并且橡膠粘結(jié)通常被限定在硬范圍里。 選擇砂輪的硬度等級(jí)應(yīng)該和選擇磨粒的大小一樣仔細(xì)。一個(gè)磨削砂輪得硬度太軟會(huì)使它的磨損非常得迅速,它的磨粒會(huì)在它的使用壽命還沒(méi)結(jié)束之前就脫落下來(lái)。另一方面, 如果砂輪的硬度對(duì)于工作太硬, 磨粒將會(huì)變鈍因?yàn)檎辰Y(jié)劑不會(huì)讓磨粒脫落, 并且砂輪的效率將被降低。 如果磨粒的大小和粘結(jié)材料的軟硬都相同，則磨粒之間距離大比磨粒間距離小的砂輪更軟。因而, 砂輪的真正硬度取決于磨粒的堅(jiān)硬和磨粒之間距離或構(gòu)造。 砂輪的結(jié)構(gòu)符號(hào) 一個(gè)砂輪的粘結(jié)強(qiáng)度不止取決于磨粒的硬度等級(jí)還取決于磨粒之間的空間構(gòu)造。即磨粒間的距離和密度。結(jié)構(gòu)和空間距離決定了每立方英寸磨粒的容量。每個(gè)砂輪由制造商標(biāo)記用蠟紙或標(biāo)簽做了標(biāo)記。制造者制定出了一套標(biāo)記的標(biāo)準(zhǔn)系統(tǒng), 用一個(gè)標(biāo)記了A36-L5-V23的砂輪舉個(gè)例子。A 提到是氧化鋁的研磨劑。36 代表粒度。L 顯示等級(jí)或程度堅(jiān)硬, 是中等。5 提到輪子的結(jié)構(gòu)并且V 提到粘結(jié)劑種類。 18 TYPES OF GRINDING MACHINES From the simplest grinding machine to the most complex, grinding machines can be classified as utility grinding machines, cylindrical grinding machines. and surface grinding machines. The average machinist will be concerned mostly with floor-mounted and bench-mounted utility grinding machines, bufting machines. and reciprocating surface grinding machines. UTILITY GRINDING MACHINES The utility grinding machine is intended for offhand grinding where the workpiece is supported in the hand and brought to bear against the rotating grinding abrasive wheel. The accuracy of this type of grinding machine depends on the operator’s dexterity. skill, and knowledge of the machine’s capabilities and the nature of the work. The utility grinding machine consists of a horizontally mounted motor with a grinding abrasive wheel attached to each end of the motorshaft. The electric-motor-driven machine is simple and common. It may be bench-mounted or floor-mounted. Generally, the condition and design of the shaft bearings as well as the motor rating determine the wheel size capacity of the machine. Suitable wheel guards and tool rests are provided for safety and ease of operation. Grinding machines come in various sizes and shapes as listed below. Floor Mounted Utility Grinding Machine The typical floor-mounted utility grinding machine stands waist-high and is secured to the floor by bolts. The floor mounted utility grinding machine shown in Figure 5-1 mounts two 12-inch-diameter by 2-inch-wide grinding abrasive wheels. The two wheel arrangement permits installing a coarse grain wheel for roughing purposes on one end of the shaft and a fine grain wheel for finishing purposes on the other end this saves the time that would be otherwise consumed in changing wheels. Each grinding abrasive wheel is covered by a wheel guard to increase the safety of the machine. Transparent eyeshields. spark arresters. and adjustable tool rests are provided for each grinding wheel. A tool tray and a water pan are mounted on the side of the base or pedestal. The water pan is used for quenching carbon steel cutting took as they are being ground. Using the 12-inch wheel, the machine provides a maximum cutting speed of approximately 5.500 SFPM. The 2-HP electric motor driving this machine has a maximum speed of 1.750 RPM. Bench Type Utility Grinding Machine Like the floor mounted utility grinding machine, one coarse grinding wheel and one fine grinding wheel are usually mounted on the machine for convenience of operation. Each wheel is provided with an adjustable table tool rest and an eye shield for protection. On this machine, the motor is equipped with a thermal over-load switch to stop the motor if excessive wheel pressure is applied thus preventing the burning out of the motor. The motor revolve at 3.450 RPM maximum to provide a maximum cutting speed for the 7 inch grinding wheels of about 6,300 surface feet per minute (SFPM). Bench-Type Utility Drill Grinding Machine The bench-type drill grinding machine is intended for drill sharpening. The accuracy of this type of grinder is not dependent on the dexterity and skill of the operator because the drill is placed in a holding device. The holding device places the drill in the correct position for the clearance and included angle. For more information on this machine refer to chapter 4. Bench-Type Utility Grinding and Buffing Machine The bench-type utility grinding and buffing machine is more suitable for miscellaneous grinding, cleaning, and buffing It is not recommended for tool grinding since it contains no tool rests, eyeshields, or wheel guards. This machine normally mounts a 4 inch-diameter wire wheel on one end. The wire wheel is used for cleaning and the abrasive wheel is used for general grinding. One of the two wheels can be removed and a buffing wheel mounted in its place for buffing and polishing. The 1/4-HP electric motor revolves at a maximum of 3,450 RPM. The maximum cutting speed of the 4-inch-diameter wheel is approximately 3,600 SFPM. Bench-Type Tool and Cutter Grinder The bench-type tool and cutter grinder, see Figure 5-4, was designed primarily to grind end mills. It can also grind a large variety of small wood and steel cutters as well as slitting saw cutters up to 12 inches in diameter using the saw grinding attachment. Capacity of the typical bench-type tool and cutter grinder is as follows: Grinding wheel travel - 7 l/2-inch vertical. Grinding wheel travel - 5 1/2-inch horizontal. Table travel - 6 inches. Slitting saws with attachment - 12-inch diameter. Distance between centers - 14 inches. Swing on centers (diameter) - 4 l/2-inch diameter. Swing in work head (diameter) - 4 l/2-inch diameter. Nonspecialized cylindrical grinding machines in the Army maintenance system include the tool post grinding machine and the versa mil attachment. Tool Post Grinding Machine The tool post grinding machine, see Figure 5-5, is a machine tool attachment designed to mount to the tool post of engine lathes. It is used for internal and external grinding of cylindrical workplaces. Refer to Chapter 7 for a description of this machine. Milling and Grinding Lathe Attachment Also called a Versa-Mil this attachment is a versatile machine tool attachment that mounts to the carriage of a lathe. It performs internal and external cylindrical grinding among its other functions. Refer to Chapter 9 for a description of this machine. SURFACE GRINDING MACHINE The surface grinding machine is used for grinding flat surfaces. The workpiece is supported on a rectangular table which moves back and forth and reciprocates beneath the grinding wheel. Reciprocating surface grinding machines generally have horizontal wheel spindles and mount straight or cylinder-type grinding abrasive wheels. RECIPROCATING SURFACE GRINDING MACHINE The reciprocating surface grinding machine is a horizontal- type surface grinding machine. Workpieces are fastened to the table and can be moved beneath the grinding abrasive wheel by hand or power feed. A magnetic chuck maybe used for fastening the workpiece to the table. This grinding machine has an internal pump and piping network for automatic application and recirculation of a coolant to the workpiece and wheel. The grinding abrasive wheel, mounted to the horizontal spindle is straight and cuts on its circumferential surface only. Grinding wheel speeds are adjustable GRINDING WHEELS STANDARD TYPES OF GRINDING WHEELS Grinding wheels come in many different sizes, shapes, and abrasives (Figure 5-7). Some of the various types are listed below. Straight Straight wheels, numbers 1, 5, and 7, are commonly applied to internal, cylindrical, horizontal spindle, surface, tool, and offhand grinding and snagging. The recesses in type numbers.5 and 7 accommodate mounting flanges. Type number 1 wheels from 0.006-inch to l/8-inch thick are used for cutting off stock and slotting. Cylinder Cylinder wheels, type number 2, may be arranged for grinding on either the periphery or side of the wheel. Tapered Tapered wheels, type number 4, take tapered safety flanges to keep pieces from flying if the wheel is broken while snagging. Straight Cup The straight cup wheel, type number 6, is used primarily for surface grinding, but can also be used for offhand grinding of flat surfaces. Plain or beveled faces are available. Flaring Cup The flaring cup wheel, type number 11, is commonly used for tool grinding. With a resinoid bond, it is useful for snagging. Its face may be plain or beveled. Dish The chief use of the dish wheel, type number 12, is in tool work. Its thin edge can be inserted into narrow places, and it is convenient for grinding the faces of form-relieved milling cutters and broaches. Saucer The saucer wheel, type number 13, is also known as a saw gummer because it is used for sharpening saws. Figure 5-4.Standard types of gringing wheels ABRASIVE MATERIALS The abrasive grains are the cutting took of a grinding wheel. They actually cut small pieces or chips off the work as the wheel rotates. The shape of each grain is irregular with several sharp cutting edges. When these edges grow dull, the forces acting on the wheel tend to fracture the abrasive grains and produce new cutting edges. ABRASIVES Most grinding wheels are made of silicon carbide or aluminum oxide, both of which are artificial (manufactured) abrasives. Silicon carbide is extremely hard but brittle. Aluminum oxide is slightly softer but is tougher than silicon carbide. It dulls more quickly, but it does not fracture easily therefore it is better suited for grinding materials of relatively high tensile strength. ABRASIVE GRAIN SIZE Abrasive grains are selected according to the mesh of a sieve through which they are sorted. For example, grain number 40 indicates that the abrasive grain passes through a sieve having approximately 40 meshes to the linear inch. A grinding wheel is designated coarse, medium, or fine according to the size of the individual abrasive grains making up the wheel. BONDING MATERIAL Bond The abrasive particles in a grinding wheel are held in place by the bonding agent. The percentage of bond in the wheel determines, to a great extent, the “hardness” or “grade” of the wheel. The greater the percentage and strength of the bond, the harder the grinding wheel will be. “Hard” wheels retain the cutting grains longer, while “soft” wheels release the grains quickly. If a grinding wheel is “too hard” for the job, it will glaze because the bond prevents dulled abrasive particles from being released so new grains can be exposed for cutting. Besides controlling hardness and holding the abrasive, the bond also provides the proper safety factor at running speed. It holds the wheel together while centrifugal force is trying to tear it apart. The most common bonds used in grinding wheels are vitrified, silicate, shellac, resinoid, and rubber. Vitrified A vast majority of grinding wheels have a vitrified bond. Vitrified bonded wheels are unaffected by heat or cold and are made in a greater range of hardness than any other bond. They adapt to practically all types of grinding with one notable exception: if the wheel is not thick enough, it does not withstand side pressure as in the case of thin cutoff wheels. Silicate Silicate bond releases the abrasive grains more readily than vitrified bond. Silicate bonded wheels are well suited for grinding where heat must be kept to a minimum, such as grinding edged cutting tools. It is not suited for heavy-duty grinding. Thin cutoff wheels are sometimes made with a shellac bond because it provides fast cool cutting. Resinoid Resinoid bond is strong and flexible. It is widely used in snagging wheels (for grinding irregularities from rough castings), which operate at 9,500 SFPM. It is also used in cutoff wheels. Rubber In rubber-bonded wheels, pure rubber is mixed with sulfur. It is extremely flexible at operating speeds and permits the manufacture of grinding wheels as thin as 0.006 inch for slitting nibs. Most abrasive cutoff machine wheels have a rubber bond. GRADES OF HARDNESS The grade of a grinding wheel designates the hardness of the bonded material. Listed below are examples of those grades: A soft wheel is one on which the cutting particles break away rapidly while a hard wheel is one on which the bond successfully opposes this breaking away of the abrasive grain. Most wheels are graded according to hardness by a letter system. Most manufacturers of grinding abrasive wheels use a letter code ranging from A (very soft) to Z (very hard). Vitrified and silicate bonds usually range from very soft to very hard, shellac and resinoid bonds usually range from very soft to hard, and rubber bonds are limited to the medium to hard range. The grade of hardness should be selected as carefully as the grain size. A grinding abrasive wheel that is too soft will wear away too rapidly, the abrasive grain will be discarded from the wheel before its useful life is realized. On the other hand, if the wheel is too hard for the job, the abrasive particles will become dull because the bond will not release the abrasive grain, and the wheel’s efficiency will be impaired. Figure 5-8 illustrates sections of three grinding abrasive wheels with different spacing of grains. If the grain and bond materials in each of these are alike in size and hardness, the wheel with the wider spacing will be softer than the wheel with the closer grain spacing. Thus, the actual hardness of the grinding wheel is equally dependent on grade of hardness and the and spacing of the grains or structure. ABRASIVE WHEEL STRUCTURE MARKINGS Bond strength of a grinding wheel is not wholly dependent upon the grade of hardness but depends equally on the structure of the wheel, that is, the spacing of the grain or its density. The structure or spacing is measured in number of grains per cubic inch of wheel volume. Every grinding wheel is marked by the manufacturer with a stencil or a small tag. The manufacturers have worked out a standard system of markings, For an example use a wheel marked A36-L5-V23. The A refers to the abrasive which is aluminum oxide. The 36 represents the grain size. The L shows the grade or degree of hardness, which is medium. The 5 refers to the structure of the wheel and the V refers to the bond type.