Grinding Machines free essay sample

GRINDING MACHINES Grinding is the process of removing metal by the application of abrasives which are bonded to form a rotating wheel. When the moving abrasive particles contact the workpiece, they act as tiny cutting tools, each particle cutting a tiny chip from the workpiece. It is a common error to believe that grinding abrasive wheels remove material by a rubbing action; actually, the process is as much a cutting action as drilling, milling, and lathe turning. The grinding machine supports and rotates the grinding abrasive wheel and often supports and positions the workpiece in proper relation to the wheel. The grinding machine is used for roughing and finishing flat, cylindrical, and conical surfaces; finishing internal cylinders or bores; forming and sharpening cutting tools; snagging or removing rough projections from castings and stampings; and cleaning, polishing, and buffing surfaces. Once strictly a finishing machine, modem production grinding machines are used for complete roughing and finishing of certain classes of work. SAFETY PRECAUTION GRINDING MACHINE SAFETY Grinding machines are used daily in a machine shop. To avoid injuries follow the safety precautions listed below. Wear goggles for all grinding machine operations. Check grinding wheels for cracks (Ring Test Figure 5-11) before mounting. Never operate grinding wheels at speeds in excess of the recommended speed. Never adjust the workpiece or work mounting devices when the machine is operating Do not exceed recommended depth of cut for the grinding wheel or machine. Remove workpiece from grinding wheel before turning machine off. Use proper wheel guards on all grinding machines. On bench grinders, adjust tool rest 1/16 to 1/8 inch from the wheel. TYPES OF GRINDING MACHINE From the simplest grinding machine to the most complex, rinding 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 offh and 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. kill, 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 motor shaft. 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 floormounted 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 ounted 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). SURFACE GRINDING MACHINE The surface grinding machine is used for grinding flat surfaces. The workpiece is supported on a rectangular table hich 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. GRINDING WHEEL 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 or 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 becaus e it is used for sharpening saws. 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 â€Å"hardness† or â€Å"grade† of the wheel. The greater the percentage and strength of the bond, the harder the grinding wheel will be. â€Å"Hard† wheels retain the cutting grains longer, while â€Å"soft† wheels release the grains quickly. If a grinding wheel is â€Å"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 ithstand 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 utoff 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 Figure 5-8 illustrates sections of three grinding abrasive the grain size. A grinding abrasive wheel that is too soft wheels with different spacing of grains. If the grain and bond will wear away too rapidly, the abrasive grain will be materials in each of these are alike in size and hardness, the discarded from the wheel before its useful life is wheel with the wider spacing will be softer than the wheel realized. On the other hand, if the wheel is too hard for with the closer grain spacing. Thus, the actual hardness of the job, the abrasive particles will become dull because the grinding wheel is equally dependent on grade of hardness the bond will not release the abrasive grain, and the and spacing of the grains or structure. wheel’s efficiency will be impaired. STANDARD SHAPES OF GRINDING WHEEL FACES Figure 5-10 illustrates standard shapes of grinding wheel faces. The nature of the work dictates the shape of the face to be used. For instance, shape A is commonly used for straight cylindrical grinding and shape E for grinding threads. SELECTION OF GRINDING WHEELS Conditions under which grinding wheels are used vary considerably, and a wheel that is satisfactory on one machine may be too hard or soft for the same operation on another machine. The following basic factors are considered when selecting grinding wheels, though it should be understood that the rules and conditions listed are flexible and subject to ccasional exceptions. GENERAL GRINDING OPERATIONS GENERAL Efficient grinding depends primarily upon the proper setup of the machine being used. If the machine is not securely mounted, vibration will result, causing the grinder to produce an irregular surface. Improper alignment affects grinding accuracy, and it is good practice to check the security and plumb of t he machine every few months. It is advisable to place a strip of cushioning material under the mounting flanges, along with any necessary aligning shims, to help absorb vibration. When a grinding wheel is functioning properly, the abrasive rains cut very small chips from the workpiece and at the same time a portion of the bond of the wheel is worn away. As long as the bond is being worn away as fast as the abrasive grains of the wheel become dull, the wheel will continue to work well. If the bond is worn away too rapidly, the wheel is too soft and will not last as long as it should. If the cutting grains wear down faster than the bond, the face of the wheel becomes glazed and the wheel will not cut freely. CLASSES OF GRINDING Precision and semiprecision grinding may the following classes: Cylindrical Grinding be divided into Cylindrical grinding denotes the grinding of a cylindrical surface. Usually, â€Å"Cylindrical grinding† refers to external cylindrical grinding and the term â€Å"internal grinding† is used for internal cylindrical grinding. Another form of cylindrical grinding is conical grinding or grinding tapered workpieces. Surface Grinding Surface grinding is the grinding of simple plain surfaces. Tool and Cutter Grinding Tool and cutter grinding is the generally complex operation of forming and resharpening the cutting edges of tool and cutter bits, gages, milling cutters, reamers, and so forth.