Grinding
Grinding It is a material cutting process which engages an abrasive tool(in the form of a wheel) whose cutting elements are grains of abrasive material known as grit. These grits are characterized by sharp cutting points, high hot hardness, and chemical stability and wear resistance. The grits are held together by a suitable bonding material to give shape of an abrasive tool. It is the most common form of abrasive machining. Grinding can be compared with milling with an infinite number of cutting edge.
Fig- cutting action of abrasive grains Advantages of Grinding Dimensional accuracy Good surface finish Good form and locational accuracy Applicable to both hardened and unhardened material
Applications of Grinding Surface finishing Slitting and parting Descaling, deburring Stock removal (abrasive milling) Finishing of flat as well as cylindrical surface Grinding of tools and cutters and resharpening of the same
Grinding If each abrasive grain is viewed as a cutting tool then in grinding operation. High Rake angle can be positive, zero, or negative ranging from +45 o to -60 o, dull, rounded grits has large negative rake angle Cutting speed is very high Very high specific energy of cutting Low Low shear angle Low feed rate Low depth of cut
Interaction of the grit and workpiece Grit with favorable geometry can produce chip in shear mode. However, grits having large negative rake angle or rounded cutting edge do not form chips but may rub or make a groove by ploughing leading to lateral flow of the workpiece material. Figure- Grits engage shearing, ploughing and rubbing
Q: How is chip accommodation volume is related to material removal rate? Volume of chip accommodation space ahead of each grit must be greater than the chip volume produced by each grit to facilitate easy evacuation of the chip from the grinding wheel. Q:Why is high velocity desired in grinding? It is desired to off set the adverse effect of very high negative rake angle of the working grit, to reduce the force per grit as well as the overall grinding force. Q:On which factors does the transverse roughness of workpiece depend during grinding? It mainly depends on the shape of the grits and overlap cuts made by the grits in the transverse direction. Lateral plastic flow of the material as a result of ploughing also influences the surface roughness.
Specific energy consumption in grinding
G Ratio The grinding ratio or G ratio is defined as volume of stock removed divided by the volume of wheel lost. In conventional grinding, the G ratio is in the range 20: 1 to 80: 1. The G ratio is a measure of grinding production and reflects the amount of work a wheel can do during its useful life. As the wheel losses material, it must be reset or repositioned to maintain workpiece size.
Parameters for specify a grinding wheel 1) The type of grit material 2) The grit size 3) The bond strength of the wheel, commonly known as wheel hardness 4) The structure of the wheel denoting the porosity i.e. the amount of inter grit spacing 5) The type of bond material 6) Other than these parameters, the wheel manufacturer may add their own identification code prefixing or suffixing (or both) the standard code.
The number 51 is manufacturer s identification number indicating exact kind of abrasive used. The letter A denotes that the type of abrasive is aluminium oxide. In case of silicon carbide the letter C is used. The number 60 specifies the average grit size in inch mesh. For a very large size grit this number may be as small as 6 where as for a very fine grit the designated number may be as high as 600. The letter K denotes the hardness of the wheel, which means the amount of force required to pull out a single bonded abrasive grit by bond fracture. The letter symbol can range between A and Z, A denoting the softest grade and Z denoting the hardest one. The number 5 denotes the structure or porosity of the wheel. This number can assume any value between 1 to 20, 1 indicating high porosity and 20 indicating low porosity. The letter code V means that the bond material used is vitrified. The codes for other bond materials used in conventional abrasive wheels are B (resinoid), BF (resinoid reinforced), E(shellac), O(oxychloride), R(rubber), RF (rubber reinforced), S(silicate) The number 05 is a wheel manufacturer s identifier.
Abrasive Material Comments and Uses Aluminium oxide Softer and tougher than silicon carbide; use on steel, iron, brass Silicon carbide Used for brass, bronze, aluminum, stainless steel and cast iron cbn (cubic boron nitride) Diamond For grinding hard, tough tool steels, stainless steel, cobalt and nickel based superalloys, and hard coatings Used to grind nonferrous materials, tungsten carbide and ceramics
Grit size The grain size affects material removal rate and the surface quality of workpiece in grinding. Large grit- big grinding capacity, rough workpiece surface Fine grit- small grinding capacity, smooth workpiece surface
Grade The worn out grit must pull out from the bond and make room for fresh sharp grit in order to avoid excessive rise of grinding force and temperature. A soft wheel should be chosen for grinding hard material.
Structure / concentration The structure should be open for grinding wheels engaged in high material removal to provide chip accommodation space. The space between the grits also serves as pocket for holding grinding fluid. Dense structured wheels are used for longer wheel life, for holding precision forms and profiles.
Bonding Material Bonding material is a very important factor to be considered in selecting a grinding wheel. It determines the strength of the wheel, thus establishing the maximum operating speed. It determines the elastic behavior or deflection of the grits in the wheel during grinding. The wheel can be hard or rigid, or it can be flexible. Finally, the bond determines the force required to dislodge an abrasive particle from the wheel and thus plays a major role in the cutting action. Bond materials are formulated so that the ratio of bond wear matches the rate of wear of the abrasive grits. Bonding materials in common use are:
Type of Bond Bonding Materials for Grinding Attributes wheels Vitrified bonds Composed of clays and other ceramic substances, porous, strong, rigid, and unaffected by oils, water, or temperature. Brittle and can not be used for high wheel speed. The operating speed range in most cases is 1500 to 5000 m/min. Resinoid, or phenolic resins Shellac bond Plastic bond, replaced shellac and rubber wheels, not with alkaline grinding fluid. For flexible cut off wheels, replaced by resin bond.
Type of Bond Rubber bond Oxychloride bond Bonding Materials for Grinding Attributes wheels For use in thin wheels, replaced by resin bond. Limited use. Metal bond Extensively used with super abrasive wheels, high toughness, high accuracy, large stock removal. Electroplated bond Used for small wheel, form wheel and thin super abrasive wheels, for abrasive milling and ultra high speed grinding. Replace by electroplated bond
Vitrified bonds. They are composed of clays and other ceramic substances. The abrasive particles are mixed with the wet clays so that each grain is coated. Wheels are formed from the mix, usually by pressing, and then dried. They are then fired in a kiln, which results in the bonding material's becoming hard and strong, having properties similar to glass. Vitrified wheels are porous, strong, rigid, and unaffected by oils, water, or temperature over the ranges usually encountered in metal cutting. The operating speed range in most cases is 1500 to 5000 m/min. Vitrified bond is suitable for high stock removal even at dry condition. It can also be safely used in wet grinding. It can not be used where mechanical impact or thermal variations are like to occur. This bond is also not recommended for very high speed grinding because of possible breakage of the bond under centrifugal force.
Why is aluminium oxide preferred to silicon carbide in grinding steel? Al 2 O 3 is tougher than SiC. Therefore it is preferred to grind material having high tensile strength like steel. Moreover, Al 2 O 3 shows higher chemical inertness than SiC towards steel leading to much improved wear resistance during grinding. Why is coarse grain and open structured wheel is preferred for stock removal grinding? Coarse grit allows large grit protrusion and open structure provides large inter grit chip space. Thus in combination those two provide large space for chip accommodation during stock removal grinding and risk of wheel loading is minimized. Why does single layer grinding wheel show progressive rise of force during grinding of high speed steel? The geometry of grit undergoes irreversible change in the form of rounding or flattening due to wear caused by rubbing action of hard carbides present in high speed steel.
Glazing With continuous use a grinding wheel becomes dull with the sharp abrasive grains becoming rounded. This condition of a dull grinding wheel with worn out grains is termed as glazing.
Loading Some grinding chips get lodged into the spaces between the grits resulting in a condition known as loaded wheel. Loading is generally caused during the grinding of soft and ductile materials. A loaded grinding wheel cannot cut properly and need dressing.
Dressing Dressing is the conditioning of the wheel surface which ensures that grit cutting edges are exposed from the bond and thus able to penetrate into the workpiece material. In dressing attempts are made to splinter the abrasive grains to make them sharp and free cutting and also to remove any residue left by material being ground. Dressing therefore produces micro-geometry.
Truing Truing is the act of regenerating the required geometry on the grinding wheel. Truing is also required on a new conventional wheel to ensure concentricity with specific mounting system. Truing and dressing are commonly combined into one operation for conventional abrasive grinding wheels, but are usually two distinctly separate operation for super abrasive wheel.
Balancing Grinding Wheels Because of the high rotation speeds involved, grinding wheels must never be used unless they are in good balance. Grinding wheel must be balanced Statically and Dynamically. A slight imbalance will produce vibrations that will cause waviness in the work surface. It may cause a wheel to break, with the probability of serious damage and injury.
Creep feed grinding This machine enables single pass grinding of a surface with a larger down feed but slower table speed than that adopted for multi-pass conventional surface grinding. In creep-feed grinding, the entire depth of cut is completed in one pass only using very small in-feed rates.
State the basic advantage of a creep feed grinder over a conventional surface Productivity is enhanced and life of the grinding wheel is extended.
Cylindrical Grinding Center-type cylindrical grinding is commonly used far producing external cylindrical surfaces. The grinding wheel revolves at an ordinary cutting speed, and the workpiece rotates on centers at a much slower speed. Grinding machines are available in which the workpiece is held in a chuck for grinding both external and internal cylindrical surfaces.
Centerless Grinding Centerless grinding makes it possible to grind both external and internal cylindrical surfaces without requiring the workpiece to be mounted between centers or in a chuck. This eliminates the requirement of center holes in some workpieces and the necessity for mounting the workpiece, thereby reducing the cycle time. Two wheels are used. The larger one operates at regular grinding speeds and does the actual grinding. The smaller wheel is the regulating wheel. It is mounted at an angle to the plane of the grinding wheel.
Centerless Grinding The regulating wheel controls the rotation and longitudinal motion of the workpiece and usually is a plastic- or rubberbonded wheel with a fairly wide face. The workpiece is held against the work-rest blade by the cutting forces exerted by the grinding wheel and rotates at approximately the same surface speed as that of the regulating wheel.
Centerless Grinding
Centerless Grinding The axial feed is calculated by the equation F = dn sin where F = feed (mm/min) d = diameter of the regulating wheel (mm) N = revolutions per minute of the regulating wheel = angle of inclination of the regulating wheel
Centreless internal Grinding This machine is used for grinding cylindrical and tapered holes in cylindrical parts (e.g. cylindrical liners, various bushings etc). The workpiece is rotated between supporting roll, pressure roll and regulating wheel and is ground by the grinding wheel.
Disadvantages of centreless cylindrical grinding machine It does not grind concentrically with centres. Large diameter short workpiece are difficult to control in the process It may not improve workpiece perpendicularity.
Surface Grinding Machines Surface grinding machines are used primarily to grind flat surfaces. However formed, irregular surfaces can be produced on some types of surface grinders by use of a formed wheel. Four basic types of surface grinding machines are: 1. Horizontal spindle and reciprocating table 2. Vertical spindle and reciprocating table 3. Horizontal spindle and rotary table 4. Vertical spindle and rotary table
FINISHING PROCESSES
Lapping Lapping is basically an abrasive process in which loose abrasives function as cutting points finding momentary support from the laps. Material removal in lapping usually ranges from.003 to.03 mm but many reach 0.08 to 0.1mm in certain cases.
Characteristics of lapping process Use of loose abrasive between lap and the workpiece Usually lap and workpiece are not positively driven but are guided in contact with each other Relative motion between the lap and the work should change continuously so that path of the abrasive grains of the lap is not repeated on the workpiece. Cast iron is the mostly used lap material. However, soft steel, copper, brass, hardwood as well as hardened steel and glass are also used.
Abrasives of lapping Al 2 O 3 and SiC, grain size 5 ~100 μm Cr 2 O 3, grain size 1 ~ 2 μm B 4 C 3, grain size 5-60 μm Diamond, grain size 0.5 ~ 5 μm
Machine oil Rapeside oil grease Vehicle materials for lapping
Technical parameters affecting lapping processes are unit pressure the grain size of abrasive concentration of abrasive in the vehicle lapping speed
Honing Honing is a finishing process, in which a tool called hone carries out a combined rotary and reciprocating motion while the workpiece does not perform any working motion. Most honing is done on internal cylindrical surface, such as automobile cylindrical walls. The honing stones are held against the workpiece with controlled light pressure. The honing head is not guided externally but, instead, floats in the hole, being guided by the work surface.
It is desired that Honing 1. Honing stones should not leave the work surface 2. Stroke length must cover the entire work length. 3. In honing rotary and oscillatory motions are combined to produce a cross hatched lay pattern. The honing stones are given a complex motion so as to prevent every single grit from repeating its path over the work surface.
Honing Fig. Honing tool Fig. Lay pattern produced by combination of rotary and oscillatory motion
The critical process parameters 1. rotation speed are 2. oscillation speed 3. length and position of the stroke 4. honing stick pressure
Buffing Buffing is a polishing operation in which the workpiece is brought into contact with a revolving cloth wheel that has been charged with a fine abrasive, such as polishing rough. The wheels are made of disks of linen, cotton, broadcloth, or canvas, and achieve the desired degree of firmness through the amount of stitching used to fasten the layers of cloth together. Negligible amount of material is removed in buffing while a very high luster is generated on the buffed surface. The dimensional accuracy of the parts is not affected by the buffing operation.
Super Finishing Fig. super finishing of end face of a cylindrical work piece in radial mode In this both feeding and oscillation of the super finishing stone is given in the radial direction.
Super Finishing Fig. super finishing operation in plunge mode In this case the abrasive stone covers the section of the workpiece requiring super finish. The abrasive stone is slowly fed in radial direction while its oscillation is imparted in the axial direction.
Process Grinding Creep feed grinding Abrasive Machining Processes Features Uses wheels, accurate sizing, finishing, low MRR; can be done at high speeds. Uses wheels with long cutting arc, very slow feed rate and large depth of cut Abrasive machining High MRR, to obtain desired shapes and approximate sizes Honing "Stones" containing fine abrasives; primarily a hole - finishing process Lapping Abrasive waterjet Fine particles embedded in soft metal or cloth; primarily a surface-finishing process Waterjets with velocities up to 1000 m/sec carry abrasive particles (silica and garnet)
Books: Workshop Technology by B.S. Raghuwanshi Production Engineering Notes bys.k.modal