MANUFACTURING SCIENCE - II
Presented by AVINASH GUPTA ME-1 , IIIrd YEAR 1 CONTENTS OUTLINE Grinding - Definition Grinding Wheel
Grinding Wheel Wear Abrasives Bond Grit Grade Grinding Machines Classification of Grinding Machines Grinding Grinding is a surface finishing operation where very thin layer of material is removed in the form of dust particles. Thickness of material removed is in range of 0.25 to 0.50 mm. Tool used is a abrasive wheel. 3 Grinding wheel Basic Functions :
A grinding wheel is an expendable wheel that carries an abrasive compound on its periphery. These wheels are used in grinding machines. Basic Functions : To remove material from a work piece in an abrasive action. Each grain acts as a cutting tool. It is a chip formation process. The only self sharpening cutting tool. Considered as abrasive machining. 4 Grinding Wheel Wear Fig: Physical model of a grinding wheel, showing is structure and wear and fracture patterns. Attritious grain wear Grain fracture Bond fracture
Grinding wheel wear is an important consideration, because it adversely affects the shape and accuracy of ground surfaces, as is the case with cutting tools. Grinding wheels wear by three different conditions : Attritious grain wear Grain fracture Bond fracture 6 Attritious wear Grain fracture
Attritious wear involves dulling of the individual grains, resulting in flat spots and rounded edges. Wear is caused by interaction of the grain with the workpiece material , involving both physical and chemical reaction. Cutting edge of a sharp grain becomes dull. Grain fracture Grain fracture occurs when a portion of the grain breaks off but the rest of the grain remains bonded in the wheel. The edges of the fractured area become new sharp cutting edges on the grinding wheel. This makes the grinding wheel self-sharpening, a unique property of a cutting tool. 7 Bond fracture Bond fracture occurs when the individual grains are pulled out of the bonding material. Bond fracture usually occurs because the grain has become dull due to attritious wear and the resulting cutting force is excessive. Sharp grains cut more efficiently with lower cutting forces; hence, they remain attached the bond structure 8 Abrasives Why Abrasive Processes are Important ?
Abrasives are intended for actual cutting action and hence they are hard substances. Abrasive machining is a material removal process that involves the use of abrasive cutting tools. Why Abrasive Processes are Important ? Can be used on all types of materials Some can produce extremely fine surface finishes, to um Some can hold dimensions to extremely close tolerances 9 Abrasive Materials Aluminum oxide (Al2O3) - most common abrasive
Used to grind steel and other ferrous high strength alloys Silicon carbide (SiC) - harder than Al2O3 but not as tough Used on aluminum, brass, stainless steel, some cast irons and certain ceramics 10 Cubic boron nitride (cBN)
very hard, very expensive Suitable for steels Used for hard materials such as hardened tool steels and aerospace alloys (e.g., Ni-based alloys) Diamond Even harder, very expensive Occur naturally and also made synthetically Not suitable for grinding steels Used on hard, abrasive materials such as ceramics, cemented carbides, and glass 11 Abrasive Material Properties
High hardness Wear resistance Toughness Friability : capacity to fracture when cutting edge dulls, so a new sharp edge is exposed 12 Hardness of Abrasive Materials
Abrasive material Knoop hardness Aluminum oxide Silicon carbide Cubic boron nitride Diamond (synthetic) 13 Grit Small grit sizes produce better finishes
Larger grit sizes permit larger material removal rates Harder work materials require smaller grain sizes to cut effectively Softer materials require larger grit sizes Grit size is measured using a screen mesh procedure Smaller grit sizes indicated by larger numbers in the screen mesh procedure and vice versa Grain sizes in grinding wheels typically range between 8 (very coarse) and 250 (very fine) 14 Bonding Material Properties
Must withstand centrifugal forces and high temperatures Must hold abrasive grains rigidly in place for cutting yet allow worn grains to be dislodged so new sharp grains are exposed 15 Wheel Grade Indicates bond strength in retaining abrasive grits during cutting Depends on amount of bonding material in wheel structure Measured on a scale ranging between soft and hard Soft wheels lose grains readily - used for low material removal rates and hard work materials Hard wheels retain grains - used for high stock removal rates and soft work materials 16 Structure 17 Grinding Wheel Specification
Standard grinding wheel marking system used to designate abrasive type, grit size, grade, structure, and bond material Example: A-46-H-6-V Also provides for additional identifications for use by grinding wheel manufacturers 18 Types of Grinding Wheel
a) straight, (b) recessed two sides, (c) metal wheel frame with abrasive bonded to outside circumference, (d) abrasive cut- off wheel Grinding machines Grinding machine is a power operated machine tool where, the work piece is fed against constantly rotating abrasive wheel to remove thin layer of material from work. 20 Principle of grinding machines
Work piece is fed against the rotating abrasive wheel. Due to action of rubbing or friction between the abrasive particles and work piece material is removed. 21 Classification of grinding machine
Surface Grinding Machine Cylindrical Grinding Machine Centre less Grinding Machine Internal Grinding Machine Centred Grinding Machine 22 Surface grinding machine
It is machine basically used to grind flat surface. Job is mounted to a table which moves longitudinally as well as in transverse direction. Manual feed or power feed. Work piece can clamped in two ways Manual clamps. Magnetic chuck. 23 Internal pump and piping arrangement for coolant.
Protective guard for safety. 24 Schematic Diagram Working Work piece is clamped to the table by operating magnetic chuck. Required grade of grinding tool is fixed to spindle. Grinding operation is carried out be operating both table traverse wheel and vertical feed hand wheel. 26 Cylindrical grinding machine
It is a process of grinding curved surfaces. Surface may be straight or tapered. Work piece is mounted on two centers, one is tailstock centre and the other is headstock centre. Head stock center may or may not revolve. 27 Working principle 28 Schematic Diagram 29 Centre less grinding machine
It is used to grind curved surface work piece which are long and slender. Work piece rests on a work-rest blade and is backed by a second wheel called as regulating wheel. Grinding wheel pushes the work piece down the work-rest blade against the regulating wheel. 30 Working Principle 31 Schematic Diagram 32 Advantages No need for maintaining centres and centre holes.
Workpieces can be loaded and unloaded from the machine rapidly. It gives maximum material removal rates. Grinding allowance required is less. Minimum wear is observed when large grinding wheels are used. Disadvantages Process is not suitable for large workpieces.
Setup time is usually large. THANK YOU