ABRASIVE PROCESS

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ABRASIVE PROCESS

ABRASIVES
An abrasive is a substance that is used for grinding and polishing operations. It should be pure and have uniform physical properties of hardness, toughness, and resistance to fracture to be useful in manufacturing grinding wheels.

They are classified into two groups.
1. Natural 2. Artificial or manufactured

Natural: Sandstone, solid quartz, emery, corundum, diamond.

Sandstone: Natural abrasive stones.
Emery: Natural Aluminium oxide containing 55 to 65% Alumina, balance of iron oxide and other impurities.
Corundum : Natural Aluminium oxide containing 75 to 95 %. Alumina and the remaining are Impurities.
Both emery and corundum have a greater hardness and better abrasive action than quartz.
Diamonds of less than gem quality are crushed to provide abrasive grains for making grinding wheels to grind cemented carbide tools and to make lapping compound.
Only a very small percentage of grinding wheels are produced from natural abrasive due to the impurities and lack of uniformity of the natural abrasives.

Artifical : a) Silicon carbide b) Aluminium oxide

Silicon carbide (SiC) abrasive is manufactured from 56 parts of silicon sand, 34 parts of powdered coke , 2 parts of salt. 8 parts of saw dust in a long, rectangular electric furnace of resistance type that is built up of loose brick work. Sand furnishes silica, coke furnishes carbon, sawdust makes the, charge porous, salt helps to fuse it and gases escape through the open joints in the brickwork. The abrasive wheels are denoted by letter ‘S’.

Types of Silicon carbide abrasive- Green grit — contains at least 97% silicon carbides.
Black grit : contains at least 95% silicon carbide . This form is harder but weaker than the later.

Silicon Carbide follows diamond in order of hardness. But it is not as tough as oxide. It is used for grinding of metals of low strength such as cemented carbide, stow and ceramic materials, grey cast iron, brass, bronze, copper, aluminium, vulcanized rubber etc.,

Aluminium oxide(A1₂0₃) manufactured by heating mineral bauxite, hydrated Aluminium oxide clay containing silicon, iron oxide , titanium oxide mixed with ground coke and iron borings in an arc type electric furnance.

Aluminium oxides is tough and not easily fractured, so it is better adopted to grinding materials of high tensile strength such as most steels, carbon steels, high speed steels, annealed malleable iron, wrought iron, tough bronze. The wheels are denoted by’ A’.

ABRASIVE PROCESS TYPES:

1. Grinding

2. Honing

3. Lapping

1. Grinding:

     Grinding is performed by means of rotating abrasive wheel which act as a tool. This is used to finish work pieces which must show a high surface quality accuracy of shape and dimension. Mostly grinding is for finishing operations because it removes comparatively little material (i.e.,) 0.02 to 0.50 mm and even to the order of 0.000025 mm.

             Grinding is also done to machine materials which are too hard for other machining method that use cutting tools.

2.0 General Classification of grinding:

2.1 Rough or non precision grinding.

2.2 Precision grinding.

2.1 Rough Grinding: Other names are snagging or off hand grinding. The work is held in the operators hand. The accuracy & surface finish, are of secondary importance. Removal of excess metal, without regard to the accuracy of the finished surface eg. excess metal on weld called snagging.

2.2 Precision grinding: Produces good surface finish and high degree of accuracy.

3.0 Clasification of grinding according to type of surface to be ground

3.1 External cylindrical Grinding..

3.2 Internal cylindrical Grinding.

3.3 Surface Grinding.

3.4 Form Grinding.

3.1 External cylindrical Grinding: To produce a straight or tapered surface. The work pieces are rotated about its own axis between centers as it passes lengthwise across the face of a revolving grinding wheel

3.2 Internal Cylindrical grinding: To produce internal holes and tapers. The work pieces are chucked and rotated about their own axis. The grinding wheel rotates against the sense of rotation of work piece.

3.3 Surface grinding: To produce flat surface. The work may be ground by either the periphery or by the end face of a grinding wheels. The work piece is reciprocated at a speed below or on the end face of a grinding wheel.

3.4 Form grinding: is done with specially shaped grinding wheels that grind the formed surfaces as in grinding gear, threads, spline shafts, holes and spheres etc.,

4.0       GRINDING MACHINES  

4.1       Classification according to quality of surface            finish.

4.1.1    Rough grinders.

4.1.2    Precision grinders.

4.1.1 Rough Grinders : Main work is to remove metal without any reference to accuracy of results.

 The main types are

                 1. Floor stand and bench grinders

                 2. Portable & flexible grinders

                 3. Swing frame grinders

                 4. Abrasive belt grinders

Grinding machine is specified according to the size of the largest work piece that can be mounted on the machine.

                                         

4.1.2 Precision grinders

Precision grinders are those, that finish parts. to a very accurate dimensions.

They are classified according to the type of surface generated or work done as noted below.

1. Cylindrical grinders

      a. Centre type (Plain)

      b. Centre type (Universal)

      C. Centre less

2 Internal grinders

      a, Chucking -  i. Plain

                             ii. Universal

      b. Planetory

     C, Centerless

3. Surface grinders

           a. Reciprocating - i. Horizontal spindle

                                        ii. Vertical spindle.

           b. Rotating Table - i. Horizontal spindle

                                         ii. Vertical spindle

4. Tool and cutter grinders — i. Universal

                                             ii. Special

5. Special grinding machines —

                                             i. Tool and cutter grinder

                                            ii. Crank shaft grinder

                                            iii. Piston grinders

                                            iv. Roll grinders

                                            v. Cam grinders

                                           vi. Thread grinders

                                           vii. Way grinders

                                          viii. Tool post grinders

PLAIN CENTRE TYPE GRINDER:

                              

                                   

               The Plain grinding machine is essentially a lathe on which a grinding wheel has been substituted for the single point tool. It consist of the following parts:

Base is the main casting that rests on the floor and supports the parts mounted on it. Guide ways are provided for the table to slide. Also houses the table drive mechanism.

Tables: The lower table slides on ways on the bed provides traverse of the work past the grinding wheel. It can be moved by hand or power within desired limits.

             The upper table is pivoted at its centre and is mounted on top of the lower table.  It has  T-slots in which headstock and tailstock can be positioned  along the table to suit the length of the work. The upper table can be swiveled upto 10^o  for grinding tapers.

Headstock supports the work piece by means of a  centre or drive the workpiece in a chuck.

Tailstock can be adjusted and clamped in various positions to accommodate different lengths of workpieces.

Wheelhead carries a grinding wheel and its driving motor. The wheelhead may be moved perpendicularly to the table ways by hand or power, to feed the wheel to the work.

SURFACE FINISHING PROCESSES

INTRODUCTION

     In a manufacturing plant, a product may be shaped, turned, milled or drilled, and left in that condition as being satisfactory for use. However, if a better finish is desired, for looks, for accuracy, for wearing qualities, or for any other reasons, one of the microfinishes that include lapping, honing, superfinishing, polishing, buffing, may be employed. In some cases other operations are done only to get durable finishes.

LAPPING
Lapping is an abrading process that is used to produce geometrically true surfaces, correct minor surface imperfections, improve dimensional accuracy, or provide a very close fit between two contact surfaces. Very thin layers of metal (0.005 to 0.01mm) are removed in lapping and it is, therefore, evident that lapping is unable to correct substantial errors in the form and sizes of surfaces. It is, however, low efficiency process and is used only when specified accuracy and surface finish cannot be obtained by other methods.

Abrasive powders (flours) such as emery, corundum, iron oxide, chromium oxide, etc., mixed with oil or special pastes with some carrier are used in lapping. Most lapping is done by means of lapping shoes or quills, called laps, that are rubbed against the work. The face of a lap becomes “charged” with abrasive particles. Charging a lap means to embed the abrasive grains into its surface. Laps may be made of almost any material soft enough to receive and retain the abrasive grains. They are made of soft cast iron, brass, copper, lead or soft steel. The method of charging a lap depends upon the shape of lap. When the lap is once charged, it should be used without applying more abrasive until it ceases to cut. Laps may be operated by hand or machine, the motion being rotary or reciprocating. Cylindrical work be lapped by rotating the work in lathe or drill press and reciprocating the lap over the work in an ever- changing path. Small flat surfaces may be lapped by holding the work against a rotating disc, or the work may be moved by hand in an irregular path over a stationary faceplate lap.
In equalizing lapping the work and lap mutually improve each others surface as they slide on each other.

HONING
Honing is grinding or an abrading process mostly for finishing round holes by means of bonded abrasive stones, called hones. Honing is therefore a cutting operation and has been used to remove as much as 3 mm of stock but is normally confined to amounts less than 0.25 mm. So honing is primarily used to correct some out of roundness, taper, tool marks, and axial distortion. Honing stones are made from common abrasive and bonding materials, often impregnated with sulphur, resin, or wax to improve cutting action and lengthen tool life. Materials honed range from plastics, silver, aluminium, brass and cast iron to hard steel and cemented. carbides. This method is mostly used for finishing automobile crankshaft journals.
When honing is done manually the tool is rotated, and the workpiece is passed back and forth over the tool. For precision honing, the tool is given a slow reciprocating motion as it rotates. Honing stones may be loosely held in holders, cemented into metal shells which are clamped into holders, cemented directly in holders, or cast into plastic tabs which are held in holders. Some stones are spaced at regular intervals around the holder, while others are interlocking so that they present a continuous surface to the bore. A typical honing tool head is shown in fig. 16.1. The honing tool may be so made that a floating action between the work and tool prevails and any pressure exerted in the tool may be transmitted equally to all sides. Coolants are essential to the operation of this process to flush away small chips and to keep temperatures uniform.
Honing is done on general purpose machines, such as the lathe, drill press, and portable drills, as an expedient. But more economical results can be obtained by honing machines for production work. There are two general types of honing machines : Horizontal and vertical. A honing machine rotates and reciprocates the hone inside holes being finished. The two motions produce round and straight holes that have a very fine surface finish of random scratches. Vertical honing machines are probably more common. Horizontal honing machines are often used for guns and large bores.