GRINDING

Grinding is a finishing process used to improve surface finish, abrade hard materials, and tighten the tolerance on flat and cylindrical surfaces by removing a small amount of material. In grinding, an abrasive material rubs against the metal part and removes tiny pieces of material. The abrasive material is typically on the surface of a wheel or belt and abrades material in a way similar to sanding. On a microscopic scale, the chip formation in grinding is the same as that found in other machining processes. The abrasive action of grinding generates excessive heat so that flooding of the cutting area with fluid is necessary. Following are the reasons for using grinding operation.

To grind means to abrade, to war away by friction or to sharpen. In manufacturing it refers to the removal of metal by an abrasive wheel rotating at high speeds and working on the external or internal surface of a metallic or other part hard enough to be abraded, rather than indented by the grinding wheel. The action of the grinding wheel is similar to that of a milling cutter. The grinding wheel is composed of many small abrasive particles bounded together, each one acting as a miniature cutting point.

Grinding removes metal from the work piece in the form of small chips by the mechanical action of abrasive particles bonded together in a grinding wheel.

Grinding machines are designed principally for finishing parts having cylindrical, flat or internal surfaces. The kind of surface machined largely determines the type of grinding machine. Following is the classification of various types of grinding machines.

It is a precision grinding machine to produce flat surfaces on a work piece. It is more economical and practical method of accurately finished flat surfaces than filling and scraping. The grinding is done on the circumference of the plain wheel. Area of contact is less. Following are the different types of surface grinders. In general, following are the parts of any grinding machine.

Base: It has a driving mechanism ( hydraulic device, tank and motor. ) It has column at the back for supporting the wheel head.

Saddle: It is the frame. It carries the table in its cross wise movement. It is used to give cross-feed to the work. It can be moved by hand feed or auto-feed.

Table: It is fitted on the saddle. It reciprocates along the guide ways to proved the longitudinal feed to the work. It has 'T' slots for clamping purposes. It is moved by hand or auto-feed.

Wheel head: It is mounted on the column. It can be moved vertically up and down to accommodate work piece of different lengths. The wheel rotates at a constant speed of 1500 m / min.

Horizontal spindle reciprocating table

Horizontal spindle rotary table

Vertical spindle reciprocating table	Vertical spindle rotary table

2. Centered Grinding:

Grinding for surfaces of rotation (axially symmetric surfaces) can be either centered or centerless. Centered grinding involves fixturing the part on a spindle axis as it is ground, as illustrated below. This configuration can be compared to fixturing a part on a lathe with or without a tail stock. The abrasive material is on a grinding wheel that rotates in a direction such that rolling or sliding contact occurs where the wheel and work piece touch. Centered grinding is accurate and stable, but set-up takes time and through-put suffers.

Center less grinding is similar to centered grinding except that there is no spindle. This allows high through-put since parts can be quickly inserted and removed from the process. Out of the two wheels the large wheel is the grinding wheel, and the smaller one is the pressure wheel. In operation, the pressure exerted by the grinding wheel on the work forces the work against the work rest and regulating wheel. The regulating wheel is of rubber bonded abrasive having the frictional characteristics to rotate the work at its own rotational speed.

            The axial movement of the work piece past the grinding wheels is obtained, by tilting the regulating wheel at a slight angel from horizontal. An angular adjustment of 0^o to 10^o is provided in the machine for this purpose. There are three main types of center less grinding.

Through-feed grinding:

            In through-feed grinding, the part rotates between the grinding wheel and a regulating wheel as shown below. For through-feed grinding, one or both wheels of the centerless grinding machine are canted out of the horizontal plane, as shown below. This imparts a horizontal velocity component to the work piece, so that outside feed mechanisms are not necessary.

            The grinding wheel is canted with respect to the other two axes so that a component of its surface velocity pushes the part in the direction shown below. This auto feeding characteristic is useful for rapidly processing many parts in quick sequence. Because of the axial movement, through-feed parts can only have right circular cylindrical ground surfaces. The wheel cannot be dressed to grind more complex shapes.

In-Feed Grinding:

It is used for jobs that, because of a shoulder or some other obstruction on the part, can only enter the machine so far and then, after the grinding is done, must be with drawn. In-feed grinding differs from through-feed grinding in that the part is not fed axially so that the ground surface does not need to be a right circular cylinder. The grinding wheel can be dressed to accommodate the part. Once the work piece part is in place, the grinding wheel is fed in radially.

Because of the set up time involved for each part, in-feed grinding does not have the high throughput of through-feed grinding. In-feed grinding is illustrated below.

End-Feed Grinding:

In end-feed grinding, the part moves in axially between the grinding wheels, stops for grinding, and then moves out again. The wheel can be dressed to form more complex shapes, but the part can only get progressively smaller in diameter. End-feed grinding is illustrated below.

Center less grinding is used when large quantities of the same part are required. Production is high and cost are relatively low because there is not need to drill center holes nor to mount the work in holding device. Almost an material can be ground with this technique. Minimum time is lost in loading and unloading. Since no axial force is acting on the work piece, long slender work pieces can be used without being distorted.

Large grinding wheels are used and hence wear is less and minimum amount of adjustment. A low order of skill is required to attend the centerless grinding much of the time.

It produces a cylindrical or conical shape on a work piece. The work piece is mounted between centers or in a chuck and the face of the grinding wheel passes over the external surface of the revolving work piece. There are two types of cylindrical grinders. They are

These are the machines that are designed for simple external grinding. The wheel head is made to operate to and from the work table but cannot be swiveled. The work table holds the work head and tail stock and can be swiveled for slight tapers. The head stock is rigidly attached to the work table and cannot be swiveled. It is located to the left of the operator. These grinders are used to produce

It is different from the above grinder in the sense that the wheel head can be swiveled on its base and can be fed to and from the table. The upper work table can be swiveled and is equipped with scales and adjusting screws for setting the table to produce slight tapers. Steep tapers may be ground by swiveling the headstock on its base. The universal grinding machine is a tool room machine.

It is designed to facilitate the finishing of holes. There are three type of internal grinders. They are

In a machine shop, many of the operations are done by single point cutting tools or multipoint cutting tools called as milling cutters. The cutting tools become blunt and becomes important to carry out re-sharpening. This is done in tool rooms where a tool and cutter grinder is sued for this purpose. A universal tool and cutter grinder is used to re-sharpen reamers, taps, single point tools dies and punches. A tool and cutter grinder is also used as a surface, grinding, cylindrical grinding and internal grinding machine with the help of certain attachments.

A grinding wheel may be considered as a multipoint cutting tool with a cutting action similar to that of a milling cutter except that the cutting points are irregularly shaped and randomly distributed over the active face of the wheel. In order to make the grinding wheel suitable for different work situations, the features such as abrasive, grain size, grade, structure and bonding materials can be varied.

Those grains which actually perform the cutting operation are called active grains. In peripheral grinding, each active grain removes a short chip of gradually increasing thickness in a way that is similar to the action of a tooth on a slab milling cutter. Because of irregular shape of the grains, there is considerable plowing action, between each active grain and the new work surface. The plowing results in progressive wear, causing the formation of worn areas on the active grains. As grinding proceeds the number and size of these worn areas increase, thus increasing the interference or friction, resulting in an increase in the force acting on the grain. Eventually this force become large enough to tear the work grain from the bond of the wheel and thus expose a new cutting edges. Thus grinding wheel has self sharpening characteristics.

A grinding wheel consists of an abrasive that does the cutting and a bond that holds the abrasive particles together. There are two types of abrasives. They are Natural and Artificial abrasives. The natural abrasives are emery and corundum. These are impure forms of aluminum oxide. Artificial abrasives are silicon carbide and aluminum oxide. The abrasives are selected depending upon the materials to be ground. Following are important criteria in grinding wheel manufacture.

Grain size: The number indicating the size of the grit represents the number of openings in the sieve used to size the grain. Larger the grit size number, finer the grit.

Grade: Indicates the strength of the bond and, therefore the hardness of the wheel. In a hard wheel the bond is strong and it securely anchors the grit in place, and therefore, reduces the rate of wear. In a soft wheel, the bond is weak and he grit is easily detached resulting in a high rate of wear.

Structure: This indicate the amount of bond present between the individual abrasive grains, and the closeness of the individual grains to each other. An open structure will cut more freely. That is, it will remove more material in a give time and produce less heat.

Bond: Is a substance which, when mixed with abrasive grains holds them together, enabling the mixture to be shaped in the form of the wheel, and after suitable treatment to take on the form of the wheel and the necessary mechanical strength for its work. The degree of hardness possessed by the bond is called as 'grade' of the wheel, and this indicates the ability of the bond to hold the abrasive grains in the wheel. There are several types of bonding materials used for making wheels.

Vitrify means to change into glass by heat and fusion. Thus when clay, feldspar or flint are mixed with the abrasive grains and heated to 1200^oC, the ceramic material melts and forms a lass like coating and bonding agent for the grains. The forming of wheels is mostly done by the puddled or pressed process.

In puddled process, the correct proportion of grain and bonding material are mixed wet and poured into a molt to dry. The wheel is then shaped on a machine operating on the principle of potters wheel. The wheel are then charged into a kiln for the burning process which takes 2 - 3 weeks. In pressed process the grains and bonding clay are mixed in a semi-dry state and the wheel moulded under pressure. But this process the wheels can be made under better control as regards density, giving a wider range of grades.

It has high porosity and strength which makes this type of wheel suitable for high rate of stock removal. It is not adversely affected by water, acid, oils at ordinary temperature conditions.

Silicate wheels have a milder action and cut with less hardness than vitrified wheels. For this reason they are suitable for grinding fine edge tools, cutlery etc.

This is used for heavy duty, large diameter wheels where a fine finish is required. These are expensive and comparatively very rare. They are used where their exceptionally cool cutting abilities are essential to prevent burn damage or to provide very fine finish. Applications include metallurgical sample cutting and Tool & Cutter grinding for reclaiming broken slot and end mills. Shellac wheels may be made to 3 mm or less in thickness. Shellac wheels posses considerable elasticity.

This is used where a small degree of flexibility is required on the wheel as in the cutting of the cutting off wheels. They produce good quality of cut with minimal of burr formation. This could be uses in places where there is polishing of metals such as ball bearing races and for cutoff wheels where burr and burn must be avoided.

This is used for high speed wheels. Such wheels are used in foundries for dressing castings. Resinoid bond wheels are also used for cutting off parts. They are strong enough to with stand considerable abuse. Resinoid bond is made from powdered synthetic resin used as phenol formaldehyde. This is mixed pressed and heated to 177o C. After cooling, this makes a wheel which is less brittle, tougher and more flexible than the vitrified bond and which can be run up to 2900 m/min.

Wheel structure defines how "open" or "closed" the wheel surface is. An "open" wheel is one with the grits spaced relatively far apart, a "closed" wheel is one with the grits spaced close together. For conventional wheels, it is assigned a number, normally between 1 [most closed] and 15 [most open]. It is a measure of the percentage of grit by volume. The less volume of grit, the more open the wheel structure is with more space for coolant and chip clearance.

Vitrified bond wheels naturally have a certain amount of porosity in their structure. The porosity level can typically be up to 50%. The structure can be artificially changed to increase the porosity level by introducing an additional material when the grit and bond are mixed together before firing. This material is in particle form of a specified size. During firing of the wheel, this material is removed to leave pores of the same size as the original particles. This type of wheel is called an induced porosity wheel. The wheel then contains natural porosity plus induced porosity as shown in the figure. Induced porosity wheels provide additional space for chip clearance and for coolant. They are particularly useful for grinding processes which have a long arc of contact between wheel and component. For this reason, they are used almost invariably for creep feed grinding. They are also used for the grinding of rubbers, plastics and polyurethane.

Each method will produce a characteristic finished determined by the lay of the surface of the work piece after the grinding operation. A straight wheel with reciprocating motion produces fine straight lines on the work piece. Where as a cup wheel with reciprocating motion will produce curving lines. A cup wheel with rotating work piece will produce concentric circles.

Standard wheel markings specify all the important wheel characteristics. The marking system comprises of seven symbols which are arranged in the following order.

A grinding wheel is specified by the marking, shape, outside diameter, bore diameter, thickness etc. A recessed wheel is specified with all the above given particulars plus the diameter of the recess and the depth of the recess.

For grinding a job the right grinding wheel is to be selected. The selection of a grinding wheel will depend on the following factors.

Material to be ground: For grinding high tensile material an aluminum oxide wheel, and for low tensile material silicon, a carbide wheel should be selected. For grinding hard materials a soft wheel and for grinding soft material, a hard wheel is chosen.

Amount of stock to be removed: When the stock of material to be removed is more with heavy cuts select a coarse grain, open structured and hard grade wheels. For removing less stock of material with light cut, select fine dense structured soft wheel.

Finish required: Rough finish requires coarse grains and open structure. High finish requires fine grain and dense structure.

Area of contact: The are of contact depends on the size of the work piece, the grinding wheel and the nature of operation. When the area of contact is more a soft grade and coarse grain wheel is to be selected. For less area of contact select hard grade and fine grain wheel.

Type of grinding operation: The selection of grinding wheel is affected by the grinding operation to be done. The wheel shape and size are to be selected on the basis of the grinding operation such as surface, cylindrical or tool grinding.

Wheel speed: Generally the speed at which a grinding wheel is to be used will be marked on the wheel by the manufacturer. Select a soft wheel for high speed and a hard wheel for low speed.

Work speed: Select a hard wheel for high work speed and a soft wheel for low work speed.

Condition of the machine: For rigid and new machines, select a soft grade and open structured wheel. For light and old machines, select a hard grade and dense structured wheel.

Personal factor: A skilled person can do the operation effectively, even if there is a slight deviation in the selection. But for a semi skilled labor, perfect selection is essential.

Method of cooling: If better cooling is required select an open structured wheel. Always the coolant should be directed at the cutting areas to minimize the heat and to wash away the grain particles.

When a new grinding wheel is used it should be checked for balancing. Most manufacturers balance their wheels before selling them. For checking the balance of the grinding wheels, it is mounted at the center of a perfect straight and round spindle, the assembly then being rested on level knife-edge ways on a lathe bed or on a special stand. For the test to be really satisfactory the wheel should be mounted on its won spindle. The wheel is then rolled a little and left. Any out of balance will result in the wheel coming to the rest with the heavy side underneath.

Balancing may be achieved by adding lead weight to the light side. This may be accomplished by removing small amounts of the wheel beneath the flanges and then filling the hole thus made with lead. The wheel is mounted on its own spindle kept on knife edge ways, and again give a slight push, allowing it to roll back and forth until it comes to rest, which it will do with the heavy portion of the wheel at the bottom. Continue adding weight from the wheel, until it is balanced. This will be evident when the wheel rolls to a gentle sop with no apparent tendency to roll backward.

There are 5 main types of grinding fluid. Of these four are water based and the other is a neat oil. With the water based fluids, the main constituent is water with a concentrate added to a specified percentage. The concentrate should always be added to the water, rather than the other way round, so that a stable emulsion will be formed.

When the surface of a grinding wheel develops a smooth and shining appearance, then it is said to be glazed. This indicates the abrasive particles on the wheel face are not sharp. These are worked down to their bond level.

When soft materials like aluminium, copper, lead etc are ground the metal particles get clogged between the abrasive particles. This condition is called as loading. The effects of glazing and loading are almost same. Following are the effects.

A glazed or loaded grinding wheel can be reused after removing the glazed or loaded particles from the grinding wheel face.

Dressing is an operation to change the cutting action of a wheel or to recondition its grinding surface. Mostly dressing and truing are done at the same time. Grinding wheels should be dressed and trued regularly to improve

*Dressing*	*Truing*
Refers to the removing of clogs and blunt abrasive grains from the surface of the grinding wheel. Dressing exposes the cutting edges which restore the correct cutting action of the wheel. Dressing is done on a glazed or loaded wheel to recondition it.	Refers to the shaping of the wheel to make it run concentric with the axis. When a new grinding wheel is mounted, it must be trued before use to remove the run out. Truing is done on the wheel which is out of shape due to long use. Sometime a wheel is also trued to change the shape of the grinding wheel face for a specific grinding operation like form grinding.

A diamond dressing tool has a hard diamond point mounted in a metal shank. The shank is fitted in a tool holder for location on the grinding machine to perform dressing. Diamond dressers are most effective for precision grinding wheels. The low feed of a diamond dresser can glaze the wheel. They are specified by their weight in carats. Usually 0.5 carat to 1 carat diamond is used for dressing up to 300 mm diameter of wheels.

Steel dressers for dressing a grinding wheel have rotary cutting surfaces made from hard steel. They are held in place against the grinding wheel by hand and moved across the face of the grinding wheel to do the dressing. The tool rest or other rigid support must be used during this operation.

When only light dressing is required abrasive sticks are used. There are abrasive materials made in the form of square or round sticks or put in metal tubes for convenient handling. This type of dresser is more common in tool and cutter grinders where truing and dressing is necessary.

There are two types of measurement. Those that are necessary to check component quality and those that can be used to check efficiency of the grinding process.

[2] Surface finish: This is often specified as a value of a surface roughness parameter. Ra is probably the most common, other parameters such as Rz and Rt are also used. As well as conforming to a measured value, visual appearance is also important in some applications. This may mean avoiding vibration or chatter marking and deep scratches.

[3] Component material condition: In many grinding applications it is essential to avoid grinding burn (also called grinding abuse). This usually means damage to the material structure of the component. There are three degrees of abuse:

Rehardening burn and temper burn are commonly assessed using a Nital etch. Temper burn shows up as a darker area. Re hardening burn shows up as a lighter area, usually surrounded by an area of temper burn. Residual stress measurement is not common, but may become more so, as component quality requirements become more stringent. Specialized equipment is needed.

The following are three ways in which grinding efficiency can be measured, additional to the quality checks above. These have not traditionally been measured, but the trend is to add these to quality checks as a way of improving the control of the grinding process and as a means of ensuring defects do not occur, rather than leaving inspection to discover them and then scrap the component.

[1] Grinding power: A measurement of grinding power will show how efficiently the wheel is cutting. A blunt or worn wheel will tend to rub so creating friction and increased grinding power. This can be used to indicate when dressing is required. Grinding power can also be used to detect if burn is likely to occur, since in some cases, the start of burn can be related to a specific level of grinding power.

[2] Grinding ratio: This is defined as the ratio of the volume of component material removed to the volume of the wheel consumed in the process. It is therefore a measure of the efficiency with which the wheel is being used. This measurement can be used to check if the wheel specification is correct. A low grinding ratio may mean the wheel is too soft and is therefore breaking down too easily under the grinding forces. Care is needed here, as too hard a wheel can sometimes give a low grinding ratio as well. Too hard a wheel encourages chips to stick to the wheel surface and this can cause grits to fall out too soon.

[3] Vibration: Vibration can be caused by many factors including a low stiffness machine, too high a work speed, too hard a wheel, faulty bearings, out-of-balance, etc. It usually leads to more efficient cutting as the vibration gives a self-dressing effect. However, it is detrimental to surface finish, wheel life and machine life. Also, it often causes excessive noise.

It is the rate of travel of the wheel surface past a point on the work piece. Wheel speed is otherwise called surface speed. It is expressed in terms of meters per second.

            V        -    Surface speed in meters / second.
            D        -    Diameter of the wheel in mm.
            N        -    RPM of the machine spindle.
            1000   -    to convert mm to meters.
            60       -    to convert RPM to revolution per second.

In grinding refers to the movement of the wheel per stroke across the work surface. The feed in grinding depends on the work speed, wheel width and the finish required. It is generally 3/4th to 2/3rd of the wheel face width for rough grinding and 1/4th to 1/8 of the wheel face width in case of the finish grinding. When feed is high the wheel wear increases surface finish deteriorates and the dimensional accuracy of the work piece is affected.

It is the thickness of the material removed in surface grinding for one cut. Depth of cut depends on the cutting load, power of the machine and finish required. Generally the depth of cut is 0.02 to 0.03 mm for rough cut and 0.005 to 0.01 mm for finish cut.