SURFACE FINISHING GRINDING MACHINES Introduction :- Grinding is a metal cutting operation which is performed by means of a rotating abrasive wheel acts as a cutting tool. Material removal is in the form of chips by the mechanical action of abrasive particles bonded together in a grinding wheel. Advantages of grinding :- 1. Extremely smooth surface finish 2. High accuracy 3. Complex profile machining 4. Hard heat treated material surface finishing 5. Easy chip removal Applications :- 1. Rough grinding 2. Finish grinding
Classification of grinding machines :- According to the quality of surface finish :- Rough or non precision grinding machines 1. Floor stand and bench grinder 2. Portable and flexible shaft grinder 3. Swing frame grinder 4. Abrasive belt grinders Precision grinding machines :- 1. External/internal cylindrical grinders 2. Surface grinders 3. Form grinders
According to the type of surface to be generated :- Cylindrical grinders :- 1. Centre type ( plain) 2. Centre type ( universal ) 3. Centreless grinder Surface grinders :- 1. Horizontal spindle reciprocating table 2. Vertical spindle reciprocating table 3. Horizontal spindle rotary table 4. Vertical spindle rotary table Centreless grinders :- 1. Through feerd grinders 2. In feed grinders 3. End-feed grinders Internal grinder :- 1. Chucking grinder 2. Planetary grinder 3. Centreless grinder
According to specialized application :- 1. Tool and cutter grinder 2. Form grinder 3. Hand grinder 4. Crankshaft grinder 5. Thread grinder 6. Cam grinder
SURFACE GRINDING MACHINES Types of surface grinders :- 1. Horizontal spindle with reciprocating table 2. Horizontal spindle with rotary table 3. Vertical spindle with reciprocating table 4. Vertical spindle with rotary table Horizontal spindle with reciprocating table :- It consists of horizontal spindle located horizontally to mount grinding wheel while the table reciprocates with a mounted work piece. Reciprocating motion of the table provides longitudinal feed and cross feed is provided by the spindle
. For taking depth of cut, grinding wheel can be fed downwards. This is called as infeed The amount of feed is very small about 0.005 to 0.15mm Horizontal spindle with rotary table :- it is used for facing circular work and narrow rings Work is clamped on rotary table and grinding wheel is mounted on horizontal spindle. The table is made to rotate in a direction opposite to that of the rotation of the grinding wheel. The vertical feed is given by moving the wheel head along the column and cross feed by the horizontal movement of wheel spindle The work table can be lowered or raised to accommodate different height work pieces
Vertical spindle with reciprocating table :- Also known as planer type grinding machines Grinding wheel spindle is vertical Work is clamped on reciprocating table and grinding is performed by the face of the wheel Infeed and cross feed is given by movement of spindle In these machines, cuped, cylindrical, or segmental grinding wheels are used The grinding area is large and large quantity of material is removed.
Vertical spindle with rotary table :- Grinding wheel is mounted on vertical spindle The work is mounted on table with clamping arrangement or having magnetic chuck The table rotates in opposite direction to that of wheel Infeed and cross feed is given by movement of spindle This type of machine gives higher production rates and comparatively surface finish is not good
CYLINDRICAL GRINDING MACHINES It is used to produce a cylindrical or conical shape on a work piece by passing the face of grinding wheel The work piece is mounted between the centres or chuck which is rotated and moved against the rotating grinding wheel
There are four movements are involved- 1. The work revolve about it s own axis 2. Wheel revolve about it s own axis 3. The wheel pass the work through feed 4. The work pass the work through feed Types 1. Centre type - (i) Plain cylindrical grinder (ii) Universal cylindrical grinder (iii) plunge type 1. Centreless type 2. Chucking type
CENTRELESS TYPE GRINDING MACHINES When the work piece is not supported by the centres but between grinding wheel and regulating wheel by a work rest blade, it is called as centreless grinding This eliminates the use of work holding devices. Also there is no need to centre the work piece. Both wheels are rotated in the same direction. The rotation of the grinding wheel forces the work piece on to the work rest blade and against the regulating wheel. The regulating wheel controls the speed of the work and longitudinal feed movement. The work rest is adjusted so that the work piece centre is kept slightly above the centreline joining the centres of grinding wheel and regulating wheel. The feed in centreless grinding is given to the work piece in one of the following methods- Through feed grinding, infeed grinding and end feed grinding
Through feed grinding 1. Used for straight cylindrical work such as rollers, pins, and straight long bars 2. Regulating wheel and grinding wheel is adjusted equal to the desired diameter of the work piece 3. The job is fed and passed through the gap between the grinding wheel and regulating wheel usually with guides at both ends In feed grinding :- 1. Used for grinding headed, stepped parts.
2. The work piece is placed on rest blade and it is moved against the grinding wheel with required feed. 3. The work rest for this operation does not have guides but adjustable stop is provided at the end. End feed grinding :- 1. Used for short tapers. Here both the wheels have the form corresponding to the shape of the work piece 2. The work piece is fed longitudinally from the sides of the wheels 3. As it advances between the revolving wheels, it s surface is ground till it s farther end touches the end stop.
Specification / Designation / Marking Scheme of grinding wheel :- It differs with the manufacturer and country. In order to bring the uniformity, Bureau of Indian Standards has suggested the marking scheme consisting of following six characters in sequence. This codification is as per Bureau Of Indian Standard Code IS 551: 1989 0 1 2 3 4 5 6 Prefix Abrasive type Abrasive grains Grade Structure ( Use Optional ) Nature of Bond Manufact urer s Symbol Prefix :- Any prefix can use manufacturer to indicate his own trade brand of abrasive used and it is optional. Abrasive type :- - Natural abrasives :- minerals, diamond, corundum, emery, sand stone or quartz. Main elements of corundum and emery is Al 2 O 3 - Artificial abrasives :- A Al 2 O 3, S Silicon carbide
Abrasive grains :- 1. It indicates the size of abrasive used for making a grinding wheel. 2. It may be classified as per the number of openings per square inch such as coarse, medium, fine, very fine - Coarse 6-24 - Medium 30-60 - Fine 70-180 - Very fine- 220-1200 Grade :- 1. It is the degree of hardness of the wheel i.e., the hardness with which the bond holds the abrasive grains in a place. 2. It is classified as Soft ( A H ), Medium - ( I P ), Hard ( Q Z ) Structure :- 1. It is the type of spacing of the abrasive grains in which the abrasive grains are distributed throughout the wheel. It is denoted by the number of cutting edges per unit area 2. Type Dense ( 1-8), Open ( 9-15 and above )
Nature of bond :- 1. It is the type of adhesives used with the abrasive grains to form the grinding wheel 2. Types Vitrified bond (V), Silicate bond (S), Shellac bond (E), Resinoid bond (B), Rubber bond (R) Manufacturers symbol :- 1. It is also optional 2. May use suitable suffix to the type of bond
Methods to remove the defects in grinding wheel :- 1. Trueing 2. Dressing Trueing :- 1. Due to continuous use of the grinding wheel, the cutting points of the abrasive grains becomes dull and it called glazing. 2. It is the operation to remove the glazing from the grinding wheel. 3. it is carried out with a diamond tool. 4. In this operation, grinding wheel is rotated and diamond tool is held against the wheel 5. The tool is reciprocated over the wheel to cover the complete width of the wheel
Dressing :- The cut particles of the material may get adhere to the open space between the cutting points Due to this the sharpness of the cutting part is lost, the face becomes smooth and looses it s cutting ability. This is called the loading of the grinding wheel. Dressing removes the loading defect It is carried out by a star wheel dresser which consists of a number of hardened steel wheel with points on their periphery. The dresser is held against the face of the revolving wheel and moved across the face to dress the wheel surface
Selection criteria / factors for grinding wheel :- Constant factors :- 1. Work piece material 2. Amount of stock to be removed 3. Area of contact 4. Type of grinding machine used Variable factors :- 1. Wheel speed 2. Work speed 3. Condition and capacity of machine 4. Personal factor
Balancing of grinding wheel :- A wheel is said to be unbalanced when some portion of it is heavier whereas the other is lighter. An unbalanced wheel greatly affect the surface finish and accuracy of the work. To obtain the good surface finish, prevent vibration and undue wear of grinding machine parts, grinding wheel must be good balanced. Unbalance may cause the wheel to break with the chances of damage to the machine and serious injury to the operator.
Methods of balancing wheel 1. Static Balancing :- - The wheel is taken off from the grinding machine - The wheel is mounted at the centre of a perfectly straight and round spindle - The assembly is then rested on a know edge balancing stand - Any out of balance will result in the wheel coming to test with the heavy side at the bottom - Correct weight then can be stuck on the opposite side to balance the wheel 2. Dynamic Balancing :- - This is done on the machine itself. There is no need to remove the wheel from the machine - It is the proper alignment of the grinding wheel with the shaft on which it is rotating.
Safety Precautions :- Use of the right wheel Test the wheel before use. Enough tightening of flange bolts Do not exceed maximum speed Use safety glasses.
Introduction :- SUPER FINISHING To reduce wear of the machine parts, friction between the machine parts, and to give the corrosion resistance properties, fatigue strength, good appearance which require the high degree of surface finish of the parts But the parts produced by the other machining processes like turning, boring, milling, shaping, drilling does not have the high degree of surface finish. To produce the parts with high degree of surface finish following fine finishing processes are used- 1. Honning 2. Lapping 3. Polishing 4. Buffing 5. Burnishing
Introduction :- HONNING It is a abrasive finishing operation where a small amount of material is removed from the internal or external surface of a part to improve the surface finish. It is used to correct the roundness, taper, tool marks and axial distortion. Mostly honning is done on internal cylindrical surfaces such as automobile cylinder walls, cylinders of IC engines, hydraulic cylinders, gas barrels, bearings etc The abrasive stone used in this process are called as hones, hence the process is known as honning. Accuracy- 0.005mm up to 50 mm diameter and 0.01mm for 50-150mm diameter. Surface finish 0.075 microns
Honning stone :- They are made from common abrasive and bonding materials often mixed with sulphur, resin, or wax to improve cutting action and lengthen tool life. Common abrasives are aluminium oxide, silicon carbide, diamond or cubic boron nitride The abrasive grain size ranges from 80-600 grit.
Principle of Honning :- It is wet cutting process which removes metal from the work piece by means of revolving tool which also reciprocates up and down usually inside the work piece. Honning Set-up and working :- It consists of (i) Honning tool (ii) Tool movement arrangement (iii) machine spindle. Honning machines are horizontal spindle type and vertical spindle type.
During honning operation, the spindle of the honning machine rotates the hone and simultaneously reciprocates it in a work piece The spindle speed is generally 2m/s for rotation and 0.5 m/s for reciprocating motion. Coolants are essential for this operation to flush away small chips and to keep temperature uniform.
Advantages of Honning :- 1. Can remove as little as 0.0025mm of stock 2. Work piece need not be rotated. 3. No chucks, face plates or rotating tables are needed. 4. It can be done on most of materials from aluminium or brass to hardened steel. 5. Several holes may be honned simultaneously on multispindle honning machines. Disadvantages of Honning :- 1. Horizontal boring creates oval holes 2. Honning stone wear out fast 3. Comparatively slow process 4. Difficult to process the slim or less thickness parts Applications :- 1. Internal honning Engine cylinders, gun barrels, long tubular parts 2. External honning Gear teeth, valve seating, races of roller and ball bearings, piston rods, piston pins, shafts
Introduction :- LAPPING It is a precision finishing process where resistance to wear of moving parts and better sealing characteristic is important It gives very high degree of accuracy and smoothness The basic purpose of lapping is to minimize the extremely minute irregularities left on work piece after machining. Principle/ set-up /working:- Abrasive powder (flours) such as emery, corundum, iron oxide, etc mixed with oil or special paste with some carrier are used in lapping. Lap which is made up of soft material to receive and retain the abrasive grains. Soft material may be like soft cast iron, brass, copper, lead or soft steel is also used in lapping process. First charging of lap is done. Charging of lap means to embed the abrasive grains into it s surface. Hence the face of the lap becomes charged with abrasive particles. 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 and the motion being rotary or reciprocating Cylindrical work may be lapped by rotating the work in lathe or drill press and reciprocating the lap over it Small flat surfaces may be lapped by holding the work against a rotating lap or the work may be moved by hand in an irregular path over a stationary lap
Advantages :- 1. Any hard or soft material can be lapped. 2. No burrs are created 3. Any size from 12 to 900 mm diameter can be lapped 4. No warping of lapped components Disadvantages :- 1. Skill and experience is needed for better result 2. Optimum result can not be ensured Applications :- 1. Press work dies 2. Moulds for castings 3. Limit gauges 4. Slip gauges 5. Surface plate 6. Piston rings 7. Crank shaft 8. Engine valve and valve seat 9. Races of ball and roller bearings
Introduction :- POLISHING It is a surface finishing operation performed by a polishing wheel to remove the tool marks, scratches from the work piece. Polishing wheels are made of leather, papers, canvas, felt or wool. The abrasive grains are set up with glue or thermosetting resin on the face of the wheel or on an abrasive belt (endless). It is done after grinding and is followed by buffing. The purpose of polishing is to remove scratches, tool marks, other defects due to machining operation and improves the surface finish. Principle :- In this process, the work is held in hand or on the machine table and the rotating polishing wheel is brought in contact with the work piece surface. An abrasive belt (endless) with very fine grade abrasives may also be used instead of polishing wheels. This belt rotates continuously on two rollers and the work piece to be polished is hand held over this rotating belt. So in both cases very thin layer is removed from the work piece
Depending on the number of pieces to be polished, the polishing may be done by hand or machine i.e., hand polishing & machine polishing. Advantages :- 1. No chips formation 2. Gives mirror type finish Disadvantages :- 1. Dimensional accuracy is not maintained 2. Narrow grooves, complex shapes are difficult to polish Applications :- 1. Polishing of small hand tools 2. Jet engine turbine blades 3. Sole plates of electric iron 4. Parts of fountain pen
Introduction :- BUFFING It is a finishing operation usually performed after polishing. It gives much higher lustrous and reflective surface finish that can not be obtained by polishing. It removes negligible amount of material. It is like a polishing operation in which the work piece is brought in contact with a buffing wheel. Buffing wheels are made of number of layers of cloth or fibre firmly sewed together in thick sections. These layers are closely fasten together by the flanges and mounted on the buffing machine arbor. The buffing wheel cloth is charged with very fine abrasives with the help of some binders. The diameter of the wheel is around 300 to 750 mm.
Principle/ Setup and working :- The buffing wheel is rotated at high speed and workpiece to be buffed is made to rub against the charged wheel The abrasives removes the minute amount of material from the work piece, removes fine scratches and gives high reflective surface finish
Advantages :- 1. Suitable for ferrous and non ferrous alloys 2. No surface preparation is required Disadvantages :- 1. Does not maintain flatness/roundness Applications :- 1. Automobile, motor cycle and bicycle parts 2. Household utensils and appliances 3. Sport items, tools, furniture, fixtures
Introduction :- BURNISHING It is a cold working process by which improvement in surface finish, dimensional accuracy and work hardening is done without removing the metal It is the non-cutting operation and uses no abrasives. It is a finishing operation and is normally done on parts which are turned, bored, milled, shaped, reamed or ground. It involves the physical displacement of surface irregularities rather than cutting or grinding. This process is used for flat, cylindrical and conical surfaces. Principle/Set-up/Working :-
. All the machined surfaces consist of a series of peaks and valleys (surface irregularities) of irregular height and spacing. The plastic deformation created by burnishing is a displacement of the material from the peaks which cold flows under pressure into the valleys. This is done by using burnishing tool which consist of tapered rollers mounted on tool holder in a retaining cage. The movement of rollers under pressure in contact with the surface with rotary motion and vertical movement to cover entire length of the surface.
It consist of (i) tool holder (ii) drill press (iii) work piece table The drill press vertical spindle is used to mount the tool holder and work piece is clamped on machine table with suitable clamping method. The tool is selected slightly larger than the work piece and allow to pass through it. While passing on the workpiece, slight pressure gets created that exceeds the yield point of softer part surface. This results in a small plastic deformation of the surface structure of the work piece. It produces accurately sized part with a mirror like finish and a tough, work hardened, wear and corrosion resistant surface.
Advantages of Burnishing :- 1. Any ductile or malleable material with hardness less than 40 HRC can be successfully burnished 2. Free from metal chips 3. Produce mirror finish in one pass with accurate sizing and close tolerances 4. Applicable to both external and internal surfaces 5. Can be done on drill press, lathe and multi spindle machines easily Disadvantages :- 1. Tooling cost is more 2. Skilled operator is required 3. Polishing is to be done before burnishing 4. Require to wear eye protection when burnishing Applications :- 1. Boring of hydraulic cylinder 2. Valve seats, steering knuckles