Grinding machine Information

GRINDING MACHINE

Grinding Machine: Grinding is a process f removing materials by the abrasive action of a revolving wheel on the surface of work piece in order to bring it the required shape and size. The rotating wheel acts as a cutting tool. Generally grinding is taken as finishing operation because it removes little material as compare to other machining process.

It removes comparatively little metal, 0.25mm to 0.5mm in most operations and the accuracy in dimension is in the order of 0.000025mm. It is also done to machine materials which are too hard for other machining methods that use cutting tools.

Purpose of grinding:

ü  To sharper the lathe tools, shaping tools and drill bits.

ü  To make points on Centre punches, scriber

ü  Grinding of threads

ü  To machine the metals which cannot be machine by other metal cutting tools?

ü  To obtain accuracy in dimensions in the order of 0.000025mm

Classification of grinding machine:

According to the quality of surface finish grinding machine can be classified as:

A.      Rough grinding (Non-precision grinding)

B.      Precision grinding

Rough grinding machine: the main purpose of rough grinding machine is to remove the excessive materials within a short time without reference to the accuracy of the results. The work is pressed hardly against the wheel or vice-versa. The accuracy and surface finish obtained are of secondary importance. They are also following types:

Ø  Floor stands and bench grinders

Ø  Portable and flexible shaft grinders

Ø  Swing frame grinder abrasive belt grinder.

Precision grinding machine: the main purpose of precision on grinding machine is to produce a work piece with accurate dimension and surface finish. Such grinding machines are used mainly for finishing operation.

According to the type of surface generated or work done they can be classified as follows:

A.      Surface grinders

1.       According to the type of table:

§  Reciprocating table

§  Rotary table

2.       According to the spindle:

§  Horizontal spindle

§  Vertical spindle

B.      Cylindrical grinders

1.       According to the types of surface:

§  External cylindrical grinders

§  Internal cylindrical grinder

2.       According to the center:

§  Centre grinder

§  Centreless grinder

§  Universal grinder

C.      Tool and cutter grinder

o   Universal

o   Special

D.      Internal grinder

1.       Chucking

§  Plain

§  Universal

2.       Planetary

3.       Centreless

E.       Special grinding machine:

o   Crank shaft grinders

o   Roll grinders

o   Thread grinders

o   Piston grinders

o   Cam grinders

Selection of grinding wheels:

Selection of grinding wheels is also a most important thing to be considered to obtain best results in grinding. The factors to be considered while selection of the grinding wheels are as follows:

Ø  The material to be ground

Ø  Quality of surface finish required

Ø  Area of contact

Ø  Amount of material to be removed (i.e. allowance provided on the work piece.)

Ø  Types of grinding machine

Ø  Dimensional accuracy machine

Ø  Method of grinding (wet/dry)

Ø  Rigidity and size of machine

Ø  Relative size of wheel and job

Ø  Types of grinding to be done

Major parts of surface grinders:

Base: The base has a column at the back for supporting the wheel head. The vase also contains the drive mechanism.

Table: table is fitted above the base. It reciprocates along ways to provide the longitudinal feed. T-slots are provided in the table surface for clamping work pieces directly on the table or for clamping grinding fixture or a magnetic chuck.

The main purpose of table is to grip or hold the work piece.

Wheel head: the wheel head is mounted on the column seared to the base. It has ways for the vertical slide which can be raised or lowered with the grinding wheel only manually by rotating a head wheels to accommodate work piece of different heights and to set the wheel for depth of cut.

Operation: The work piece is placed on the tale and it reciprocates under the wheel, and the wheel or the table feeds axially between passes to produces a fine flat surface. Wheel down feed or table up feed determines depth of cut and final height of the piece.

The amount of feed must only be equal to a few hundredth of a millimeter.

Specification of grinding wheels:

The method of specifying the grinding wheels are different in different countries. According to the Indian standard system the grinding wheel can be specified depending upon the following factors:

1.       Abrasives

2.       Bond and binding process

3.       Structure of wheels

4.       Grade

5.       Grain size of grit

Abrasives: it is that material of grinding wheel which does the cutting action. An abrasive is a substance used for making grinding wheels it should be pure and have uniform physical properties of hardness, toughness, and resistance to fracture to be useful if manufacturing of grinding wheels.

Abrasives may be classified in two types:

Ø  Natural

Ø  Artificial or manufactured

Natural: they are directly obtained from mines. There are various types of natural abrasives. It includes sandstone or solid quartz, emery, Garnet diamond corundum, Glist, rotten stone etc.

Abrasives artificial: they are more important than natural abrasives because they have uniform hardness and structure. The main types of artificial abrasives are: silicon carbide (Sic), Boron oxides, aluminum oxide.

Uses of Abrasives:

Use of abrasive	Symbol used
Aluminum oxide	A
Silicon carbide	C
Boron oxide	B
Diamond	D

Grain size and grit: it indicates the size of cutting teeth or the size of abrasive particle used in making or grinding wheels. The grain size or grit is denoted by number which indicates no. of meshes per square inch (25.4mm* 25.4mm) of the screen through which the grains pass when they are graded after crushing. The sizes are given below:

Types of grain	Grain number	Application
Coarse	8 to 24	Heavy material removal for shaft and ductile material.
Medium	30 to 60	Average material removal and finish.
Fine	80 to 180	For finish work and also used to grind hard and brittle material
Very fine	220 to 600	Fine and very high surface finish

Grade: the term grade indicates the strength of bond in wheel i.e. the power of the abrasive particle to hold together and resists disintegration under the cutting pressure. ‘A’ denoting the softest and ‘Z’ the hardest grade they are grouped in standard size which are mentioned below.

Soft	A     B     C   D   E   F   G  H
Medium	I   J  K   L   M  N  O   P
Hard	Q   R   S   T  U   V   W   X    Y    Z

Bond: A bond is an abrasive substance which is used to hold the grains of abrasives together to form grinding wheels. It is necessary to use bond to give an effective and contains cutting action because using bond the grain of abrasives are held together forming series of cutting edge.

There are many types of bonds which are given below:

Bond type	Represented by letter	Composition	Application
Vitrified bond	‘V’	Ceramic clays, porcelain and wear	Suitable for max. metal removing and 75% of tool cutting
Silicate	‘S’	Silicate of soda water or glass power shellac	Suitable for small metal removal as knife, tool etc.
Shellac	‘E’	Power shellac	Suitable in light work used in chilled G.I. steel
Rubber	‘R’	Sheets of pure rubber	Used for good surface finish
Oxy-chloride	‘B’	Oxide and chloride of magnesium.	Rapid stock removal used for precision

Structure of wheels: the abrasive grains in the wheel are not tightly packed but distributed through the bond. The structure indicates the space between the abrasive grains or in other words density of wheel as the quantity of bond increases, the distance between grains also increases which is referred as open structure whereas lower position of bond indicates the distance between grains is less which is referred as close structure or dense.

Structure commonly used as is denoted by member as follows:

Dense	1   2   3   4   5   6   7   8
Open	9   10   11   12   13   14   -   - - -higher.

Wheel shapes and sizes:

Grinding wheels are made in many different shapes and sizes to adapt them for use in different types of grinding machine and on different classes of work. They are different shape and sizes.

Dressing and truing grinding wheel:

Dressing means remove loading and breaks away the glazed surface so that sharp abrasive particle are again presented to the work.

Truing is the process of changing the shape of the grinding wheel as it becomes worn from an original shape, removal of abrasives and bond material from the cutting face. This is down to make the wheel true and concentric with the bore to change the face contour for from grinding.

Truing and dressing are done with the same, but not for the same purpose. For precision and high finish grinding single point diamonds are used for dressing and truing.

Comparison between truing and dressing:

Truing of grinding wheel	Dressing of grinding wheel
1.       It is the operation to create a true and concentric surface with the wheel spindle axis.	1.       It is the operation to remove gate or other adhesive particles and to expose fresh cutting particles.
2.       It is necessary to really manufactured wheel taken from kilns for truing to shape and dimension	2.       It is necessary to nearly manufactured or the dulled crystals closed with some foreign materials after it is being used for some time on the wheel face for the purpose of exposing new cutting particle
3.       Commercial diamond tool or conical point tungsten carbide inserted on a shank of cold rolled steel.	3.       Same as truing tool
4.       To rotate the wheel in a true and concentric path.	5.       To remove the blunt surface on wheels for efficient cutting.

Cutting speed (v):

The cutting speed is the relative speed of the wheel vω (peripheral speed) and the work piece. It is expressed in meter per second with sufficient approximation.

So V= vω = ˮDωNω/1000

Where Dω is the diameter of the cylinder wheel in mm and Nω is speed of wheel in r.p.s.

Feed:

It is in cylindrical grinding in the longitudinal movement of the work piece per revolution. It is expressed in mm. per revolution of the work piece. It is usually from 0.6 to 0.9mm. Increased feed greatly reduce cutting time but also greatly reduces the tool or grinding wheel life.

Depth of cut:

It is the thickness of the later of metal removed in pass. It is expressed in mm. the depth of cut is taken in a range from 0.005 to 0.4 mm but it depends upon grinding types and process.

Machining time in grinding:

Time = Li K/ si Np minute

Where

·         L – length of longitudinal travel in mm

·         i – Number of pass

·         Si = longitudinal feed in mm per revolution

·         Np = speed of the work piece in r.p.s.

And K is a coefficient depending on the specified grade of accuracy and class of surface finish.

v  Value of K for rough grinding K= 1 to 1.2

v  Value of K for finish grinding K= 1.3 to 1.7

Cutting fluid:

It is referred to as lubricants or coolants as liquid and gas applied to the tool and work piece to assist in the cutting operation.

Purpose of cutting fluid:

·         To cool the tool

·         To cool the work piece

·         To lubricate and reduce friction

·         To improve surface finish

·         To protect the finish surface form corrosion

Properties of cutting fluids:

Ø  High head absorption for readily absorbing heat developed

Ø  Good lubricating qualities to produce lo-coefficient of friction

Ø  High flash point so as to eliminate the hazard of fire

Ø  Stability so as not to oxide in nature

Ø  Neutral so as not to react chemically

Ø  Odorless as as not to produce any bad smell even when heated

Ø  Harmless to skin of the operators

Ø  Harmless to bearings

Ø  Non-corrosive to the work piece and machine

Ø  Transparency so that the cutting action of the tool may be observed

Ø  Low viscosity to permit free flow of liquid

Ø  Low price to minimize production cost

Choice of cutting fluid:

v  Types of operation

v  The rate of metal removal

v  Type of material of the work piece

v  Material of the tool

v  Surface finish required

v  Cost of cutting fluid

Balancing of grinding wheel:

Wheel that are out of balance not only produce poor work but may put undue strains on the machine also cause faster wheels breakdown, poor surface finish etc. wheel balance is also affected by the machine spindle and the means of tightening the wheel on the machine.

Wheels generally should be statically balanced before putting them of the machine. They should be dressed into concentricity taken off rebalanced and put on the grinder and operated.

Generally balancing is done by shifting weight from the following steps should be taken to balance a wheel.

1.       Remove the wheel and place the wheel on balancing ways permit the heavy slide of the wheel to come to rest. Place the chuck mark at the bottom (the heavy point) turn the wheel 90 degree first to one side and then to another side, to check the location of the heavy point.

2.       Draw a line horizontally through the axis on the mount of the wheel insert the two weights. Placing them above and equidistance from the horizontal line, and repeat the operation of locating the heavy point. If it is again at the original mark move the two weights closer together towards the top if it is opposite the original mark move he weights further apart towards the horizontal axis line at some point between the top and horizontal line the proper balance should be found. If not that is if between weights are opposite the heavy mark and touching each other the wheel is too far out of balance for normal corrective measures and a third or fourth balance weight should be added.

3.       Process as before to located the heavy point of the wheel and draw the horizontal live place he wheel and draw at approximately 90 degree apart. Then move the two top weight closer together if this is not sufficient move the bottom weights upward towards he horizontal line turn the wheel 90 degree and check the location of the heavy point which is to be in this case 90 degree away from the bottom if this is the case wide off the original, horizontal lines and draw a new line horizontal to the new heavy point and proceed as before to change weight.