Grinding machine: Grinding is metal cutting operation by means of rotating abrasive wheel that acts as a tool. This is used to finish work pieces which must show a high surface quality, accuracy of shape and dimension. The act of grinding goes back many centuries. Grinding is metal cutting or removing processes performed with the help of grinding wheel. Mostly grinding is the finishing operation because it removes comparatively little metal 0.25 to 0.50 mm in most operations and the accuracy in dimensions is in the order of 0.000025 mm.
Another material removal process, in which abrasive particles are contained in bonded grinding wheel, that operates at very high surface speed. The grinding wheel is usually in disk shaped and is precisely balanced for high rotational speeds. The grinding process consists of removing material from the work piece by the use of a rotating wheel that has a surface composed of abrasive grains. Grinding is considered to be the most accurate of the existing machining processes. Grinding processes are used when high accuracies, close dimensional tolerances, and a fine surface finishes are required. Grinding processes also allow for high production rates. This allows for a lowered cost of production. Hard materials can also be machined.Grinding may be classified as non-precision or precision, according to purpose and procedure.
The various operations performed on a grinding machine:
1. Grinding flat surface
2. Cutting of blanks
3. External and internal cylinder grinder.
4. Taper and completed surface grinder
5. Gear tooth grinding
6. Screw thread grinding
7. Cutting tools grinding
Grinding means abrasion by frication in grinding the material is removed by means of a rotating wheel.
Purpose or use of grinding: To remove a small amount of metal from a work pieces and finish those to closed tolerances, to obtain a better surface finish, To machine hard surface that cannot be machine by high speed steel, Sharpening of cutting tool, Grinding of thread, Sometimes, it is used in removing bigger stroke of metal.
Kinds of grinding machine: Grinding may be classified into two groups are:-
Rough grinding: The common forms of rough grinding are snagging and off- hand grinding where the work is held in the operations hand rough grinder are those grinding machine whose chief work is the removal of stick without any reference to the accuracy of the result.
There are mainly of the following types:
Main features: The work pieces is usually held between dead centers and rotated by a dog and driver on the face plate. The work also be rotated about its own axis in a chuck. There are four movements involved in a cylindrical entre type grinding.
1. The work must revolve.
2. The wheel must revolve.
3. The work must pass the wheel.
4 . The wheel must pass the work.
The operation may be stopped automatically when the work piece grinding is from 20 to 30 s.m.p.m. (surface speed in meter per minute.). Thread grinding requires work speed as low as 1 to 3 s.m.p.m wheels speed usually range from 1500 to 2000 s.m.p.m. Plunge grinding requires very low speed. The in feed or depth of cut, at each reversal should be from 0.025 to 0.125 mm for roughing and from 0.125 to 0.o625 mm for finishing. The longitudinal feed should be from ¼ to ¾ of the width of the wheels face, depending on the power and rigidity of the machine. In machines of cylindrical type, two distinct types of grinding operations are done:-
1) Traverse and 2) plunge grinding.
According to the type of surface generated or work done they may be classified as follows:-
1. Cylindrical grinders: is also called center-type grinding and is used in the removing the cylindrical surfaces and shoulders of the work piece. Both the tool and the work piece are rotated by separate motors and at different speeds. The axes of rotation tool can be adjusted to produce a variety of shapes.
2. Plain–centre type grinders: In these grinders, the work pieces is usually held between two centers. One of these centres is in the headstock and the other in the tailstock. In operation, the rotating work is traversed across the face of the rotating grinding wheel. At the end of each traverse, the wheel is fed into the work by an amount equal to the depth of cut. The tailstock and headstock both can be moved along the table to suitable the work. The table is usually made in two parts. The upper table carries the tailstock, headstock and the workspaces and can be swiveled in a horizontal plane to a maximum of 10 degree an either side, along the circular ways provided on the lower table. The wheel head is usually mounted on horizontal cross ways on the bed and travels along there to feed the wheel to the work. This movement is known as in feed.
Principle parts are:
Base: The base or bed is the main castings that rests on the floor and supports the parts mounted on the top of the base are precision horizontal ways set at right angles for the tables to slide on. The base also houses the table drive mechanism.
Table: There are two table –upper table and lower table. 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 that is pivoted at its centre is mounted on the top of the sliding tables. Setting for taper up to =-10 degree can be made in this way. Steep taper are ground by swiveling the wheel head.
Head stock: The headstock supports the work piece by means of a dead centre and drives it by means of a dog or may hold and drive the work piece in a chuck.
Tailstock: The tailstock can be adjusted and clamped in various positions to accommodate different lengths of work pieces.
Wheel head: The wheel head carries a grinding wheel and driving motor is mounted a slide at the top and near of the base. The wheel head may be moved perpendicular to the table ways by hand or power to feed the wheel to the work.
Cross feed: The +grinding wheel is fed to the work by hand or power as determined by the engagements of the cross- feed lever.
Chuck: for holding the crankshaft to be ground are installed on the face plates of the headstock and tailstock. The accuracy of setting up can be checked by a dial indicator.
Coolant system: to cool the work piece during grinding operation, the machine is provide with an electric coolant pump which delivers coolant from the reservoir located in the base. The coolant pump starts automatically when wheels head spindle rotation is switched on
Electric equipment: the machine is generally equipped with four –three phase asynchronous squirrel –cage electric motors for 220/380 volts, 50 cycle’s power supply.
3. Universal centre type grinders: Universal grinder are widely use in tool rooms for grinding tools etc. a universal machines has the following additional features:-
1. The headstock spindle may be used alive or dead, so that the work can be held and revolved by a chucks as well as ground between centers.
2. The headstock can be swiveled at an angle in a horizontal plane.
3. The wheel head and slide and slide can be swiveled and traversed at any angle.-
4. There is an auxiliary wheel head for doing internal grinding.
4. Center less grinding: is when the work piece is supported by a blade instead of by centres or chucks. Two wheels are used. The larger one is used to grind the surface of the work piece and the smaller wheel is used to regulate the axial movement of the work piece. Types of center less grinding include through-feed grinding, in-feed/plunge grinding, and internal center less grinding. Center less grinding is a method of grinding exterior cylindrical, tapered and formed surface on work pieces that are not held and rotated on centers. The principle elements of an external center less grinders are the grinding wheel, regulating or back up wheel, and the work rest. Both wheels are rotated in the same direction. The work rest is located between the wheels. The work is placed upon the work and the latter together with the regulating wheels, is fed formed, forcing the work against the grinding wheel.
The actual feed (S) can calculate by the formula:
[ S = /-\dn sin Alfa}
Where,
S = Feed in mm/minute.
n = Revolution /minute.
d = face of regulating wheel in mm.
Alfa = angle if inclination of wheel.
Center less grinding may be done in one of the three ways:
Through feed: The work is passed completely through the space between the wheel and regulating wheel useful for grinding long cylinders shafts or bores. The layers of metal removed by the grinding wheel of the work piece by 0.02 to 0.3 mm.
In feed grinding: Which is similar to plunge grinding or form grinding the regulating wheel is drawn back so that work pieces may be placed on the work rest blade. Then it is moved in to feed the work against the grinding wheel. If is used to grind shoulder and formed surface.
End feed: It is used to produce taper either the grinding wheel or regulating wheel or both are formed to a tapper, the wheel is fed length wise between the wheels and is ground as it advances until it reaches the end stop.
Advantage of center less grinding:
1. The work piece being supported throughout its entire length as grinding takes place.
2. The processes in continuo and adapted for production work.
3. No centre holes no chucking or mounting of the work on mandrels or other holding devices are required.
4. The size of the work is easily controlled.
5. A low order of skill is needed in the operation of the machine.
Disadvantage:-
In hallow work there is no certainty that the outside diameter will be concentric with the inside diameter.
Work having multiple diameters is not easily handled.
No comments:
Post a Comment