The main purpose of grinding is finish work pieces which must show a high surface quality, accuracy of shape and dimension. The art of grinding goes back many centuries. Over 5000 years ago the Egyptians abraded and polished building stones to hairline fits for the pyramids. Many different types of grinding machines have now been developed for handling various kinds of work to which the grinding process is application
Kinds of grinding:
1. Rough or non-precision grinding
2. Precision grindining
Grinding machines, according to the quality of surface finish may be classified as:
Centre type grinders :
- Cylindrical center type grinders
- Plain center type grinders
- Universal center type grinders
Cylinderical centre type grinder :
Cylindrical center type grinders are intended primarily for grinding plain cylindrical parts although they can also be used for grinding contoured cylinders, fillets, and even cams and crank shaft.
The work piece is usually held between dead centers and rotated by a dog and driver on the face plate as shown in fig.10.1 the work may also be rotated about it s own axis in a chuck. There are four movements involved in a cylindrical center- type grinding:
1) The work must revolve,
2) The wheel must revolve,
3) The work must the wheel,
4) The wheel must pass the work.
Plain centre-type grinder :
A plain grinding machine is essentially a lathe on which a grinding wheel has been substituted for the single point. Tool, it consists of the following parts.
The base of bad 7 is the main casting that rests on the floor and supports the parts mounted on if. On the top of the base are precision horizontal ways set at right angels for the table to slide on. The base also houses the table drive mechanism.
There are to table’s lower table 6 and upper table 5. The lower table slides on ways on the bad provides traverse of the work past the grinding wheel. It can be moved hand or power within desired limits.
The upper table that is pivoted at its centers is mounted on the top of the sliding table. It has t slots for securing the headstock and tailstock and can be positioned along the table to suit the length of work. The upper table can be swiveled and clamped in position to provide adjustments for grinding straight or tapered work are desired.
The head stock 1supports the work piece by means of a dead center and drives it by means of a dog or it may hold and drive the work piece in a chuck.
The tail stock 4 can be adjusted and clamped in various positions to accommodate different lengths of work pieces.
The wheel head 3 carries a grinding wheel 2 and its driving motor is mounted on a slide at the top and rear of the base. The wheel head may be moved perpendicularly to the table ways, by hand or power to feed the wheel to the work.
The grinding wheel is fed to the work by hand or power as determined by the engagement of the cross feed control lever.
On the plain grinding machines, the operation may be stopped automatically when the work piece has been finished to size.
Universal centre type grinders :
Universal grinders are widely used in tool rooms for grinding tools, etc. a universal machine has the following additional features:
- The headstock spindles may be used alive or dead, so that the work can be held and revolved by the chuck as well as ground between centers.
- The head stock can be swiveled at an angle in a horizontal plane.
- The wheel head and slide can be swiveled and traversed at any angle. The wheel head can also be arranged for internal grinding by the addition of an auxiliary wheel head to revolve small wheels at high speeds.
Centre less grinders:
- Center less grinding is a method of grinding exterior cylindrical, tapered and formed surface on work pieces that are not held and rotated on center.
- The principal elements of an external center less grinder are the grinding wheel, regulating or back up wheel, and the work rest. Both wheels are rotated in the same direction the work rest in located between the wheels. The work is pleased upon the work rest, and the latter, together with the regulating wheel, is fed for word, forcing the work against the grinding wheel.
- The axial movement of the work past the grinding wheels obtained by tilting the regulating wheel at the slight angle from horizontal. An adjustment of 0 to 8 or 10 degrees is provided in the machine for this purpose.
- The actual feed (s) can calculated by formula:
S=Πdn sin X
S=feed in mm per minute.
N=revolution per minute.
D=diameter of revolving wheel in mm.
X=angle if inclination of wheel.
Center less grinder may be done in one of the three ways:-
- A) Through feed
- B) Infeed
- c) End feed.
- Through grinding:-the work is passed completely through the space between the grinding wheel and regulating wheel, usually with guides at both ends. This method is used when there are no shoulders or others forms to interfere with the passes of the work. It is used for grinding long, slender shaft or bars the layer of metal removed by the grinding wheel in one pass reduced the diameter of the work pieces by 0.02 to 0.3 mm.
- Infeed grinding:-which is similar to plunge grinding or from grinder, the regulating wheel is drawn back so that work pieces may be pleased on the work rest blade. Then it is moved in to feed the work against the grinding wheel. This method is useful to grind shoulders and formed surfaces.
- End feed grinding:-it is used to produce taper, either the grinding wheel or regulating wheel or both are formed to a taper. The work is fed lengthwise between the wheels and is ground as it advances until it reaches the end stop.
Internal center less grinding the external Centerless grinding priceless also applied to internal grinder. In internal Centerless grinding the work is supported by three rollers one is the regulating roll, and the other is a pressure roll to hold the work pieces firmly against the support and regulating rolls. The grinding wheel connects the inside diameter of the work pieces directly opposite the regulating roll,
The advantages of center less grinding:
- As the true floating condition exists during the grinding process, less
Metal needs to be removed
- The work piece being supported throughout its entire length as grinding takes place, there is no tendency for chatter or deflection of the work and small, fragile or slender work pieces can be ground easily.
- The process is continuous and adapted for production work.
- No center holes, no chucking or mounting of the work on mandrels or others holding devices are required.
- The size of the work is easily controlled.
- The low order of skill is needed in the operation of the machine.
The disadvantages of center less grinding:
- In hollow work there is no certainty that the out side diameter will be
Concentric with the inside diameter.
- Work having multiple diameters is not easily handled.
Surface grinding machines are employed to finish plane or surfaces.
They are also capable of grinding irregular, curved, convex, and concave surfaces.
Surface grinder may also be classified according to whether they have horizontal or vertical grinding wheel spindles. There may be four different types of surface grinders:
- Horizontal spindle reciprocating table
- Horizontal spindle rotary table
- Vertical spindle reciprocating table
- Vertical spindle rotary table
The majority of surface grinders are of the horizontal table type. In the horizontal type of machine, grinding is normally done on the periphery of the wheel. The area of contact is small, and the speed is uniform over the grinding surface. Small grain wheels can be used, and the finest finishes obtained. In the vertical, surface grinder apply the face or side of the wheel, and cupped, cylindrical, or segmented wheels are used. The area of contact may be large, and stoke can be removed rapidly. But a crisscross pattern of grinding scratches is left on the work surface.