SPECIAL TYPES OF GRINDING MACHINES

These machines are designed to do some specialized operations. Some common types of these machines are the following:

1. Roll Grinders.

2. Cam shaft Grinders.

3. Disc Grinders.

4. Crank shaft Grinders.

5. Piston Grinders.

6. Thread Grinders.

7. Tool post Grinders.

1. Roll Grinders:

These are very heavy duty types of plain cylindrical grinders. They carry all the features of plain cylindrical grinder, but are made relatively heavier and more rigid.

Normally roll grinders do not carry any provision for grinding the tapered work, but some carry, a set over type tail stock, similar to that used on a lathe, in order to deal with such type of work. The main use of these grinders is in grinding various types of heavy cylinders, like hydraulic rams, turbine shafts and rolls used in various industries like paper mills, steel mills, textile mills, printing presses, rubber industries and flour mills etc.

The specific equipment of these machines includes the journal rests and a cambering device. The journal rests are used to receive the previously turned journals of the rolls and support the rotating rolls on them during the operation. The cambering device is a special attachment used for crowning or cambering the rolls at the centre. The term crowning has similar meaning here also as applied to the pulleys. It means, increasing the size of the roll at its centre as compared to the dimensions at the ends. A reverse of it is called cambering.

Rolls used in cold working of metals are crowned, as they are likely to be subjected to heavy forces. Against this, the rolls which work on hot metal are cambered as they are likely to expand more at the centre than the ends during hot working. The above features in the rolls neutralize the bad effects of cold and hot working and enables the products to have a uniform thickness. A common type of cambering and crowing attachment involves the use of a cam which works in conjunction with the table traverse to direct the grinding wheel in or out to produce the desired curvature at the centre.

2. Cam Shaft Grinders:

It is a special type of cylindrical grinder. In this, the work piece is mounted between the centers. A separate base is provided, which carries the tail stock and head stock and the complete unit can oscillate about a centre below the work piece. A template, which is a small facsimile of the cam shaft, is mounted on the head stock. It rotates along with the cam shaft to be ground and actuates a hardened steel roller, which makes the whole unit to oscillate in and out to produce the desired shape. The work revolves very slowly. All such machines are made automatic.

3. Disc Grinders:

The operation of disc grinding is used for large scale grinding of flat surfaces. A surface grinder does the same work, with more close dimensional tolerances, but a disc grinder does it more rapidly. The abrasive discs used in disc grinding are fairly large in size and less in thickness as compared to the standard grinding wheels. As such they are backed by steel plates to with stand the grinding pressure. The main reason for faster a grinding is that the discs present more abrasive area to the work than the wheels.

The main types of disc grinders are the single and double spindle. Single spindle grinders are used for off-hand grinding and double spindle for grinding both sides of the work simultaneously. The size of the machine is designated by the diameter of the disc it uses.

Some machines carry horizontal spindles and some others vertical spindles.

4. Crank Shaft Grinder:

These machines are basically cylindrical grinders, involving the principle of plunge cut grinding, for grinding of crank shafts for different types of automobiles and air craft engines, compressors etc. With large scale increase in the use of automobiles, engines, pumps etc. the use of these machines has also considerably increased in the country.

Main parts of a crank shaft grinder:

The machine consists of the following principal units:

1. Bed

2. Table

3. Hydraulic system

4. wheel head

5. wheel head spindle cross feed and axial movement mechanisms

6. table hand traverse mechanisms

7. head stock

8. tail stock

9. coolant system and

10. Electricals

5. Piston Grinders:

A majority of pistons used in I.C. Engines are not of true cylindrical shape. Most of these pistons carry slightly elliptical outer surface. At times, the outer surface may be slightly tapered also. The grinders used in grinding of these pistons, therefore, carry suitable mechanism to automatically regulate and synchronize the inward and out ward movements of the revolving piston and the cutter feed in such a way that the required type of outer surface is ground on the piston together with taper, if desired.

6. Thread Grinders:

Thread grinding is basically, a generating process, in which the desired thread profile is generated on a solid cylindrical object through grinding. The machines used in this process operate on the principle of cylindrical grinders. These machines carry a lead screw which is connected to the head stock, in the same way as in a centre lathe, in order to establish a definite ratio between the speed of rotation of the work and the longitudinal traverse of the grinding wheel. Thus the grinding wheel, which is given the shape of thread profile on its face, follows the desired helix path similar to a single point thread cutting tool on a lathe. The helix angle is provided to the grinding wheel by tilting its spindle to the required angle.

If the thread length is small, a better and quicker alternative to the above method is plunge cut grinding. In this, the cylindrical grinding wheel chosen should have its face wider than the length of the threads. The desired thread profile is provided on its periphery and, after mounting, the spindle is tilted to contain the required helix angle. The work piece is revolved as usual and the shape of revolving grinding wheel is fed straight across the work axis. There is no need for longitudinal traverse of wheel spindle in this case, because the entire thread length is covered by the wheel face.

7. Tool Post Grinder:

It consists of a bracket, which is mounted on the cross-slide, a grinding wheel and a separate motor. The grinding wheel is driven by a separate motor. The job is held in a chuck or between the centers and the rotating grinding wheel is fed against the job. The attachments may be for external or internal grinding.

Some tool post grinders carry provisions such that the same attachments with a little change can be used for internal as well as external grinding.