CYLINDRICAL GRINDERS

The principle of cylindrical grinding involves holding the work piece rigidly on centers in a chuck or in a suitable holding fixture, rotating it about its axis and feeding a fast revolving grinding wheel against the work.

If the work surface to be ground is longer than the face width of the grinding wheel, the work is traversed past the wheel or the wheel past the work. Traversing of wheel or work is done either by hydraulic or mechanical power or by hand. Feed is given to the work or the wheel at the end of each traversing movement. In case the width of wheel face is more or equal to the length of the work surface to be ground, the wheel may be fed in with no traversing movement of it or that of the work. This is known as “Plunge Grinding”. The simplest and commonly used type of cylindrical grinder is a tool post grinder used on lathes. When wheels of large diameters are used, they can be mounted directly on the motor shaft. For mounting small wheels an auxiliary shaft is provided, which runs at a relatively much higher speed than the motor. Both external and internal cylindrical grinding can be done on lathe by this equipment.

Cylindrical grinding machines are mainly of the following three types:

1. Plain Cylindrical Grinders.

2. Universal Cylindrical Grinders.

3. Centre less Grinders.

1. PLAIN CYLINDRICAL GRINDERS:

On these grinders, the work piece is usually held between two centers. One of these centers is in the head stock and the other in the tail stock. 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 in to the work by an amount equal to the depth of cut. While mounting the work between centers, the head stock centre is not disturbed. It is the tail stock centre which is moved in or out, manually or hydraulically, to insert and hold the work. Tail stock and head stock both can be moved along the table to suit the work.

The table is usually made in two parts. The upper table carries the tail stock, head stock and the work piece and can be swiveled in horizontal plane, to a maximum of 10⁰on either side along the circular ways provided on the lower table. This enables the grinding of tapered surfaces.

The lower table is mounted over horizontal guide ways to provide longitudinal traverse to the upper table, and hence the work. The table movements can be both by hand as well as power.

Hydraulic table drives are usually preferred. The wheel head is usually mounted on horizontal cross ways on the bed and travels along these to feed the wheel to the work. This movement is known as in feed. The wheel and work are so adjusted that the grinding force is directed downwards to ensure proper stability.

2. UNIVERSAL CYLINDRICAL GRINDERS:

A universal cylindrical grinder carries all the parts and movements of a plain cylindrical and in addition, carries the following advantageous features:

1. The head stock can be made to carry alive or dead spindle, as desired, the former (live centre) being needed, when the work is held in a chuck.

2. The head stock can itself be swiveled in a horizontal plane.

3. Its wheel head can be raised or lowered and can also be swiveled to ±90⁰to grind tapered surface having large taper angles.

All these factors contribute towards the greater versatility of these grinders.

All the modern universal type cylindrical grinders carry hydraulic drive for wheel head approach and feed, table traverse and elimination of back lash in the feed screw nut.

Most of the modern universal grinders are provided with necessary extra equipment like work rest to support slender work, wheel truing device, arbor for balancing the wheel, internal grinding spindle and three jaw self centering chuck etc.

3. CENTRELESS GRINDERS:

These grinders are also a type of cylindrical grinders only, but the principle of centre less grinding differs from centre type grinding in that the work, instead of being mounted between centers, is supported by a combination of a grinding wheel, a regulating wheel and a work rest blade. The relative movements of the work piece and two wheels, the principle of centre less grinding is used for both the external as well as internal grinding. Many hallow cylindrical and tapered work pieces, like bushes, pistons, valves tubes and balls etc, which either do not or cannot have centers, are best ground on centre less grinders.

It carries a heavy base and two wheel heads, one carrying the grinding wheel (larger one) and the other regulating wheel (smaller one). The work piece rests on the blade of the work rest between these two wheels. Each head carries a separate wheel truing mechanism for the wheel it carries. Housing is provided on one side of the machine body to house the main driving motor. There are two control panels on the front.

The left hand panel carries controls for speed adjustments of the two truing mechanisms and the in feed grinding mechanism. The right hand panel carries controls for hydraulic mechanism speed adjustment of regulating wheel, automatic working cycle switch, start and stop switches etc.

In operation, grinding operation is performed by the grinding wheel only while the function of the regulating wheel is to provide the required support to the work piece while it is pushed away by the cutting pressure of the grinding wheel. At the same time, required support from bottom is provided by the work rest as the work piece, while rotating rests on the blade of the work rest. The regulating wheel essentially carries rubber bond and helps in the rotation of the work piece due to friction. The directions of rotation of the two wheels are the same. The common methods used feeding the work is:

1. Through feed

2. In feed

3. End feed

1. Through feed Grinding:

In this method of centre less grinding, the work piece is supported and revolved as described above but is simultaneously given an axial movement also by the regulating wheel and guides so as to pass between the wheels. For this, the axis of the regulating wheel is inclined at 2⁰to 10⁰with the vertical.

The amount of the stock to be removed determines as to how many times a work piece has to pass between the wheels. This method is used for straight cylindrical objects.

The actual feed (f) can be determined by the following relationship:

Where f = feed in mm/min

d = dia. of regulating wheel in mm

n = Revolutions/min

α = Angle of inclination of regulating wheel

2. In feed Grinding:

Both regulating wheel and grinding wheel are more in width than the work length to be ground. Axis of the regulating wheel is inclined about half a degree from the horizontal.

This method is used for grinding shouldered or formed components.

Before the operation, the regulating wheel is drawn away to accommodate the work piece. After placing the work piece on the blade of the work rest, the regulating wheel is again pushed in to press against the work. In this operation, the work rest does not carry guides.

Instead, it is made to have an end stop at the rear end.

4. End feed Grinding:

This method, in a way, a sort of form grinding. It is because both the wheels i.e. the grinding wheel and regulating wheel, and dressed to contain the required shape or form. The work is fed longitudinally from the side of the wheels.

As it advances between the revolving wheels, its surface is ground till its farther end touches the end stop. This method can be used for grinding of both spherical and tapered surfaces, but it suits best to the grinding of short tapered surfaces.

Advantages of centre less Grinding:

1. Need for centering and use of fixtures etc is totally avoided.

2. It can be applied equally to both external and internal grinding.

3. Once a set up has been made, it is a faster method than centre type grinding

4. In feed method also no chucking of work is needed and idle time is negligible.

5. Since there is no end thrust, there is no any spring action or distortions in long work pieces.

6. The work is rigidity supported during the operation, heavy cuts can be taken, rapid & economical.

7. Making and making of centre holes totally eliminated and smaller grinding allowance, less time.

8. Large grinding wheels are used and errors due to wheel wear are reduced.

9. Very little maintenance is needed for the machine.

10. Very high skilled operators are not needed.

11. Direct adjustment for sizes can be made, resulting in high accuracy.

12. A fairly wide range of components can be ground.