SURFACE GRINDING MACHINE

SURFACE GRINDING MACHINE

Surface grinding machine
It is a precision grinding machine to produce flat surfaces on a work piece. It is a more economical and more practical method of accurately finishing flat surfaces than filing and scraping.

TYPES OF SURFACE GRINDERS
There are four types of surface grinders.
▪ Horizontal spindle reciprocating table (Fig 1)
▪ Horizontal spindle rotary table (Fig 2)
▪ Vertical spindle reciprocating table (Fig 3)
▪ Vertical spindle rotary table (Fig 4)

A horizontal spindle type surface grinder is widely used.

Horizontal spindle reciprocating table surface grinder
MAIN PARTS
• Base
• Saddle
• Table
• Wheel head

FUNCTIONS

Base
It has a driving mechanism (hydraulic device, tank and motor). It has a column at the back for supporting the wheel head

Saddle:
It is the frame. It carries the table in its crosswise movement.  It is used to give cross feed to the work.  It can be moved by hand feed or auto feed.

Table:
It is fitted on the saddle.  It reciprocates along the guide ways to provide the longitudinal feed to the work.  It has T’s slots for clamping purposes it is moved by hand or auto feed.

Wheel head:
It is mounted on the column. It can be moved vertically up and down only manually, to accommodate work pieces of different heights and set the wheel for depth of cut. The wheel rotates at a constant speed. (1500 m/min.)
The grinding is done on the circumference of the plain wheel. Area of contact is less. A small wheel is also used and a good finish obtained. In a vertical spindle surface grinder, the area of contact may be large.

Types of lay
Each method will produce a characteristic finish determined by the lay of the surface of the workpiece after the grinding operation.
A straight wheel with reciprocating work will produce fine straight lines on the work surface. (Fig 6)

 

A cup wheel with reciprocating work will produce curving lines. (Fig 7)

A segmental wheel on a vertical spindle will produce radial lines on a rotating work piece. (Fig 8)

A cup or segmental wheel with rotating work will produce concentric lines on the work surface.(Fig 9)

A particular type of finish may be considered unsuitable on certain works. Hence the suitable finish and the convenient method of grinding should be determined

Specification of a surface grinder
1 Maximum dia of the wheel that can be held on the spindle
2 Maximum size of the job that can be ground. . (Length x width x height) (150 x 150 x 400)
3 The type of drive of the worktable: hydraulic/ electrical
Detailed specifications and dimensions of a surface grinder are fumished by the manufacturer in the operator's instructional manual. Refer to one such manual of your section with the help of your instructor.
Eg. Praga, 175 wheel, 400 mm strokes, hydraulic surface grinder. Model 451.
Detailed specifications and dimensions of a surface grinder are fumished by the manufacturer in the operator's instructional manual. Refer to one such manual of your section with the help of your instructor.

Surface grinding operations
The figures given here show the various types of surface grinding operations.
∙ Grinding flat surface (Fig 10)
∙ Grinding vertical surface (Fig 11)
∙ Grinding Slot (Fig 12)
∙ Grinding angular surfaces (Figs 13 & 14)
∙ Grinding a Radiou (Figs 15 & 16)
∙ Cutting off (Fifg 17)

∙ Grinding flat surface (Fig 10)

∙ Grinding vertical surface (Fig 11)

∙ Grinding Slot (Fig 12)

∙ Grinding angular surfaces (Figs 13 & 14)

∙ Grinding a Radious (Figs 15 & 16)

∙ Cutting off (Fifg 17)

Centreless grinding:
This method is applied in mass production. It is performed on the centreless grinding machine. The workpiece can be ground without centres between two grinding wheels and is ground by the large wheel. The small wheel is the feed wheel. It runs with lower circumferential speed than the large wheel and breaks the workpiece, which is rotated by the large wheel to the desired number of revolutions. The workpiece is pushed along the large wheel by the inclination of the feed wheel.

STANDARD MARKING SYSTEM FOR GRINDING WHEELS
Introduction
Standard wheel markings specify all the important wheel characteristics. The marking system comprises of seven symbols, which are arranged in the following order.

Example
Marking system
51 - A46 H5V8
Specification of grinding wheels
A grinding wheel is specified by the:
- Standard wheel markings Example:
- Diameter of the wheel 2 A46 H8V
- Bore diameter of the wheel 250 x 20 x 32
- Thickness of the wheel Straight wheel
- Type (shape) of the wheel.

Position 0	Position 1	Position 2	Position 3	Position 4	Position 5	Position 6
Manufacturers' symbol for abrasive	Type of abrasive grit	Grain size	Grade	Structure (optional)	Type of bond (optional)	Manufacturers' own mark
51	A	46	H	5	V	8

CONSTRUCTION OF A GRINDING WHEEL
In order to make the grinding wheel suitable for different work situations, the features such as abrasive, grain size, grade, structure and bonding materials can be varied.
A grinding wheel consists of an abrasive that does the cutting, and a bond that holds the abrasive particles together Abrasives
There are two types of abrasives.

The NATURAL ABRASIVES are emery and corundum. These are an impure form of aluminum oxide. ARTIFICIAL ABRASIVES are silicon carbide and aluminium oxide. 'Brown' aluminium oxide is used for general purpose grinding of tough materials. Aluminium oxide is used for grinding die steels. 'Green' silicon carbide is used for very hard materials with low tensile strength such as cemented carbides.


Grain size (Fig 1)

The number indicating the size of the grid represents the number of openings in the Sieve used to size the grain.  The larger the grit size number, the finer the grit.

Grade (Fig 2)
Grade indicates the strength of the bond and, therefore, the ‘hardness’ of the wheel.  In a hard wheel the bond is strong and it securely anchors the grit in place, and therefore, reduces the rate of water. In a soft wheel, the bond is weak and the grit is easily detached resulting in a high rate of wear.

Structure (Fig 3)
This indicates the amount of bond present between the individual abrasive grains, and the closeness of the individual grains to each other. An open structured wheel will cut more freely. That is, it will remove more metal in a given time and produce less heat. It will not produce such a good finish as a close structured wheel.

Bond
The bond is the substance which, when mixed with abrasive grains, holds them together, enabling the mixture to be shaped to the form of the wheel, and aftera suitable treatment to take on theform of the wheel and the necessary mechanical strength for its work. The degree of hardness possessed by the bond is called the 'grade' of the wheel, and this indicates the ability of the bond to hold the abrasive grains in the wheel. There are several types of bonding materials used for making wheels.

TYPES OF BONDS AND THEIR USES
Vitrified bond (V)
This is the most widely used bond. It has high porosity and strength which makes this type of wheel suitable for high rate of stock removal. It is not adversely affected by water, acid, oils at ordinary temperature conditions.

Silicate bond (S)
Silicate wheels have a milder action and cut with less harshness than vitrified wheels. For this reason they are suitable for grinding fine edge tools, cutlery etc.

Shellac bond (E)
This is used for heavy duty, large diameter wheels where a fine finish is required. For example, the grinding of mill rolls.

Rubber bond (R)
This is used where a small degree of flexibility is required on the wheel as in the cutting of the cutting off wheels.

Resinoid bond (B)
This is used for high-speed wheels. Such wheels are used in foundries for dressing castings. Resinoid bond wheels are also used for cutting off parts. They are strong enough to withstand considerable abuse.