grinding

Production Processes 4-1 Grinding

C h a p t e r

Syllabus:

Introduction, classification of grinding machines, grinding wheel composition, types and shapes of grinding wheels, designation as per IS 551-1954, types of grinding operations

Statistical Analysis

BTE Papers Marks allotted for this Chapter

Summer 2004 20

Winter 2004 20

Summer 2005 20

Grinding

4


4.1 Introduction :

Grinding and finishing operations are used for the final finish and super finish. There are various types of grinding. The most common one is surface grinding. This process is accompanied by a certain amount of metal removal. This process can give a surface finish in the range of 0.75 to 1.25 m.

Fig. 4.1.1 : Grinding operation

Fig. 4.1.1 shows the basic arrangement of surface grinding. In this process, the conventional cutting tool is replaced by an abrasive tool, whose cutting elements are of abrasive materials. In old days emery and corundum were used as abrasives. Modern grinding wheels are made with the artificial abrasives like silicon carbide and aluminous oxide. The abrasives used have high resistance to wear the heat and possess large hardness.

4.1.1 Features of Grinding Process :

(i) Grinding wheel has a self-sharpening character. This means that the dull or worn-out grains of the grinding wheel are removed either by fracture or tearing of the bond, thus exposing fresh new grains.

(ii) The operation is intermittent in nature and produces discontinuous chips.(iii) The grain geometry is highly random and the time of contact between the chip and an

abrasive grain is very small.(iv) The high temperatures (1000-1400C) are usually encountered in grinding resulting into

rapid grain wear and high induced surface in the workpiece.(v) The grinding action depends strongly upon the characteristics of the grinding wheel.

4.1.2 Advantages of Grinding Process :[ Asked in Exam : W – 04 !!! ]

The different advantages of grinding over other cutting operations are :(i) Extremely smooth finish desirable at contact and bearing surfaces can be produced

by grinding operation due to large number of cutting edges on the grinding wheel.(ii) No surface marks because the wheel has considerable width.(iii) Very accurate dimensions and smoother surface finish can be achieved within short

time.(iv) Complex profiles can be produced accurately with relatively inexpensive turning

templates.


(v) Grinding is the only method of removing material from materials after hardening.

4.1.3 Applications of Grinding Wheel :

(i) To remove a very small amount of metal from the workpiece to bring its dimensions within very close tolerances.

(ii) To obtain better finish on the surface.(iii) To machine hard surfaces which are otherwise difficult to be machined by the high

speed steel tools or carbide cutters.(iv) To sharpen the cutting tools, etc.

4.2 Classification of Grinding Machines:

Different types of grinding machines are based on the operations to be performed with the help of grinding wheel. Those are

Rough grinders

1) Floor grinders 2) swing frame grinders3) portable and flexible shaft grinders 4) belt grinders

Cylindrical grinders

1) Plain cylindrical grinders 2) universal cylindrical grinders

3) Centre less grinders

Surface grinders

1) Reciprocating table type 2) rotary table type

Special type grinders

1) Internal grinders 2) tool and cutter grinders

3) Crankshaft grinders 4) thread grinders

4.3 Types and shapes


Shape Name Applications

Straight (Type 1)

Cylindrical, centerless, internal, cutter, surface, and offhand grinding operations.

Cylinder (Type 2)

Surface grinding on horizontal and vertical spindle grinders.

Tapered (both sides) (Type 4)

Snagging operations; tapered sides lessen the chance of the wheel’s breaking.

Recessed

(one side) (Type 5)

Cylindrical, centerless, internal and surface grinders; recess provides clearance for the mounting flange.

Straight cup (Type 6)

Cutter and tool grinder and surface grinding on vertical and horizontal spindle machines.

Recessed (both sides) (Type 7)

Cylindrical, centerless and surface grinders; recesses provide clearance for mounting flanges.

Flaring cup (Type 11)

Cutter and tool grinder; used mainly for sharpening milling cutters and reamers.

Dish (Type 12)

Cutter and tool grinder; its thin edge permits it to be used in narrow slots.

Saucer (Type 13)

Saw gumming, gashing milling cutter teeth.

6. Idle running for 5-10 minutes

Table 4.4.1 : Common grinding wheel shapes and applications


4.4 Wheel designation[ Asked in Exam : S – 04, W – 04 !!! ]

A standard marking system for the grinding wheel provides convenient shorthand notation to describe wheels in terms that are meaningful to those with at least a little grinding experience.

Properties of a grinding wheel marking system :(i) Abrasive type (ii) Size of grain(iii) Grade (iv) Structure(v) Type of bond (vi) Manufacturers symbol for identification.

4.5 Types of grinding operations

Surface Grinding :

The types of wheels used are determined by the spindle position. Each type can further be sub-divided into

Sr. No.

Spindle Table

1. Horizontal Reciprocating

2. Horizontal Rotary

3. Vertical Reciprocating

4. Vertical Rotary


Cylindrical Grinding:[Asked in Exam: W – 04!!!]

A typical centre type cylindrical grinding consists of cylindrical workpiece mounted between the two either revolving ends or two dead ends resting on the machine bed as shown in Fig. 4.6.2

The workpiece is capable of revolving with the help of an independent motor and the grinding wheel also rotates around its own axis by another independent motor.

In all, four movements are essential in this type of grinding(i) The work – must revolve around its own axis.(ii) The wheel – must revolve around its own axis.(iii) The wheel must pass over the work and (iv) The wheel must be fed to the work by hand or automatically.

Centreless Grinding :[ Asked in Exam : S – 04, W – 04, S – 05 !!! ]

In centreless grinding, the workpieces are held between the grinding wheel and the regulation wheel and a work rest blade, as shown in Fig. 4.6.3. By tilting the rotational axis of the regulating wheel with respect to the grinding wheel. The workpiece is given a longitudinal force, creating movement or through feed. No manual handling of the part is required.

In this method, rate of production is high and cost of production is less.

Internal Grinding :

Internal grinding is used to obtain very accurate sizes and good surface finish on holes made by other operations. Machines are made, that are able to finish holes from a few thousands of an inch in diameter to 5 feet or more .

Here the grinding wheel diameter is smaller in size than that of the hole being ground. Grinding wheels of small diameter wear out rapidly and require more frequent dressings.

Grinding machines are designed properly for finishing parts of cylindrical, flat surfaces. The kind of surface machined largely determines the type of grinding machines.

Tool and Cutter Grinding :

Tool and cutter grinder is designed for sharpening a variety of tools and cutters, such as milling cutters, drills, hobs, broaches etc.

A cutter grinder is a machine which supports the cutter while a rotating abrasive wheel is applied to the cutting edge for the purpose of sharpening it. These machines vary in design from single purpose grinders used to sharpen cemented carbide tipped tool bits to complex universal machines capable of doing any type of cutter grinding.


(a) Tool and cutter grinding (b) Block diagram of a tool and cutter

Fig. 4.6.4

Special Grinding processes

Special purpose machine tools are the one which are typically used for a particular type of operation, for a specific job namely crankshaft grinding, cam grinding, thread grinding, etc. The above mentioned operations are carried out for the specific job, they mention i.e. crankshaft grinding is carried for crankshaft, cam grinding for cams and so on.

The significant feature of these type of grinding machines is that, they are manufactured to do highly specialized work. They also can be categorized as SPM (Special purpose machines).

(a) Single wheel grinding (b) Plunge cut grinding

Fig. 4.6.5 : Thread grinding


A typical thread grinding machine is shown in Figs. 3.9.1(a) and (b). Fig. 3.9.1(a) shows circular disc with the thread form being given the feed equal to the pitch of the thread under consideration. Again, Fig. 3.9.1(b), depicts a figure wherein the threads are ground using a circular drum which has the desired thread profile and the height equal to the complete length the threads. This is then plunged to achieve the objective.

Way grinders : These are large, heavy single purpose machine used to grind the various surfaces of the ways and beds of machines. The wheel is cup, ring or segmented mounted on vertical spindle and can be tilted at any angle as shown in Fig. 3.9.2.

Fig. 4.6.6 :Way grinding

4.6 Mounting of Grinding Wheel


4.7 BTE Questions

Summer 2004 – Total Marks 20

Q. 1 What are the abrasives? Name artificial abrasives used for manufacturing grinding wheel. (Section 4.3) (4 Marks)

Q. 2 Explain working principle of centreless grinding, stating its advantages. Describe the different methods of feeding the job on centreless grinding machine. (Section 4.6)

( 8 Marks)

Q. 3 Explain the main factors to be considered in selection of a grinding wheel.

(Section 4.3) (4 Marks)

Winter 2004 – Total Marks 20

Q. 5 Identify the following grinding wheel

W S 30 R 7 V 17 (Section 4.5) (4 Marks)

Q. 6 List, sketch and state uses of any four types of grinding wheels. (Section 4.3)

(4 Marks)

Q. 9 Explain with neat sketch working principle of centreless grinding. State any four

advantages. (Section 4.6) (4 Marks)

Summer 2005 – Total Marks 20

Q. 10 What materials are used in the manufacture of grinding wheels ? (Section 4.3)

(4 Marks)

Q. 11 What are the different types of bonds used in the manufacture of abrasive wheels ?


(4 Marks)

Q. 13 What are the advantages of centreless grinding over centre type grinding ?


grinding

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