In this article we will discuss about:- 1. Introduction to Grinding 2. Origin of Grinding 3. Cutting Action in Grinding 4. Mechanics of Cutting Action in Grinding 5. Temperature in Grinding 6. Self-Sharpening Characteristics of Grinding Wheel 7. Residual Stresses in Grinding 8. Causes of Wheel’s Wearing too Rapidly 9. Causes of Wheel’s Glazing 10. Operating Conditions and Other Details.
Contents:
- Introduction to Grinding
- Origin of Grinding
- Cutting Action in Grinding
- Mechanics of Cutting Action in Grinding
- Temperature in Grinding
- Self-Sharpening Characteristics of Grinding Wheel
- Residual Stresses in Grinding
- Causes of Wheel’s Wearing too Rapidly
- Causes of Wheel’s Glazing
- Operating Conditions
- Use of Cutting Fluids during Grinding
- Safety in Grinding
- Grinding Faults
- Thermal Effects of Grinding
- Factors Affecting Surface Roughness in Grinding Operation
- Trouble Shooting in Grinding
1. Introduction to Grinding:
Grinding can also be considered as a machining process, i.e. process of removing metal, but comparatively in smaller volume. To grind means ‘to abrade’, to wear away by ‘friction’ or ‘to sharpen’. In grinding, the material is removed by means of a rotating abrasive wheel. The action of grinding wheel is very similar to that of a milling cutter.
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The wheel is made up of a large number of cutting tools constituted by projected abrasive particles in the grinding wheel. Definite elongated metal chips varying in size from 0.4 to 0.8 mm can be seen by examining the material removed under the microscope.
Nowadays, grinding is mainly used for the following purposes:
(i) To remove a very small amount of metal from the workpiece to bring its dimensions within very close tolerances after all the rough finishing and heat treatment operations have been carried out. It is thus basically a finishing process employed for producing close dimensional and geometrical accuracies.
(ii) It is sometimes used to obtain better finish on the surface.
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(iii) Sometimes it is used for machining those hard surfaces which are otherwise difficult to be machined by the high speed steel tools or carbide cutters.
(iv) It is also used for sharpening the cutting tools.
(v) It is also used for grinding threads in order to have close tolerances and better finish.
(vi) Sometimes it is also applied for higher material removal rates (abrasive machining).
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Grinding is one of the extreme important processes in production work. It possesses certain advantages over other cutting processes.
Some of the advantages are:
(i) It is very suitable for cutting hardened steels etc. Parts requiring hard surfaces are first machined to shape in annealed condition, only a small amount being left for grinding depending upon the size, shape and tendency of material to warp during heat-treating operation.
(ii) Extremely smooth finish desirable at contact and bearing surfaces can be produced only by grinding operation due to large number of cutting edges on the grinding wheel.
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(iii) No marks as a result of feeding are there, because the wheel has considerable width.
(iv) Very accurate dimensions and smoother surface finish can be achieved in a very short time.
(v) Complex profiles can be produced accurately with relatively inexpensive truing templates.
(vi) Very little pressure is required in this process, thus permitting its use on very light work that would otherwise tend to spring away from the tool. This characteristic permits the use of magnetic chunk for holding the work in many grinding operations.
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(vii) Abrasives have very high hardness; are less sensitive to heat compared to other materials and can sustain high temperatures. Thus these can be worked at higher cutting speeds. Grinding wheels have self-sharpening properties due to releasing of dulled grains and exposing new sharp ones.
(viii) Grinding is the convenient method of removing material from materials after hardening.
(ix) Grinding unlike conventional machining need not cut through the hard skin of forgings, etc.
2. Origin of Grinding:
In the early stages, ‘chisel’ was thought of as the most convenient tool for removing metal. In chisel there is only one cutting edge and more material can be removed by it but with very poor finish. For getting better finish on the materials man started using file. In file, there are multi cutting edges.
With it the material removed is less, but better finish can be obtained. With the advancement of technology, chisel was replaced by a single point cutting tool in order to have controlled removal of metal and the operation of metal removal is carried out on various machine tools like lathes, shapers, milling machines etc.
Similarly, in order to control the metal removal and obtain better finish by multi-cutting edge tool, grinding is used. The grinding process results in an improvement in geometric accuracy of a component (± 0.02 mm) and an improvement of surface finish (0.1 µm Ra).
3. Cutting Action in Grinding:
It will be observed from Fig. 20.1 that a grinding wheel consists of abrasive particles, bonding material and voids. The projecting abrasive particles act like cutting tool tips and remove metal. A properly selected grinding wheel exhibits self-sharpening action.
As cutting proceeds, the abrasive particles at cutting edge become dulled, and eventually these become cracked along the cleavage planes due to resistance offered by workpiece material which resists the cutting action. Thus new cutting points are produced which carry out further cutting action.
This process continues till the abrasive grains get worn down till the level of bond. At this point the bond allows the remainder of the worn grains to be torn from the wheel, exposing new grains which were previously below the surface of the wheel and the new grains do further cutting action.
Two problems often encountered either by wrong selection of grinding wheel or by improper cutting conditions are wheel glazing and wheel loading. Wheel glazing refers to the condition when the grains are worn down to the level of bond and held for too long for efficient cutting. This results due to use of a hard wheel (wheel with a strong bond strength and too fine grains).
The problem can be remedied by changing the wheel and sometimes by changing the cutting conditions. Wheel loading occurs when workpiece chips are embedded in the cutting face of the wheel, thereby reducing the rate of cutting because the depth of penetration is reduced. It occurs due to too small voids and can be cured by increasing the wheel speed or using different wheel even.
Thus the selection of the grinding wheel for correct, continuous and efficient cutting demands the correct selection of the type of abrasive, the size of the grains, the type of bonding agent and its strength, and the size of the voids. Further the behaviour of the grinding wheel is affected by the workpiece material, cutting speed, depth of cut and the feed rate.
Though diamond is the hardest material but because of its high cost, its applications are restricted. Al2O3, SiC and B4C have high hardness in comparison to hardened steel and thus can be used for metal removal by plastic deformation. It may be mentioned that cutting tool material has to be harder for material removal by plastic deformation and also to maintain its shape and for less wear.
Since it is not possible to make usual shape of cutting tool with these materials use is made of them in the form of grains, the form in which they are available in natural form. The grains of these materials (abrasives) are bonded with some bonding material in the shape of wheel. The abrasive grains on the surface of wheel act as cutting edges. These are randomly distributed and randomly oriented.
4. Mechanics of Cutting Action in Grinding:
Fig. 20.2 (a) shows the cutting action of grains in a grinding process. For simplification, all grains can be assumed to be identical.
Fig. 20.2 (b) shows the elaborated view of scheme of chip formation during surface grinding. The cross-section of uncut chip is found to be approximately triangular having thickness t and width w. However, the uncut thickness and width vary and let their maximum values be tmax and wmax. Average value may be half of these. The average length of chip l = D/2 x θ (D = grinding wheel diameter and θ is very small)
If f is the feed (typical value is 0.2 to 0.6 m/sec) and W = width of cut in mm, total volume of material removed per unit time = fdW
Average volume of one chip = f(1/6) wmax tmax l.
If N is the rpm of grinding wheel, ρ = surface density in grains/mm2, then number of active grains on boundary of wheel and thus number of chips produced per unit time = πNDW ρ.
It will be seen from here that wheel will appear to be softer, if N, D, or ρ decrease, or f or d increase, because the value of Fav will increase and cause a more frequent dislodging of the abrasive grains. In surface grinding operation, radial force FR = 2F. (Refer Fig. 20.3)
5. Temperature in Grinding:
A very high temperature is attained by the tip of the abrasive particle when cutting. However no serious heating of the wheel occurs because such high temperature is only for a very short duration and the temperature gradient at the cutting grains is very steep.
The approximate theoretical mean chip/tool interface temperature is given by:
∴ For fine grinding, chip/tool temperature can be reduced by decreasing both wheel speed and chip thickness.
For normal grinding, temperature can be reduced by lowering wheel speed but not by decreasing chip thickness. In fact thermal damage can result for light finishing cuts.
By using fluid in grinding, not only workpiece temperature decreases and wheel wear decreases, but wheel is less loaded which reduces frequency of wheel dressing. However fluid can’t prevent surface damage to workpiece due to high momentary temperature.
6. Self-Sharpening Characteristics of Grinding Wheel:
In a grinding wheel, the cutting tools (points) are irregularly shaped and randomly distributed. The sharp edges on the periphery of the wheel take part in material removal process and gradually they become blunt i.e., worn out (dull). Due to greater forces on them during machining, they either fracture to present new sharp cutting edge, or get lodged out and new grains below it become exposed and take part in material removal.
This process imparts grinding wheels the characteristic of self-sharpening. It would be realised that the strength of bond (called its grade) decides the maximum force an abrasive grain can withstand and this is an important characteristic of grinding wheel. A wheel with a strong bond is called hard.
The small and hot chips produced in grinding operation have tendency to weld to wheel or workpiece. Further a large number of grains may have a large negative rake angle due to random grit orientation, and these instead of cutting, may rub. These factors make grinding process to be inefficient and consume high specific energy.
7. Residual Stresses in Grinding:
The temperature at the grain-chip interface during grinding reaches very high value (around 1500°C). Due to high temperature, the micro structural changes may take place due to rapid heating and quenching (due to cutting fluid). The thermal and mechanical effects can affect the ground surface to a depth of about 0.2 mm.
These would result in development of high residual tensile stresses and if these attain high values, surface cracks may occur. Fig. 20.7 shows how the residual stress may occur at various depths with different speeds of wheel in a workpiece after surface grinding. Grinding temperature can be assumed to be proportional to the energy spent per unit surface area ground,
Thus temperature and the defects caused by high grinding temperature can be reduced by decreasing d, D, ρ or N, or by increasing f.
The time during which a grit remains in contact with the chip:
which is of the order of 0.0001 sec.
The grain chip interface temperature is found to be:
where V = wheel surface speed
R = thermal conductivity of work material
ρc = volume specific heat of the work material.
8. Causes of Wheel’s Wearing too Rapidly:
i. Too soft wheel
ii. Too narrow face of wheel
iii. Too slow speed of wheel
iv. Too fast speed of work
v. Crowding of wheel
vi. Presence of holes or grooves in the work.
9. Causes of Wheel’s Glazing:
i. Too hard wheel
ii. Too fine grain
iii. Too fast speed of wheel
iv. Too slow speed of work
v. Wheel loaded with chips
10. Operating Conditions:
The proper selection of various operating conditions is very important for the success of any grinding operation.
The various operating conditions and their effect on grinding operation are given below:
(i) Wheel Speed:
The increase in wheel speed (with constant feed rate) results in reduction of the chip size removed by a single abrasive grain, thereby reducing the wear of the wheel. Higher wheel speed is limited by the wheel design, type of bond, grinding operation, power and rigidity of the grinding machine, etc. Wheel speed normally varies between 20 to 40 m/sec depending on type of bond and different grinding operations.
(ii) Work Speed:
Increase in work speed increases the wheel wear, but decreases the heat produced. High work speed is limited by premature wheel wear and vibrations induced by wear. Low work speed results in local overheating, which deforms/tempers the hardened workpiece and affects its mechanical properties.
In order to decrease wheel wear, the work speed should be reduced. If the heat produced is more, clogging occurs, particularly with hard wheels, the work speed should be increased. For roughing operation, work speed varies from 11 to 50 m/min and for finishing operation from 6 to 30 m/ min in case of cylindrical grinding. Work speed for internal grinding varies between 15 to 30 m/min and for surface grinding between 8 to 15 m/min.
(iii) Feed:
Material removal rate is increased by increasing the down feed or infeed rate, bit it results is greater wheel wear and poor finish, thereby affecting dimensional and geometrical accuracy.
Increase of traverse-feed or cross-feed increases the wheel wear and produces poor surface. Usually its value is adjusted to 2/3 to 3/4 of the wheel width in case grinding of steel and 3/4 to 5/6 of wheel width in the case of cast iron workpiece.
(iv) Area of Grinding Contact:
When area of contact is large (as in the case of internal grinding, surface and with larger diameters of work with small diameter wheel), unit pressure is low, and for continuous free cutting action a soft grade wheel is used. Coarser grit is used to provide adequate chip clearance between abrasive grains. Finer grit and harder grade wheels are used when area of contact is small.
11. Use of Cutting Fluids during Grinding:
A lot of heat is generated at the contact of grinding wheel and the workpiece during grinding operation, majority of which is transferred to the workpiece. Grinding fluids help in preventing excessive heating of workpiece and flush the wheel.
Grinding fluids containing sulphur or chlorine additives help in reducing the cutting force and improving the surface finish and increasing the life of the grinding wheel. Usually water based emulsions and grinding oils in ample quantity (15-20 litres/min for normal medium sized grinding machine) are used for this purpose.
The fluid is directed to the interface between wheel and workpiece so that it can create a film of low shear strength between the wheel and the work. The fluid is supplied under pressure using special nozzles, so that air film around the wheel surface due to high speed, is penetrated. In order to prevent clogging in the wheel due to fine particles, the grinding fluid is finely filtered.
12. Safety in Grinding:
Any unsafe practices in grinding can be hazardous for operation and deserve careful attention.
Various important aspects are:
(i) Mounting of Grinding Wheels:
The wheel should be correctly mounted in the spindle and enclosed by a guard. Wheel bore should not be a tight fit on the sleeve.
(ii) Wheel Speed:
The maximum wheel speed is determined by the ultimate bursting strength of the wheel and it depends on the abrasive used, grit size, bond, structure, grade, shape and size of the wheel. Its value is specified by the manufacturers which should never be exceeded.
(iii) Wheel Inspection:
Wheels before mounting should be checked for damage in transit, cracks and other defects. Ringing test is good enough for vitrified bond. Sound wheels, when tapped lightly at 45° from the vertical line with a plastic hammer sound like a clear metallic ring but the cracked wheel will not ring.
Wheels, when not in use, should be stored in a dry place and placed on their edges in racks.
(iv) Wheel Guards:
These should always be used during grinding, and periodically adjusted to compensate for wheel wear.
(v) Dust collection and Health Precaution:
When grinding dry, provision for extracting grinding dust should be made. Protective covers of machine should never be removed while machine is in use. Operator should wear safety devices to protect his eyes and body from flying abrasive particles and dust.
(vi) Wheel Operation:
Adequate power is essential in grinding machines. If power is not adequate then wheels will slow down and develop flat spots, making the wheel to run out-of-balance.
During wet grinding, the wheel should not be partly immersed, as this would seriously throw the wheel out balance.
13. Grinding Faults:
Two common faults due to incorrect choice of wheel or incorrect grinding condition are:
(i) Loading and
(ii) Glazing.
Loading occurs when spaces between the abrasive grains become clogged with particles of the metal being ground. As such grains do not project sufficiently to promote efficient cutting. It occurs due to grinding of soft metals with open structured wheel. Glazing is easily recognised by shiny appearance on the face of the wheel.
It occurs due to abrasive grains becoming dull and not breaking away from the bond. This happens when wheels are too hard for the material being ground. Glazing can be reduced by increasing wheel or work speed.
Surface finish and specific power requirement could also be incorporated to assess the overall performance of grinding wheel. In that case, grinding ratio is equal to the ratio of the amount of material ground per amount of wheel wear and the product of specific grinding power and surface finish on test piece.
14. Thermal Effects of Grinding:
During the process of grinding a lot of heat is generated between the cutting tool and workpiece. A major portion of the heat is dissipated in the workpiece and the remaining is retained by the grinding wheel.
Two thermal effects of grinding are:
1. Effect on Grinding Wheel:
Due to the generation of heat, cracks are developed which are called grinding cracks. The cracks are perpendicular to the grinding marks.
2. Effect on Workpiece:
(a) Discoloration:
Oxidation of surfaces takes place at 200°C producing metallic oxide. These oxides have different colours unlike the parent metal. In other words, we can say that this leads to discoloration of workpiece. The generation of heat is due to the dull grains which will lead to burning of surface.
(b) Mechanical Damage:
Due to the sharpness of grains, scratches are formed on the metallic surface.
(c) Metallurgical Damage:
Due to generation of heat the brittle cracks are formed on the surface.
(d) Chemical Damage:
Due to generation of heat chemical oxides are formed.
15. Factors Affecting Surface Roughness in Grinding Operation:
Surface roughness in grinding depends on grinding wheel (its diameter, abrasive, hardness, dressing, wear) and grinding conditions (wheel speed, workpiece speed, longitudinal feed, workpiece diameter). Figs. 20.14 shows variation of surface roughness in grinding with change in various parameters.
16. Trouble Shooting in Grinding:
Various faults that could be encountered during grinding and various solutions to overcome them are given below:
(i) Fast Wheel Wear:
This can be taken care of by:
(a) Using harder wheel,
(b) Increasing wheel speed,
(c) Reducing rate of traverse and work speed, and slightly decreasing the depth of cut.
(ii) Wheel Glazing:
This occurs due to incorrect dressing, wrong wheel selection and using slow traverse and high work speed. It can be taken care of by keeping the wheel sharp, using softer wheel or coarser grit, reducing wheel speed and fast traverse, using a greater depth of infeed and increasing depth of cut.
(iii) Chatter Marks:
These can be taken care of by:
(a) Properly balancing the wheel,
(b) Using proper dressing tool,
(c) Using softer grade or coarser grit,
(d) Reducing machine vibrations by checking bearings and foundations, and adjusting spindle bearings,
(e) Tightening pulley,
(f) Using suitable supports or clamps for large jobs.
(iv) Coarse Finish:
This could be due to using either too coarse wheel or too soft wheel.
(v) Wheel Loading:
This can be taken care of by using softer or porous structure wheel; using sharper dresser, using a copious quantity of clean coolant. Irregular marks of different lengths and widths could occur due to dirty coolant. Deep irregular marks occur due to loose wheel flanges.
(vi) Overheating of Workpiece:
This occurs due to wrong selection of wheel. To overcome it, softer wheel should be used and sufficient coolant used.