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Grinding requires a very high energy per unit volume of the material removed during the process. Basically the energy dissipated will be in the form of heat and it will be produced at the interaction of the grinding wheel and the work piece. This in fact leads to the high temperature generation which usually is the main reason for thermal damage. Thermal damages for example transformations in metallurgical phase, tempering or the softening, burning of the surface of the work piece. This intern results in cracks, unfavourable residual tensile stresses, re-hardening and also fatigue strength reduction (Thermally Induced Damage in Grinding, 1978) [1, 2]. Thermal damage is the main factor which decides the quality of the work piece produced. It also the affects and limits the rate of production. Henceforth it is a vital to understand the factors affecting the temperature of grinding.
Most of the damages occur during grinding are thermal in origin. Generation of heat is due to the energy expended by the grinding process. The consumed energy or the power is nothing but the controlled output of the grinding process. Generally the temperature measuring techniques do not give any kind of practical means to find out and to control the temperature of the grinding process in any production environment. But anyways their application is restricted to laboratory. In-process monitoring of the grinding power, when coupled with a thermal analysis of the grinding process, can provide a much more feasible approach to estimating grinding temperatures and controlling thermal damage. (Thermal Analysis of Grinding, 2007)
In grinding based upon the application of the moving heat source theory, thermal analysis will be done. Due to this, the grinding zone is modelled as a source. It moves along the work piece surface. In grinding process, the energy expended is considered to be converted to heat exactly at the grinding zone. To calculate the temperature, a vital parameter is considered. The temperature response is the energy partition to the work piece, which is the fraction of the total grinding energy transported to the work piece as heat at the grinding zone. The partition of the energy depends on the factors such as the type of the grinding, the work piece materials, the wheel and the operating conditions of the grinding process. (Thermal Analysis of Grinding, 2007)
Cutting fluids are used in many types of metal removing process. The purpose of using these fluids is to improve the life of the tool used during the process, to decrease the thermal deformation of work piece, for better surface finish and to flush away the chips produced during the grinding zone from the grinding zone. In practical, the cutting fluids used in today's world generally fall in the below mentioned categories.
Synthetic fluids and
Semi synthetic fluids. (Engineering, 2005)
Figure : Applying cutting fluid on the Machining Process (Engineering, 2005)
Cutting fluids made of soluble oil when mixed with water generally forms an emulsion. It contains a mineral oil base and emulsifiers which helps in producing a stable emulsion. These fluids are used in diluted form whose concentration will be 3 to 10 %. It increases heat transfer performance and provides good lubrication. These type of cutting fluids are used in industries and the less expensive among all type of cutting fluids.
Straight oils are usually used in machining operation and are non-emulsifiable. These oils can be used in diluted form. They mainly contains of petroleum oil or base mineral. It often consists of polar lubricants like fats, vegetable oils and esters. It also includes pressure additives such as Sulphur, Chlorine and Phosphorus. It provides the poorest cooling characteristics and the best lubrication. (Engineering, 2005)
The other type of oil is Synthetic fluids which contains no mineral oil base or petroleum. But these are formulated from alkaline organic and inorganic compounds along with the additives which inhibits corrosion. Like the soluble oils, these types of fluids are also used in their diluted form whose concentration lies between 3 and 10 %. These are the best cooling cutting fluids among all the available fluids.
The semi synthetic fluids are combination of the soluble oil and the synthetic oil, because of which it possess the properties of the both fluids. For example, its properties such as heat transfer performance and the cost lies in between those of soluble oil and synthetic fluids.
The choices made for selecting neat cutting fluids used in metal cutting processes impact on the many aspects of the modern production process.
The health and comfort of operational staff, the manufacturing environment, the machine tool cleanliness, the quality of components being manufactured, the tool cost, the materials being machined, and the disposal costs are all impacted on the correct selection of a soluble cutting fluid.
Neat cutting oils are fluids usually based on mineral oils and used for cutting without further dilution i.e. as supplied by the manufacturer. They are generally blends of mineral oils and other additives.
Neat oils can be used for applications from light machining to heavy-duty operations such as gear hobbing, broaching, turning, honing and drilling.
Historically extreme pressure (EP) additives such as chlorine and sulphur have been added at various levels to neat oils to provide increased tool life and improve surface finish, however environmental and improved performance requirements have lead to the development of neat cutting oils with more sophisticated environmentally friendly synthetic ester based cutting oils which are chlorine and sulphur free. (AZoM, 2000)
PROPERTIES OF CUTTING FLUIDS
A cutting fluid should have the following properties: -
1) It should have high heat absorbing capacity
2) It should have good lubricating action to reduce the friction generated at tool work interface
3) High flash point to damp the fire which may occur (usually in rare cases)
4) It should be chemically stable such that oxides should not be generated.
5) These should not react with work piece or tool material.
6) Cutting fluids should not produce any odour (or) bad smell
7) Cutting fluids should not affect the machine operator.
8) Bearings of the machine should not get affected due to employing of cutting fluids
9) Cutting fluids should be in transparent nature such that the cutting action is to be visible to operator
10) These should be non- corrosive to both work piece and the tool
11) Cutting fluid should have low viscosity. It should be very free to flow.
12) Cost of the cutting fluid is also to be taken into consideration (ISC, 2006)
CUTTING FLUID APPLICATION STRATEGIES
There are many methods of applying the cutting fluid on the grinding zone. Some of them are listed below.
Flood Application of Fluid: - In this type of fluid, a flood of cutting fluid is directly applied on the work piece. The figure shown below illustrates the flood application of fluid.
Figure : Flood type application of fluid (Engineering, 2005)
Jet Application of Fluid: - In the jet application of fluid, a jet of cutting fluid is applied on the grinding zone. The mentioned below shows the jet application of fluid.
Figure : Jet type application of fluid (Engineering, 2005)
Mist Application of Fluid: - This type of application of fluid is atomised by a jet of air and its mist is directed at the grinding zone.
Figure : Mist type application of fluid (Engineering, 2005)
FLUID FLOW IN GRINDING
Flood application refers to low pressure application of grinding fluid from a nozzle. This method is commonly used for shallow cut grinding. For those type of operations, where the grinding area may not be completely enclosed, the flow rate is often kept small enough so as to limit splashing of the fluid. For straight surface grinding, some of the applied fluid usually hits the wheel and then falls on to the work piece, and some of the remaining fluid may not even reach the wheel but fall directly on to the work piece. This seemingly undesirable situation may not lead to poor grinding, since the grinding zone temperature, even with higher flow rates of the fluid carefully directed to the wedge between the work piece and the wheel, which would be probably is well above the burn out limit, thereby limiting the potential for cooling at the grinding zone. Low flow rates with flood application are normally sufficient to provide lubrication and bulk cooling of the work piece. (Stephen Malkin, 2008)
CUTTING FLUID EFFECTS IN MACHINING
The primary functions of any cutting fluids are as follows: -
Cooling the work piece especially at high speed of cutting involved,
At low cutting speeds, lubricating the machining process,
To flush away the chips formed during the process
Secondary functions of the cutting fluid includes: -
Protection of the surface from corrosion,
To enable the part handling by the cooling the hot surface.
Process effects of using cutting fluids in machining include:
Longer Tool Life
Reduced Thermal Deformation of Work piece
Better Surface Finish (in some applications)
Swarf Handling and
Ease of chip
Figure : Formation of chips in grinding process (Engineering, 2005)
CRITERIA FOR CUTTING FLUID SELECTION
The principal criteria for selection of a cutting fluid for a given machining operation are:
Process performance :
Heat transfer performance
Fluid mist generation
Fluid carry-off in chips
Corrosion inhibition Fluid stability (for emulsions)
Health Hazard Performance
CUTTING FLUID MAINTENANCE AND DISPOSAL
Cutting fluid maintenance involves checking the concentration of soluble oil emulsions (using refractometers), pH (using a pH meter), the quantity of tramp oil (hydraulic oil leaking into the cutting fluid system) and the quantity of particulates in the fluid. Action taken to maintain the fluid includes adding make-up concentrate or water, skimming of tramp oil, adding biocides to prevent bacterial growth and filtering the particulates by centrifuging:
Figure : Cutting fluid maintenance method (Engineering, 2005)
The cutting fluid within a coolant system degrades with time due to bacterial growth and contamination with tramp oil and fine metal swarf from the machining operation. Usually the fluid is dumped, when it becomes uneconomical to use and maintain the general make up operations. Prior to letting the fluid flow into a sewer system, it should be treated to bring the fluid composition to safe disposal levels. (Engineering, 2005)
Measurement of fluid jet thickness for grinding application
CHARACTERISTICS OF A LUBRICANT
The physical characteristics of any lubricants generally affect their selection for various applications. Explanations of some of the characteristics are mentioned below.
Viscosity is the most important characteristics of lubricating oil. It is nothing but a measure of the thickness of the lubricating oil at any given temperature. Viscosity is indirectly proportional to the thickness of the oil. Accurate determination of the viscosity is done by measuring the rate of flow in capillary tubes. Viscosity is measured in centistokes (cSt). Unit of viscosity is always accompanied with temperature as oils become thicker on cooling and thinner on heating. Viscometer is used to measure viscosity. Kinematic viscometers available are more consistent and sensitive.
VISCOSITY INDEX (VI)
Viscosity Index generally referred as VI. It is the way of expressing the rate of change of viscosity with temperature. Oils become less viscous with the increase in temperature and vice versa. Viscosity Index varies with different oils and it mainly depends on the crude oil from which it is derived and also on the method of refining. Viscosity Index is important characteristic in applications where the operating temperature is subjected to change. Higher the value of viscosity Index, lower is the variation in viscosity relative to temperature.
This is a rough measure of a limiting viscosity. It is the temperature 2.5°C above that at which the oil ceases to flow when the vessel in which it has been cooled is held horizontally for 5 s. The pour point is a guide to behaviour and care should always be taken that operating temperatures are above the figure specified by the oil manufacturer as the pour point of given oil.
The flash point is oil is the temperature at which it gives off, under specified conditions, sufficient vapour to form a flammable mixture with air. This is very different from the temperature of spontaneous combustion. The test is an empirical one and the result depends upon the instrument used and the prescribed conditions. For example, the flash point may be 'closed' or 'open', depending on whether the test apparatus has a lid or not. As far as lubricating oils are concerned, the test is of limited significance, although it can be indicative of contamination (for example, the dilution of crankcase oil by fuel).
PENETRATION OF LUBRICANT
The most important physical property of lubricating grease is its consistency, which is analogous to the viscosity of a liquid. This is determined by an indentation test in which a weighted metal cone is allowed to sink into the grease for a specified time. The depth, to which the cone penetrates, in tenths of a millimetre, is a measure of the consistency. There is a widely accepted scale, that of the American National Lubricating Grease Institute The penetration test is used mainly to control manufacture and to classify greases and is, within limits, a guide to selection. Penetrations are often qualified by the terms 'worked' and 'unworked'. As greases are thixotropic, that is, they soften as a result of shear but harden again after shearing has stopped, the worked penetration for particular grease may be appreciably greater than the unworked penetration. The difference between these two figures may be a useful guide to the selection of greases for operating conditions that involve much churning - as small a difference as possible being desirable.
The drop point of grease is an indication of change from a soft solid to a viscous fluid; its value depends completely on the conditions of test, particularly the rate of heating. The grease sample, which is held in a small metal cup with an orifice, is heated at a predetermined rate. The drop point is the temperature at which a drop of the sample falls from the cup. The drop point is of limited significant as far as the user is concerned, for it gives no indication of the condition of the grease at lower temperatures or of change in consistency or structure with heat. It is a very rough indication of grease's resistance to heat and a guide to manufacture. The difference between the highest temperature at which grease can be used and the drop point varies very much between types. It is at its maximum with some soda greases and much smaller with multi-purpose lithium products and modern complex greases.
Lubrication exists in one of three conditions:
1. Boundary lubrication
2. Elasto hydrodynamic lubrication
3. Full fluid-film lubrication
Boundary lubrication is perhaps best defined as the lubrication of surfaces by fluid films so thin that the friction coefficient is affected by both the type of lubricant and the nature of the surface, and is largely independent of viscosity. A fluid lubricant introduced between two surfaces may spread to a microscopically thin film that reduces the sliding friction between the surfaces. The peaks of the high spots may touch, but interlocking occurs only to a limited extent and frictional resistance will be relatively low.
A variety of chemical additives can be incorporated in lubricating oils to improve their properties under boundary lubrication conditions. Some of these additives react with the surfaces to product an extremely thin layer of solid lubricant, which helps to separate the surfaces and prevent seizure. Others improve the resistance of the oil film to the effect of pressure.
Elasto hydrodynamic lubrication provides the answer to why many mechanisms operate under conditions which are beyond the limits forecast by theory. It was previously thought that increasing pressure reduced oil film thickness until the asperities broke through, causing metal-to-metal contact. Research has shown, however, that the effect on mineral oil of high contact pressure is a large increase in the viscosity of the lubricant. This viscosity increase combined with the elasticity of the metal causes the oil film to act like a thin solid film, thus preventing metal-to- metal contact.
Full fluid-film lubrication can be illustrated by reference to the conditions existing in a properly designed plain bearing. If the two bearing surfaces can be separated completely by a fluid film, frictional wear of the surface is virtually eliminated. Resistance to motion will be reduced to a level governed largely by the viscosity of the lubricating fluid.
To generate a lubricating film within a bearing, the opposed surfaces must be forced apart by pressure generated within the fluid film. One way is to introduce the fluid under sufficient pressure at the point of maximum loading, but this hydrostatic method, although equally effective at all speeds, needs considerable power and is consequently to be avoided whenever a satisfactory alternative exists.
DATA ACQUISITION SYSTEMS
The products and/or processes used to collect information to document or analyze some phenomenon are called as Data acquisition systems. Recording of the temperature of an oven on a piece of paper by an expert is said to be performing data acquisition. This type of process has been simplified and made more accurate, versatile, and reliable through electronic apparatus now-a-days as the technology improved. The apparatus facilities form simple recorders to sophisticated computer systems. Data acquisition products tying together a wide variety of products such as sensors that indicate temperature, flow, level, or pressure and also used as a focal point in a system. (Engineering, 2009)
TYPES OF DATA ACQUISITION SYSTEMS
Figure : Data Acquisition System (Engineering, 2009)
Wireless Data Acquisition Systems
Wireless data acquisition systems can be less costly and time consuming is also less. Data back to a wireless receiver connected to a remote computer are sending through one or more wireless transmitter. To find the ambient temperature and relative humidity, thermocouples, RTDs, pulse output sensors, 4 to 20 mA transmitters and voltage output transducers wireless transmitters are used. Receivers can be linked to the USB or Ethernet port on the PC. (Engineering, 2009)
UWTC Wireless Thermocouple Connector System
Figure : Wireless Data Acquisition Systems (Engineering, 2009)
Serial Communication Data Acquisition systems
When the measurement needs to be made at a location which is distant from the computer serial communication data acquisition systems are a good choice. RS232 is the most common but only supports transmission distances up to 50 feet. RS485 is superior to RS485 and supports transmission distances to 5,000 feet these are the different communication standards.
iDRX Series Serial Port Data Acquisition System
Figure : Serial Communication Data Acquisition systems (Engineering, 2009)
USB data acquisition systems
PCs are connected to the peripheral devices such as printers, monitors, modems and data acquisition devices using Universal Serial Bus (USB). USB is ideal for data acquisition applications and also the advantages using USB are conventional serial and parallel connections, including higher bandwidth (upto 12 Mega bytes/s) and these have the ability to provide power to the peripheral device. To connect data acquisition device to the PC only one cable is enough from USB connections supply power and these have at least one USB port. (Engineering, 2009)
OMB-DAQ-3000 1-MHz, 16-Bit USB Data Acquisition Modules
Figure : USB type Data Acquisition System (Engineering, 2009)
Data Acquisition Plug-in Boards
Computer data acquisition boards plug directly into the computer bus. Since they are connected directly to the bus the boards are speed and cost because the expense of packaging and power is provided by the computer. Because of the number and type of inputs (voltage, thermocouple, on/off), outputs, speed and other functions provided the characteristics supplied by the cards can vary and also the boards offered are basically for IBM PC and also for compatible computers. Each board installed in the computer is addressed at a unique input/ output map location. The processor uses to gain access to the specific device as required by its program.
Computer data acquisition board's plug directly into the computer bus the I/O map in the computer provides the address locations. (Engineering, 2009)
OME-PCI-1002 data Acquisition Plug-in Board
Figure : Data Acquisition using Plug-in Boards (Engineering, 2009)
For the pressure measuring instruments attained demand with the steam age. For the first time the pressure instruments are Bourdon tubes or bellows, where mechanical displacements were moved to an indicating pointer and these are still in use today.
Figure : Pressure sensors
Converting pressure into an electrical quantity this technology is termed as pressure metrology. These resistive values changes when the pressure induced strain. In general a diaphragm construction is used with strain gauges either bonded to, or diffused into it, acting as resistive elements.
Under pressure-induced displacement the capacitor changes its value when the pressure diaphragm is one plate in capacitive technology
Figure : Pressure sensing using the difference in pressure
The difference in pressure of the two sides of the diaphragm by applying the pressure sensing using diaphragm technology. An ABSOLUTE term is used when it is depending upon the relevant pressure. When the reference is vacuum, GAUGE, where the reference is atmospheric pressure, or DIFFERENTIAL, to measure two different pressures the sensor has two ports.
Coolant must be delivered from the pump through the valves, pipes and finally via nozzle and the necessary pressure to adjust with the velocity of the wheel and in a laminar flow. Any kind of turbulence or the entrained air will create dispersion causing a rapid loss of momentum and preventing the coolant from overcoming the momentum of air generated by wheel drag-the so called air barrier. Turbulence is a common problem governed by its Reynolds number which is given by
Re= v. d/ ® . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Eq. 1
Where, v = velocity of the fluid,
d = jet or the pipe diameter
® = kinematic viscosity of the fluid. (Ioan D. Marinescu, 2007)
Cooling and lubrication are the commonly referred actions of the cutting fluid in metal cutting tribology. Lubrication is most intensively tested using various tests. While the cooling did not much attract besides the perception of the testers that the water soluble cutting fluids possess higher cooling capability than the waterless ones. There are no characteristics of the cooling
Basically all the mechanical energy associated with the chip formation gets converted into the thermal energy. The heat balance equation is of prime concern in metal cutting process. According to the energy flow in the machining operation, the equation can be given as
Pc = Fzv = Q“ = Qch + Qw + Qct . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Eq.
Pc is the cutting power,
Fz is the power component of the cutting force,
v is the cutting speed,
Q“ is the total thermal energy generated in the cutting process,
Qch is the thermal energy transported by the chip,
Qw is the thermal energy conducted into the work piece and
Qct is the thermal energy conducted into the tool.
Experimentally it is proved that most of the thermal energy which is generated during the cutting operation is carried away by the moving chip. At the higher cutting speeds, almost 80 -85 % of the thermal energy is carried away by the moving chip. In equal amounts, the total thermal energy generated is conducted into the work piece and into the tool. This proportion varies greatly with the cutting speed. Thus increase in speed results in greater portion of the thermal energy is conducted into the tool.
Much of the thermal energy generated during the cutting process is carried away by the moving chip. But it can be said that the chip temperature is higher than the tool temperature generally which is much lower. This is because of the reason that the chip moves and results in mass transfer. So that the elementary volume of the moving chip is not exposed to high contact temperatures for adequate time to increase the chip temperature significantly. It happens the same with the work piece as the tool moves on the surface spreading the thus generated thermal energy over all machined surface.
On the other hand, the tool contact surfaces such as tool -work piece and tool-chip interfaces do not move. Because of which, the tool contact temperatures are generally normal than those of the chip and the work piece.
Some of the observations of the cooling action of a cutting fluid are as follows: -
Slightly reduces the cutting temperature.
Reduces the radius of curvature of the chip
Improves the accuracy of machining
Reduces the tool-chip contact length
Increases thermal shock in interrupted cuts.
COOLANT SUPPLY- NOZZLE TYPES
It is not only the coolant type and composition, but the effectiveness of the supply of coolant to the contact zone by nozzles and diffusers that governs the efficiency of the cooling and lubrication in respect of work piece quality. It is critically important that the coolant is forced into the contact zone.
To achieve the highest possible cooling and lubrication effect, several nozzles have been developed for different grinding applications. Figure shows some examples of the coolant supply strategies. (Friction, Cooling and Lubrication in Grinding, 1999)
Figure : Applying fluid using different type of nozzle and their effectiveness
(Friction, Cooling and Lubrication in Grinding, 1999)
In grinding, the free jet nozzles as well as the shoe nozzle, both belonging to the group of flooding nozzles, are most common. During supply with a conventional flooding free jet nozzle or with a spot jet nozzle the coolant jet is pointed tangentially at the contact zone. The grinding wheel is wetted with coolant which is dragged into the contact zone. Webster carried out experiments for optimal design of free jet nozzles. He suggested a concave instead of a convex form for the nozzles, because the concave form, similar to special elements in front of the nozzle that homogenize the flow, produces a longer coherent jet.
Sometimes, to ensure better wetting, 'scrapers' are employed. These reduce the air flow at the wheel circumference (the so-called 'air barrier'), so that the grinding wheel surface can be more effectively wetted with coolant. Ebbrell and Rowe simulated by means of finite element calculations the air flow near the grinding wheel shortly before entering the contact zone, and then went on to confirm the calculations by Laser- Doppler-Anemometer-measurements. The actual influence of the air barrier on coolant supply in the grinding arc is not clear. It plays an only minor role. In contrast to this, in its influence - especially in high-speed grinding - is heavily emphasized. Masaki specified in that the air barrier is generated through two major air flows around the wheel surface: Air flow in the circumferential direction and air spouting in the normal direction to the wheel surface induced by centrifugal force of wheel rotation. These two air flows are merged into an air barrier along the ground groove thus hindering effective coolant entry. The influence of the air barrier is clearly pictured in Rowe's photo. Because of the air barrier, the coolant is 'dammed up' in front of the grinding wheel and not carried into the grinding arc. (Friction, Cooling and Lubrication in Grinding, 1999)
Figure : Coolant supply and air barrier in grinding 
(Friction, Cooling and Lubrication in Grinding, 1999)
The obvious condition for the coolant to penetrate the air barrier and consequently reach the contact zone is that the momentum of the supplied coolant overcomes the momentum of the air barrier along the ground groove. Inasaki 1993, calculated this critical limit for coolant penetration.
Because of high costs and the danger of mist development, high coolant pressures and resulting high jet velocities for penetration of the air barrier cannot always be used. Therefore, methods to reduce the air barrier effect are being researched. One simple method for diversion of the air barrier is the use of a scraper, but this must be continually adjusted, so that its use is questionable. Ott 1991, tried to get around this problem by using a two-component nozzle, in which the air barrier is diverted by a second cooling lubricant jet, perpendicular to the grinding wheel.
Elimination or reduction of the air barrier is the subject of a large number of patents and papers open to public inspection. The patented appliances extend from scraper systems attached to the grinding wheel to air barrier suction systems.
A special form of free jet nozzle is the spot jet nozzle which is defined by a small nozzle outlet cross section and therefore (at the same flow rate) higher jet velocities. Spot jet nozzles provide a much higher coolant ram pressure in front of the grinding arc, so that coolant, due to the drag effect of the grinding wheel, is drawn into the grinding arc. The special jet geometry of the spot jet nozzles enables the nozzle to be placed further from the contact zone. Another advantage is the much more economical use of coolant. In a coolant supply with a spot jet nozzle at very high jet velocities of up to 365 m/s and maximum pressures at the nozzle of up to 3800 bar is described and compared to a conventional flooding nozzle.
In free jet nozzles with different external coolant guiding systems are compared. Here, as well as photographically shown in and confirmed in, the jet shows a good adhesion tendency to the grinding wheel when fed to it tangentially. Ott 1991 suggests, for free jet nozzle use, coolant jet velocities VCL of 0.6 to 1 .O times v, (v, = cutting speed) to achieve good jet adhesion to the grinding wheel. Additionally, it is recommended to achieve a laminar flow. It could be shown that laminar flows at the current jet velocities are not possible, and that turbulence and air swirls must be taken into account. Kdnig (et al.) found that at high coolant pressures, contractions within the free jet nozzle cause a significant pressure decrease which negatively influences the cooling and cleaning effect. Experiments show that the elimination of sudden cross-section changes leads to an improvement of surface roughnesses by 20%.
The use of a shoe nozzle (or diffuser) leads to wetting of the grinding wheel directly in front of the contact zone over a comparably large area. In addition, when using porous wheels, the coolant can penetrate the grinding wheel; the porous layer at the circumference fills with coolant. By the rotation of the wheel and the resulting centrifugal force to the coolant in the porous layer, a part of this coolant leads to cooling and lubrication in the contact zone, providing another transport mechanism for the coolant into the contact zone, in addition to the drag effect by the grinding wheel rotation.
Adaptation of nozzle geometry to the grinding wheel profile is a possible further way to optimize the coolant system. Noichl 1992 showed in his investigations that an adjustment of nozzle geometry to the grinding wheel, with an opening cross section dimensioned according to flow rate, can lead to a significant increase of the material removal rate. The modification of nozzle geometry, for instance a permanent adjustment of the nozzle geometry to the decreasing grinding wheel diameter is also the subject of several patents and papers open to public inspection. K6nig (et al.) showed in experiments for dressing, that the use of a ring nozzle, enclosing a non-rotating dresser, instead of a conventional free jet nozzle, leads to a significant decrease of dresser wear.
In contrast to external supply of coolant by nozzles, some studies address strategies for 'internal' supply through the grinding tool. Here Graham distinguishes between radial holes in the grinding spindle and side holes in the grinding wheel flange. Another form of 'internal' supply is radial supply from outside via a shoe nozzle, by which the porous layer of the grinding wheel is filled with coolant. This particular supply strategy presupposes a porous wheel structure. For nonporous grinding wheels such as electro-plated or vitrified bonded CBN wheels, radial holes in the grinding wheel basic body can provide an 'internal' supply of coolant. In this context, slots in the wheel circumference is another possible way to increase cooling and lubrication effects in the grinding arc - although in this was shown in combination with an external supply nozzle. The 'internal' supply strategies can provide economic benefits, because of lower coolant supply.
Several investigations deal with the comparison of different coolant nozzle types. Often, important characteristic data such as the coolant flow rate, the pressure directly in front of the nozzle or the outlet cross section are not reported, so that comparability of results is not always possible. But the following points can be made:
In comparison to free jet nozzles, shoe nozzles lead to lower tool wear and less burning with generally lower coolant flow rates and lower pressures. A disadvantage is that shoe nozzles are difficult to adjust and must normally be readjusted to the grinding wheel.
With internal coolant supply strategies, the coolant flow rate can be reduced in comparison to external flooding, leading to a similar process quality.
A combination of internal and external supply leads to an enhancement of quality and economy.
In contrast to flooding nozzles, nozzles for minimum quantity lubrication (MQL), that ensure drastically reduced coolant flow rates, are the subject of recent research projects. Currently, these coolant supplies are mainly used for machining processes with a geometrically defined cutting edge in the form of spot jet or spray mist nozzles and are also being examined for processes in the area of grinding technology.
COOLANT FLOW RATE AND NOZZLE POSITION
Many papers give advice on the necessary coolant flow rates and dimensions of the flood coolant appliance, depending, for example, on the grinding power available. Taking a closer look, it becomes clear that increasing machining efficiency demands are often met by an over-supply of coolant, instead of by an optimized coolant supply. This is especially the case when free jet nozzles are used.
Okuyama 1993 and Engineer 1992 report on the effect of coolant supply depending on different influencing parameters, such as nozzle output flow rate. In the influence of different parameter variations is examined by measuring the heat transfer coefficient close to the contact zone. It can be shown that increasing coolant jet velocity can lead to a digressive incline of the heat transfer coefficient and therefore to a lower cooling efficiency. This is due to a geometrical limitation of the flow rate through the grinding arc. Vits confirmed that the contact zone limits the coolant flow rate and that in result the depth of the heat influenced subsurface layer of the work piece is also limited. Engineer describes similar examinations by measurement of flow rates through the grinding arc. He analyzed for example the effects of work speed, of supplied coolant flow rate and nozzle position. In confirmation of other work, he found that above a certain flow rate, saturation takes place. Accordingly, excess coolant is rejected resulting in a reduction of usefully delivered coolant.
Most studies indicate that an increase of coolant flow rate with otherwise identical process parameters in flooding, gives enhanced surface quality of the work piece. This means both reduced thermal subsurface damage and lower roughness and tool wear. In addition K6nig (eta/.) found a reduction of non-rotating dresser wear by use of increased coolant flow rate during preparation of the grinding tool. Until now, the effect of increase of coolant flow rate on residual stress in the work piece has hardly been addressed. Vansevenant and Treffert carried out experiments on this. Both noticed that at low coolant flow rates (0.1 to 0.8 I/ (min x mm)), residual stresses at the work piece surface decrease with increasing flow rate. Czenkusch investigated the effect of coolant flow rate and nozzle cross section on residual stresses. Although nozzles with the smallest cross section give the lowest flow rates, they achieve low tensile residual stresses because higher jet velocity provides better coolant penetration into the grinding wheel pores so that more coolant is carried in the grinding arc.
The process perpendicular force usually increases with increasing coolant flow rates at constant nozzle output cross section. This is because of pressure building hydro dynamical effects in the area of the contact zone. Increase in process tangential force and therefore of spindle power, is related to higher flow rate. Enhanced spindle power at higher flow rates is a result of the necessary acceleration of the coolant by grinding wheel rotation and the related drag effect into the contact zone. For this reason, this part of power is similar to the no-load power of the grinding spindle (idle grinding) and is therefore lost power. This power loss can amount to 80% of the total power, and increases with increasing grinding wheel circumferential speed. Minke 1993 were able to show that at constant flow rate and increasing circumferential speed, process perpendicular forces, caused by hydro dynamical effects, do not constantly rise, but reach a maximum and then decrease with further increase of the circumferential speed. The position of the maximum depends on the flow rate, and increases with higher circumferential speeds at higher flow rates.
As has already been stated, use of minimum quantity lubrication during grinding is a subject of current research. Here, 500 to 20000 times lower coolant flow rates in respect to flooding coolant supply can be employed. Brunner 1995 showed that with 4mVmin ester oil, as compared to 11Wmin mineral oil, during grinding of 16 Mn Cr 5 (SAE - 51 15) with micro-crystalline aluminium oxide reduces process perpendicular and tangential forces to one third, but increases grinding wheel wear and surface roughness by 50%. Investigations by Brinksrneier, Brockhoff 1995 and Walter confirmed these results and showed in addition that the type of coolant used during MQL (ester oil or emulsion) can considerably influence the process result. KIocke and Beck 1997 also corroborated the influence of coolant composition on the process result when using MQL. These studies indicate that, in grinding, MQL can be used only for fine grinding, because of the reduced cooling and lubrication effect. Otherwise, there is a danger of thermal subsurface damage. The same applies to dry grinding of hardened steels.
One mainly dry grinding process that makes use of the heat generated in grinding is grind-hardening, developed by Brinksrneier and Bronckhoff 1995. This method utilizes the developed process heat for a short-term surface layer heat treatment, based on the martensitic transformation of the material structure and the associated increase of hardness in the surface layer. The grind-hardening process is characterized by comparably high process forces, as they are known from other dry grinding processes, as well as high tool wear and high surface roughness's, making necessary a finishing operation in a second process step.
Not only coolant flow rate, but also the position of the nozzle plays a decisive role with respect to efficiency of the coolant system, especially when using free jet nozzles. Engineer 1992 showed that the nozzle position has a significant influence on the useful part of supplied coolant flow rate. Corresponding to Okuyama, Mindek and Webster 1994, he noticed that the nozzle should be positioned as close to the contact zone as possible to ensure optimum use of the coolant. Mindek and Webster 1994 used a test stand to examine the optimal nozzle position and were thus able to simulate the cooling effect of different coolant system configurations in the grinding arc. In addition to the distance between nozzle and grinding arc, orientation of the jet in relation to the grinding wheel is important. Vits and Ott recommended that the jet should flow in a tangential direction to the grinding wheel. The free jet, however, should not be directed exactly at the grinding arc, but should hit the grinding wheel tangentially, at approximately 10" to 25" in front of the grinding arc. These results were confirmed by Brucher. 1995
HIGH PRESSURE COOLING
Lauer-Schmaltz 1979 focused on the mechanisms of loading of conventional grinding wheels. He differentiated between 'snarl' chips, 'grain covering' and 'layer' chips and showed that changes of the cutting edge geometry that influence the grinding process significantly, are mainly caused by layer chips. To reduce loading of conventional grinding wheels, high pressure cleaning of grinding wheels with coolant jets can be employed. On the one hand, increasing the coolant pressure with optimal nozzle geometry can help to decrease the work piece surface roughness, whereas, using several cleaning nozzles reduces grinding wheel wear by more than 40%. In addition, Spur 1995 reported an increase in material removal limit by 400% during screw thread plunge grinding with high pressure cleaning nozzles, employed at 25 bar and 90 I/ min, in comparison to conventional cooling with 5 bar and 60 I/ min. The criterion he used was the appearance of burning.
In similar tests are described. Here, one coolant nozzle and one cleaning nozzle, each impinging with a pressure of up to 10 bars were used for generative grinding. Removing chips loaded in the grinding wheel provided a damage-free tooth flank surface and higher material removal rates.
Off also described high pressure cleaning of conventional grinding wheels for removing loaded chips. Cleaning nozzles attached in tangential or radial direction to the grinding wheel, have to be adapted to the grinding wheel profile. Whereas the radial arrangement of cleaning nozzle requires pressures of more than 8 bar, a tangential arrangement with anti rotation permits lower pressures. Grabner 1987 noticed that the specific material removal rate during internal grinding can be increased by 50% by cooling the grinding wheel at 5 bars instead of 2 bars. At the same time, grinding wheel wear and residual stresses at the work piece surface are significantly reduced. Tawakoli 1990 reached an optimal result using a flow rate of 60 I/ min during high power grinding with a coolant cleaning pressure of 15 to 20 bars. The nozzles had a diameter of 0.8 to 1.0 mm and were attached perpendicularly (radial) to the working circumferential grinding wheel surface. In [Ill standard values of cleaning pressure and flow rate for high power grinding are given with 60 to 90 bars and 60 to 100 I/ min, respectively.
Detailed quantitative relationships between grinding process parameters and necessary adjustment conditions in high pressure wheel cleaning are not yet established. Kovacevic 1995 reported on the use of a free jet high pressure coolant supply. He used a free jet nozzle with a diameter of 0.46 mm, from which a water jet was emitted at a maximum pressure of 380 MPa (=3800 bar), with a velocity of 365 m/ s and thus a flow rate of only 3.6 I/ min. In comparison to conventional flooding coolant supply, process forces decreased by 25% and surface roughness decreased by 50%.
Grinding, in most operations, takes place above the boiling temperature of the fluid. The fluid still plays an important lubricating role but cooling capability is greatly reduced (W B Rowe, 2003). In creep grinding, continuous dressing is sometimes employed to prevent 'burn-out'. A recent discovery excited the scientific community. Extremely high cooling can take place within the grinding arc of contact with high-efficiency fluid application (W B Rowe, 2003). It was previously thought that fluid convection within the grinding contact zone was negligible since usually no cooling effects were measured under the normal burn-out condition. It has been found that convection cooling within the contact arc is dramatically more efficient in reducing grinding temperatures than bulk cooling outside the contact arc and the extremely high convection coefficients apply as long as the flow-rate is 'useful'. This discovery implies a new approach to fluid delivery. It is required to measure and investigate the achievement of useful flow rate and correlate useful flow rate with grinding performance. The initial finding was for extremely high-removal rates - two orders of magnitude greater than conventional grinding processes. Further work is needed to explore potential gains to be achieved in improving productivity in high-precision high-removal rate grinding operations.