Waste Streams in Machining
Introduction:

During the manufacturing of a product any material or excess energy generated in addition to the final product can be termed as waste. The waste generated due to machining is a major environmental concern for manufacturers. The form and state of waste streams generated, and their transportation mechanisms differ with the process used and also vary within the process. The impact due to each waste stream differs too.

Four different waste streams have been considered:
 

Figure 1 is a schematic representation of a machining process. A machining process involves the processing of material to produce a finished or a semi-finished product. This is done using tools, accessories, machines and other inputs that are pertinent to the process. The processes considered for purpose of this work involve machining of material using tools and fluids to produce parts and products. The output of the process includes the product and the waste streams. The waste streams consist of material in the form of
 
Fig1: Waste Streams in Traditional Machining


Material Stream:

The manufacturing section of the product life cycle begins when raw material enters the manufacturing system to be converted to a product. Loss of virgin material during production has a significant environmental impact in terms of
 

It is important to point out at here that most forms of wastes are difficult to control. The chips generated in machining constitute one such material stream. Material losses during machining occur due to chip formation. Since chip formation is an inherent characteristic of machining processes, the generation of chips is unavoidable.
However the physical form of the material waste stream can be controlled.

Chip morphology is determined by the physical properties of the work material and the machining conditions. Handling and disposal of the chips is dependent on their shape, size and texture.
Chip shape influences the amount of cutting fluid carried away.
It is more economically viable to recycle large volumes of chips rather than small volumes.
Another constraint in recycling is the contamination of the stream due to the presence of dissimilar materials.

Continuous chips are formed while cutting ductile metals such as iron, steels, copper and aluminum.
Shearing is the primary phenomenon involved in continuous chip formation.
Chip formation takes place in the zone extending from the cutting edge to the junction of the work and tool surfaces. This is called the primary deformation zone.
Forces transmitted to the chips at the tool-chip interface are sufficient to slide the chips into the secondary deformation zone (along the tool face). At low cutting speeds, due to the increased friction between chip and tool, the chips may weld to the tool. This phenomenon is known as built up edge (BUE) formation. The BUE continues to grow and breaks down when it becomes unstable. The broken pieces are carried away by the chips and also distributed over new work surface causing poor surface finish.

 In case of brittle materials the fracture occurs in the primary deformation zone. This is due to severe strain in the material. Segmented chips thus formed are called discontinuous chips. Machining of brittle materials such as cast iron, cast brass and machining of ductile materials at extremely low speeds and high feeds causes formation of discontinuous chips.

 As discussed above characterization of the material stream involves characterization of chips. There are no established models to do this. For the purpose of this study the environmental impact due to material loss has been quantified in terms of chip characteristics such as material removal rate, chip width, and chip thickness.

 Material Removal Rate:

 The amount of material removed and the rate at which it is removed is discussed in detail for the relevant machining operations. Material removal rate is related to machining time which in turn is constrained by the production rate, and indicates the rate at which material is being lost or disposed.

Chip Width:

 Chip width is another chip characteristic that may used to understand the material stream. For simplicity, the chip width is approximated to be equal to the depth of cut

where kr is the lead angle and is given by the side cutting edge angle in straight turning. It is equal to the end cutting edge angle in boring operations.


 Fig: Turning Operation 
In case of milling operations the uncut chip thickness [Boothroyd, 1989] is specific to each type of operation. The uncut chip thickness for slab milling is given by Equation and Equation for face milling and end milling. In the case of drilling the feed engagement (af) is defined in terms of the feed per tooth. The uncut chip thickness is given by
equation.

 Energy Stream:
Energy is a basic concept familiar to all science and engineering disciplines. Energy can neither be created nor destroyed - it only changes from one form to another. With depleting sources, energy conservation is a major issue today. Machine tools which perform the metal cutting operation are driven by motors which in turn require some source of energy.

The power required for the cutting operation is given by Equation:

where Fc is the cutting force and V is the cutting speed.

Specific energy (unit power) is used to compute the amount of energy required for the machining operation. Unit power is defined as the amount of power required to machine a unit volume of material. The rate of energy consumption (power) is thus given by:

where ps is the specific cutting energy or the unit power and mrr is the material removal rate. The average value of ps is obtained from Machining Data Handbook [American Society of Metals, 1989] and for a given operation is dependent on the work and tool material type. The energy consumption has an impact on the heat generated. Heat generated and energy usage are major environmental concerns. These issues are dealt with detail in subsequent sections.


Fig: Heat Generation in Metal Cutting


 

High temperatures are generated during metal cutting. These have an influence on the performance of the process in terms of the tool wear rate and surface finish. For the purpose of this work it is assumed that the heat generated, H, during machining is equal to the energy consumed, E, as given in equation:

According to Boothroyd [1989], Equation is valid as elastic deformation forms only a small portion of the total deformation. In case of elastic deformation the energy required is stored as strain energy and no heat is generated. On the other hand in case of plastic deformation most of the energy is converted to heat. The rate of heat generated is thus obtained from the rate of energy consumed as given by Equation.

The conversion of energy into heat occurs in two regions: the primary deformation zone or the shear zone and secondary deformation zone or the friction zone. The rate of heat generated is approximated as the sum of the rate of heat generation in the primary (Ps) and secondary (Pf) zones.

where Pf is given by Equation.
 

Ff is the frictional force and Vf is the velocity of the chip flow given by Equation.
 

where rc is the cutting ratio defined as the ratio of uncut chip thickness to the actual chip thickness.

The rate of heat generated in the shear zone can be determined from the relationship given in equation
 when the total rate of energy consumption and the rate of heat generation in the friction zone are known.

Heat Generation in Machining:

The heat generated during machining causes a change in the temperature of the workpiece, fluid used and the tool. The heat generated is dissipated to the workpiece (Fw) tool (Ft) and the chip(Fc).
 

Virtually all of the heat generated in machining is dissipated to the chip and the workpiece. The rate of heat flow to the tool forms a small portion of the total rate of heat flow and is neglected.

Of the total heat generated in the shear zone (Ps), a fraction of it, G, is conducted into the workpiece. The rest is transferred to the chips.
 

 The fraction of the heat transfer to the workpiece, G, is a function of the processing conditions.

 The chip also flows through the friction zone and hence the maximum temperature of the chip occurs when the material leaves the secondary deformation zone. The temperature at this point is given by the following relationship.

 Cutting Fluid Heat Transfer
 One important energy issue considered is the heat transfer between the fluid and the workpiece. The amount or rate of heat transfer is a direct measure of cutting fluid performance. Cutting fluid heat transfer in machining operations depends on the nature of cutting fluid used and the fluid application strategy.

Heat transfer between the fluid and the workpiece is assumed to be primarily due to convection.
 
 

 The convective heat transfer coefficient is a function of the Nusselt (Nu) number which in turn is a function of the Reynolds number (Re), the Prandtl number (Pr), and the geometry. For instance, consider the case of a turning operation with flood application of cutting fluid. The correlation of the Nusselt number with the Prandtl number is given by the following equation:

where Rer and Ret are the two Reynolds numbers that take into account the effect of the rotation of the workpiece and the transverse flow of the fluid respectively.

 The convective heat transfer coefficient of the fluid, h, is given by the following relationship.

where m and kf are the fluid properties as defined earlier and D is the diameter of the workpiece.

Once the Prandtl number and the Reynolds numbers are known, the Nusselt number and the convective heat transfer coefficient of the cutting fluid can be determined. Consider the turning of a workpiece of 100 mm diameter. The workpiece is being rotated at 1000 rpm. The cutting fluid used is a water soluble oil (10% concentration) with a 20 liter/min flow rate. The Prandtl number is found to be 6.63 and the Nusselt number as given by Equation (3.27) is 1653.70. The convective heat transfer coefficient is found using Equation (3.28) and is equal to 10137 W/m2K.

 Cutting Fluids Waste Stream:
 Metalworking fluids are engineering materials that are employed in metal working processes [Byers, 1994]. In manufacturing activities metalworking fluids used for material removal processes are known as cutting and grinding fluids.

 Types of Cutting Fluids:
A variety of cutting fluids are available to satisfy the requirements of machining processes. Although there is no-all purpose cutting fluid, some offer considerable versatility while some are for specific applications. Preferred fluids have long life and require very little changing or modification. The four basic types of cutting fluids are as follows:

Straight Oils:
A straight oil is a petroleum or vegetable oil that is used without dilution with water. Soluble oils may or may not be compounded with additives. Paraffin oils, napthenic oils, vegetable oils are some examples. It is claimed that straight oils provide excellent lubrication. For environmentally favorable requirements, vegetable oils are preferable due to their ease of biodegradation and disposal.

Soluble Oils:
 Soluble oils or emulsifiable oils are the largest type of cutting fluids used in machining operations. These are composed of an oil added with emulsifiers and additives and is diluted with water. Dilution levels vary from 1 - 20% with 5% being the most common dilution level. Emulsifiers are chemical substances that cause suspension of oil droplets in water to make a translucent solution in water. The predominant emulsifier is sodium sulfonate, which is used with fatty acid soaps, esters, and coupling agents that provide a white emulsion with no oil or cream separating out after mixing with water.

Synthetic Fluids:
 Synthetic fluids are water based fluids and contain no mineral oil. They have a typical particulate size of 0.003mm. Water provides excellent cooling properties. However this results in a compromise on the lubricity and also causes corrosion. To inhibit rust formation multiple rust inhibitors are added. Synthetic fluids are widely used in applications that require long tank life and modest lubrication

Semi-Synthetic Fluids
Semi-Synthetic fluids are similar to soluble oils in that they are emulsions and to synthetic fluids in that they are water based fluids. About 5 - 20% mineral oil is emulsified with water to produce a microemulsion. The particle size is in the range of 0.1 to 0.01 mm. This is small enough to transmit all incident light. These types of fluids are preferred largely due to their advantages of both soluble oils and synthetics. Some major advantages are rapid heat dissipation, cleanliness of the system, resistance to rancidity and bioresistance (due to the small particle size).

 Functions of Cutting Fluids
The role of cutting fluids in machining operations has been questioned by many researchers. It is claimed that cutting fluids play many roles in metal machining. This section provides an insight into these claims and reasons behind the use of the cutting fluids. When properly applied cutting fluids are said to increase productivity and quality and reduce costs by making it possible to apply higher cutting speeds, higher feed rates, and greater depth of cuts.

 Depending upon the machining operation being performed a cutting fluid has one or more of the following claimed functions:
cooling the tool, workpiece and chip,
lubricating (reducing friction and minimizing the erosion of the tool),
controlling the built-up-edge (BUE) formation on the tool,
flushing away chips, and
providing corrosive resistance to the workpiece.
The relative importance of each function depends upon the work material, the tool material, the cutting conditions, and the finish required. The selection of the fluid is governed by these functional requirements. Figure 3.4 outlines the various decisions and criteria involved in cutting fluid selection.



 
 
 Fig:  Cutting Fluid Selection Criteria

 
Environmental Issues of Cutting Fluids:
The advantages of the use of cutting fluids are offset by the associated environmental, health and safety consequences. The waste streams emanating from the use of cutting fluids have an impact on the environment and on the operator. Fumes, odor, and smoke from the fluid result in both dermal and eye irritation. Some of the waste streams are also toxic and carcinogenic.

 Mist is one of the waste streams generated from cutting fluids. Cutting fluids vaporize due to the heat generated during machining. The vapors subsequently condense to form mist droplets. The non-aqueous components of the cutting fluid, such as biocides end up as fine aerosol. Cutting fluid mists are also formed by atomization mechanisms. As a cutting fluid stream impacts a stationary surface, the fluid may splash to form a mist (converting mechanical energy in the stream to droplet surface energy). The drops formed by either vaporization/condensation/atomization are under the influence of buoyancy force, aerodynamic drag force and gravity force.

 The mist formed, if not collected, remains suspended in air the and finally settles on a large area of the work floor. Most industries employ collection systems to ensure that the workers have minimum exposure to the fluid mist. This is also due to the regulations laid down by the Occupational Safety and Health Administration (OSHA). One such standard is the 5 mg/cubic meter specification for airborne particles (largely due to fluid mist). However the mist collectors and machine enclosures are expensive to install and maintain.

Mist models developed [Yue et al., 1996] so far assume atomization to be the main cause of fluid mist formation. The droplet size is dependent on the type of the fluid application and also on the state of motion of the workpiece.

For situations in which a cutting fluid interacts with a rotating component, the surface energy required to form mist droplets is provided by the rotating component. An empirical relationship gives the values for the mean drop size [Yue et al., 1996].
 

where N is the rotational frequency,
Q is the flow rate,
R is the radius of the workpiece,
r is the density of the cutting fluid,
is the surface tension, and
m is the viscosity of the cutting fluid used.

Tooling Stream:
Most manufacturing processes use some form of cutting tools and dies. These processes often require regular replacement of tools. At the same time there are processes that involve minimal or no tool losses. Most joining processes are of this type.

Worn tools contribute significantly to the waste streams in the form of wear particles and a worn tool at the end of its useful life. The wear particles usually are carried away by the fluid. From an environmental perspective the tools remaining at the end of the tool life are of importance as they are often disposed off and hence are a burden to the environment. It is thus required that the tools have a long tool life so that the number of tools expended is minimized.

Tool life is one of the most important economic considerations in metal cutting. High grinding and replacement costs act as restrictions to selection of process parameters that lead to shorter tool life. On the other hand the use of low feeds and speeds result in improved tool life but also reduce the production rate. Tool failure is usually classified into two types [Lindbeck et al., 1990]:

1. Slow-death: The gradual or progressive wearing away of rake face (crater wear) or flank (flank wear) of the cutting tool or both, and
2. Sudden-death: Failures leading to premature end of the tool

 The sudden-death type of tool failure is difficult to predict. Tool failure mechanisms include plastic deformation, brittle fracture, fatigue fracture or edge chipping. However it is difficult to predict which of these processes will dominate and when tool failure will occur.

 As the tool wears its geometry changes, thus influencing the process performance in terms of the forces, energy consumed, and surface finish. Tool wear in machining operations occurs by adhesion, abrasion and diffusion wear. The wear process is to a large extent dependent on the cutting temperatures. Abrasion is the common phenomenon in the low temperature regime. At higher cutting speeds, that result in higher temperatures, the chemical solubility of the material determines the wear rate. At still higher cutting temperatures diffusion wear occurs.

 In 1907, F. W. Taylor published his now famous tool life equation wherein the tool life was related to the cutting speed (V) and the feed rate (f). The equation had the form

which over the years took the more widely published form,
 

 For analysis the time required for the flank wear to reach a pre-specified level is defined as the tool life. Experimentally and analytically developed tool life models can be found in literature. The following equation is used to model the tool life for this research work.

where the tool life is expressed in minutes
 V is the cutting speed expressed in feet per minute,
 f is the feed rate in inches per revolution, and
 A, n1, n2 are constants.

 Summary:
 This chapter presented the models, approaches, and measures used to represent the environmental impact of machining operations. Four waste streams: Material, Energy, Fluid and Tooling were identified. The issues related to each of them have been discussed. The material removal rate, chip width, and chip thickness are the measures used to provide an estimation and understanding of the material waste stream. Energy issues were dealt with from several perspectives. The rate of energy consumption, heat generation and the heat transfer to the workpiece, chip and cutting fluid are the issues taken into account. Great concern is associated with the use of cutting fluids. While characterizing the impact of the cutting fluids, a background to cutting fluids has been provided. The information includes the classification of the fluids, their functions, and selection criteria. Mist generation is one of the major health hazards associated with the use of fluids. The mist size is an indication of the harmful effect. Models that have been recently developed have been incorporated. Empirical models used to determine tool wear and thereby the rate of tool disposal have been presented. With the identification of the waste stream characterization models, the next step is to develop an operation planning methodology to support environmentally conscious decision making.