power and heat generation in machining operations (Part I)

ABSTRACT

Power and heat generation in machining operations in machining is very crucial and its importance cannot be overemphasized. This paper seeks to critically examine and explain in detail the generation of heat as well as power in machining operations which include turning, milling, drilling, grinding, planning and shaping. Even though these different machining processes differ in their mode of power and heat generation, they still possess the same fundamentals and basics.

This work goes a long way in carefully analyzing the steps and methods involved in heat generation during machining. It shows how heat affects tool wear, tool life and thermal deformation, amongst other environmental problems. It's hard to predict the intensity and distribution of the heat sources in any machining operation. Especially, because the properties of materials used in machining vary with temperature, the mechanical process and the thermal dynamic process are tightly coupled together. Since early this century, many efforts in theoretical analyses and experiments have been made to understand this phenomenon, but many problems are still remaining unsolved. This write up tries as much as feasibly possible to get a handle on this heat generated during the various machining operations.

Another essential part of machining is power generation. This work looks into it as it affects the machining efficiency of a machine. This work enlightens one adequately on the excessive forces and torques that cause tool failure, spindle stall and undesirable deflections. Thus, at the end of this study, one can easily ascertain the amount of cutting forces, torque and power necessary for proficient machining whilst eliminating the excesses.

INTRODUCTION

The growing demand for higher productivity, increased product quality and overall economy in manufacturing by machining, particularly to meet the challenges thrown by liberalization and global cost competitiveness, insists high material removal rate and high stability and long life of the cutting tools. [11]

Machining operations (e.g., drilling, milling, etc) are shape transformation processes where metal is removed from a stock of material to produce a part. The objective of these operations is to produce parts with specified quality as productively as possible. Many phenomena naturally occur during machining operations which are detrimental to this objective. [13]

However, high production machining with high cutting velocity, feed and depth of cut is inherently associated with generation of large amount of heat and high cutting temperature. Such high cutting temperature not only reduces dimensional accuracy and tool life but also impairs the surface finish of the product. In the actual cutting or machining activity, some form of force is involved. This force gives rise to torque and this torque generates power.

For proper understanding of the generation of heat and power in machining, one has to sufficiently understand and appreciate the intricacies involved in these machining operations. Consequently, we have to introduce some of the vital machining operations.

 Turning is a machining process in which a cutting tool, typically a non-rotary tool bit, describes a helical toolpath by moving more or less linearly while the workpiece rotates. The tool's axes of movement may be literally a straight line, or they may be along some set of curves or angles, but they are essentially linear (in the nonmathematical sense). Drilling is a cutting process that uses a drill bit to cut or enlarge a hole of circular cross-section in solid materials. A milling machine is a machine tool used to machine solid materials. Planing is a manufacturing process of material removal in which the workpiece reciprocates against a stationary single-point cutting tool producing a plane or sculpted surface. Grinding is an abrasive machining process that uses a grinding wheel as the cutting tool.[13, 12]

Basically, all machining operations involve some form of material removal, movement or cutting. For instance, drilling involves material removal to create a hole or crevice, turning involves the movement of a material with significant amount of force, so that it assumes its new position, and so on as subsequently explained in the previous paragraph. Essentially, on critical examination, anyone can easily deduce that all basic machining operations have two things in common. They are heat generation and power generation.

Generally, the intensity of heat sources in real machining operations can be determined approximately by the external work applied, however, the distribution of the heat sources are hard to obtained by either theoretical or experimental methods.

BODY

MACHINING

In fully understanding the intricacies involved in power and heat generation during machining, one must adequately understand what is involved in these individual machining processes. The basic ones include but are not limited to the following.

Turning is a machining process in which a cutting tool, typically a non-rotary tool bit, describes a helical tool path by moving more or less linearly while the work piece rotates. The tool's axes of movement may be literally a straight line, or they may be along some set of curves or angles, but they are essentially linear (in the non-mathematical sense). Usually the term "turning" is reserved for the generation of external surfaces by this cutting action, whereas this same essential cutting action when applied to internal surfaces (that is, holes, of one kind or another) is called "boring". Thus the phrase "turning and boring" categorizes the larger family of (essentially similar) processes. The cutting of faces on the work piece (that is, surfaces perpendicular to its rotating axis), whether with a turning or boring tool, is called "facing", and may be lumped into either category as a subset.

The figure above shows in details how turning takes place. Turning can be done manually, in a traditional form of lathe, which frequently requires continuous supervision by the operator, or by using an automated lathe which does not. Today the most common type of such automation is computer numerical control, better known as CNC. (CNC is also commonly used with many other types of machining besides turning.) When turning, a piece of relatively rigid material (such as wood, metal, plastic, or stone) is rotated and a cutting tool is traversed along 1, 2, or 3 axes of motion to produce precise diameters and depths. Turning can be either on the outside of the cylinder or on the inside (also known as boring) to produce tubular components to various geometries. Although now quite rare, early lathes could even be used to produce complex geometric figures, even the platonic solids; although since the advent of CNC it has become unusual to use non-computerized toolpath control for this purpose. The turning processes are typically carried out on a lathe, considered to be the oldest machine tools, and can be of four different types such as straight turning, taper turning, profiling or external grooving. Those types of turning processes can produce various shapes of materials such as straight, conical, curved, or grooved work piece. In general, turning uses simple single-point cutting tools. Each group of work piece materials has an optimum set of tools angles which have been developed through the years. The bits of waste metal from turning operations are known as chips (North America), or swarf (Britain). In some areas they may be known as turnings.[2]

Drilling is a cutting process that uses a drill bit to cut or enlarge a hole of circular cross-section in solid materials. The drill bit is a rotary cutting tool, often multipoint. The bit is pressed against the work piece and rotated at rates from hundreds to thousands of revolutions per minute. This forces the cutting edge against the work piece, cutting off chips from what will become the hole being drilled.

A milling machine is a machine tool used to machine solid materials. Milling machines are often classed in two basic forms, horizontal and vertical, which refers to the orientation of the main spindle. Both types range in size from small, bench-mounted devices to room-sized machines. Unlike a drill press, which holds the workpiece stationary as the drill moves axially to penetrate the material, milling machines also move the workpiece radially against the rotating milling cutter, which cuts on its sides as well as its tip? Work piece and cutter movement are precisely controlled to less than 0.001 in (0.025 mm), usually by means of precision ground slides and lead screws or analogous technology. Milling machines may be manually operated, mechanically automated, or digitally automated via computer numerical control. [6]

  

Milling machines can perform a vast number of operations, from simple (e.g., slot and keyway cutting, planing, drilling) to complex (e.g., contouring, die sinking). Cutting fluid is often pumped to the cutting site to cool and lubricate the cut and to wash away the resulting swarf.

Planing is a manufacturing process of material removal in which the work piece reciprocates against a stationary single-point cutting tool producing a plane or sculpted surface. Planing is analogous to shaping. The main difference between these two processes is that in shaping the tool reciprocates across the stationary work piece. Planing motion is the opposite of shaping. Both planing and shaping are rapidly being replaced by milling. The mechanism used for this process is known as a planer. The size of the planer is determined by the largest workpiece that can be machined on it. The cutting tools are usually carbide tipped or made of high speed steel and resemble those used in facing and turning. [7]

Grinding practice is a large and diverse area of manufacturing and toolmaking. It can produce very fine finishes and very accurate dimensions; yet in mass production contexts it can also rough out large volumes of metal quite rapidly. It is usually better suited to the machining of very hard materials than is "regular" machining (that is, cutting larger chips with cutting tools such as tool bits or milling cutters), and until recent decades it was the only practical way to machine such materials as hardened steels. Compared to "regular" machining, it is usually better suited to taking very shallow cuts, such as reducing a shaft's diameter by half a thousandth of an inch (thou) or 12.7 um. Grinding is a subset of cutting, as grinding is a true metal-cutting process. Each grain of abrasive functions as a microscopic single-point cutting edge (although of high negative rake angle), and shears a tiny chip that is analogous to what would conventionally be called a "cut" chip (turning, milling, drilling, tapping, etc.). However, among people who work in the machining fields, the term cutting is often understood to refer to the macroscopic cutting operations, and grinding is often mentally categorized as a "separate" process. This is why the terms are usually used in contradistinction in shop-floor practice, even though, strictly speaking, grinding is a subset of cutting. [7, 2]

It is noteworthy that machining processes generally involve the use of a cutting tool to perform some form of cutting or material removal activity on a workpiece. Thus, the same basic method of heat generation is involved as well as power generation during machining. This is critically examined in the following paragraphs.

HEAT GENERATION

Heat has critical influences on machining. To some extent, it can increase tool wear and then reduce tool life, get rise to thermal deformation and cause to environmental problems, etc. But due to the complexity of machining mechanics, it's hard to predict the intensity and distribution of the heat sources in an individual machining operation. Especially, because the properties of materials used in machining vary with temperature, the mechanical process and the thermal dynamic process are tightly coupled together. Since early this century, many efforts in theoretical analyses and experiments have been made to understand this phenomena, but many problems are still remaining unsolved. [12, 13]

Theoretical Analysis Review

Assumptions

Due to the complexity of heat problem in machining, the following assumptions are generally imposed:

First, almost all (90%-100%) of the mechanical energy consumed in a machining operation finally convert into the thermal energy.

Second, there are three major sources of thermal energy in orthogonal cutting with a sharp tool: plastic deformation in the so-called primary zone and secondary zone, and the frictional dissipation energy generated at the interface between tool and chip. But if the tool is with a round tip, part of heat may be generated at the interface between tool and workpiece due to friction. In pure theoretical analysis, more assumptions are needed: usually, the plane heat sources at the shear plane and the tool-chip interface are assumed as being uniformly distributed.

Third, even with the above assumptions, the problem of estimating the mean temperatures on the shear plane and tool face is complex. This is because part of the thermal energy will convect away by the chip; part will conducted into the workpiece and tool, i.e., a partition criterion is needed. [11]