Machinability of Advanced Materials

J. Paulo Davim

• Publisher: Wiley-ISTE
• Year: 2014
• Print ISBN: 9781848213968
• eBook ISBN: 9781118576793
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Machinability of Advanced Materials addresses the level of difficulty involved in machining a material, or multiple materials, with the appropriate tooling and cutting parameters. A variety of factors determine a material's machinability, including tool life rate, cutting forces and power consumption, surface integrity, limiting rate of metal removal, and chip shape. These topics, among others, and multiple examples comprise this research resource for engineering students, academics, and practitioners.

Chapter 1

Machinability: Existing and Advanced Concepts

This chapter first analyzes the known concepts, definition and multiple methods, including the standard, of the assessment of machinability. It argues that having been developed a long time ago when cutting data for various tools were not widely and readily available, machinability, as a concept, became obsolete. As a result, the old notion of machinability means “all things to all men” and thus does not have any practical value nowadays, a fact admitted by leading tool suppliers, despite its colorful name which is still used in book, journal and paper titles.

This chapter presents an introduction to the basic ideas for the development of a new concept of machinability, arguing that the notion of machinability has a dual meaning: firstly, the machinability of work material which should be considered as an inherent property of the work material related to its physico-mechanical properties, and secondly, the process machinability which relates to a specific machining operation. The meaning, physical background and improvement of both machinabilities are discussed. It is revealed that the existing methods of enhancing the process machinability work well when their application reduces the specific energy of fracture of the layer being removed in machining. The role of tool geometry and the application of workpiece pre-heating (hot machining) and advanced plastic deformation (APD) of the work material are considered.

1.1. Introduction

In the literature on metal machining, machinability of the work material is defined as the ease with which it can be machined [SCH 02]. It is often pointed out that machinability depends on the properties of the work material, as well as on the cutting conditions. Therefore, it is not clear if machinability is an inherent property of a material or a property of the material in the particular machining system of components. This is the first and foremost issue that should be resolved for the proper definition of machinability and its assessment.

On the other hand, process developers, manufacturing engineers and practitioners in the machine shop may ask some logical questions: why do we need to know and understand the concept of machinability? What exactly can we gain using this knowledge? These are reasonable questions as no one has so far come up with a methodology that can calculate machinability yet also clearly show the gains one can obtain using the calculated result(s).

One problem is that machinability is a response variable that has not been clearly defined as it does not even have unit(s) with which to measure this qualitative notion. What exactly is it that one is trying to measure in any assessment of machinability? Traditionally, in the assessment of machinability, four “basic” factors are considered:

– Tool wear: tool life defined either by the number of machined parts, i.e. process economy, or by the cutting speed at which the criterion of tool life is achieved over the defined time, for example 20 minutes.

– Magnitude of the cutting force: tool and machine abilities to withstand this force, i.e. process feasibility.

– Chip shape: chip transportability from the machining zone, i.e. process feasibility.

– Roughness of the machined surface: quality of machining, i.e. process suitability.

Generally, the harder the work material or the higher its tensile strength, the more difficult it is to machine. However, copper is very soft, but difficult to machine because it is very ductile and chips do not break away, often leading to tool breakages. A higher carbon and alloy content usually makes steel more difficult to machine. Alloying elements in steels added for hardening characteristics (i.e. chromium, molybdenum, tungsten, etc.) increase the material strength and cause the material to work to harden, generally decreasing machinability. Nickel and aluminum tend to adhere to the cutting tool, causing a built-up edge which causes chipping and poor edge retention. The addition of some elements to alloys improves machinability. These include sulfur, phosphorous, lead, graphite, etc.

On the other hand, a harder work material may have greater machinability. For example, a material of hardness HRc 47 is the starting point of hard turning, while it is regularly performed on parts of hardness HRc 60 and even higher [AST 11A]. If the hardness of the work material is less than HRc 47 then hard tuning is not feasible. Another example is the machinability of a relatively hard, gray cast iron that is normally much greater than that of a soft austenitic stainless steel. Therefore, the hardness of the work material is not always a relevant parameter in comparisons of the machinability of various work materials.

The application of metal working fluid (hereafter called MWF), also known as coolant, aims to improve machinability, but sometimes actually inhibits it [AST 12]. The MWF type, brand, clearness, pH, flow rate and many other characteristics may affect machinability dramatically.

The cutting tool material is another factor that greatly affects machinability. For example, if a high-speed steel drill is used to machine a high-silicon aluminum alloy widely used in the automotive industry then machinability is low since the tool life and quality of drilled holes will be poor. On the other hand, if a PCD drill is used, then mirror-shining holes of close tolerance and a tool life measured in a hundred thousand holes are the direct results, so that the machinability in this case is excellent. Therefore, the tool material cannot be excluded from any machinability considerations.

Other factors that can affect machinability are the machine and its workholding fixture as they may define the ranges of available speeds and feeds as well as the range of vibration-free performance.

1.2. Traditional concepts of machinability and methods for its assessment

1.2.1. Common perceptions

The most common way to grade the machinability of various work materials is the so-called machinability comparison chart where the machinability of a given work material is measured as a percentage relative to the machinability of steel 1212 chosen to be 100% [TOO 83]. For example, the machinability of AISI steel 4140 is 55% according to this chart. A number of questions arise from this:

– What is the true meaning of this 55%? Is it related to the cutting speed, tool life, surface finish, chip control, etc.?

– Is this 55% valid for all machining operations from sawing to gear manufacturing?

– Is this 55% valid for any metallurgical state of steel 1040 in terms of hardness, grain size, etc.?

– Is this 55% still the same for any cutting tool, including the tool material and tool geometry, that can be used to machine this work material?

– How was this 55% obtained? What kind of tool (make, tool material, geometry), machining regime (cutting speed, feed, depth of cut), MWF (brand, chemical composition, concentration, flow rate, delivery system, etc.), machine (static and dynamic rigidity, alignment, accuracy of motions, etc.) was used in such a determination? What was actually measured and how?

Clearly, no answers to these practical questions can be found in the known machinability charts, which makes them worthless in the author’s opinion.

More accurate data can be found in multiple machinability books developed by specialized manufacturing companies; for example, Metcut Co., which was founded in 1948, with the objective of developing and disseminating technical information in the science of machinability as claimed by its website¹. The company has published a number of editions of Machining Data Handbook (e.g. [MAC 80]). This book contains machining recommendations including tool geometry, MWF, tool materials, surface finish and surface integrity. It provides guidelines for various machining processes. Using these data, the company developed an online data machinability database, CUTDATA, approximately 20 years ago. However, there are still some major concerns about the results obtained:

– Although it is called a machinability database, it has a little to do with this concept as it presents the recommended cutting data.

– It is not clear how this database was compiled. Obviously, the company developer did not conduct the cutting tests, results of which are included. Instead, these data were collected from various sources with no clear conditions of how they were obtained.

– The inputs are much too general. For example, the specification of the cutting tool material as HSS or carbide is not sufficient to determine the cutting data as there are great varieties in HSSs and carbides. For example, the operational cutting speed for two different grades of HSS (including proper coating) can differ by ten times for the same operation and work material. The tool life for the same operation and work material can differ by 5–10 times depending on the particular carbide grade and its coating.

It is no wonder that this database did not have any further development and was thus gradually abolished.

Nowadays, manufacturing engineers and shop practitioners have different ways of selecting the proper tool (tool design, materials, coating geometry) and machining regime for any practical applications. One of the most common ways is through direct assistance from cutting tool manufacturers (catalogs and field application specialists), who recommend tools and machining regimes for particular jobs. Moreover, detailed and easy to use paper and online catalogs of major tool manufacturers and suppliers are available where known work materials are classified into machinability groups.

The leading cutting tool materials and cutting tool manufacturer Sandvik Coromant Co. finally admits in its latest catalog “Materials” that “Machinability has no direct definition, like grades or numbers. In a broad sense it includes the ability of the workpiece material to be machined, the wear it creates on the cutting edge and the chip formation that can be obtained.” It is further explained that low alloy steel is considered to have a better machinability compared to stainless steel. The concept of “good machinability” usually means an undisturbed cutting action and a fair tool life. Most evaluations of the machinability for a certain material are made using practical tests, and the results are determined in relation to another test in another type of material under approximately the same conditions. In these tests, other factors, such as micro-structure, smearing tendency, machine tool, stability, noise, tool life, etc. will be taken into consideration. Other companies are still using the old-style table rating of machinability as shown in Figure 1.1. As such, no reference to 100%-machinability work material and no criteria for the listed percentages are given.

1.2.2. Non-standardized tests for machinability assessment

Mills and Redford published the only book on machinability of a wide variety of work materials [MIL 83]. In this book, machinability, considered as a property of the work material, has no generally accepted parameter for its measurement. As a result, machinability tends to remain a term which means “all things for all men”. However, Mills and Redford suggested that consideration of the cutting energy should not figure in the definition of machinability and that this term should be understood to be some measure of the way in which a material wears away a cutting tool when it is being machined.

Figure 1.1. Fragment of the Seco Co. machinability table

In the author’s opinion, Mills and Redford [MIL 83] built a logical trap for themselves because:

– Considering machinability as a property of the work material, Mills and Redford were forced to find a specific characteristic of the work material responsible for tool wear. Moreover, this characteristic should be measurable.

– Having selected a measure of the way in which the work materials wear away the cutting tool, Mills and Redford assumed that this measure is a property of the work material so all known tool materials ranging from high carbon tool steel to polycrystalline diamond (know as PCD) should be subjected to the same wear or wear type, not to mention a great range of cutting conditions resulting in a great variety of contact pressures and temperatures at the tool-chip and tool-workpiece interfaces.

– Mills and Redford shifted their attention to the place of wear (flank wear, crater wear, etc.) instead of the physics of wear. They admitted, however, that a fundamental understanding of the process of tool wear is lacking so it is not possible to combine basic properties of the work and tool materials as well as the cutting conditions to arrive at a measure of machinability.

– Having realized that the experimental determination of tool wear is highly uncertain, Mills and Redford concluded that the known tests and experimental data are valid only for the test conditions. As there a great variety of machining conditions in terms of tool materials and coatings, work materials and their metallurgical state, MWF parameters and grades, machining regimes, machine tool properties, part design etc., the machinability index becomes next to meaningless. Mills and Redford pointed out that even if the machinability test does attempt to compare the machinability of two different work materials for a given set of cutting conditions, there is no guarantee that when cutting conditions change the ranking will remain the same.

Mills and Redford [MIL 83] subdivided machinability tests into two basic categories: those which do not require one to carry out the actual machining and those which do. A parallel subdivision includes two more categories: those tests that merely indicate, for a given set of conditions, the relative machinability of two or more work-tool combinations (ranking tests) and those which indicate the relative merits of two or more work-tool combinations for a range of cutting conditions (absolute tests). A simple analysis, however, shows that for the results of the absolute test to be of any use, both the time spent and cost of the test tend to infinity.

Although Mills and Redford described the known non-machining tests to assess machinability [MIL 83], they did not present the critical analysis of these tests and their advantages and obvious drawbacks. Moreover, these tests are rather old and never considered as serious tests for standards or for any practical industrial applications. Nowadays, none of these tests have any practical use. Below are some obvious drawbacks to these tests:

– Chemical composition test is to correlate the chemical composition of the work materials with the 60-minute tool-life cutting speed. In the author’s opinion, this test is meaningless as it is next to impossible to correlate the composition of the work material even with its mechanical properties. For example, the addition of a small percentage (or even a fraction of a percent) of manganese, as often used in the automotive industry, does not change the mechanical prope...

Table of contents

• Cover Page
• Table of Contents
• Title Page
• Copyright
• Preface
• Chapter 1. Machinability: Existing and Advanced Concepts
• Chapter 2. Milling Burr Formation and Avoidance
• Chapter 3. Machinability of Titanium and Its Alloys
• Chapter 4. Effects of Alloying Elements on the Machinability of Near-Eutectic Al-Si Casting Alloys
• Chapter 5. The Machinability of Hard Materials – A Review
• Chapter 6. An Investigation of Ductile Regime Machining of Silicon Nitride Ceramics
• List of Authors
• Index