26.2 Mechanics of Turning

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26.2.1 General Terms

A typical turning operation is schematically shown in Figure 26.1. The cutting tool moves parallel to the

workpiece and spindle, and hence reduces the diameter of the shaft. The most important machining

parameters are:

* Cutting speed (usually expressed in m/min)

* Feed rate (usually expressed in mm/rev)

* Depth of cut (usually expressed in mm)

The force response on the tool tip due to the turning operation consists of three components: Fx, Fy,

and Fz. These forces consist of a static and a dynamic part, as shown in Figure 26.2. The static forces are

governed by the static pressure between the tool and workpiece, and are a function of the machining

26-2 Vibration and Shock Handbook

© 2005 by Taylor & Francis Group, LLC

parameters. The dynamic forces are governed by forced and free vibrations due to excitation from the

cutting operation. Analytical models exist that can describe the static forces for basic machining

operations (Merchant, 1945). The dynamic behavior is more difficult to model theoretically, although

there is also continuous research in this area (Kapoor et al., 1998).

One of the main difficulties of monitoring tool wear with vibration is to identify the frequency range

that is influenced by tool wear, since machining processes entail various mechanisms that produce

vibrations that are not related to tool wear. The frequency range of vibrations produced during ordinary

machining operations usually falls between 0 and 10 kHz. From the literature, it can be concluded that

the frequency range sensitive to tool wear depends entirely on the type of machining operation, and must

be determined experimentally for each individual case. There are two important vibration frequencies

present during cutting:

* The natural frequencies of the tool holder and its components

* The frequency of chip formation

Dynamic tests should be conducted to identify the dynamic properties of tool holders (Scheffer and

Heyns, 2002a). However, the interaction of the

working tool engaged into the rotating workpiece

complicates the situation, and as a result the

dynamic behavior during cutting could be different

from the expected behavior obtained from offline

tests. Scheffer and Heyns (2004) compared

continuous cantilever models with modal hammer

tests for different tool holder overhang lengths.

The natural frequency of the first mode as a function

of overhang length is plotted in Figure 26.3

(for a specific tool holder). It can be seen that a

continuous fixed-free cantilever beam model

corresponds well with the results obtained with

hammer tests.

FIGURE 26.1 Turning operation.

dynamic cutting

force

0

time [s]

force [N]

static cutting force

FIGURE 26.2 Static and dynamic forces.

Vibration-Based Tool Condition Monitoring Systems 26-3

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The chip formation frequency can be calculated

with simple equations that take the machining

conditions into account (Lee et al., 1989). The tool

holder natural frequencies and chip formation

frequency are independent. Generally, tool wear

has a larger effect on the vibration amplitudes at

the tool holder natural frequencies but can

influence chip formation frequencies as well.

26.2.2 Chatter Vibrations

Another phenomenon important to machining

operations is tool chatter vibrations. These are

self-excited vibrations resulting from the generation

of different chip thicknesses during machining.

Initially, cutting forces excite a structural

mode of the machine – workpiece system. This

leaves a wavy surface finish on the workpiece.

During the next revolution, another wavy surface

is produced in the same way. Depending on the

phase shift between these two waves, the maximum chip thickness can grow and oscillate at a particular

frequency that is close to that of a structural mode. This is called the regenerative chatter frequency.

Chatter cause a poor surface finish and can also lead to tool breakage.

The analysis and prediction of chatter has been the subject of research for many years. Morimoto et al.

(2000) developed a piezoelectric shaker/actuator to regenerate the vibrations of the cutting process. In

this way, unwanted vibrations such as chatter can be attenuated. The system is also helpful to determine

the dynamic properties of the machine tool. Koizumi et al. (2000) used a very interesting approach called

the correlation integral in the time domain to identify chatter onset. Lago et al. (2002) designed a sensor

and actuator integrated tool for turning and boring to control chatter. The tool holder shank vibrations

are sent to the actuator via a digital controller. An adaptive feedback control system is used to perform

broadband vibration attenuation up to 40 dB at different frequencies simultaneously.

26.2.3 Tool Wear

26.2.3.1 Tool Failure Mechanisms

Tool wear is caused by mechanical loads, thermal loads, chemical reactions, and abrasive loads. The load

conditions are in turn influenced by the cutting conditions and materials. The different loads can cause

certain wear mechanisms that may occur in combination. These mechanisms have either a physical or

chemical characteristic that causes loss or deformation of tool material. Tool wear mechanisms can be

classified into several types, summarized as follows (Du, 1999):

* Abrasive wear resulting from hard particles cutting action

* Adhesive wear associated with shear plane deformation

* Diffusion wear occurring at high temperatures

* Fracture wear due to fatigue

Other wear mechanisms are plastic deformation and oxidation, which are not very common in

industry. It is estimated that 50% of all tool wear is caused by abrasion, 20% by adhesion, 10% by

chemical reactions and the remaining 20% by the other mechanisms (Kopac, 1998). Abrasion is basically

the grinding of the cutting tool material. The volume of abrasive wear increases linearly with the cutting

forces. Higher hardness of the tool material can reduce the amount of abrasive wear. During adhesion,

the high pressures and temperatures on the roughness peaks on the tool and the workpiece cause

7

6

5

4

3

2

1

0

40 45 50 55 60

1st mode frequency [kHz]

overhang length [mm]

fixed-free cantilever beam

hammer tests

FIGURE 26.3 Frequency of first tool holder mode.

(Source: Scheffer, C. and Heyns, P.S., Mech. Syst. Signal

Process, Elsevier, 2004. With permission.)

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welding. These welding points are broken many times every second due to the workpiece movement and

as a result cause removal of the tool material (Kopac, 1998). Diffusion wear occurs at even higher cutting

speeds, where very high temperatures are present (especially when using hard metal tools).

26.2.3.2 Tool Failure Modes

Tool wear will generally occur as a combination of a number of wear modes, with one mode predominant.

The dominant mode will depend on the dominant wear mechanism, which in its turn is influenced by the

machining conditions and the choice of tool and workpiece material. For a given tool and workpiece

combination, the dominant wear mode can be determined at different cutting speeds using the product of

the cutting speed and the undeformed chip thickness (Dimla, 2000). The common wear modes are:

* Nose wear

* Flank wear

* Crater wear

* Notch wear

* Chipping

* Cracking

* Breakage

* Plastic deformation

Figure 26.4 is a graphical representation of the different tool failure modes. The consequences of tool

wear are deviations in shape and roughness of the machined part that cause the part to be discarded because

it is out of tolerance. Most wear modes cause an increase in cutting forces, although this is not always the

case for all tool and workpiece combinations. The most widely researched tool failure modes for turning

with single point tools are flank wear, breakage (fracture), and crater wear. Flank and crater wear are

accepted as normal tool failure modes, because the other failure modes can be avoided by selecting the

proper machining parameters. The growth of flank and crater wear is directly related to the total cutting

time, unlike some of the other failure modes, which can occur unexpectedly even with a new tool.

FIGURE 26.4 Tool failure modes. (Source: Scheffer, C. and Heyns, P.S. 2001a. COMA ’01, University of Stellenbosch.

With permission.)

Vibration-Based Tool Condition Monitoring Systems 26-5

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26.2.3.3 Tool Wear Measurement

Wear measurements of tool inserts are done

through the implementation of an appropriate

international standard, ISO 3685. Flank wear is

quantified in terms of VB, which is the mean of the

wear height on the tool flank. The length of flank

wear is also measured in terms of b. Crater wear is

quantified in terms of the crater depth, K. The

parameters are depicted in Figure 26.5, which is a

scanning electron microscope (SEM) picture of a

worn turning insert.

26.2.3.4 Tool Wear Stages

It is assumed by most authors that tool wear

consists of an initial, a regular, and a fast wear stage

(Zhou et al., 1995). Some authors divide tool wear

into five distinct stages (Bonifacio and Diniz, 1994):

1. Initial stage of wear

2. Regular stage of wear

3. Microbreakage stage of wear

4. Fast wear stage

5. Tool breakage

It has been established by various researchers

that the initial and fast (before tool breakage)

stages occur more rapidly than the regular stage.

Bonifacio and Diniz (1994) explain that, during

the fast wear stage with coated carbide tools, the

tool loses its coating and the tool substrate (which

has less resistance) begins to perform the cut and

wears faster. During the initial stage, the tool edge

loses its sharp edge rapidly, after this the process

stabilizes for a given time. Flank wear in relation to

total cutting time will typically appear as depicted

in Figure 26.6.

The geometrical growth and rate of wear is

unique for every tool insert, even those used with

the same machining parameters. Wear measurements

conducted on the shop floor of a piston

manufacturer by Scheffer and Heyns (2004) are

shown in Figure 26.7. It was found that the tools

last between 1000 and 6000 components, which

makes the optimal use of the tool extremely

problematic if the wear is not monitored on-line.

The reason for this behavior is mainly attributed to

fluctuating conditions on the shop floor, for

example, the rate at which components are

manufactured. If the time allowed for the tool to

cool down between workpieces is not constant,

large variations in the tool life can be expected.

FIGURE 26.5 Tool wear parameters.

1 2 3

4

initial regular fast 5

cutting time

flank wear FIGURE 26.6 Flank wear in relation to cutting time.

0.25

0.2

0.15

0.1

0.05

0 2000 4000 6000

flank wear VB [mm]

number of workpieces

FIGURE 26.7 Typical variations in tool life. (Source:

Scheffer, C. and Heyns, P.S., Mech. Syst. Signal Process,

Elsevier, 2004. With permission.)

26-6 Vibration and Shock Handbook

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