Tool wear

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Seminar Report on

Prepared By:

Limbasiya Girish – 08DME005

Diploma in Mechanical Engineering

Semester – VIYear 2011

Nirma University

INSTITUTE OF DIPLOMA STUDIES

AHMEDABAD (GUJARAT- INDIA) 382481

Tool wear

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C E R T I F I C AT E

This is to certify that following students of Diploma in Mechanical

Engineering Semester-VI have completed their Seminar Work titled

TOOL WEAR satisfactorily in partial fulfillment of requirement of

Diploma in Mechanical Engineering in the year 2010-11

Roll No.Name of the Student08DME005Limbasiya Girish

Seminar Guide:-Prof Darshan Vaishnav

NIRMA UNIVERSITY

INSTITUTE OF DIPLOMA STUDIES

AHMEDABAD (GUJARAT- INDIA) 382481

Tool wear

Index

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No Topic Page No.

1. Tool wear definition 6

2. Tool wear phenomena 7

3. Types of tool wear 8

4. Causes of tool wear 11

5. Effect of tool wear 13

6. Remedies of tool wear 14

7. Measurement of tool wear 17

Tool wear

ACKNOWLEDGMENT

We wish to express our heartfelt appreciation to all those who have contributed to

this project, both explicitly and implicitly, without the co-operation of whom, it

would not have been possible to complete this Seminar.

We would like to thank our institute for giving us the opportunity to have some

feel about the Seminar. We would like to thank our H.O.D Mr. Suresh Pareek and

our Faculty Darshan Vaishnav for constantly guiding and showing us the correct

path to reach towards our desired goal.

Last but not least I would like to thank our family members and friends for their

constant encouragement and support throughout the duration of our seminar.

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Tool wear

1. TOOL WEAR DEFINITION

Tool wear describes the gradual failure of cutting tools due to regular operation. It is a term often associated with tipped tools, tool bits, or drill bits that are used with machine tools.

The change of shape of the tool from its original shape, during cutting, resulting from the gradual loss of tool material

During machining, cutting tools remove material from the component to achieve the required shape, dimension and surface roughness (finish). However, wear occurs during the cutting action, and it will ultimately result in the failure of the cutting tool. When the tool wear reaches a certain extent, the tool or active edge has to be replaced to guarantee the desired cutting action.

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Tool wear

2. TOOL WEAR PHENOMENA

The shear stress and normal stress involved in metal cutting is much higher . The high contact stress between the tool rake-face and the chip causes severe friction at the rake face, as well, there is friction between the flank and the machined surface. The result is a

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Tool wear

variety of wear patterns and scars which can be observed at the rake face and the flank face.

3. TYPES OF TOOL WEAR

1. Crater wear

2. Flank wear

3. Notch wear

Crater wear

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Tool wear

Crater wear: The chip flows across the rake face, resulting in severe friction between the chip and rake face, and leaves a scar on the rake face which usually parallels to the major cutting edge. The crater wear can increase the working rake angle and reduce the cutting force, but it will also weaken the strength of the cutting edge. The parameters used to measure the crater wear can be seen in the diagram. The crater depth KT is the most commonly used parameter in evaluating the rake face wear.

Flank wear

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Tool wear

Wear on the flank (relief) face is called Flank wear and results in the formation of a wear land. Wear land formation is not always uniform along the major and minor cutting edges of the tool. Flank wear most commonly results from abrasive wear of the cutting edge against the machined surface. Flank wear can be monitored in production by examining the tool or by tracking the change in size of the tool or machined part. Flank wear can be measured by using the average and maximum wear land size VB and VBmax.

Notch wear

This is a special type of combined flank and rake face wear which occurs adjacent to the point where the major cutting edge intersects the work surface.

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Tool wear

The gashing (or grooving, gouging) at the outer edge of the wear land is an indication of a hard or abrasive skin on the work material. Such a skin may develop during the first machine pass over a forging, casting or hot-rolled workpiece. It is also common in machining of materials with high work-hardening characteristics, including many stainless steels and heat-resistant nickel or chromium alloys. In this case , the previous machining operation leaves a thin work-hardened skin.

4. CAUSES AND EFFECTS OF TOOL WEAR

Causes of Tool Wear

Abrasion

Adhesion

Chemical

Diffusion

Thermal degradation

Vibration and Rigidity

Abrasion :

Abrasion wear is caused by the friction between the cutting tool and the work piece.

Adhesion :

When particles of work piece become welded to a cutting tool, they form a built up edge. Eventually a built up edge is broken off the cutting tool and some times the part of the tool is also broken.

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Tool wear

Chemical :

Some times chemical reaction can occur between the cutting fluid and cutting tool. The cutting fluid can cause oxidation of cutting tool , which may result in premature tool failure.

Diffusion :

This occur when particles of material form a built up edge on the work piece. The material in the built up edge and the cutting tool material start to alloys which can result in tool being weakened.

Thermal Degradation :

This is caused by severe temperature gradients during machining operations. The dramatic changes in temperature can cause crack to form near the cutting edge which ultimately leads to tool failure.

Vibration and Rigidity :

The machine condition and rigidity which affect the quality of the surface texture produced. Excessive wear of the spindle bearing’s feed mechanism can result in poor surface texture. Without proper adjustment and maintenance of machine, vibration can develop causing poor tool life and surfaces.

Vibration can also be causes by other variables such as :

Spindle overhung

Work piece support

Effect of tool wear

Increase the cutting force;

Increase the surface roughness;

Decrease the dimensional accuracy;

Increase the temperature;

Vibration;

Lower the production efficiency, component quality;

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Tool wear

Increase the cost.

Reduction in tool wear can be accomplished by using lubricants and coolants while machining. These reduce friction and temperature, thus reducing the tool wear.

5. TOOL FAILURE AND REMEDIES

Thermal cracks

Causes:

Small cracks running across the cutting edge, caused by thermal shock loads in interrupted cutting operations, particularly in milling, Danger of breakage !

Remedies:

use grade  with greater resistance to thermal shock check use of cutting fluid; Cutting fluid should not generally be used  for interrupted cuts if cutting fluid essential, use copius flow of the same

chip control

causes:

Effective chip control is essential for trouble-free operation. Key factors are work material, feed and depth of cut.  Too short chips result in vibrations and cutting edge overloading.  Danger of breakage! Too long chips tends to coil around the workpiece and also the tool.

Remedies

avoid too small depths of cut below 1 x radius,  except in finishing

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Tool wear

if chips are too long; select chipbreaker geometry for smaller chip sections or increase feed

if chips are too short; select chipbreaker geometry for larger chip section or reduce feed when form turning shoulders check sequence of operations Surface finish

causes:

Surface roughness is a tool-life citerion often applied in finishing operations. It is affected by the configuration and condition of the cutting point, the cutting conditions and the rigidity of the machining setup

Chatter marks or surface damage due to unfavourable chip flow call for special measures.

Remedies

increase cutting speed increase radius use cermets where possible when cutting steel avoid vibrations use cutting fluid vary feed slightly change approach angle select different chipbreaker geometry check rigidty of tool and holding system

vibration, instability

Causes;

Vibrations in the workpiece usually occur with thin-walled parts and non-rigid setups.

Unbalance and excessive cutting forces also cause problems

Remedies

select larger approach angle for the tool use positive geometries use smaller radii change turning frequency

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Tool wear

reduce chip cross sections

Burring

Causes

Burring cannot always be avoided when machining steel workpieces. Chamfering operations should therefore be planned wherever possible

Remedies select inserts with positive geometry use insert with sharp cutting edges, eg. cermets reduce approach angle check sequence of operations

part to part variations

Causes

Varying cutting load

Excessive tool wear

Operation error

Mechanical looseness

Remedies

Provide uniform allowance for finish cut over entire contour.

Use separate tool for finish cut, check proper feed and speed.

Eliminate operator setting.

Plastic deformation

Change to high thermal resistance grade.

Reduce the cutting speed and feed.

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Tool wear

6. TOOL WEAR MEASUREMENT

MEASURING TOOL WEAR WITH MACHINE VISION

In this section the approach which was developed to measure flank wear is described.Implementation details are discussed in Section 6. The theory behind the methods used inthe algorithm is explained to clarify the actual algorithm. A brief introduction to machinevision in general is also given.4.1 Machine VisionMachine vision is used to "Recover useful information about a scene from its two-dimensionalprojections", as quoted from [24, p. 1]. This quote embodies the goal of any machinevision system. The quote also sets some constraints on the machine vision system.To get useful information, it must first be decided what is useful information and to get theinformation at all, vision must be used so that the information is available to be recoveredfrom the projection, i.e. the image. In machine vision, information recovery is usually leftto a dedicated processor or a computer, but the processor can be thought of as a humandoing the same processing, e.g. measuring wear from the tool, like in the context of thisthesis. Compared to a human, machine based vision processing has many benefits, likerepeatability, which has led to many applications used in industry and military.

The components of a basic machine vision system can be seen in Figure 8. The Basiccomponents of any machine vision system are: Camera, frame grabber, lighting, objectand processor. These basic components allow the system to image the object, i.e. capture2D representation of the object, and extract the useful information from the image.

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Tool wear

The camera is combined with a lens. Using a lens, the camera can be focused to differentdistances. Each lens has a specific focal length or a range of focal lengths, if the lens isa zoom lens. Focal length determines object’s size in the 2D projection. The camera andlens are usually modelled with a perspective projection [25, p. 4], which will be used inthis thesis as well to model the camera and lens.

Figure : General machine vision system

The Perspective projection is a model that applies when a pinhole camera is used. Thepinhole is presumed to be so small that only one light ray for each point in the scene isable to pass the pinhole. This is not true in the real world, but perspective projection is a19convenient model for both real pinhole cameras and cameras with lenses. [25, p. 4]A frame grabber is used with a camera to convert an analog signal from the camera to digitalimages. This process involves analog-to-digital converters which quantize the analogsignal received from the camera. In modern cameras, the process is usually integrated tothe camera, which can output the digital image through a digital bus.The lighting set-up depends entirely on application. There are multitude of different lightsources. Two often used light sources are point light sources and uniform light sources.Point light sources can be used to create specular reflections, as in many of the measurement applications discussed in Section 2.2. Uniform light sources create uniform lighting, removing any specular reflections, which can be useful in many applications, e.g. detecting texture of a surface.The object represents the observed object. Other components of the system, lighting andcamera, have to be chosen such that the information can be recovered. For example, ifthe object is large, a wide-angle lens and powerful lighting might be needed to image thewhole object with sufficient lighting.

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Tool wear

Finally, the processor takes the image and performs image processing to recover the requiredinformation from the image. The processor is usually a computer attached to thecamera, but the image processing can be done in the camera as well.

2.1 Cutting ToolWear and Measurement of ToolWear

ISO standard 3685:1993 [1] is the standard for measuring the wear for wear experimentswhen using a single-point turning tool. The word "single-point" refers to the fact that thetool cuts the material with a single point. Figure 1 depicts a cutting tool.Figure 1: Image of a cutting tool

From the Figure 1, it can be seen that the tool consists of three important sides: themajor flank, the minor flank and the face at the top. However, the ordering of the sidesis dependent on the application. Major flank is the cutting edge, while minor flank facesthe newly cut surface and the face receives the material being cut and forms chips. Thetype of cutting tool presented in Figure 1 has four usable cutting edges, one on each side3of the tool. Of these, the most useable for measuring are the major flank and the face asthe standard [1] gives threshold values for wear experiments on the wear types occurringon the two sides.The standard [1] also defines four definitions which will be used in this thesis as they aredefined in the standard:Tool wear: The change of shape of the tool from its original shape, during cutting, resultingfrom the gradual loss of tool material or deformation.Tool wear measure: A dimension to be measured to indicate the amount of tool wear.Tool-life criterion: A predetermined threshold value of a tool wear measure or the occurrence

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Tool wear

of a phenomenon.Tool life: The cutting time required to reach a tool-life criterion.According to the ISO standard 3685:1993, there are multiple types of wear and phenomena,which can cause tool-life criterion to be fulfilled. Most important of the wear typesare flank wear and crater wear. Flank wear is present in all situations and it is the bestknown type of wear. It can be found on the major flank of the tool. Crater wear appearson the face of the tool as a crater. Crater wear is the most commonly found wear on theface of the tool.The wear process itself changes under the influence of different conditions. However,three main factors contributing to the wear are known: adhesion, abrasion and diffusion.Adhesion occurs when the work material, that the tool is cutting, welds onto the tool. This happens because of the friction between the tool and work material, which generates heat. When these welds are broken, small pieces of the tool are lost. Abrasion is mechanicalwear resulting from the cutting action, where the tool grinds itself on to the work material.Diffusion wear occurs on a narrow reaction zone between the tool and work material. Indiffusion wear the atoms from the tool move to the work material. This usually acceleratesthe other two wear processes as the tool material is weakened. [2, pp. 141-142]Figure 2 shows the general curves of flank wear. The wear is typically characterized byrapid initial wear, a linear increase of the wear in the middle of the tool life and finally arapid increase of the wear rate before the tool breaks completely. [2, p. 143]

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Tool wear

Figure 2: Flank wear progress as a function of time [1]While the general shape of the curve stays the same, cutting conditions or cutting parametersaffect the tool life, i.e. the gradient of the curve, especially the linear section. Mostimportant cutting parameters in relation to tool wear are cutting speed, denoted by V , andcutting feed, denoted by f. Of these speed is considered to have the most effect on thetool life. However, Astakhov [3] argues that there are conflicting results and that feed hassignificant effect under optimal cutting temperature. The importance of cutting speed canalso be seen from the Taylor’s equation as the formula relies only on the cutting speed toestimate tool life. Taylor’s equation is described in Section 2.3. [4, pp. 60-62]The effects of cutting speed can be seen from Figure 2. The Cutting speed is usuallyexpressed in meters per minute and can range up to 500 m/min [4]. The standard [1]gives a limit to the cutting speeds by limiting the minimum tool life to 5 minutes or to2 minutes when using a ceramic tool. The cutting feed is expressed as millimeters perrevolution, ranging from 0.1 to 0.9 mm/rev [4, p. 61]. Standard [1] has defined themaximum feed to 0.8 times the corner radius of the tool. The corner radius can be seen in

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Tool wear

Figure 3.To be able to measure the wear, especially flank wear, a few variables must be known.Most important variables are r_, corner radius, and b, depth of cut. These are representedin Figure 3. This figure is depicting the tip or nose of the tool viewing it from the top.The arrow in the figure represents the cutting direction.

two measures are the most important, as they are designated in the standard[1] to be tool-life criterions. These measures are VBB and VBB max. Besides these mostimportant measures, also VBC is measured, which is considered to be the maximum wearwidth in zone C. VBB and VBB max. govern the tool-life by following criteria:6a) the maximum width of the flank wear land VBB max. = 0.6 mm if the flank wear landis not regularly worn in zone B.b) the average width of the flank wear land VBB = 0.3 mm if the flank wear land isconsidered regularly worn in zone B.Commonly the measurements have been done by hand using a microscope. In the standard[1] manual measurement is defined as the way to measure the wear. So to follow thestandard, measurements done with an automatic system need to be close to the manualmeasurements.Kurada et al. [5] divide the available methods for tool condition monitoring into twoparts: direct and indirect sensors. Direct sensors are sensors which measure the toolwear directly. These sensors include vision, proximity and radioactive sensors. Proximitysensors estimate the wear by measuring the distance between the workpiece and tool edge.Radioactive sensors measure the amount of radioactive material transferred from the tool,

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which has been infused with radioactive material, to the chips of the work-piece. Visionsensors will be dealt separately in Section 2.2 as they are in the scope of this thesis. Directmeasurements can usually be taken only between the machining runs, because the majorflank of the tool is not visible during actual machining. Indirect sensors can only estimate the wear as they have no direct way to measure the actual wear, but instead they rely on secondary effects of the wear, such as increase in cutting forces. To indirectly measure the wear, measurements of cutting force, vibration and acoustic emission during the machining have been proposed. Also surface texture of the work-piece has been used to estimate the wear of the tool. These methods will also be explained in Section 2.2 as the texture can be captured using machine vision. Indirect measurements have the advantage of being on-line measurements, i.e. they canbe measured during machining.

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