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Influence of Tool Wear and Chip Forming Mechanism

on Tool Vibration

Aco Antic, Milan Zeljkovic, Jozef Novak-Marcincin

AbstractThe paper presents an experimental research related to the chip formation mechanism, chip type and chip shape

dependant on the tool wear process in turning. The performed experimental investigations measured the influence of the

degree of tool wear on the morphology and microstructural alterations in a chip. In dependence to the tool wear degree, thealterations were monitored in the chip structure and the character of vibrations originating after a certain degree of toolwear, as well as chip segmentation type in the defined processing conditions. The conducted research presented in the paperhas the objective to contribute to better understanding of mechanisms, shape types and segmentation of chips formed in

dependence to the tool wear degree in turning.

Keywords: chip segmentation, vibrations, tool monitoring, turning

1 IntroductionIn order to achieve the satisfactory tool life in the con-

ditions of contemporary production, researches have notbeen directed only on achieving tool cutting characteristicsin high temperatures in the sense of resistance to high tem-

perature and wearing. The understanding of chip formationmechanism and tool wear in processing hard and improvedmaterials has an important role in the process, revealing animportant influence and the appearance of attrition in criti-cal places during the cutting process, as well as determiningoptimal cutting conditions. Generally, with the application

of conventional (slower) cutting speed, the dominant wearmechanism includes adhesive and abrasive wear, while infaster cutting speed, the chemical influences of diffusion

and oxidation have a leading role in tool wear appearance.Kopa et al. [2] state that cutting in lower temperatures pro-duces a high pressure, due to which welding occurs, which

then results in layers on the cutting edge, while the incre-ased cutting temperature occurring as a consequence of fastcutting speed increases the distribution of the oxidation pro-

cess to the larger tool surface area. The process of diffusionoccurring between the chip and the front tool surface resultsin crater wear, while the reaction of the oxidation in the

environment produces alterations on the cutting edge [2]. Inthe conditions of aggressive cutting, with fast processingspeed and fast additional movement speed, on the location

of the contact between the tool and the chip, material mer-ging in individual zones in the front surface occurs, whichcan spread to the entire surface due to the large contact and

rapid spreading. This paper deals in determining the influ-ence of tool wear on chip formation mechanism, i.e. themeasures in which the tool wear development influences the

type and shape of the chip originating in the cutting process[1].

2Tool wear and chip formation

mechanismThe type of chip originating during processing can be

classified as continuous, segmented, localized, broken (saw-

tooth) and discontinuous chip. Generally speaking, the chipgenerated in processing hard and improved materials is inmost cases the sawtooth type of the chip. Currently, accor-

ding to the available literature data, there are two theoriesdefining the origin and formation of the sawtooth chip. One

theory is referred to as the crack theory; the beginning ofthe chip formation is attributed to the appearance of a crackon the free workpiece surface, and the crack begins to grow

rapidly towards the tool cutting edge until the certain lengthwhere it stops due to strong plastic deformations in thematerial under the influence of high pressure stresses of the

processing tool. Segment, or the chip lamella, occurring bet-ween the front tool surface and the crack moves foreword inthe direction of tool movement, while the material in the

region beneath the plastic deformation of the initial crack isspread along the tool front surface in such a manner as toform the sawtooth shape of the chip [1, 3, 4, 5]. The secondtype of shearing appearance in the primary cutting zone is

referred to as the adiabatic theory of chip formation whe-re the thermal-plastic material instability occurs inside the

primary shearing zone and the material deformation mecha-nism occurring at that time due to thermal softening is she-ared following the action of forces appearing in thermal-

plastic material strengthening due to large material tensionin the shearing zone [4, 5]. Adiabatic shearing can precedethe generation of the initial crack and its spreading inside

the non-cracked area in the primary shearing zone, depen-ding on the processing conditions. Barry and Gerald [4]considered the chip formation mechanisms in processing theimproved steel and they concluded that the primary insta-bility inside the primary shearing zone during the sawtoothchip formation initiates the adiabatic shape of the chip

formation, where the dominance is linked to the shearingstresses and their spreading towards the free surface of theworkpiece material. The deformation of the upper region of

the primary zone close to the free chip surface is the con-sequence of both theories, the crack theory and the adia-batic theory, depending on the processing conditions. Pro-

cessing in hard conditions, like high hardness of the work-piece material and fast cutting speed, material degradationand chip formation, cause the appearance of the ductilefracture; on the opposite, material degradation leads to greatplastic deformation. The increase in the workpiece materialhardness, cutting speed, cutting depth (non-deformed chip

thickness) and tool wear band, as well as the increase in thenegative rear angle, can result in the origin of the sawtoothchip [1]. It is determined that the deformed part on the chip

cross section surface decreases with the increase in the cut-

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ting speed.

The main frequency of the tool vibrations is the resonantsystem frequency induced by the friction on the cuttingedge. Vibration acceleration is the best vibration measure

when occurring on high frequencies. Since the cutting toolvibrations are high frequency vibrations (i.e. more than 1kHz), tool acceleration is selected as a parameter for tool

wear monitoring [3].

3 Overview of experimental results

Formation of the sawtooth chip

Testing performed within the paper provides a clear in-sight into the basis of metallurgic instability responsible forthe origin of sawtooth type of the formed chip. MicroscopicFigure 1 presents an example of the formed chip in pro-cessing low alloy carbon steel, thermally processed to the

hardness of 45 HRC with the cutting speed of 200 m/min. Itcan be clearly observed that the lamella teeth during thecutting process presented in the Figure are generated from

the cycle process when the initial segment has significantalterations due to physical action (or the deformation due tohigh pressure on the tool tip). Likewise, grain elongation inthe material structure which, due to great elongation and

pressure compresses into a line and is spread from theprimary shearing zone, are all visible on the cross section ofthe formed chip after teething, Figure 1.

Fig. 1 Sawtooth chip

This shape is not the same as the one occurring as the

result of sliding via the fracture surface, which is evidentfrom the fact that the force is maximal on the location offorce sliding [4]. Instead of fracture crack spreading down-

wards through the primary shearing zone, the generatedband is a deformation localized as a band formed due toadiabatic material sliding. This conclusion is based on the

fact that its thickness is connected to the type, i.e. toolgeometry and shape, assuming the appearance of shearingdue to the action of the cutting force and the spreading of

the initial crack towards the tool cutting edge. The lack ofthe expressed shearing in the upper segment of the primaryshearing zone in the material demonstrates the appearance

of deformation and pressure fall due to the influence of thethermal strengthening of the basic material, i.e. the condi-tion of the processed material.

The band along the right chip edge demonstrates she-aring along the sample edge formed within the secondarycutting zone. The remaining elements of the elongated grain

in the material structure are formed within the primaryzone; nevertheless, it is evident that each occurrence ofcracks and initial chip lamellas in the basic shearing zone

forms a discrete segment. For the conditions in which the

sample in Figure 1 is formed, one can observe decrease inthe advancement and spreading of the initial crack in thelower segment of the primary shearing zone (where the

lamella shearing localization occurs). Within the observedchip segment, one can identify two mechanisms of chip for-mation within the primary cutting zone in forming sawtooth

chip shape, which can be attributed to strict processingconditions, having as a consequence the greater working

time for material removing and greater cutting speed due tothermal material improvement.

Formation of the continuous chip

It is well known that the continuous chip presents theclear evidence in the formation of lamella shearing in theprimary cutting zone on the free surface, even though, ob-served on the frequency scale, the frequency is for one

value order smaller than in the formation of a sawtooth chip[4]. The more important difference in the nature of defininglamella shearing in continuous and sawtooth chip is the

ratio between the span of the cutting front D presented inFigure 1 and d presented in Figure 2 and the non-deformed

chip thickness (cutting depth). The teeth of the sawtoothchip, i.e. the distance between lamellas, is similar in size tothe thickness of the non-deformed chip and, generally ob-served, it is within the 50% of the thickness.

Fig. 2 Continuous chip

Figure 2 presents a lamella characterized by the free sur-face of the continuous chip. This type of chip is produced inthe orthogonal cutting of the low alloy tool steel with the

hardness of 45 HRC, cutting speed 200 m/min, and feed f =

200 m. If one increases the material hardness and/or cut-ting speed, as well as the thickness of the non-deformedchip (cutting depth) in such a manner as the beginning of

fracture spreading for the formation of teeth in the chip isclose to the transition of the free lamella surface into thecontinual chip structure, the result is referred to as the

wrinkled type of structure. In this phase, the main mannerof the chip flow over the front tool surface is sliding. After acertain period of time and cutting, the tool insert is worn,

the chip shape begins to alter and becomes smoother, whilethe lower chip surface becomes wavy and rough in com-parison to those surfaces obtained by utilizing new tool.

4 Behaviour of vibration signals during

chip formation

The formation of the initial crack within the primaryshearing zone during the teeth formation of the sawtoothchip results in fast release of elastic energy and interior

stresses, while with continuous chip this energy remainstrapped between chip lamellas and results in the increase of

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deformation energy and adiabatic material shearing. Howe-

ver, the important difference between these two types ofchips is related to the shearing zone width, the area onwhich it is localized and the distribution.

The conducted research demonstrate that the upper partof the vibration acceleration spectre measured on the tur-ning knife handle in the zone close to the cutting wedge

enables the indirect identification of the alterations in theconditions of chip formation process induced by the altera-

tion of the cutting tool wear condition, i.e. the degradationof its cutting edge by wearing. In the range between 1 kHzand 50 kHz, there is a larger number of personal tool fre-quencies, creating a space for the appearance of resonance

under the influence of the stimulated force created by thechip fragmentation and lamella formation. The formation ofthe segmented chip can be observed as a process of discrete

stimulus of the processed system by a set of energy packa-ges whose frequency can be calculated with the toleratederror, and further more, that stimulus can be recognized in

the response of the processing system, especially the toolhandle. The analysis of the performed tests can lead to theconclusion that the type of the formed chip in the cutting

process is not dominantly influenced only by the state andcharacteristics of the material and cutting speed, but also thetool insert wear condition, i.e. tool geometry. From the tests

it is observed that, in maintaining the constant cutting con-ditions: speed, feed and cutting depth, as well as materialcharacteristics, the alteration in the tool wear degree in a

certain moment leads to the transition from one chip typeinto another. The direct responsibility is attributed to thecutting geometry which is altered with the tool wear degree.

The change in the cutting geometry, and hence in the chiptype, directly influences the observed parameters in theanalysed area of the high frequency segment of the vibra-

tion spectre.Figure 3 presents the signal strength spectres of indivi-

dual worn inserts expressed in the linear scale. Frequency

spectre is limited to 50 kHz. The utilized instrumentationcan realistically guarantee the reliability for a wider frequ-ency spectre as well, up to 100 kHz; however, due to the

features of the utilized sensor, the wider frequency spectreis not considered.

Fig. 3 Type and shape of chip segmentation dependingon the tool wear degree

Fig. 4 Signal strength spectre expressed in the linear scale

5 ConclusionsThe formed chip in the initial phase is in the elongated

shape with the smooth lower segment in contact with thefront tool surface. By the advancement of the wear band and

crater tool wear, the chip alters the shape, becomes rougher,wavier and chipped, while the chip segmentation type altersinto the sawtooth shape with very distinctive teeth on the

free surface. On increasing the lateral wear, the chip seg-mentation becomes smaller, i.e. the lamella formation fre-quency decreased, while the plastic deformations in theprimary cutting zone become larger and more distinctive.With the formation mechanisms of the sawtooth chip, forthe alternating cutting conditions, one can conclude that the

thermal-plastic deformation due to tool action is dominantuntil the appearance of the initial crack and, in that segment,the cutting occurs following the adiabatic theory of she-

aring, while the cracks in the upper area on the free surfaceof the primary cutting zone are formed following anothermechanism, the so-called crack theory.

Acknowledgements

This paper presents a segment of the research on theproject Contemporary approaches in the development ofspecial solutions bearing in mechanical engineering and

medical prosthetics, project number TR 35025. Financedby the Ministry of Education and Science of the Republic ofSerbia. University of Novi Sad, Faculty of Technical Scien-

ces.

Antic Aco, M.Sc., Dr.,University of Novi Sad, Faculty of Technical Sciences,

Trg Dositeja Obradovica 6, 21000 Novi Sad, Serbia,

E-mail: antica@uns.ac.rs

Fax: +381 21 454 495

Phone: +38121450366

Milan Zeljkovi

University of Novi Sad, Faculty of Technical SciencesTrg Dositeja Obradovica 6, 21000 Novi Sad, Serbia

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