ORIGINAL ARTICLE

Effect of cutting energy on fatigue behavior of threadedspecimens

Hamza K. Akyildiz & Haydar Livatyali

Received: 1 May 2013 /Accepted: 25 August 2013 /Published online: 18 September 2013# Springer-Verlag London 2013

Abstract Thread cutting is a form of cutting process, andduring the process, energy is transferred into the workpieceby the tool that generates the surface layer of the threaded part.Cutting conditions are heavier in thread cutting than cylindri-cal side cutting operations, and energy consumption is moreimportant at the thread root during thread cutting. Total cuttingenergy is mainly affected by cutting forces and cutting veloc-ity. Variation of cutting forces, chip thickness ratios, and shearangles in thread machining were investigated. Effects of thethread height and tool wear on the chip compression ratiowere determined experimentally. Main cutting forces at thethread root were calculated by using Zorev’s force calculationapproach to evaluate the cutting power and specific cuttingenergy during thread cutting process. Fatigue strengths ofthreaded specimens produced by machining were evaluateddepending on the cutting power and specific cutting energy atthe thread root. It is observed from the experimental resultsthat when specific cutting energy is increased, fatiguestrengths of the threaded specimens are also increased signif-icantly, while cutting velocity and chip thickness are keptconstant in the specified range.

Keywords Machining . Thread cutting . Cutting power .

Specific cutting energy . Fatigue

1 Introduction

The fatigue strength of machine elements under fluctuatingloads heavily depends on the surface layer properties, as wellas the geometry, working temperature, size, and load type[1–3]. Endurance limits obtained experimentally include ef-fects of all machining and testing conditions. When fatigue isconsidered, machining processes impose three critical effectsin the surface layer of the machined part, which are residualstress, surface hardening (or rarely softening), and surfaceroughness.

Surface-layer residual stress during machining is caused bymechanical and thermal effects. Thermal effects are claimed tocause tensile, while mechanical effects usually lead to com-pressive residual stresses on the part surface [4–6]. A well-known fact is that increasing the surface residual tensile stressdecreases the fatigue strength, and increasing compressiveresidual stress increases fatigue strength of the part [1–3, 6–8].

Cutting tool wear is reported to increase the thermal effectssignificantly, and, thus, it generates tensile residual stresses.When the cutting tool is sharp, plastic deformation is the maincause of the residual stresses; thermal field plays a minor role;however, it becomes increasingly significant with flank wear[9]. It has also been reported that tensile residual stresses areincreased by increasing feed and cutting velocity [10–12].Strain hardening of the surface layer, as well as thermalhardening or softening is also observed during machining[10, 13]. Fatigue strength increases as the surface hardnessof the part increases.

In external thread machining on a lathe, the effect of thetool wear on the surface roughness is at a very low level. Thus,the effect of the axial feed on the surface roughness is differentthan smooth cylindrical parts. The increase or decrease of theaxial feed means the increase or decrease of the thread pitch inthread cutting. It is also claimed that the effect of the cuttingvelocity on the surface roughness has no significant effect

H. K. Akyildiz (*)Department of Mechanical Engineering, Bozok University,TR-66200 Yozgat, Turkeye-mail: hkemal.akyildiz@bozok.edu.tr

H. LivatyaliDepartment of Mechanical Engineering, Istanbul Tech. University,TR-34437, Gumussuyu Istanbul, Turkey

Int J Adv Manuf Technol (2014) 70:547–557DOI 10.1007/s00170-013-5278-1

between 50 and 150 m/min cutting velocity [14]. Consequent-ly, surface hardening or softening and residual stresses aremore influential on the fatigue strengths of the parts [9].

Determining the effects of the residual stresses and strainhardening on the fatigue strength of the parts is a very difficultprocedure because of measurement difficulties of the residualstresses and harnesses in surface layer of the part in practice. Itis also difficult to characterize and produce the depth ofeffective residual stress and strain hardening in the machinedpart for highest fatigue strength in practice.

When considering the present knowledge of fatigue, it canbe seen that predicting the fatigue strength of the partdepending on machining parameters or surface layer proper-ties is very difficult in practice. More easy methods are neededto determine fatigue strengths of machined parts. Duringmachining, it is easier to determine the cutting power andspecific cutting energy than to determine the residual stressand strain hardening in the surface layer of the part. Thus, inthis study, the relationship between cutting power, specificcutting energy, and fatigue strength of threaded test specimensproduced by machining was investigated.

2 Experimental method and cutting force prediction

2.1 Specimen design

Threaded fatigue test specimens were machined from AISI4340 steel bars (Fig. 1). This steel is one of the widely usedhigh-strength steels in the drill pipe industry especially inrotary shouldered threaded connections in drill pipe joints.The material of the specimens were hardened in oil at 800–850 °C and tempered at 550–600 °C twice. The mechanicalproperties and chemical composition of the steel are given inTables 1 and 2, respectively. The chemical and mechanicalproperties are given as an average of ten tests.

Threaded fatigue test specimen was designed for machin-ing in the universal lathe. During fatigue testing, according toexperimental results, the designed fatigue test specimen wasmodified for ensuring more reliable test results. Details of thedesign process of threaded fatigue test specimen for machin-ing were given by the authors previously [15]. The threadedfatigue test specimen given in Fig. 1 was used in all fatigueexperiments. However, among the specimens machined in the

universal lathe, two of three specimens were broken at thetransition part of the critical test sections, where the threadteeth were not fully developed, and machining conditionswere also not fully obtained. When fatigue test results andcutting force simulations are considered, the fracture shouldtake place in the critical test section at any point along the fullydeveloped teeth. Consequently, this specimen design was noteffective, and, thus, a new design that has more effectivecritical minor test diameter at the critical threaded section isneeded. Fatigue test results of the specimens that were frac-tured at the region which have fully developed threads wereconsidered in this study.

The thread tooth form machined on the specimens is thestandard API V-0.040 thread form used in the rotary shoul-dered connections (pin-box) [16]. In the machining of thethread form on the specimens, Kennametal LT-22ER 5API403 KC5025 grade threading tool was used. The tool was0.5 mm chamfered at the tool tip. Rake angle and flank anglewere −10 and +10°, respectively, at the tool tip. The threadingtool is chamfered at the tool tip. The chemical composition ofthe ISO-P type sintered carbide tool was 78%WC, 14% TiC,and 8 % Co. Specimens were machined in the universal lathebetween chuck and tail stock center. In all conditions, coolantwas used. Coolant liquid (10 % boron oil) was used duringmachining of specimens.

2.2 Fatigue testing

When threaded fatigue test specimens are tested, critical testregions of the specimens cannot be restricted to a specificlocation, as in the notched specimens in cantilever type fatiguetest machine, because the thread is a continuous helical notchon the specimen. Thus, a four-point rotary bending typefatigue machine that can maintain constant pure bendingmoment over the threaded zone is the most appropriate testingequipment for threaded specimens.

In Fig. 2, schematic of a Moore type four-point rotarybending fatigue test machine is given. Maximum bendingmoment supplied by the fatigue testing machine was240 nm, and maximum specimen diameter allowable was25 mm.

Six sets of fatigue experiments given in Table 3 wereconducted in this study. While evaluating fatigue strengthsfor sets of threaded specimens, 12–16 specimens in total were

Fig. 1 Threaded fatigue testspecimen

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used for one set fatigue experiment. Seven to ten specimens ineach set were tested in the infinite life region of the experimentto determine the endurance limits clearly. In the fatigue exper-iments, fatigue strengths of the specimens for 2.106 cycleswere experimentally determined, and fatigue strengths ofthreaded specimens were evaluated for 90 % reliability forall experiments. Misalignment (eccentricity) of the specimenswas measured by using an analogue dial indicator at thetransition region of each specimen before starting the experi-ment on the testing machine. In all fatigue tests, totalmisalignment of the specimen for each experiment was keptless than 0.125 mm during rotation in the testing machine.

2.3 Cutting force model, cutting power, and energy

Cutting power and specific cutting energy was determined inthread root for each set of cutting condition. Compoundstraight and swiveled feeding methods were used duringmachining of threaded specimens. Because it is not possibleto measure cutting forces separately at the thread root, forevaluating cutting forces at the thread root (machined regionby the tool circular nose radius), Zorev’s force calculationmodel was used and experimentally verified [13].

Cutting force measurements for the experimental verifica-tions on the universal lathe during thread cutting were madeby using one component mechanical dynamometer. To deter-mine chip thickness ratio during thread machining, threadchips were collected and prepared for optical microscope tomeasure chip thicknesses and determine chip thickness ratiosfor thread chips (Fig. 3 and Fig. 4).

According to Zorev’s force model, the following equationwas used to calculate the main cutting forces at the thread rootduring cutting:

Fe ¼ τKa−SinγCosγ

þ TanC

� �a1l ð1Þ

In this equation, C is a material constant and has littlevariation depending on the mechanical properties of the ma-terial, and it is assumed as C ≈46∘ for steels containing 0.15–0.5 % carbon [13]. τ is the shear stress on the shear plane, γ isthe rake angle, a1 is the uncut chip thickness, l is the width ofcut, and Ka is the chip thickness or compression ratio. The(chip) compression ratio, Ka (or its reciprocal the chip ratio),is determined as the ratio of the chip thickness, a2, to the uncutchip thickness, a1 [17, 18], that is as follows:

Ka ¼ a2a1

ð2Þ

In this investigation, τ ≈0.9σu is used as recommended byBobrov for alloyed steels [10].

The length of the cutting edge is related to the edge forces(friction forces), and edge forces are very small in comparisonto the shear forces while tool is sharp. Edge forces in this studywere ignored. It is clearly seen from Fig. 5b that for both thefine and coarse threading passes, lengths of the edges are thesame and must be considered that edge forces are not affectedfrom the uncut chip thickness.

After evaluating the main cutting force Fc, the energy forunit time P or power required for cutting can be calculated asfollows:

P ¼ Fc⋅V ð3Þ

In the above equation, V is the cutting velocity. The specificcutting power Ps, or unit cutting energy, is the power required

Table 1 Chemical composition of the AISI 4340 steel used in the experiments

Element C P Mo Si S Ni Mn Cr Al W Sb Fe

% 0.429 0.0161 0.047 0.232 0.0186 1.11 0.65 0.531 0.022 0.074 0.049 Remaining

Table 2 Mechanical properties of the AISI 4340 steel used in theexperiments

σy,0.001

[MPa]σy,0.002[MPa]

Tensilestrengthσu

[MPa]

Hardness[HRC]

Maximumelongation[%δ]

Cross-sectionalreduction[%ψ]

E[GPa]

1,199 1,220 1,300 33 12 48 209

9 2 1 3

8 7

6 4

5

10

Fig. 2 R.R Moore four-point rotary bending fatigue testing machine, 1specimen, 2 gripping part, 3 gripping part, 4 load arm, 5 loads, 6 fixedjoint, 7 bearing, 8 bearing, 9 electric motor, and 10 flexible coupling

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to machine a unit volume of the work material and can becalculated as follows:

Ps ¼ P

MRRð4Þ

MRR is the volume of the work material machined for unittime and can be calculated by using chip cross-sectional areaand cutting velocity during cutting.

3 Experimental results and discussions

3.1 Machining experiments

Detailed results of cutting force simulations in thread cuttingoperations were given by the authors previously [19, 20].

Experimental verification for the force model was obtainedfor 0.1 mm constant radial depth of cut per pass for 16threading passes in this study. Difference between experimen-tal and simulated cutting forces for all machining conditionswas less than 10%. Simulated and measured cutting forces for0.1 mm constant radial depth of cut per pass up to H=1.6 mmthread height are given in the Fig. 6. As the cumulative radialfeed increases, the main cutting forces increase due to theincreases in the uncut chip cross-section area. Thread cuttingforces depending on the thread height are measured for 88 m/min cutting velocity and 0.1 mm constant uncut chip thicknessper pass at thread root (Fig. 6). It is shown from the results thatcalculated cutting forces are in good agreement with theexperimental results.

As it is known, cutting forces varies depending on chip cross-sectional area and length of cuts. Uncut chip cross-sectionalareas depending on the numbers of threading passes are depictedin Fig. 4. Chip cross-sectional areas and lengths of cuts duringthread cutting depending on the successive threading passeswere analyzed and given in Fig. 5a, b, respectively.

Throughout the sides, chip thickness is constant. Chipthickness has the highest value at the thread root and decreasestoward the sides and become half of the value of the root at thesides for the specified tool geometry.

The area of uncut chip cross-section for each cutting passchanges as the cumulative radial feed changes from 0.05 to2 mm (0.05×40 threading passes or 0.1 mm×20 threadingpasses). While the cumulative radial feed increases, linearlyuncut chip cross-section areas increase exponentially until theradial feed reaches to the value of 0.25 mm (Hr, for used tool,fifth pass for Sr=0.05 mm). Thereafter, chip cross-sectionalareas vary linearly. This is the consequence of the effect of thetool nose radius on the chip cross-section as the cumulativeradial feed reaches the engagement point of the linear sidesand circular nose radius. After that critical point, the chipcross-sections increase linearly when the nose radius and sideedges of the threading tool engaged.

Because thread cutting is a form of cutting operation, thereis interference of chip flow on the rake face of the tool duringthread machining. It is reported that specific cutting forcedecreases in the beginning of the threading and then increases

Table 3 Thread cutting parameters and fatigue strength of the threadedspecimens

Thread cutting parameters(cutting fluid 10 % boron oil inall the experiments )

Main cutting force atthread root Fc (N)

Fatiguestrength(MPa)

Sr=0.1 ;V=88 m/min; sharp toolCompound straight tool feedmethod

396 217

Sr=0.05 ; V=88 m/min; sharptool

Compound straight tool feedmethod

212 198

Sr=0.1; V=44 m/min; sharp toolCompound straight tool feedmethod

415 166

Sr=0.1; V=88 m/min; flankwear= 0.3 mm

Compound straight tool feedmethod

491 242

Sr=0.1;V=88 m/min; flank wear= 0. 6 mm

Compound straight tool feedmethod

641 280

Sr=0.1;V=88 m/min; sharp toolCompound swiveled tool feedmethod

254 182

Fig. 3 Thread chip profile beforeand after machining H =threadheight, Sr=chip thickness inthread root, Hr=thread height ofthe circular root radius, and H1=heights of the sides

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with the cumulative radial feed, which explains the effect ofchip interference [21].

When fatigue is considered, the most important region ofthe threaded part is the thread root. Theoretically, the notchfactor has the highest value, and diameter of the threaded parthas the lowest value at the root. Maximum nominal stressoccurs in thread root during cyclic loading of the part. Whenthread cutting operations were compared with the side cutturning operations, it has been found that the main cuttingforce components are nearly twice as large as in the side cutturning because the interference of the chip flow occurring inthe threading complicates the deformation [21].

Variation of the chip thickness ratio at the thread rootdepending on the cumulative radial feed (thread height) wasgiven in Fig. 7. Chip thickness ratio, on the contrary to shearstrain, represents the true plastic deformation in metal cutting.The chip compression ratio can be used to calculate the totalwork done by the external force applied to the tool and thenmight be used for optimization of the cutting process [22]. It isseen from Fig. 7 that with increasing thread height, chipthickness ratio increases significantly at the thread root. Chipinterference during thread cutting also may be one of thereasons of the increasing chip compression ratio at the threadroot beside constrains of thread sides during thread cutting.

Fig. 4 Chip cross-sectional areasduring thread cutting (compoundstraight feed method) A1 cross-sectional area of the first pass, An

=cross-sectional area of the nthpass, Sr=radial feed given perpass, and H=thread height(cumulative radial feed)

0 5 10 15 20 25 30 35 400

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Number of the Threading Pass

Cro

ssec

tiona

lare

a [m

m2 ]

[Sr = 0.05 mm]

[Sr = 0.1 mm]

0 5 10 15 20 25 30 35 400

1

2

3

4

5

6

Number of the Threading Pass

Leng

th o

f C

uttin

g E

dge

[mm

]

[Sr = 0.05 mm]

[Sr = 0.1 mm]

a b

Fig. 5 Variation of the chip cross-sectional areas and length of the cutting edge depending on the number of the threading passes in thread machining(compound straight tool feed method, API V-0.040 thread form, Sr=radial feed per pass (depth of cut at the tool tip))

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In thread sides, there is no significant change in compressionratio (and probably temperature) because with increasing threadheight, only the chip width changes, and chip width does notinfluence the compression ratio. On the other hand, compressionratio increases towards a saturation level with increased cumu-lative radial feed due to tighter constraint induced by the sides onthe root. Higher chip thickness ratio means higher plastic defor-mation in the chip, and this means higher adiabatic heating and,thus, higher temperature. So it is shown from Fig. 7 that effi-ciency of machining process at thread root decreases dependingon the thread height during thread machining.

Variation of the shear angle is determined according tocumulative radial feed. Figure 8 shows that the shear angledecreases by increasing cumulative radial feed (constant radialfeed per pass) in thread cutting. This figures show that whenthread height is increased, machining conditions are becomingheavier, and specific cutting energy is increasing at the threadroot. Thread tooth height is very important for the formationof the surface layer at the thread root. The level of the plasticdeformation at the thread root is determined by the threadheight. Effect of the uncut chip thickness on the chip thicknessratio in thread machining was also investigated. For differentuncut chip thicknesses at last passes in two thread heights,variations of the chip thickness ratios were determined exper-imentally (Fig. 9). It is seen from Fig. 9 that chip thickness

ratio decreases with increasing uncut chip thickness. It is alsoshown that the effect of the uncut chip thickness on the chipthickness ratio is more influential in lower thread heights. Ifthread height is increased, the effect of uncut chip thickness onthe chip thickness ratio decreases. This results show that tincalibration passes are also useful for increasing fatiguestrength of the part besides increasing geometrical precision.

When thread height is increased, the energy spent for theplastic deformation in thread root is also increased. Thiscauses severe residual stress and strain hardening at threadroot. Tool wear is one of important effects producing heat inthe machining. Tool wear also increases cutting forces. Effectof the tool wear on the main thread cutting force was deter-mined for the Hr=0.25 mm thread height depending on theradial feed (Fig. 10). Effects of the 0.3 and 0.6 mm tool flankwear on the main cutting forces were determined experimen-tally in three different radial feeds.

It is seen from Fig. 10 that tool wear increases the maincutting forces in significant amounts. Because friction forcesare not affected by the uncut chip thickness, when uncut chipthickness is decreased, the effect of tool wear on the maincutting force becomes more influential. Increasing frictionforces produce more increase in the specific cutting energy.

When the effect of the tool wear on the chip compressionratio is investigated in the thread root, similar effect can be

0

200

400

600

800

1000

1200

0 2 4 6 8 10 12 14 16 18

Mea

sure

d A

nd S

imul

ated

Mai

n C

uttin

g F

orce

s [N

]

The number of the passes , Radial Feed =0.1 mm Thread Heigth)

Experimentally measured maincutting forcesSimulated main cutting Forces

Fig. 6 Simulated andexperimentally measured mainthread cutting forces duringthread cutting. Radial feed (Sr)=0.1 mm per pass, total threadheight=1.6 mm, sharp tool

1

1.5

2

2.5

3

3.5

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Chi

p T

hick

ness

Rat

io A

t the

T

hrea

d R

oot [

Ka]

Cumulative Radial Feed (thread Heith) H,[mm]

H

Fig. 7 Variation of the chipcompression ratio (Ka) at thethread root during tread cutting(V=88 m/min, Sr=0.1 mm,cutting fluid 10 % boron oil)

552 Int J Adv Manuf Technol (2014) 70:547–557

seen from Fig. 11. With increasing tool wear from 0.05 to0.6 mm, chip compression ratio increases from 1.75 to 2.4 forthe 0.1 mm uncut chip thickness in 0.25 mm thread height.Chip thickness ratio increases from 2.35 to 4.0 when the uncutchip thickness decreased to 0.025 mm depth of cut at the lastpass for the same thread height. Experimental results showthat tool wear increases chip thickness ratio in the significantamount in smaller depth of cuts. When worn tool duringfinishing passes is used, fatigue strength of machined partsmay increase.

3.2 Fatigue experiments

Threaded fatigue specimens were machined at the definedthread machining conditions, and fatigue strengths of thefatigue specimens for different machining conditions weredetermined (Table 3). Detailed results of the fatigue testsconducted with threaded specimens were presented by theauthors previously [23]. In this study, effects of the cuttingpower for unit cross-sectional chip area and specific cuttingenergy for unit volume of material on the fatigue behavior ofthreaded fatigue test specimens were investigated. Residualstresses, strain hardening, and hardening or softening of thematerial during machining are caused by the total cutting

energy. Therefore, cutting power and specific cutting energyin thread root were calculated for different thread cuttingconditions and compared with the fatigue strengths of thespecimens (Table 3). Variation of the fatigue strengths of thethreaded specimens depending on the cutting power and spe-cific cutting energy is given in Fig. 12a, b, respectively.

It is seen from Fig. 12a that with increasing cutting powerfrom 3,741 to 11,556 W/mm2, fatigue strengths of the speci-men increased from 166 to 280 MPa. An increase in fatiguestrengths of the specimens with increasing specific cuttingenergy can also be seen, while cutting velocity and uncut chipthickness are kept constant (Fig. 12b). It is seen from Fig. 12bthat specific cutting energies are about at the same level for the44 m/min cutting velocity, 0.1 and 0.05 mm uncut chipthicknesses, but fatigue strengths are different for these cuttingparameters. The lowest fatigue strength was obtained for thelowest cutting velocity. Fatigue strength of the 0.05 mm uncutchip thickness is lower than the fatigue strength of the 0.1 mmof uncut chip thickness.

It can be seen from Fig. 12b that the share of the energyspent for different surface modification mechanisms are af-fected significantly by the cutting velocity and uncut chipthickness during cutting. Decreasing cutting velocity maydecrease the thermal effects, and increasing uncut chip

14

16

18

20

22

24

26

28

30

0.1 0.3 0.5 0.7 0.9 1.1 1.3 1.5 1.7

She

ar a

ngle

[Deg

ree]

Cumulative Radial Feed (H=Thread Heigth)[mm]

H

Fig. 8 Variation of the shearangle at the thread root (V=88 m/min, Sr=0.1 mm, cutting fluid10 % boron oil)

Fig. 9 Effect of the radial feed onthe chip compression ratio at thethread root in two different threadheights (V=88 m/min, a1=chipthickness, and a2=thickness ofdeformed chip)

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thickness may decrease strain hardening effects. Increasingstrain hardening may increase fatigue strength. Increasingthermal effects may increase or decrease fatigue strengthbecause of the formation of tensile residual stresses.

When cutting velocity and uncut chip thickness are keptconstant, and machining conditions given in the Table 3 isconsidered, it is seen that the main cause of the increase in thespecific cutting energy is the tool wear. Worn tool increasedchip thickness ratios and cutting forces. Increase in the chipthickness ratios increased the plastic deformation. It is knownthat the cutting edge sharpness exerts a great influence on themachined surface hardness also. The hardness of the surfacemachined with a sharp tool is smaller than that machined witha dull tool [24, 25]. Increasing of hardness in the surface layerof the machined part may be caused by phase transformationor deformation hardening. But both of the hardening mecha-nisms need energy. It is seen from Fig. 12b that fatiguestrengths of the specimens increased with increasing specificcutting energy, while cutting velocity and uncut chip thicknessare kept constant. But it should be consider that the limits ofthis increase in fatigue strength should be investigated withmore detailed researches.

It is also seen from the SEM pictures of the machined withworn tool (0.6 mm flank wear) and fractured threaded surfacethat there is significant plowing and burnishing effect on themachined surface because of the energy spent for plasticdeformation is increased when worn tool is used (Fig. 13). It

is seen from Fig. 13 that significant increases in surfaceroughness and consequently theoretical notch factor wereobtained when worn tool in thread machining is used. Toolwear changes the cutting edge radius. It is found that theresidual stress of the machined surface is largely affected bythe tool sharpness. At the same depth of cut, the absolute valueof residual stress varies with the cutting edge radius. A largecutting edge radius corresponds to high residual stresses.When the depth of cut is at the same order of the cutting edgeradius, the workpiece is actually removed by the tool with alarge negative rake angle. In this case, the cutting process isaccompanied by severe rubbing or burnishing action. Thespecific machining energy will increase dramatically, and thematerial near the vicinity of the tool tip will be subject to alarge plastic deformation. As a result, the magnitudes ofresidual stress and strain hardening will increase greatly [25].

Larger tool-edge radius induces higher tensile residualstresses in the near-surface layer, while the thickness ofstressed layer is unaffected. Higher tensile stresses are attrib-uted to the increase in workpiece temperature with edgeradius, as more heat is generated when the friction contactarea between the tool tip and workpiece increases. Larger tool-edge radius induces higher compressive residual stress, farfrom the surface (for balancing the surface residual tensilestresses), andmoves the location of their maximummagnitudedeeper into the workpiece. This is attributed to higher materialplastic deformation and more material being ploughed into the

0

50

100

150

200

250

300

350

400

450

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

Cut

ting

For

ces

[N]

Flank wear [mm]

Sr=0.025 mmSr=0.05 mmSr=0.1 mm

Hr

Fig. 10 Experimentallymeasured cutting forces. Effect ofthe tool wear on the cutting forcesin three different radial feeds atlast passes=88 m/min (H =0.25 mm)

1

1.5

2

2.5

3

3.5

4

4.5

0 0.1 0.2 0.3 0.4 0.5 0.6

Chi

p th

ickn

ess

ratio

Flank wear [mm]

Sr=0.025 mmSr=0.05 mmSr=0.1mm

Hr

Fig. 11 Effect of tool wear on thechip thickness ratio V=88 m/min,10 % boron oil cutting fluid, Sr=0.025, 0. 05, and 0.1 mm, andHr=0.25 mm

554 Int J Adv Manuf Technol (2014) 70:547–557

new machined surface. A stagnation zone is developed fornon-sharp tools underneath the tool edge, which acts as thefirst effective cutting edge. The location of the stagnation-zone tip moves away from the machined surface as the tool-edge radius increases. This explains the increase in the amountof ploughed material and material plastic deformation, whichconsequently explains the increase in the plowing force com-ponent with edge radius [26].

When plastic deformation increases during machining re-sidual stresses, strain hardening increase too. But in the com-bined manner of the residual stresses and strain hardening,information about their effects on the fatigue behavior of thepart is not yet clearly obtained. The cause of the increasing ofthe fatigue strength of the parts machined by using worn toolmay be strain hardening on the thread root. According toexperimental results, it is shown that, if residual stresses andstrain hardening take place in a surface layer, strain hardeningis more important for the fatigue strength of the part. Mechan-ical properties of the strain-hardened material also arechanged. In modified material, residual stresses are formedaccording to the modified materials mechanical and thermalproperties. On the other hand, increasing the friction producesexcessive heat. This heat causes undesirable high temperaturein the tool workpiece contact area which leads to softening ofthe tool material and phase transformation on the surface layerof the part. During this transformation in the workpiece, thesurface may be hardened or softened. If it is hardened, thenfatigue strength of the part may be increased. But tool life isalways reduced by the excessive heat. The manufacturingresearchers’ challenge has been to decrease the cutting forceand move the heat toward the chip with better tool geometrydesign [24]. But this is impossible in thread cutting because ofthe fixed tool geometry by the relevant standard. Researchers[19–21] have investigated cutting forces experimentally andtheoretically during thread cutting and shown that cuttingforces are excessively higher than the cutting forces of

cylindrical side cut operations in the thread cutting. Thus,selecting of cutting parameters during thread cutting becomesa critical research subject.

Understanding the formation of thread profile and the V-shaped chip is crucial because the mechanical performance ofthe precision screw thread, particularly the fatigue behavior,depends on the deformation hardening and residual stresses onthe surface [23, 27].

During the process of thread machining in similar condi-tions given in Table 3, significant residual stress and strainhardening at the thread root is observed by Fetullazade et al.[24]. Tensile residual stresses are reported in both tangentialand axial direction at thread root, and it was observed thatstrain hardening took place at the surface layer in all machin-ing conditions. Tool wear increased the strain hardening in thesurface layer of the thread root. Fetullazade et al. showed that

Fig. 12 Variation of the fatigue strength depending on the thread cutting power end energy

Fig. 13 Burnishing and plowing effects on the machined thread rootsurface of the specimen

Int J Adv Manuf Technol (2014) 70:547–557 555

when using worn tool in thread machining, tensile residualstresses increased significantly as opposed to using sharp tool.

Later, Akyildiz and Livatyali [23] have conducted fa-tigue tests with threaded specimens machined in the similarconditions with Fetullayev’s specimens. They experimental-ly confirmed that fatigue strengths of the threaded speci-mens did not decrease depending on the tensile residualstress which is induced by the machining process. Also,they produced threaded fatigue test specimens by usingworn tool and conducted fatigue tests with them. Thestrengths of the specimens machined by using worn toolincreased unexpectedly.

Residual stresses and strain hardening in the surfacelayer of the machined part are very important factors forevaluating fatigue strength. But determining the interactiveeffects of these factors on the fatigue strength of the part is avery difficult process. For that reason, in this study, theeffect of specific cutting energy on the fatigue strengths ofthe threaded fatigue test specimens was evaluated. It is seenthat fatigue strength of the specimen increased linearly withincreasing specific cutting energy at thread root. It is shownfrom Fig. 12 that any effect that increases specific cuttingenergy also increases fatigue strength of the machined partin the specified range. In machining total, cutting energy isspent for friction heating, plastic deformation, and phasetransformation. All of these effects determine the finalmechanical properties of the part surface layer. From theview point of fatigue, the amount of energy consumed bythese mechanisms is very important. In the presented study,this situation is not considered.

4 Conclusions

In this study, threaded fatigue test specimens were machinedat various conditions and tested to evaluate their fatiguestrength as a function of specific cutting energy. Variationsof the chip thickness ratio and shear angle during threadmachining were determined at the thread root depending onthread height. Specific cutting energies for different threadcutting conditions were calculated. Fatigue strengths ofthreaded specimens were determined depending on the spe-cific cutting energy.

Based on the results, the following may be concluded:Fatigue strength of the specimen increased with increasing

cutting power for unit cross-sectional chip area.When specific cutting energy is increased, fatigue strength

of the threaded specimens also increased, while cutting veloc-ity and uncut chip thickness are kept constant. This is becauseof the energy spent for plastic deformation increases in threadroot in the studied conditions.

To determine the limits of increase, different machiningconditions should be studied. It should be studied also to

determine the amount of the energy that is used for plasticdeformation in thread machining. Chip thickness ratio is avery good indicator of this ratio.

When thread height (cumulative radial feed–number ofthreading passes) during thread machining is increased, chipthickness ratio increases and shear angle decreases in threadroot. When thread tooth height is increased, thread cuttingconditions in thread root become more severe. During achiev-ing of thread height in machining, the amount of the plasticdeformation at thread root should be considered. Transforma-tions at the thread root may cause weakening of the surfacelayer mechanically because of crack initiation on the surface(overhardening).

When the thread height is increased, the effect of the uncutchip thickness on the thread root decreases. In the lower threadheights, the effects of the uncut chip thickness on the chipthickness ratio at the thread root are more influential. Thecause of this effect may be edge forces. In the higher threadheights, the length of the cutting edge is higher.

The effect of the tool flank wear on the chip thickness ratioincreases when the uncut chip thickness reduced. For thatreason, calibration passes in the thread cutting are very im-portant for the formation of the surface layer of the threadedparts.

Variations of shear angles and chip thickness ratiosdepending on the cumulative radial feed at the thread rootwere experimentally determined, and severe conditions atthread root were experimentally observed.

Acknowledgments This paper is summarized from H.K. Akyildiz’sdissertation at the Istanbul Technical University. The authors express theirultimate gratitude and prayers of mercy and grace to late Assoc. Prof.Eldar Fetullayev(Fetullazade) of the Bozok University who co-supervised the original doctorate and passed away in October 2008.

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