Research ArticleA Mathematical Modeling to Predictthe Cutting Forces in Microdrilling
Haoqiang Zhang,1,2 Xibin Wang,1 and Siqin Pang1
1 Key Laboratory of Fundamental Science for Advanced Machining, Beijing Institute of Technology, Beijing 100081, China2Hebei United University, Tangshan 063009, China
Correspondence should be addressed to Haoqiang Zhang; [email protected]
Received 4 June 2014; Revised 18 July 2014; Accepted 19 July 2014; Published 6 August 2014
Academic Editor: Zhen-Lai Han
Copyright Β© 2014 Haoqiang Zhang et al. This is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.
In microdrilling, because of lower feed, the microdrill cutting edge radius is comparable to the chip thickness. The cuttingedges therefore should be regarded as rounded edges, which results in a more complex cutting mechanism. Because of this, themacrodrilling thrust modeling is not suitable for microdrilling. In this paper, a mathematical modeling to predict microdrillingthrust is developed, and the geometric characteristics of microdrill were considered in force models.The thrust is modeled in threeparts: major cutting edges, secondary cutting edge, and indentation zone. Based on slip-line field theory, the major cutting edgesand secondary cutting edge are divided into elements, and the elemental forces are determined by an oblique cutting model and anorthogonalmodel, respectively.The thrustmodeling of themajor cutting edges and second cutting edge includes two different kindsof processes: shearing and ploughing. The indentation zone is modeled as a rigid wedge. The force model is verified by comparingthe predicted forces and the measured cutting forces.
1. Introduction
There has been an increasing requirement for high-accuracymicroholes in the microelectronic, automotive, computercomponents, and sensor industries. Microdrilling is experi-encing a very rapid growth in precision production indus-tries. Inmany aspects, microdrilling has fundamentally iden-tical features with conventional drilling, but the downsizingof the dimensions of the drill introduces many problems,which has a major influence on the microdrilling process,such as cutting edge radius, increased web thickness, largevibrations due to high rotation speed, and high ratio of drillbreakage. There are many factors influencing the microdrill-ing process, such as drill geometry, drill materials, drillingforces, workpiece materials, machining parameters, and vib-ration. Drilling forces are related to drill life, holes quality,and productivity.Therefore, drilling forces are one of themostimportant factors affecting the drill performance.
In general, there are four methods of modeling cuttingforces in metal machining: analytical method, experimentalmethod, mechanistic method, and numerical method [1].Many models have been developed by researchers in the
past several decades. In the study of macrodrilling models,Shaw and Oxford [2] were the pioneers. Armarego andCheng [3, 4] presented a model in which a series of obliquecutting slices was used to the drilling process with flat rakeface and conventional twist drills. Watson [5β8] produceda more detailed model of material removal in both cuttingedges and chisel edge. Stephenson and Agapiouβs model [9]simulated arbitrary drill point geometries. Chandrasekharanet al. [10, 11] developed a mechanistic model of the cuttinglips and chisel edge to predict the cutting force systemfor arbitrary drill point geometry. Strenkowski et al. [12]developed a thrust force model based on analytical finiteelement technique in drilling with twist drills. In their model,the cutting lips were regarded as a series of oblique sections,and the cutting of the chisel region was treated as orthogonalcutting. Wang and Zhang [13] presented a predictive modelfor the thrust in drilling operations usingmodified plane rakefaced twist drills. Their models were based on the mechanicsof cutting approach incorporating many tools and cuttingprocess variables. There was less literature on force modelinginmicrodrilling. Sambhav et al. [14]modeled the thrust by theprimary cutting lip of a microdrill analytically and modeled
Hindawi Publishing CorporationMathematical Problems in EngineeringVolume 2014, Article ID 543298, 11 pageshttp://dx.doi.org/10.1155/2014/543298
2 Mathematical Problems in Engineering
Indentation zoneSecondary
cutting edges Rind
Figure 1: Regions of the chisel edge.
shearing forces and ploughing forces of the major cuttingedges. Hinds and Treanor [15] analyzed the stresses occur-ring in microdrills using finite element methods in printedcircuit board drilling process, but they did not produce anymathematical model for cutting forces of microdrills.
Slip-line field theory was often used to analyze the cuttingprocess. Manymachining parameters can be predicted by theslip-line field model, such as cutting force, chip thickness,shear strain, and shear strain-rate. Merchant [16] was thefirst one who presented a mathematical model to determineshear angle by using the minimum energy principle, andhis model was the basis of all subsequent models. Lee andShaffer [17] developed a slip-line field model which wasan approximation method under certain cutting conditions.Dewhurst and Collins [18] presented a matrix technique fornumerically solving slip-line problems. Oxley [19] proposeda parallel surface shear zone model of orthogonal cuttingthat considered the change of material flow stress. Waldorfet al. [20] developed a slip-line model for ploughing by acutting tool with a definite cutting edge radius. Fang [21]presented a generalized slip-line field model for cuttingwhen edge was rounded. Fangβs model included nine effectsthat commonly occurred in machining. Then Fang [22]quantitatively analyzed orthogonal metal cutting processesbased on his slip-line model. Manjunathaiah and Endres [23]developed a new orthogonal process model that included theeffects of edge radius. Jin and Altintas [24] simplified Fangβsmodel, and they considered the effects of strain, strain-rate,and temperature on the cutting process.
In microcutting applications the uncut chip thicknessis very small, typically within the range of 25 πm. Sincethe cutting edge radius is typically ground with a 5β20πm,the assumption of having a perfectly sharp cutting edge inmacrodrilling is not valid, so the cutting edge radius shouldnot be taken to be zero in microcutting operations. Paststudies have found that if the uncut chip thickness is belowthe minimum chip thickness π‘cmin, elastic deformation ora mixed elastic-plastic deformation will take place. Abovethis value, chip formation starts taking place. This is knownas the minimum chip thickness effect. However, due to the
extrusion of the material by the chisel edge region of thedrill, the drilling process can still take place, even if the chipthickness is very small.
The cutting edge is made up of the major cutting edgesand the chisel edge of the microdrill.Themajor cutting edgesare formed by the intersection of the flute surface with theflank surface of the microdrill, while the intersection of theflank surfaces forms the chisel edge. Although the length ofchisel edge is very small relative to the cutting edge of themicrodrill, the thrust created by the chisel edge is significant,and it exceeds even the thrust created by the cutting edges.In the region around the center of the chisel edge, materialremoval is by extrusion. This region is called the indentationzone, as shown in Figure 1. The portion of the chisel edgeoutside the indentation zone is termed as the secondarycutting edges. Material removal of secondary cutting edge isby orthogonal cutting with large negative rake angles.
During microdrilling, both shearing and indentingactions are happening. When the microdrill contacts theworkpiece, the drill point rubs workpiece first. Under theextrusion force of microdrill, material is squeezed around thedrill point; at the same time, the secondary cutting edges onthe chisel edge perform cutting.Then, the major cutting edgeenters intoworkpiece and begins to cut.The central portion ofthe chisel edge performs the indenting action, and the secondcutting edges on the chisel edge and the major cutting edgeson the fluted portion perform shearing.
In this paper, the thrust is modeled in three parts of amicrodrill: major cutting edges, secondary cutting edge, andthe indentation zone. The major cutting edge and secondarycutting edge force models are based on the slip-line fieldtheory, and the indentation zone is modeled as a rigid wedge.The model is, then, verified by comparing predicted thrustforce with measured data including the effects of microdrillgeometric and machining parameters.
2. Major Cutting Edge Cutting Force Models
The cutting behavior of the major cutting edge is an obliquecutting process. The cutting edge is divided into elementsand each element is approximated as a straight line, shownin Figure 2. The magnitude of the total drilling thrust (πΉ
1) is
obtained by summing the forces at all the cutting elements oneach edge and all the cutting edges on the drill.
The direction of the elemental cutting force ππΉcut isopposite to the velocity direction; ππΉcut is resolved intoππΉπΆ
and ππΉπΏ. The direction of ππΉ
πΆis along the actual
cutting direction. ππΉπΏis the elemental lateral force, which
is orthogonal to the cutting force and the elemental obliquecutting thrust force ππΉ
π.The thrust force ππΉ
πis normal to the
plane that contains the velocity vector and the cutting edge.The magnitude of those forces is given by
ππΉπΆ= ππΉcut β cos ππ
ππΉπΏ= ππΉcut β sin π
π
ππΉ1=cos π sinπ
cos ππ
β ππΉπβ tan π
π β cosπ β ππΉ
πΆ,
(1)
Mathematical Problems in Engineering 3
XY
Z
V
X
Y
V
Major cutting edge
r
rw
Fz
Fx
Fy
dFL
dFT
dFcut
dFC
π
πs
π
Figure 2: Forces at an element on the major cutting edge.
where π is the half point angle and the inclination angle ππ
and angle π can be obtained by the following equations:
ππ = sinβ1 (
ππ€
πsinπ)
π = sinβ1 (ππ€
π) ,
(2)
where ππ€is half the web thickness and π is the distance from
a point on the cutting edge to the drill axis.The normal rake angle at any point on the cutting edge is
πΎπ= tanβ1 (
(π/π ) tanπ½ cos πsinπ β (π
π€/π ) tanπ½ cosπ
)
β tanβ1 (tan π cosπ)
= tanβ1((π/π ) tanπ½β1 β (π2
π€/π2 )
sinπ β (ππ€/π ) tanπ½ cosπ
)
β tanβ1(ππ€cosπ
βπ2 β π2π€
),
(3)
where π½ is the helix angle of the drill and π is the drill radius.The magnitude of the total drilling thrust along the axis
of the drill can be obtained by summing the forces at allthe cutting elements on each cutting edge and all the cuttingedges on the drill, so themagnitude of the total drilling thrustforce πΉ
1is
πΉ1= 2β«ππΉ
1
= 2β«(cos π sinπ
cos ππ
β ππΉπβ tan π
π β cosπ β ππΉ
πΆ) .
(4)
Thus, if we know the forces for each cutting element in thecutting and thrust direction in the plane perpendicular to thecutting edge, the total drilling thrust force can be calculated.
Due to the technological and material constraints inmicrodrill preparation, the major cutting edge has a definiteradius, and the uncut chip thickness is very small, so themajor cutting edge cannot be seen as completely sharp. Theslip-line field model of microcutting process for each cuttingelement of major cutting edges is shown in Figure 3.
Thematerial deformation region consisted of three zones:primary shear zone [AIBB
1I1A1A2], secondary shear zone
[π»πΈπ΅πΊπΌπ½], and tertiary zone [BSCD1B1]. The shape of the
slip-line field was originally proposed by Fang [21]. In Fangβsmodel, the slip lines HJ and JI are defined as two basic sliplines; after their shapes are obtained, all other slip lines inthe secondary shear zone can be determined using Dewhurstand Collinsβs matrix technique [18].Then, the slip-lines in theprimary and tertiary shear zones can be easily determinedfrom relevant slip-line relationships. The primary shear zoneincluded three regions: triangular region AA
1A2, convex
region AII1A1, and concave IBB
1I1. In region AA
1A2, line
AA2is a stress-free boundary; all of the slip lines in AA
1A2
intersect with AA2at a 45β angle. Both region AII
1A1and
region IBB1I1consist of circular arcs and straight radial lines.
Point S is the separation point for the upward and downwardmaterial bifurcating. Part of the materials flows downwardsfrom point S to point C along the rounded edge, while otherparts of the materials flow upwards from point S to point B.In order to simplify the mathematical formulas of the slip-line problem with a curved boundary, the tool edge BC isapproximately represented by two straight chords BS and SC.
BS and SC are considered to have rough surfaces; theincluded angles between them and the slip lines are π
2and
π1, respectively. The intersection angle of AA
2and horizon is
πΏ. The separation angle ππ, the tool edge radius π
π, and the
tool rake angle πΎπdetermine the position of the stagnation
point π on the rounded tool edge. Geometric analysis givesthe following set of equations:
πΌ1=
5π
4β π2β
πΎπ
2β
ππ
2β π2+ π1,
4 Mathematical Problems in Engineering
Workpiece
Tool primary rake face
Rounded cutting edge
Chip
H
B
C
S
G1
I1
A1
A2
A3
A
O
I
V
45β π1
D1
B1
E
G
πΏ πΌ2
πΌ1
π2
rn
B
S πc
OπΎe
π2
π1
C
πb
πs
π4
πΎn
B1
D1
J
π3
Figure 3: Slip-line field model of each element cutting process of major cutting edges.
πΏ = πΌ1β
3π
4,
ππ = sinβ1 (β2 sin πΏ sin π
1) ,
ππ=
π
2 + πΎπβ ππ
,
π =1
2cosβ1 (π
π) ,
ππ΅π
= 2ππsin(
π
4+
πΎπ
2β
ππ
2) ,
πππΆ
= 2ππsin
ππ
2,
(5)
where π is the frictional shear stress and π is the material flowstress.The tool-chip frictional shear stress along the rake facewas assumed to be constant.
Jin and Altintas evaluated the total cutting forces byintegrating the forces along the entire chip-rake face contactzone and the ploughing force caused by the round edge. Thedetailed process can be found in [24]. According to theircomputing methods, the cutting forces on the major cuttingedges of microdrill can be derived as follows.
After thematerial passes through the shear zones, the chipbegins curling freely, so the resulting force along the slip linesπ΄πΌ, πΌπ½, and π½π» should be zero. Consider
πΉπ₯π΄πΌ
+ πΉπ₯πΌπ½
+ πΉπ₯π½π»
= 0,
πΉπ¦π΄πΌ
+ πΉπ¦πΌπ½
+ πΉπ¦π½π»
= 0.
(6)
Line π΄π΄2is a stress-free boundary. The distribution of
hydrostatic pressure and shear flow stress along slip line π΄πΌ
can be calculated by dividing π΄πΌ into several differentialelements, such as 100 differential elements, as shown in Figure4. The total force along slip line π΄πΌ is obtained by summingall of the elemental forces in theπ and π directions.
H
B
C
S
A
O
I
J
Chip
E
G
X
Y
G1
I1
A1
A2
A3
π1
D1B1
π2
π
π
N1kN1
N3
N4
N2N5
pN1
Figure 4: Stress analysis in the primary shear zone.
The forces on point π1of slip line π΄πΌ in the π and π
directions are calculated as
ππΉπ₯= (ππ1
sin π + ππ1
cos π) Ξππ΄πΌπ€,
ππΉπ¦= (ππ1
cos π + ππ1
sin π) Ξππ΄πΌπ€,
(7)
where ππ1
is the hydrostatic pressure and ππ1
is the shearflow stress on the element, π is the angular coordinate of theelement, Ξπ
π΄πΌis the length of the element, and π€ is the width
of cut. Consider
Ξππ΄πΌ
=ππ΄πΌ
π, (8)
where π is the number of divided differential elements.So the hydrostatic pressure π
πΌand shear flow stress π
πΌ
of point πΌ can be concluded. After the total forces along slipline π΄πΌ are calculated, the forces along πΌπ½ and π½π» can bedetermined. In the second shear zone, slip line π΅πΌ is dividedinto 100 angular elements in the same way; then the samenumber of slip lines is formed in the secondary shear zone.For any slip line π
2π3, the shear flow stress π
π2and the
hydrostatic pressure ππ2
at point π2are obtained from the
Mathematical Problems in Engineering 5
stress distribution in the primary shear zone. Then, the shearflow stress and hydrostatic pressure at pointπ
3are calculated
as
ππ3
= ππ2
,
ππ3
= ππ2
+ 2ππ2
β (2π) .
(9)
The elemental force is projected into the π₯ and π¦ direc-tions, and then the elemental forces in the π₯ and π¦ directionsat pointπ
3are
ππΉπ₯π»π΅
= [(βππ3
β ππ3
sin (2π)) Ξππ»π΅
π€] cos πΎπ
β [(ππ3
cos (2π) Ξππ»π΅
π€)] sin πΎπ,
ππΉπ¦π»π΅
= [β (βππ3
β ππ3
sin (2π)) Ξππ»π΅
π€] sin πΎπ
β [(ππ3
cos (2π) Ξππ»π΅
π€)] cos πΎπ.
(10)
The elemental force at other points along the tool rake faceπ΅π» is calculated following the same procedure as point π
3,
and then the total force along π΅π» is obtained by summing allof the elemental forces in theπ and π directions.
In tertiary shear zone, line π΅π΅1is divided into 100 small
elements. The shear flow stress and hydrostatic pressure atpointπ
4can be concluded from point π΅.Then, the shear flow
stress and hydrostatic pressure at pointπ5are calculated as
ππ5
= ππ4
ππ5
= ππ4
β 2ππ4
β (2π) .
(11)
The elemental forces along line π΅π in the π₯ and π¦ direc-tions at pointπ are
ππΉπ₯π΅π
= [(βππ5
β ππ5
sin (2π)) Ξππ΅ππ€] cos πΎ
π
β [(βππ5
cos (2π) Ξππ΅ππ€)] sin πΎ
π
ππΉπ¦π΅π
= [(βππ5
β ππ5
sin (2π)) Ξππ΅ππ€] sin πΎ
π
+ [(βππ5
cos (2π) Ξππ΅ππ€)] cos πΎ
π.
(12)
Similarly, along line ππΆ,
ππΉπ₯ππΆ
= [(βππ5
β ππ5
sin (2π)) ΞπππΆπ€] cos πΎ
π
+ [(βππ5
cos (2π) ΞπππΆπ€)] sin πΎ
π,
ππΉπ¦ππΆ
= [(βππ5
β ππ5
sin (2π)) ΞπππΆπ€] sin πΎ
π
β [(βππ5
cos (2π) ΞπππΆπ€)] cos πΎ
π.
(13)
The element forces in the plane perpendicular to themajor cutting edge along the cutting direction and the thrustdirection are obtained on the major cutting edge as
ππΉ1π
= (ππΉπ₯π»π΅
+ ππΉπ₯π΅π
+ ππΉπ₯ππΆ
) ΞπΏ,
ππΉ1π‘
= (ππΉπ¦π»π΅
+ ππΉπ¦π΅π
+ ππΉπ¦ππΆ
) ΞπΏ,
(14)
whereΞπΏ is the length of differential element ofmajor cuttingedge.
Therefore, the magnitude of the total drilling thrust alongthe axis on the major cutting edge of the drill πΉ
1is
πΉ1= 2β«ππΉ
1
= 2β«(cos π sinπ
cos ππ
β ππΉ1π‘β tan π
π β cosπ β ππΉ
1π) .
(15)
3. Chisel Edge Cutting Force Model
Mauch and Lauderbaugh [25] obtained the indentation zoneradius π ind (Figure 1) for a conical drill based on the pointangle. Paul et al. [26] suggested that the dynamic clearanceangle becomes zero at the indentation zone radius. So theradius π ind of the indentation zone is given by the followingequation:
π ind =π
2π tan πΎπ
, (16)
where πΎπ is the static clearance angle of the chisel edge.
3.1. Secondary Cutting Edge Cutting Force Model. Since thechisel edge has a definite radius and the uncut chip thicknessis comparable in size to the edge radius, the chisel edge cannotbe seen as completely sharp but should be as a roundededge. The chip thickness at the elements on the chisel edgeis equal to half of the drill feed. The secondary cutting edgesare divided into elements and the elemental drilling thrust isdetermined; then the magnitude of the total drilling thrustalong the axis of the drill can be obtained by summing theforces at all elements for the secondary cutting edges.
Because the flank surfaces of microdrill are plane, theslip-line model of secondary cutting edge is different fromthe major cutting edge. Figure 5 shows the analytical slip-line model for machining with secondary cutting edge. Theintersection angle of π΄π΄
2and horizontal line is πΏ. Consider
πΏ =π
4β ππ
πΏ2= ππ β πΎπβ π2,
πΏ3= π1+ π2+ πΎπβ ππ .
(17)
The element forces in the plane perpendicular to thechisel edge along the thrust direction on the chisel edge areobtained as
ππΉ2= πΞπΏ {(cosπ
π β sinπ
π ) ππ»π΅
+ [cos 2π2cos πΎπ
β (1 + sin (2πΏ2+ 2π2)) sin πΎ
π] ππ΅π
+ [(1 + sin (2πΏ2+ 2πΏ3+ 2π1)) cos π
π
β cos 2π1sin ππ ] πππΆ} ,
(18)
6 Mathematical Problems in Engineering
Chip
Workpiece
Chisel edge
Secondary cutting edge
A
A2A3
45β
45β
πΏ45β
I
B2
D2
B
S
C
H
V
πΎn
O
rn
πΏ2 πΏ3
B
S
πc
OπΎe
π2
π1
C
πb
πsA1
I1 B1
D1
VChip
πs
tc
Figure 5: Slip-line field model for machining with secondary cutting edge.
where ΞπΏ is the length of differential element, and
ππ»π΅
=π‘π+ ππ΄π΄2
sin πΏ β ππ(1 + sin πΎ
π)
sinππ
,
ππ΅π
= 2ππsin(
π
4+
πΎπ
2β
ππ
2) ,
πππΆ
= 2ππsin
ππ
2,
(19)
where π‘πis the uncut chip thickness, π‘
π= π/2, and π is feed.
Consider
ππ΄π΄2
= β2 (ππ΅πcos π2+ πππΆ
sin π1) . (20)
Themagnitude of the total drilling thrust can be obtainedby summing the forces at all elements for the secondarycutting edges. So the magnitude of the total drilling thrustforce πΉ
2is
πΉ2= 2
πΏπ/2
β
π ind
ππΉ2
= 2π
πΏπ/2
β
π ind
{(cosππ β sinπ
π ) ππ»π΅
+ [cos 2π2cos πΎπ
β (1 + sin (2πΏ2+ 2π2)) sin πΎ
π] ππ΅π
+ [(1 + sin (2πΏ2+ 2πΏ3+ 2π1)) cos π
π
β cos 2π1sin ππ ] πππΆ} ΞπΏ,
(21)
where πΏπΆis the length of chisel edge.
3.2. The Indentation Zone Cutting Force Model. In micro-drilling processes, the ratio of web thickness to drill diameteris larger than that of macrodrilling, so the indentation zoneis quite important, and the contribution to the total drilling
Z
Xf/2
Workpiece Plastic region
Indentation zone model
π
2πΎind
Figure 6: Indentation zone model schematic.
thrust force by the indentation zone needs to be considered.In microdrilling, although the chisel edge is circular edge,due to the major effect of the indentation zone on extrudematerial, the indentation zone can be regarded as a rigidwedge. The material is extruded on both sides of the wedge.The indentation zone model schematic is shown in Figure 6.According to the slip-line field theory, the force normal to thesurface of the wedge can be determined. Consider
ππ1
= 2π (1 + π) , (22)
where π is the solution of the slip line and is given by thefollowing equation:
2πΎind = π + cosβ1 [tan(π
4β
π
2)] , (23)
where 2πΎind is the included angle of the wedge, which is equalto twice the magnitude of the static normal rake angle at thechisel edge and is given by
πΎind = βtanβ1 [tanπ cos (π β π)] , (24)
where π is chisel edge angle of microdrill.
Mathematical Problems in Engineering 7
(a) (b)
(c)
Figure 7: Experimental setup and microdrill. (a) Experimental setup diagram; (b) CNS7d CNC machine; (c) diameter 0.5 mmmicrodrill.
The load acted on unit length of the wedge is
ππΉ3= 2πππ΄
ππ1sin πΎind, (25)
where
πππ΄
=π
2 [cos πΎind β sin (πΎind β π)]. (26)
So the total drilling thrust force of the indentation zonecan be expressed as
πΉ3= 2 β
π
2 [cos πΎind β sin (πΎind β π)]β 2π (1 + π)
β sin πΎind β 2π ind
=4π (1 + π) π sin πΎindπ ind
cos πΎind β sin (πΎind β π).
(27)
4. Experimental Validation of the ThrustForces Model of Microdrills
4.1. Experimental Work. To calibrate the thrust forces modelof microdrills, the microdrilling processes were performed
on a DMG DMU 80 monoBLOCK machining center. Theexperimental setup was shown in Figure 7(a). Workpiece isAISI 1023 carbon steel plate with a thickness of 1.5mm.Workpiece is mounted on a multicomponent dynamome-ter (Kistler, model 9257B). The material of microdrills iscemented carbide of ultrafine grain (AF K34 SF, made byGermanyAFHartmetall Group), and its performance is listedin Table 1. Microdrills were fabricated on a Makino SeikiCNS7dCNCmicrotool grindingmachine, as shown in Figure7(b). The basic parameters of microdrills are shown in Table2. The microdrill was observed under a laser microscope(KEYENE vk-x100 Series) and a stereoscopic microscope(Zeiss). Figure 7(c) shows an example of microdrills.
The material shear flow stress π is 282.7MPa, the coeffi-cient of coulomb friction is 0.15, and the shear stress ratio π/π
is 0.95. The separation angle ππon the major cutting edges is
56β and 58.5β on the second cutting edges. The spindle speedis 22,000 r/min and the feed is 0.5 πm/r, 1.0 πm/r, 2.0πm/r,3.0 πm/r, and 5.0 πm/r, respectively. The following equationis used to evaluate the shear angle when the second cuttingedges are cutting [14]:
ππ = 31.48 + 0.32πΎ
π. (28)
8 Mathematical Problems in Engineering
Table 1: Mechanical and physical properties of microdrills material.
Mechanical and physical properties ValueCo Content (%) 9WC including doping (%) 91Density (g/cm3) 14.3HV 30 (N/mm2) 2000HRA 94.4Transverse rupture strength (N/mm2) 4000Tungsten carbide particle size (πm) 0.2
The experimental thrust force signals weremeasuredwitha dynamometer. The results are shown in Figures 8(a)β8(e).
A typical thrust profile is shown in Figure 9. In zone A,the chisel edge has contacted and extruded the workpiece; atthe same time, the second cutting edge is cutting. In zoneB,the major cutting edges are entering the hole gradually andbegin to cut. The thrust forces in zoneB consist of two partsof forces, the force generated by the chisel edge and the forcegenerated by the major cutting edges. The latter increasesgradually in zoneB, but it is always smaller than the formereven at its maximum. In zone C, the major cutting edgeshave completely entered the hole and the entire microdrill isexerting the thrust. In zone D, the chisel edge of microdrillis just out from the bottom of workpiece and the majorcutting edges are still cutting.The force in zoneD is generatedwithout the contribution of the chisel edge. Therefore, thethrust in zoneD is significantly smaller than that in zoneB.In zone E, workpiece has completely drilled through, andthere exists friction between drill and hole wall. Then, themicrodrill withdraws from the hole.
The chisel edge and cutting edges forces must be sepa-rated in order to compare them to the values predicted bythe model. The approach is to use a blind pilot hole with adiameter exactly equal to the web thickness of the microdrillused for the validation. For pilot holes, 0.15mm drills wereused, and the depth of pilot holes was kept at 0.5mm. Thetypical thrust profile for the operation is shown in Figure 10.
The experimental thrust force results are compared withthe corresponding predicted results in Figure 11.
4.2. Results and Discussion. As seen in Figure 8, the shape ofcurve in Figure 8(a) is completely different from the others,and the trend of the curve is basically the same as in Figures8(b)β8(e). At very low feed, the chip thickness is less thanthe minimum chip thickness; chips are not formed, andonly ploughing takes place. However, because the indentationzone keeps extruding the work material, the cutting processdoes take place. Figure 8(a) shows the thrust of this case; itis the main ploughing forces. As the feed increases, whenthe chip thickness exceeds theminimum chip thickness, bothshearing and ploughing take place in the cutting, so the thrustforces include shearing forces and ploughing forces, as shownin Figures 8(b)β8(e). By comparing Figures 8(a) and 8(b),we can see that the value of the minimum chip thickness isbetween 0.25 πm and 0.5 πm.
Table 2: Basic parameters of microdrill.
Geometric feature ValueDiameter (mm) 0.5Flute length (mm) 5.0Helix angle (β) 25Web thickness (mm) 0.15Web taper (mm/mm) 0.03Point angle (β) 130Primary face angle (β) 12Chisel edge angle (β) 42.8Major cutting edge radius (πm) 2Second cutting edge radius (πm) 3
As seen in Figure 11, almost all the predicted valuesare lower than the experimental ones. The data shows thatthe cutting force model of chisel edge including secondarycutting edge and indentation zone can correctly predict thethrust, and the average error is less than 5 percent. Theaccuracy of major cutting edges cutting force is relativelylower. The experimental results show that the average errorin the predicted steady state major cutting edges thrust is lessthan 10 percent.When the feed is between 0.5 and 1.0, amixedelastic-plastic deformation happens to the material; a transi-tion from the ploughing mechanism to shearing mechanismcan be seen. In general, the total drilling thrust (cutting edgesand chisel edge) is predicted with an average error of less than7 percent. Inmajor cutting edges cutting forcemodel, becausethe hydrostatic pressure and shear flow stress along tool-chip contact zone are calculated by dividing some differentialelements, the number of divided differential elements has cer-tain effect on the accuracy of themodel. On the other hand, inorder to simplify themathematical formulas, the tool circularedge is approximately represented by two straight chords,which lead to lower accuracy to some extent. Other sourcesof deviation might include the wear or local fracture of themajor cutting edges in the cutting process; these factors canlead to the increasing of the thrust during the drilling process.
The predicted and experimental results show that thethrust created by the chisel edge is quite significant. It exceedsthe thrust created by the cutting edges and represents about60β70 percent of total thrust. In this paper, the chisel edgeangle ofmicrodrill is relatively low at 42.8β, which causes boththe length of chisel edge and the cutting force to increase.
5. Conclusions
Themathematical models to predict the microdrilling thrustare developed. The thrust is modeled in three parts: majorcutting edges, secondary cutting edge, and the indentationzone. Major cutting edge and secondary cutting edge forcemodels are based on the slip-line field theory, and theindentation zone is modeled as a rigid wedge. The majorcutting edges and secondary cutting edge are divided intoelements and the elemental forces are determined from anoblique cuttingmodel and an orthogonalmodel, respectively.Shearing and ploughing are included in the models of the
Mathematical Problems in Engineering 9
0 2 4 6 8 10β0.2
0.0
0.2
0.4
0.6
0.8Th
rust
(N)
Time (s)
(a)
0 1 2 3 4 5β0.2
0.0
0.2
0.4
0.6
0.8
Thru
st (N
)
Time (s)
(b)
0 1 2 3 4β0.2
0.0
0.2
0.4
0.6
0.8
Thru
st (N
)
Time (s)
(c)
0 1 2 3 4β0.2
0.0
0.2
0.4
0.6
0.8Th
rust
(N)
Time (s)
(d)
0 1 2 3 4β0.2
0.0
0.2
0.4
0.6
0.8
Thru
st (N
)
Time (s)
(e)
Figure 8: The thrust force profile for microdrilling. (a) Feed: 0.5 πm/r; (b) feed: 1.0 πm/r; (c) feed: 2.0 πm/r; (d) feed: 3.0 πm/r; (e) feed:5.0 πm/r.
10 Mathematical Problems in Engineering
0 2 4 6 8 10β0.2
0.0
0.2
0.4
0.6
0.8
Thru
st (N
)
Time (s)
1 2 3 4 5
Figure 9: A typical profile of the thrust force formicrodrilling (feed:1.0πm/r).
0 1 2 3 4 5β0.2
β0.1
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Edge
s thr
ust
Maj
or cu
tting
Thru
st (N
)
Time (s)
Tota
l thr
ust
Figure 10: Thrust profile using pilot hole (feed: 1.0 πm/r).
major cutting edges and second cutting edge. The model isapplied to a 0.5 mm ultrafine grain cemented carbide micro-drill, and the experimental and predicted values of forces arecompared.
The main conclusions from the study are as follows.
(i) Almost all the predicted values are lower than theexperimental ones.Thismight be attributed by factorssuch as drill vibrations, drill wandering, the friction ofdrill, and hole wall.
(ii) On the chisel edge, the forces of secondary cuttingedge can be modeled based on slip-line theory, andthe indentation zone can bemodeled as a rigid wedge.The model of chisel edge shows a good conformitywith the experimental results.
(iii) The accuracy of major cutting edges cutting forceis low relatively, and the average error is about 10percent. This may be due to the fact that some ofthe constants such as shear stress ratio and separationangle as well as others are calibrated for other process-ing methods and not for drilling.
Expt (a) Pred (a)Expt (b) Pred (b)Expt (c) Pred (c)
0 1 2 3 4 5 60.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Thru
st (N
)
Feed (πm/r)
Figure 11: Comparison of experimental and predicted value. (a)Major cutting edges thrust; (b) chisel edge thrust; (c) total drillingthrust.
Future work should aim at two aspects: improving theaccuracy ofmajor cutting edges cutting force and consideringthe effect of the chisel edge angle and length on total drillingthrust.
Nomenclature
πΉ1: Total thrust of major cutting edges
πΉ2: Total thrust of second cutting edge
πΉ3: Total thrust of the indentation zone
ππΉcut: Elemental cutting forceππΉπΏ: Elemental lateral force
ππΉπ: Elemental oblique cutting thrust force
π: The frictional shear stressπ: The material flow stressπ: The hydrostatic pressureπ: Half the drill point angleππ : Cutting edge inclination angle
πΎπ: Normal rake angle of major cutting edge
π½: Helix angleππ: The separation angle
πΎπ: Effective rake angle
ππ: Effective shear angle
π: The chisel edge angle2πΎind: The included angle of the wedgeπΎπ : The static clearance angle of the chisel edge
ππ: The tool edge radius
ππ€: Half the web thickness
π: The distance from the selected point onthe major cutting edge to drill axis
π : Drill radiusπ ind: The radius of the indentation zoneπ: Feed
Mathematical Problems in Engineering 11
πΏπΆ: The length of chisel edge
π‘π: The uncut chip thickness
π‘cmin: The minimum chip thickness.
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper.
Acknowledgment
Theauthors would like to thankTheNational Natural ScienceFoundation of China (Key Program, no. 50935001) for theirfinancial support.Without their support, this workwould nothave been possible.
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