59

ISSN 1392 - 1207. MECHANIKA. 2008. Nr.5(73)

The wear of single flute gun drill and tool life tests
I. Sihvo*, J. Varis**
*Lappeenranta University of Technology, Skinnarilankatu 34, P.O. Box 20, 53851 Lappeenranta, Finland, E-mail:
inga.sihvo@lut.fi
**Lappeenranta University of Technology, Skinnarilankatu 34, P.O. Box 20, 53851 Lappeenranta, Finland, E-mail:
juha.varis@lut.fi

1. Introduction 2. Gun drills and tool wear

Gundrilling is widely used, e.g. in automobile Gundrilling is a deep-hole drilling process. The
manufacturing for machining deep holes in engine parts. In process produces deep, onepass, high quality holes. In
the automotive industry, there is always room for im- gundrilling, the cutting speed is high and the feed rate is
provement in the gundrilling process to extend tool life, relatively low, but the penetration rate is higher than that of
obtain sufficient quality in holes and, at the same time, a twist drill. The method allows the tool to drill the full
increase productivity. In the car industry, production runs length of the deep hole without retraction. In the method,
are extensive and the time used to machine one part has to cutting fluid is injected with high pressure through the hol-
be as short as possible while chip flow must be maximized. low shank. Once the fluid has lubricated the cutting edges
Changing the work piece material or tool geometry is al- of the tip, it escapes along the v-shaped flute of the shank,
ways a challenge, and tool wear monitoring helps in these taking the chips out of the hole. A gun drill needs guidance
situations. in the beginning of the drilling. When gundrilling in an NC
Wear can be measured directly (direct monitoring machine, a pilot hole is used to guide the drill [9]. Drill
method) from the tool for example between chancing the manufacturers usually recommend the use of a lower feed
work piece into the machine. This visual tool condition at the beginning of the drilling and raising the feed after
monitoring method is used especially during drilling tests the drill tip has penetrated the material.
in laboratory circumstances, for example when testing Drill wear has an influence on the hole quality,
drills which have different geometry or coating or when surface finish, straightness of holes, and also on the tool
testing new cutting parameters. Tool wear is also measured life. Tool life is usually determined by one criterion or sev-
in order to find the relationship between cutting forces and eral depending upon the drilling conditions and the opera-
tool wear. [1-3] However, it is quite time consuming to tional requirements. The most common indicators of the
measure all forms of tool wear and it is also more difficult end of tool life are fracture or chipping, excessive wear,
to draw conclusions on the test results while monitoring and poor surface quality or accuracy of holes [10]. In turn-
several wear types. Moreover, it is difficult to decide how ing operation, common criteria for sintered carbide tools
often the tool should be measured (measuring frequency). are, for example, the maximum width of the flank wear
Flank wear is present in all cutting operations. It land (if the flank is not considered to be regularly worn),
is the best known type of tool wear and it is also relatively the average width of the flank wear land (if the flank is
easy to measure. At least for a single-point turning tool, the considered to be regularly worn), and the depth of the cra-
width of the flank wear maximum is a suitable wear meas- ter [4]. It has been suggested that outer corner wear should
ure, and a predetermined value of the flank wear maximum be used as a performance index in drilling when categoriz-
is regarded as a good tool life criterion [4]. ing the drill condition [3]. In several studies, the average
In many drilling tests, flank wear is used as an in- flank wear of a twist drill has been measured from a few
dicator of tool condition, and several attempts have been points on both cutting edges and possibly an average value
made to predict the flank wear, e.g. by drilling force sig- is then calculated [1, 5, 7, 11]. It also has been recom-
nals [1, 5-8]. Measuring the wear of the tool has still re- mended to measure, as a performance index in a standard
mained one of the best tool condition monitoring methods drilling test, three types of outer corner wear and crater
in drilling tests. During the years, many reports have been wear (width), two types of flank wear (mean and maxi-
written concerning the wear of twist drills [2]. The wear of mum wear), two types of chisel wear (width and length),
a gun drill differs a bit from the wear of a twist drill [2, 9] and margin wear (length). Other wear types have also been
due to the special geometry of the gun drill tip, Fig. 1. The used in drilling tests. Researchers have also used the width
wear of a gun drill is less investigated. of the chisel, crater, margin and flank wear and the length
The object of this study was to find out the wear- of the outer corner wear as drill condition and drill life
ing of the gun drill and which of the measured wear types indicators [3, 8].
the best indicate the tool condition. For comparing the cho- The amount of chipping is evaluated to a certain
sen wear types, gundrilling tests were conducted by using extent by the maximum width of the flank wear. A deep,
two different kinds of drill geometries. After these tests, wide crater far from the cutting edge may be less danger-
the best wear types for predicting the tool condition and ous to the tool than a less deep, narrow crater close to the
suitable measuring frequency were discovered. cutting edge.
The distance from the cutting edge to the crater is
sometimes a useful criterion which, if limited can eliminate
catastrophic failure [4].
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In principle the drill wear has been suggested to In this paper, the same kinds of symbols are used
act as an accelerating process which takes place at the for different wear types as have been used generally for
outer margin of the flute(s) of the drill due to the intimate wear types of twist drills [3]. Different wear types in Fig. 1
contact and elevated temperatures at the tool work piece are marked as follows: outer corner wear W, wear in the
contact. It is also pointed out how there is a period of ini- drill length WL, mean width of the crater wear KB, two
tial wear, followed by a period of moderate wear and in the types of flank wear VB (mean wear) and VB (maximum
third phase a period of excessive wear [12]. wear), and flank wear in the drill tip CT.
Gundrilling tests were carried out in the Horizon-
3. Research tal Machine Center. Cylindrical billets (bars) were used as
work pieces. Microalloyed steel, which is widely used in
In this study, the tool wear was evaluated in tool the automotive industry, was used as test material. Car
condition monitoring. Wear types detected in the gun drill engine parts like crankshafts can be made from microal-
are presented in Fig. 1. The selection of wear types meas- loyed steel. The average Vickers hardness of the test mate-
ured in this research is based on different wear types de- rial was 260 HV/30 (KV+20C). Chemical composition of
tected in gundrilling tests preceding these tests (with two the test material is shown in Table.
microalloyed steels, and drill different sizes, diameter 5 Holes were drilled through the bar to the cross-
and 8 mm), and also the performance of the measuring section. The diameter of the billet was 87 mm and the
equipment used (measuring software and a microscope length was 187 mm. A total length of seven meters was
which was attached to a tool presetting device). The meas- drilled into each bar (40 holes). The holes were positioned
ured wear types were: into the cross-section of the bar in the way that the mini-
- maximum (VB) and mean (VB) flank wear of the mum distance to the edge of the work piece was 5 mm and
inside and outside edge; between holes at least 2 mm [14].
- flank wear in the drill tip (CT); Single flute gun drills with a solid carbide tip
- wear in the drill length (WL); (carbide type H15) and with two different kinds of grind-
- wear in the outside corner (W); ings (drill A and B) were used. In drill A, the inside angle
- mean crater wear (KB); X is 20 and outside angle Y is 30. In drill B, the X is 25
- width of the build-up edge, BUE (average of and the Y 36, see Fig. 2. The diameter (D) of the drill was
width of the BUE in the inside and outside edge, 8 mm and the total length 270 mm.
measured separately).

Fig. 2 Geometry of the gun drill

a b Cutting parameters from the recommended area

were used so that the data corresponded to conventional
drilling conditions in the industry. Cutting conditions were
such that no fractures or chipping would occur before 100
meters (the drill could be regrinded) and the quality of the
holes was good (e.g. arithmetical surface roughness Ra
abroximately 1 m). Surface roughness was measured by a
stylus instrument for controlling the quality of the holes.
Also chip formation (length, color and form of the chips)
was controlled. The coolant used in these test was deep-
hole drilling oil (as a base molecularly converted oil,
40 cSt/+20C).
The cutting parameters were:
- guide hole: length 12 mm and diameter 8 mm;
c - gundrilled hole: depth 187 mm;
Fig. 1 Various wear types detected in the gun drill (a) outer - spindle speed: n = 3200 1/min (cutting speed
corner, (b) flute and (c) top of the drill tip [13] vc = 80 m/min);
- feed: f1 for the first 50 mm of the hole 80 mm/min
The minimum distance from the front edge of the (0.025 mm/r), and after 50 mm depth f2 =
crater to the cutting edge was not included in this study =96 mm/min (0.03 mm/r);
because it did not change in the previous test. - oil pressure: p = 6 MPa (60 bar).
 61

Table
Chemical composition (%) of the test material
C Si Mn P S Cr Ni Mo V Ti Cu Al N
0.36 1.26 1.08 0.01 0.059 0.2 0.14 0.03 0.117 0.014 0.18 0.016 0.011

Tool wear was measured and the tool was photo- V B1 +VB1 + V B 2 +VB 2
graphed regularly after drilling. Because of the adhering Average flank wear = (1)
4
material, many of the wear types in the drill are difficult to
measure. Therefore, pictures of the drill were taken and where VB1 is the mean and VB1 the maximum flank wear
used to help to compare the tool wear. The drilling was on the inside edge and VB2 is the mean and VB2 the maxi-
stopped after each bar, and the drill was taken out of the mum flank wear on the outside edge. During the first 49
machine to measure the tool wear. After measuring, the meters there was no clear maximum flank wear value on
test continued with a new work piece. the inside edge.
With both drills, 49 meters was drilled. After that,
the drills were compared and the drilling continued with a
less worn drill until the maximum flank wear (VB) was
0.3 mm in the flank wear land.

4. Results

After 49 meters of drilling, tool A was found to be

less worn. Pictures of the worn tools A and B are presented
in Fig. 3. Drilling continued with drill A. The total drilled
length with drill A was 105 meters (a flank wear limit of
0.3 mm was achieved in the drill tip). Fig. 4 includes fig-
ures of drill A after 105 meters drilling.

Fig. 4 Drill A when 105 meters was drilled (right). (a)

outer corner, (b) flute and (c) top of the drill tip

5. Discussion

The greatest differences between the wear of the

two drills, A and B, were in the width of the flank wear of
the tip (CT) and the wear in the drill length (WL). The flank
wear measuring was not always easy. In the flank wear
area there was not always a clear point for the maximum
(VB) and mean flank wear (VB), especially when the drill
Fig. 3 Drills A (a) and B (b) when 49 meters was drilled. was quite new (less than 49 meters was drilled). Compari-
Flute (right) and top of the drill tip (left) son of the wear of different drills was easy with the pic-
tures taken.
Figs. 5 and 6 present the progress of the drill The average flank wear (VB average) seemed to be
wear. Wear in the outer corner (W) increased quickly at a more reliable indicator of the tool condition than a single
first (during the first 7 meters), but later increased slowly value measured from the wear area. Hence the significance
(Fig. 5). Other wear forms presented in Fig. 5 increased at of a single value is smaller, and for example small dimen-
a constant rate. sional errors do not have a significant effect on the wear
The maximum values of flank wear in the drill curve. The maximum (peak) values of flank wear are im-
tip, in Fig. 5, were measured after there were clear maxi- portant and those measurements should be included when
mum peaks in the worn area. calculating the average flank wear, like in this study. As it
In addition to the mean and maximum flank wear, can be seen from the results, the calculated flank wear av-
the average flank wear was calculated. It consisted of four erage value indicates the tool condition sufficiently.
values: mean and maximum flank wear values on outside Some wear types, for example crater wear (KB)
and inside edges. The average flank wear was calculated as and wear in the drill length (WL), were difficult to measure
follows: accurately mostly because of the BUE (build-up edge)
 62

Fig. 5 Mean (VB) and maximum (VB) flank wear on the edges, flank wear in the drill tip (CT), longitudinal wear (WL) and
outer corner wear (W)

Fig. 6 Crater wear (KB), build-up edge and maximum (VB) and average (VB average) values of flank wear

formed on the inside and outside edges. The amount of difficult to measure because of the build-up edge. It is a
build-up edge changed slightly during drilling, but no sta- suitable measure for cases where BUE forming is small.
ble increase in the drawn curve was detected. On the inside The width of the margin of a new tool seemed to affect the
edge the width of the crater wear did not increase much width of the corner wear. A solution to this problem could
after 7 meters of drilling. The depth of the crater was not be found by using a non dimensional parameter: the ratio
measured because measuring of the crater wear depth is of width of the wear land on the outer corner to the width
not possible using only a microscope. The pictures show of the margin of a new drill [15, 16].
that the depths of the craters increased when the drilling
length increased. The depth of the crater wear on the out-
side edge should not be forgotten because it affects the
regrinding. Gun drill manufacturers recommend that the
flank wear limit for regrinding should be 0.3 mm. After the
limit is passed, regrinding is not cost-effective. However,
the depth of the crater and the distance from the cutting
edge to the crater can, in some cases, grow very fast and
cause tool failure. That is why inspecting the size and posi-
tion of the craters of a drill few times during the drilling
tests is important. Measuring it constantly does not bring
additional value to tool condition monitoring. The drill has
to be inspected so that there is no chipping or preliminary
failure in the tool and at same time craters can be checked.
Wear in the outside corner (W) grew quickly at
first (during the first 14 meters), but later increased slowly.
Fig. 7 Corner wear from another direction (marked as X)
Measuring the outside corner wear was one of the most
challenging tasks. Corner wear, from the direction seen in In the present study one type of corner wear was
Fig. 7, could be a good indicator of tool condition but is detected in addition to the one which was measured, num-
 63

ber 2 shown in Fig. 8. Based on the measurements and wear is easier. Other difference can appear in the width of
pictures taken in this study, the outer corner wear should the crater wear. In the case of this study the crater wear
be measured from two places, as in Fig. 8, or at least value was quite wide. As concluded earlier, in the end the crater
1 should be measured. In this research, wear type 2 in- wear is not a good parameter for predicting the tool condi-
creased quickly to a nearly constant value and thereby it tion and other wear forms are more significant. Otherwise
did not give any additional information on the tool condi- the wearing of the drill should be similar when drilling
tion when the drilling continued. From the pictures taken it different steels.
could be seen that wear type 1 grew steadily until the In these tests the measuring distance was 7 me-
drilled length of 105 meters was reached. ters, and especially when the drill was slightly worn the
wear did not increase significantly. The measuring fre-
quency could be longer with a new drill and shorter with a
worn drill. If the cutting parameters had been higher, the
wear could have been more aggressive already with a new
drill, making the 7-meter measuring frequency appropriate.
It can be said that a 5-10-meter measuring distance in gun-
drilling tests is adequate.

References

1. Lin, S.C., Ting, C.J. Tool wear monitoring in drilling

using force signals. -Wear, 1995, 180, p.53-60.
2. Jantunen, E. A summary of methods applied to tool
condition monitoring in drilling. -Int. J. of Machine
Fig. 8 Outer corner wear types Tools and Manufacture, 2002, 42, p.997-1010.
3. Kanai, M., Kanda, Y. Statistical characteristics of drill
The surface roughness did not change signifi-
wear and drill life for the standardized performance
cantly when the drilled length increased. The Ra -value was
tests. -Annals of the CIRP, 1978, 27, p.61-66.
mainly below 0.8 m. The forms and shapes of chips with
4. International Standards Organization ISO 3685, 1993.
both drills were about the same with the new and the worn
5. Liu, T.I., Anantharaman, K.S. Intelligent classifica-
drill.
tion and measurement of drill wear. -J. of Engineering
for Industry, 1994, 116, p.392-397.
6. Conclusions
6. Liu, T.I., Wu, S.M. On-line detection of drill wear. -J.
of Engineering for Industry, 1990, 112, p.299-302.
Based on this study, the best wear types for pre-
7. Subramanian, K., Cook, N.H. Sensing of drill wear
dicting the condition of a gun drill when the work piece
and prediction of tool life. -J. of Engineering, 1977, 99,
material is microalloyed steel are flank wear in the drill tip
p.295-301.
(CT), the average flank wear (VB) and the mean and maxi-
8. Abu-Mahfouz, I. Drilling wear detection and classifi-
mum flank wear (VB ,VB) on the outside edge. If calculat-
cation using vibration signals and artificial neural net-
ing the average flank wear then the mean and maximum
work. -Int. J. of Machine Tools and Manufacture, 2003,
flank wear values are not needed when monitoring tool
43, p.707-720.
condition.
9. Ketter, L.C. The Gundrilling Handbook. -USA: Cam-
The width of the crater wear (KB) on the outside
bell Viking Press, 2004.-112p.
edge and wear in the drill length (WL), the mean and
10. Griffiths, B.J. Guidelines for planning a deep hole
maximum flank wear on the inside edge (VB, VB) and the
drilling operation. -In Proc. of the 2nd Int. Conf. on
type of outside corner wear measured in this study are not
Deep Hole Drilling and Boring. -Uxbridge (U.K):
such good indicators of the end of tool life or did not give
Brunel University, 1977, p.22.
any more valuable data than those mentioned above. Al-
11. Lee, B.Y, Liu, H.S., Tarng, Y.S. Modeling and opti-
though the maximum flank wear value on the inside edge
mization of drilling process. -Int. J. of Materials Proc-
would, in some cases, be an accurate tool condition indica-
essing Technology, 1998. 74, p.149-157.
tor, is not a very suitable variable for example when com-
12. Thangaraj, A., Wright, P.K. Computer-assisted
paring the maximum flank wear of two drills because dur-
prediction of drill-failure using n-process measure-
ing the first 49 meters there was no clear maximum value
ments of thrust force. -J. of Engineering for Industry,
on the inside edge.
1988, 110, p.192-200.
The build-up edge and width of the crater wear
13. Jaako, I., Varis, J. Tool condition monitoring in gun-
(KB) on the inside edge were the least informative meas-
drilling using feed force and torque measurements.
ured parameters in predicting tool life. None of the meas-
-19th Int. Conf. on Production Research.-Valparaiso
ured wear types in this study is alone adequate to indicate
(Chile). 29.07-02.08.2007, p.6.
the tool condition.
14. Jaako, I., Varis, J. Tool condition monitoring system
In the case of drilling other steels than microal-
and deep-hole drilling tests. -Academic J. of Manufac-
loyed steel, the tool wear can be slightly different. The
turing Engineering, 2007, 5, p.47-52.
difference would probably be in build-up edge formation
15. Kador, S., Lenz, E. Investigation on tool life of twist
which depends mainly on the hardness of the material (the
drills. -Annals of the CIRP, 1980, 29, p.2-27.
lower the hardness is, the more build-up edge tends to
16. Lenz, E., Mayer, J.E., Lee, D.G. Investigation in
form). When there is less BUE formation the measuring of
drilling. -Annals of the CIRP, 1978, 27, p.49-53.
 64

I. Sihvo, J. Varis The object of this study was to ascertain the wear-
ing of the gun drill and the measured wear types that the
VIENAMENIO AUTUV GRTO best indicate the tool condition. In the present study, dif-
NUSIDVJIMO IR ILGAAMIKUMO TYRIMAS ferent tool wear types were found in the gun drill. To test
these wear types, gundrilling tests were carried out by us-
Rezium ing two different kinds of drill geometries. The best wear
types for predicting the condition of a gun drill seem to be
Main gamyboje didelis naumas pasiekiamas flank wear in the drill tip (CT), the average flank wear (VB)
gerinant pjovimo parametrus, kuriant naujus rankius, dar- and the mean and maximum flank wear (VB, VB) on the
bines mediagas. Grimo slyg patikra padeda parinkti outside edge.
tinkamus pjovimo parametrus esant ypatingoms apdirbimo
slygoms, pavyzdiui, apdirbant naujas technologines me-
diagas. rankio nusidvjim laboratorinmis slygomis I. Sihvo, J. Varis
galima imatuoti gana lengvai.
i tyrim tikslas nustatyti autuv grto nusi-
dvjim ir imatuot nusidvjim tipus, kurie geriausiai
apibdina rankio bkl. Tyrim metu buvo nustatyti vai-
rs autuv grto nusidvjimo tipai. Ibandant juos, buvo
naudojami dviej skirting geometrini form autuv
grtai. Atrodo, kad autuv grto bkl geriausiai apib-
dina jo antgalio upakalinio paviriaus nusidvjimas (CT), ,
vidutinis upakalinio paviriaus nusidvjimas (VB) ir pa- .
grindinio upakalinio paviriaus iorins briaunos nusid- -
vjimo vidutin reikm (VB, VB). -
, -
.
.
I. Sihvo, J. Varis -
, -
THE WEAR OF THE SINGLE FLUTE GUN DRILL , -
AND TOOL LIFE TESTS .
. -
Summary . -
, -
In the machining industry, the demand for higher -
productivity can be gained with higher cutting parameters (CT), (VB)
and also by developing new tools and work materials. Drill
B

condition monitoring assists in choosing suitable cutting (VB, VB) .
parameters for specific applications, e.g. for new work
material. In addition, tool wear can be measured quite eas-
ily in laboratory conditions. Received June 13, 2008