A Study on Straightness of Deep Hole in Small-Diameter Drilling ofStainless SteelKoichi Okuda1, a *, Toshiya Yamaguchi1 , Taizo Yamamoto2,bTsubasa Takeda and Hiroo Shizuka3,c1

Graduate School of Engineering, University of Hyogo, Shosha 2167, Himeji, Hyogo, Japan
2

Yamamoto Metal Technos co, Setoguchi, Hirano, Osaka, Japan

3

Graduate School of Engineering, Shizuoka University, Nakaku, Hamamatsu, Shizuoka, Japan
aokuda@eng.u-hyogo.ac.jp, byamakin@mx1.alpha-web.ne.jp, cthshizuka@ipc.shizuoka.ac.jp

Keywords: Small-diameter drill, Stainless steel, Thrust force, Straightness, Step feedAbstract. This paper describes influence of the cutting conditions on the hole shape accuracy in
deep-hole machining of stainless steel with small-diameter drill. The drilling tests were carried out bychanging the step feed, the rotational speed and the feed rate in order to investigate the relationshipbetween the straightness of the hole and the cutting conditions. As a result, it was found that the thrustforce decreased with a decrease in the step feed amount and accordingly the straightness wasimproved. By changing the drill in incremental step from short drill length to long drill length, thestraightness was improved.
Introduction
Pressure vessels used in large-scale facility such as an electric power plant is mainly fabricated bywelding the thick stainless steel plates. As the residual stress is present in the welded portions, thestress needs to be measured to satisfy the safety criterion. For this measurement, DHD (Deep HoleDrilling) method is employed. A hole with a degree of diameter of 2mm and depth of 200mm isrequired in order to inspect the residual stress generated in the pressure vessel.
However, it is extremely difficult to drill hole with an aspect ratio (ratio of the hole depth to thetool diameter) of 100. With an increase in the aspect ratio, the chips become hard to be evacuated tothe outside of hole and the higher cutting heat is generated at the cutting point. As a result, the shapeaccuracy of the hole is deteriorated and the tool life is shortened [1, 2]. Furthermore, in the deep-hole
drilling with small diameter, the tool rigidity decreases and the high speed revolution is requiredcomparing with the conventional aspect ratio. Thus, the buckling and the failure of the tool are easy tooccur.
Authors successfully machined the hole with diameter of 2 mm and depth of 200mm through atrial and error process and the ingenious attempt such as the way of oil supply and step feed drilling[3]. However, a cause-and-effect relationship between the machining accuracy and the cutting
conditions is not completely confirmed.
In this study, the drilling tests were carried out by changing the step feed, the rotational speed andthe feed rate in order to investigate the relationship between the straightness of the hole and thecutting conditions. It is discussed how the cutting conditions for drilling small-diameter deep holes instainless steel affect the straightness of the hole.
Experimental Method
The tapping center (TV-30, Mori Seiki Co., Ltd.) was employed for the drilling experiments.
High-speed steel (HSS) and tungsten carbide drills were used for the deep-hole drilling. The work
material is stainless steel (SUS304) which is commonly used for pressure vessels. The thrust force,the surface roughness and the straightness of the hole were measured. A three-component force
transducer (9378B, Kistler) was used to measure the thrust force. Water-soluble cutting lubricant wasapplied on the surface of workpiece. The step feed drilling technique which is commonly performedin the deep-hole drilling was adopted.
Experimental Results and Discussion
Drilling of Hole with L/D=100. Authors have been investigating the appropriate cutting conditions
for machining of the hole with diameter of 2mm and depth of 200mm [4]. Eventually, the targeteddeep hole was successfully machined through a trial and error process and the ingenious attempt.
Figure 1 shows a photograph of drilling method and the chips evacuated out of hole. The oil pool wasfixed on the top side of workpiece and the drilling operation was performed in the oil to prevent air
bubble from entering the hole. The chips were evacuated out of the pool through the outlet hole on thewall surface to prevent the chips from piling up.
Table 1 shows the cutting condition when the targeted hole was successfully machined. Two
different tungsten carbide drills were used. Firstly, the pilot hole with depth of 6mm was drilled andthen the hole with depth of 40 mm was drilled by another commercially available drill with flute
Table 1 Cutting conditions.
Hole depth
Cutting speed
Feed rate
Step-feed[mm][m/min][mm/rev][mm]

0-40
400.040.01

40-200
80.02
0.01

Drill
Diameter 2mm

Oil pool
Workpiece
Chips in oil pool

Cemented carbide drill

Hole entrance
diameter 2mm
Cross-section of machined holeFig. 2 Cross-section of deep hole and long drill used.

length of 50mm. After that, the drill wasexchanged to specially designed drill with flutelength of 205mm and the special long drill waspenetrated to 200mm in depth with step feed of0.01mm.
Figure 2 shows the specially designed longdrill and the cross-section of the hole which wasmachined by the milling cutter. The cross section
shape was measured by CCD laser displacement
sensor and the straightness of the hole wasevaluated based on the cross section shapemeasured at the position of 10mm interval fromthe hole entrance. Figure 3 shows the deflectionof the hole obtained from the cross-section shapewith reference to the hole entrance. The
deflection increases with an increase in the holedepth, while the straightness is good in the range

0.40.3
Deflection of hole [mm]Fig. 1 Deep hole drilling method.

200mm

0.20.1YX0Tool change-0.1050100150
Depth from hole entrance [mm]Fig. 3 Deep hole drilling method.

200

of 50mm or less in depth. After exchanging to the long drill, the deflection steeply grows. It is
understood that the tool length has an influence on the hole deflection or the straightness.
As mentioned above, the hole with diameter of 2mm and depth of 200mm was successfullymachined based on the knowledge obtained by the experiments and through a trial and error process.
However, a cause-and-effect relationship between the machining accuracy and the cutting conditions
is not completely confirmed. In the following section, the relationship between the thrust force andthe step feed is examined and the effect of the cutting conditions on the straightness the hole isdiscussed.
Relationship between thrust force and step feed. A magnitude of thrust force was strongly
responsible for the buckling and failure of tool in the drilling with small-diameter and long drills [4].
Then, authors have attempted to drill a long hole in the conditions of smaller feed rate and step feed
than the conventional drilling conditions. Figure 4 shows the signal wave of the thrust force measured
when drilling under the cutting conditions of spindle rotational speed 2400 min-1, feed rate 0.02mm/rev and step feed 0.01mm with high speed steel drill of diameter 2mm and flute length 50mm.
Thrust force increases at the instant of the tool engagement in the workpiece and decreases at theback-feed of the tool. Stationary wave patterns are cycled in synchronization to the tool engagementand disengagement.
The relationship between the step feed and the thrust force is shown in Fig. 5. The cutting
conditions are same as Fig.4. Values of the plots were obtained by the averaging procedure of thefluctuating values in the top of the signal wave. The thrust force drastically increases between
0.05mm and 0.05mm in the step feed. Notable change in the thrust force values is not seen in therange of 0.1mm or more and 0.01mm or less.
Figure 6 shows the signal wave of the thrust force measured when drilling without the step-feed
technique under the same cutting conditions as Fig.4. The force values shown in Fig.5 are alsosuperimposed in the continuous force signal of Fig. 6. The times corresponding to the horizontal axesof the graph were calculated based on the spindle rotational speed and the feed rate as the times
required in the movement between the tool engagement and disengagement. In this cutting condition,
it is understood from the continuous force signal wave that the time of approximately 0.1 second isrequired for a steady cutting process and the cutting process is in the unsteady cutting state till 0.1
second. Therefore, the magnitude of the step feed should be determined in consideration of the time
for the unsteady cutting process, the spindle rotational speed and the feed rate.
Influence of step feed on hole straightness. It was mentioned in the previous section that themagnitude of the thrust force influenced on the buckling deformation and the failure of the tool. Then,the hole straightness is deteriorated with an increase in the hole depth. In this study, the holestraightness was evaluated as the amount of the hole curvature or deflection. As shown in Fig. 7, the1207011060

100

Thrust force [N]Thrust force [N]504030201090807060050-1000.2

0.4

0.60.8Time [sec]1400.001

0.010.1

Step feed [mm]Fig. 4 Signal wave of thrust force in drilling withstep feed.

Fig. 5 Relationship between step
feed and thrust force.

1

200

S.F=0.1mm
S.F=0.05mm

Thrust force [N]

150

100

S.F=0.5mm
500S.F=0.01mm
S.F=0.005mm
S.F=0.001mm
00.1

0.20.30.40.50.6

0.7

0.8

Time [sec]Fig. 6 Signal wave of thrust force in drilling without step feed.
workpiece was machined so that the cross section of the hole was visible. The cross section wasmeasured by CCD laser displacement sensor and the reference point was fixed on the cross section at
10mm from the hole entrance. The displacement of the reference point at another cross section weredefined as the deflections of X and Y directions. These measurement results were compared with theresults measured by X-ray CT scanner and it was confirmed that both results were almost same.
Figure 8 shows the relationship between the deflection ofhole and the depth from the hole entrance. The cutting
conditions are spindle rotational speed 6000 min-1, feed rate
0.02 mm/rev and step-feed 0.01mm with tungsten carbide drill(TiSiN coating) of diameter 2mm and flute length 70mm. Up to0.06mm of deflection occurs at 60mm depth from the hole
10entrance. It is seen that the hole does not always curve in one
direction. Then, in order to evaluate the magnitude of thedeflection, we defined the accumulation of the deflection asshown in the shaded area of the graph. Figure 9 shows therelationship between the step feed and the accumulation of Fig. 7 Measurement of holedeflection by CCD laserhole deflection. The accumulation of deflection increases withdisplacement sensor.
an increase in the step feed. Especially, the deflection rapidly
6.0

Accumulation of deflection [mm2]Deflection of hole [mm]0.08
X direction
Y direction

0.06
0.04
0.02
0-0.02
-0.04
-0.06

5.5
5.0
4.5
4.0
3.5
3.00102030

40506070

Depth from hole entrance [mm]Fig. 8 Relationship between depth anddeflection of hole.

00.01

0.02

0.03

0.04

0.05

0.06

Step feed [mm]Fig. 9 Relationship between step feed anddeflection hole.

increases in the condition of step feed 0.05mm and the best straightness is obtained in the case of step
feed 0.01mm within this experimental condition.

Thrust force [N]Influence of tool length on hole straightness. In this section, an influence of the tool length on thethrust force and the hole straightness was examined. The drilling tests were carried out with three
types of drills which have the flute length of 40mm, 50mm and 70mm. The rest of cutting conditions
are the same as the experiments of the previous section. Figure 10 shows the changing process of thethrust force with and without the tool change. The dashed line indicates the thrust force change when
the hole of 60mm depth was drilled by the drill of 70mm flute length without the tool change. Themagnitude of the thrust force is120
approximately 100N and tends to increase in
the depth of 40mm or more. The solid line
Tool change
110
indicates the thrust force change. Firstly, thehole of 30mm depth was drilled by the drill
No tool changeof 40mm flute length and the drill was
100changed to the drill of 50mm flute length.
After drilling till 40mm depth, the drill was
90changed again to the drill of 70mm flute
length and finally the hole of 60mm depth80
was drilled. The magnitude of the thrust force
(3)
(2)
(1)
is approximately 85N in the case of using the70
short drill (40mm flute length), while theChange Change
thrust force was approximately 100N in thecase of using the long drill. By exchanging to600102030
40506070the second drill and the third drill, the thrust
Depth from hole entrance [mm]force increased to approximately 95N and110N, respectively. The accumulation of thedeflection was 2.85mm2 in the case of the Fig. 10 Change of thrust force by tool length andtool change.
tool change and 3.45mm2 (see Fig. 9) without
the tool change.
Influence of feed rate and spindle rotational speed on hole straightness. Figure 11 and 12 show
the relationship between feed rate and thrust force, the relationship between spindle rotational speed
and thrust force, respectively. The magnitude of the thrust force tends to increase with an increase in
the feed rate and with a decrease in the spindle rotational speed, respectively. However, the thrust
160

130

Spindle rotational speed : 6000 min-1

150
140

Thrust force [N]Thrust force [N]

120

110

130
120
110
100

100

90

90

00.02

0.04

0.06

0.08

0.1

Feed rate [mm/rev]

Fig. 11 Relationship between feed rate
and thrust force.

80
1000

Feed rate : 0.02 mm/rev
2000

3000

4000

5000

6000

7000

Spindle rotational speed [min-1]

Fig. 12 Relationship between spindle
rotational speed and thrust force.

force increases in the condition of feed rate 0.01mm/rev. Too small feed rate leads to a work
hardening of the work material and a strong burnishing process due to small effective depth of cut.
That results in the increase of the thrust force.
Figure 13 and 14 show the relationship between feed rate and the accumulation of the holedeflection, the relationship between spindle rotational speed and the accumulation of the holedeflection, respectively. These results correspond to the results of Fig. 11 and Fig. 12. It is understoodthat the magnitude or the change of the thrust force is totally responsible for the deflection of the holeor the hole straightness.
Accumulation of deflection [mm2]Accumulation of deflection [mm 2]

7.0
Spindle rotational speed : 6000 min-1

6.5
6.0
5.5
5.0
4.5
4.0
3.5
3.0

0

0.02

0.04

0.06

0.08

0.1

0.12

Feed rate [mm/rev]

Fig. 13 Relationship between feed rate
and accumulation of deflection.

9876543

Feed rate : 0.02 mm/rev

21000 2000 3000 4000 5000 6000 7000

Spindle rotational speed [min-1]

Fig. 14 Relationship between spindle rotational
speed and accumulation of deflection.

Summary
The drilling tests were carried out by changing the step feed, the rotational speed and the feed rate inorder to investigate the relationship between the straightness of the hole and the cutting conditions.
The findings which were obtained from cutting tests are presented. It was found that the thrust force
decreased with a decrease in the step-feed amount and accordingly the straightness was improved. Bychanging the drill in incremental step from short drill length to long drill length, the straightness wasimproved.
References[1] Sakurai. K, Adachi.K, Okita.K, Hanasaki.S: Effect of Content of Austenitic Stainless Steel onDrill Life, Transactions of the Japan Society of Mechanical Engineers, 62 (1996), pp.362-366 (inJapanese).
[2] Wei-Liang Ku, Ching-Lien Hung, Shin-Min Lee, Han-Ming Chow: Optimization in ThermalFriction Drilling for SUS 304 Stainless Steel, International Journal of Advanced Manufacturing
Technology, 53 (2010) ,pp.935-944.
[3] Hiroo Shizuka, Yoshiyuki Masuta, Koichi Okuda and Masayuki Nunobiki: Improvement ofDrilled Hole Accuracy of Stainless Steel with Small Diameter Drill, Proc. of Spring Annual
Meeting of JSPE, (2013), pp.185-186.
[4] Yoshiyuki Masuta, Koichi Okuda, Hiroo Shizuka and Masayuki Nunobiki: A Study ofDeep-Hole Machining of Stainless Steel with Small-Diameter Drill, Advanced Materials
Research, 565 (2012), pp376-381.