- Email: [email protected]
Wear studies in Boring Trepanning Association drilling
Wear,
(1988)
124
WEAR
33 .43
STUDIES
IN BORING
P. K. RAMAKRISHNA Department (Zndia)
33
TREPANNING
ASSOCIATION
DRILLING
RAO and M. S. SHUNMUGAM
of Mechanical Engineering,
Indian Institute of Technology,
Madras 600036
(Received July 22, 1987; revised November 13, 1987; accepted November 26, 1987)
Summary
Boring Trepanning Association (BTA) drills are capable of machining holes having a large length-to-diameter ratio in a single pass. This is a unique self-piloted operation in which cutting and burnishing are both carried out at the same time. Wear studies on BTA drills reveal quite useful information regarding their life and performance. This paper deals with wear studies on 20 mm diameter BTA drills (solid boring) of the Heller design using En9 steel work samples. The studies include examination of the flank and crater wear on the cutting edge, circular land wear and the supporting pad wear. The variation in wear with respect to cutting time for different speeds and feeds is reported.
1. Introduction
When the length-to-diameter ratio of a hole exceeds 10, gun drilling and deep hole boring become more economical [l]. The Boring Trepanning Association (BTA) method of deep hole machining basically utilizes a singlecutting-edge tool with an internal chip removal system. The drilling oil is fed to the cutting region under pressure into the annular space between the drill and the hole. It flows over the cutting edge and through the duct above the edge into the centre of the tubular boring bar, carrying the swarf with it. The BTA drilling head presents an asymmetrical cutting edge balanced by supporting pads at approximately 90” and 180” behind the cutting edge to provide guidance after the tool has entered the workpiece as shown in Fig. 1. Like any other cutting tool, tool wear is a major index of the performance of a BTA drill since it limits the useful life. Acceptable surface quality and integrity, dimensional accuracy and the overall economics of the process are directly influenced by tool wear. For a single-point tool, the most important forms of wear are flank and crater wear. On a BTA drill, in addition to the wear at the cutting edge, wear also occurs on the supporting pads and circular land. Figure 1 shows three distinct zones of wear on a BTA 0043-1648/88/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
34
i---
-.- Inner
cutting
edge
ntermediote
cutting
Outer cutting
edge
\
Cutting
edge
edge
Crater
Fig. 1. BTA drill showing typical areas of wear.
drill: (a) cutting edge wear on the rake and clearance faces; (b) supporting pad wear on the 90” pad and 180” pad; (c) circular land wear. Comey and Griffiths have studied the self-guiding action of BTA drills and concluded that the maximum burnishing pressure occurs in front of the pad [2]. Osman and Latmovic have emphasized the need to maintain a hydrodynamic lubricating film between the bore and the supporting pad in order to reduce pad wear [ 31. Fink has considered a tool life criterion based on flank wear or cutting edge chipping, whichever occurred earlier, to arrive at the optimum cutting conditions in deep hole mac~n~g [4]. Syrett and Bult have considered drill failure which corresponds to a maximum flank wear of 1 mm together with chip formation and surface finish in their investigation to evaluate the relative performance of cutting fluids [ 51. Griffiths has used a drill life criterion based on 0.5 mm mean flank wear at the cutting edge. He has pointed out that the choice of cutting data for deep drilling operations depends on tool wear, cutting forces and power, chip forma~on, hole quality and surface finish [6]. In an attempt to evalwte different tool materials, Sakuma et al. have considered the growth of flank wear at each cutting edge together with the cutting torque, thrust and surface finish [ 71. These studies were carried out under constant drilling conditions. Osman et al. have compared the wear occurring at the cutting edge, pad and circular land and formulated a relative performance index which would be useful in evaluating different cutting fluids [S]. Grieve and Griffiths have brought out the significance of wear on the cutting edge and the supporting pads and incorporated this wear into a cost model to carry out economic analyses [9].
35
From the literature, it is seen that wear occurring at all three zones, namely cutting edge, supporting pad and circular land, is important. Excessive wear at any one zone may limit the life of BTA drills, necessitating regrinding. It may be necessary to study variables affecting the wear at these zones and to adjust the variables to avoid excessive wear at any one zone. In this paper, a systematic study of wear at the cutting edge, supporting pad and circular land is reported. The machining trials have been carried out on En9 steel work samples using a 20 mm diameter BTA drill under different machining conditions. The wear time curves with respect to speed and feed have been included.
2. Experimental details 2.1. Machining trials BTA machining trials were carried out using 20 mm diameter BTA drills (solid boring) of the Heller design. As shown in Fig. 1, the cutting tip is divided into three sections. The outer and intermediate cutting edges separated by a step are at 18” and 12” respectively. The inner edge is reversed so that an edge rather than a point is at the centre of rotation. The point offset is approximately 20% of the radius and the outer and intermediate cutting edges are equal in length. The rake angle for the inner cutting edge is -30” compared with 0” for outer and intermediate edges. The types of carbides used for the cutting tip and supporting pads, in the present work, were P20 and PlO carbides respectively. The work material was En9 steel of 42 mm diameter and 450 mm length. The BTA machine used for the trials was based on a rotating tool system. The cutting fluid used was a mineral oil with extreme pressure additives having the trade name TRIPONOL-H (Bharath Petroleum Corporation). It contains reactive sulphur (0.8% by weight) and chlorine (0.1% by weight). The kinematic viscosity of the cutting fluid is (10 - 15) X 10m6 m2 s-’ at 40 “C and the flash point is 160 “C. The flow rate was maintained at 150 1 min-’ at a pressure of 34.5 bar. The machining trials were carried out over a range of feed and speed combinations obtained by maintaining a feed of 0.1 mm rev-’ and varying the speed and maintaining a speed of 64 m min-’ with different feed rates. A fresh tool was used for each cutting condition. 2.2. Measurement and characterization of wear The wear measurements were carried out at hole depths of 400, 1200 and 2000 mm for different machining conditions. Figure 2(a) shows crater formation on the intermediate and outer cutting edges. The depth of the crater was measured by taking profile traces on the rake face with a Perth-OMeter. Figure 2(b) shows a typical trace of a crater on the intermediate cutting edge. The flank wear at the cutting edge is shown in Fig. 3(a). A flank wear plot developed with measurements obtained using a tool-maker’s microscope is shown in Fig. 3(b). Figures 4(a) and 4(b) show the wear
36
(a)
----I-loop ZSpm
/-
r
Fig. 2. Crater wear (drilling depth, 2000 mm; speed, 64 m min-‘; feed, 0.1 mm rev-‘): (a) photograph showing crater wear; (b) profile trace of a crater on the intermediate cutting edge.
l.O@I
Fig. 3. Flank wear (drilling depth, 2000 mm; speed, 64 m min-‘; (a) photograph showing flank wear; (b) wear contours on flank.
feed, 0.1 mm rev-‘):
Fig. 4. Supporting pad wear (drilling depth, 2000 mm; speed, 64 m min-I; feed, 0.1 mm rev-‘): (a) SO” pad wear;(b) 180” pad wear; (e) profile trace on the 90” pad.
on 90” and 180” pads respectively. As it was difficult to characterize the pad wear on the basis of such an irregular pattern, it was decided to measure the depth of pad wear using profile traces. Figure 4(c) shows a typical trace taken on the 90” pad using a Perth-O-Meter. A trace was also obtained on the circular land as shown in Fig. 5(b). Figure 5(a) shows the nature of wear on the circular land. Figures 2 - 5 are for a speed of 64 m min-’ and a feed of 0.1 mm rev-’ after machining a depth of 2000 mm. For s~gle-post tools, various parameters are used to characterize the crater and flank wear occurring at the cutting edge [lo]. The BTA drill under investigation also exhibits wear on the rake and flank surfaces at the cutting tip. The depth KT of the crater on the rake face and the average flank wear VB shown in Figs. 6(a) and 6(b) are used in the present work to study the progress of wear on the cutting edge of the BTA drill with time. In addition to the cutting edge wear, both pad and circular land wear occur at the front end of the BTA drill. The depths Wp and WL of these wear zones are taken to characterize the wear pattern as shown in Fig. 6(c).
contour
3. Results and discussion At the cutting edge, wear was observed on the rake face and flank surface. Wear on the rake face is characterized by the formation of a crater
10pm
Groove
Fig. 5. Circular land wear (drilling depth, 2000 mm; speed, 64 m min-‘; feed, 0.1 mm rev-‘): (a) photograph showing land wear; (b) profile trace on circular land.
(c)
(a)
Crater
(b)
Flank
Pad/circular
wear
wear
land
wear
Fig. 6. Characterization of tool wear in BTA machining: (a) crater wear; (b) flank wear; (c) pad-circular land wear.
and it results from the action of the chip flowing along the face. Wear on the flank is caused by the rubbing action of the freshly cut surface. The crater wear on the intermediate and outer cutting edges was measured for various machining conditions. It was observed that the crater wear was predominant on the intermediate edge. Similarly, the flank wear on the clearance face of the intermediate cutting edge was also found to be more predominant than
39
that on the outer edge. This is due to the fact that, near the axis of a BTA drill, the cutting edge is subjected to unfavourable cutting conditions. Figures 2(b) and 3(b) show a typical crater profile of the intermediate cutting edge and the flank wear contour respectively. The variations in crater depth KT and the average flank wear VB on the intermediate cutting edge with cutting time are plotted in Fig. 7. The trends of these curves could be explained from the well-established wear character-
(a)
ib)
Cutting
Cutting
-
Flank
---
Crater
time,
wear depth
VS KT
min
-
Flank
---
Crater
wear depth
VS KT
time, min
Fig. 7. Wear-time curves for the intermediate cutting edge: (a) different speeds at a feed of 0.1 mm rev-l; (b) different feeds at a speed of 64 m min‘-l.
40
istics of a single-point carbide tool machining steel. For a given feed rate, crater wear increases with increase in speed (Fig. 7). As the speed increases, the temperature at the tool-chip interface increases. This leads to enhanced diffusion wear in which the metal and carbon atoms of the tool diffuse into the work material and are carried away in the chip [ 111. The effect of feed rate on the crater wear is shown in Fig. 7(b). The main effect of increased feed appears to be an increase in length of contact between the chip and the tool with increase in the size of the heated area from the edge and deeper below the rake face accompanied by an increase in the maximum temperature. However, at very high feed rates the cutting edge may be chipped owing to high stresses. The higher value of the flank wear at low speeds is mainly due to macro transfer of tool material involving mechanisms of abrasion and adhesion [ 121. Because of the slenderness of the BTA drill and the resulting chatter and vibration, the metal flow past the tool may be very uneven. This unevenly flowing metal imposes localized tensile stresses which break fragments away from the tool [13]. However, as the speed is increased the cutting temperature increases and softening occurs at the interface, reducing the adhesion and the intensity of wear [14]. The supporting pads balance the cutting forces generated at the cutting edge and burnish the machined surface by severe plastic deformation. The burnishing action over a period of time causes the pads to wear. The fact that both pads wear at their edges suggests that high pressures are present at the leading edge of the burnishing pads together with the high strain rate which gives rise to high temperatures. Figure 4(a) shows a typical contour of wear on a 90” pad. It may be noted that the wear is irregular. Therefore, the depth WP of wear is taken as a parameter to characterize pad wear (Fig. 4(c)). Figure 8 shows the trend of pad wear with respect to speed and feed for the 90” and 180” pads. The pad wear WP attains a minimum value at an intermediate speed and increases with increasing feed. As the feed increases, the amount of burnishing occurring at the front of the pads also increases. This results in a high pressure on the pad and the wear increases. The circular land wear is also characterized by its depth W, as shown in Fig. 6(c). A groove due to the previously cut surface which is a strainhardened layer is observed on the land. The variation in land wear with cutting speed and feed is shown in Fig. 9. Like flank wear, land wear also decreases with increasing cutting speed. However, higher feed rates increase the load on the land, thereby increasing the wear.
4. Conclusion A systematic approach to the measurement and characterization of tool wear in BTA machining has been presented in this paper. Three critical zones of wear have been identified: cutting edge, supporting pad and circular
41 18 14
-
180’ Pad wear __. _ 90 Pad wear
---
t
0
8
(a)
12 Cutting
16 time,
20
24
:
min
18_ E =
180. Pad wear
---
wear
90. Pad
-
I (431
I
G
I
I 8
I
I 12
Cutting
I
time,
I 16
I
I 20
I
II 24
min
Fig. 8. Wear-time curves for supporting pads: (a) different speeds at a feed of 0.1 mm rev-l; (b) different feeds at a speed of 64 m min-‘.
land. Study of the variation in wear at these zones with respect to speed and feed reveals the following. (1) The crater wear and flank wear are predominant on the intermediate cutting edge. (2) Crater wear increases with increasing speed and feed. (3) Flank and circular land wear decreases with increasing cutting speed. (4) Supporting pad wear is less at intermediate speeds. (5) Flank wear attains a minimum value at an intermediate feed whereas crater wear, circular land wear and supporting pad wear increase with increasing feed.
42
40
32 E x $
2L-
I 2
I L
I1 6
8
I 10
III
Cutting
(b)
Cutting
I time,
time,
III 20
I
I
min.
min
Fig. 9. Wear-time curves for circular land: (a) different speeds at a feed of 0.1 mm rev-‘; (b) different feeds at a speed of 64 m min-‘.
Acknowledgment The present work was conducted in the Design and Development Division of Widia (India) Ltd., Bangalore. The authors are grateful to the management of Widia and its Design and Development Division for their continuous support and encouragement.
43
References 1 H. J. Swinehart, Gundrilling, Trepanning and Deep Hole Machining, American Society of Tool and Manufacturing Engineers, Dearborn, MI, 1967. 2 T. Corney and B. J. Griffiths, A study of the cutting and burnishing operation during deep hole drilling and its relation to drill wear, Znt. J. Prod. Res., 14 (1) (1976) 1 - 9. 3 M. 0. M. Osman and V. Latinovic, On the development of multi-edge cutting tools for B.T.A. deep hole machining, ASME Trans., J. Eng. Znd., 98 (May 1976) 474 - 480. 4 P. G. Fink, Optimum cutting speed and feed rate in deep hole drihing, Proc. 2nd Znt. Conf. on Deep Hole Drilling and Bon’ng, Brunei University, 1977, 0.1-l 1. 5 R. J. Syrett and H. J. Bult, The reiative performance of cutting fluids in deep drilling, Proc. 2nd Znt. Conf, on Deep Hole Drilling and Boring, Brunei University, 1977, P.Q.l-9. 6 B. J. Griffiths, Guidelines for planning a deep hole drilling (self piloting drilling) operation, Proc. 2nd Znt. Conf. on Deep Hole Drilling and Boring, Brunei University, 1977, P.D.l-11. 7 K. Sakuma, K. Taguchi and S. Kinjo, Tire effect of tool materials on the cutting performance, Bull. JSME, 21 (153) (1978) 532 - 539. 8 M. 0. M. Osman, V. Latinovic and B. Greuner, On the performance of cutting fluids for B.T.A. deep hole machining, Znt. J. Prod. Res., 19 (5) (1981) 491 - 503. 9 R. J. Grieve and B. J. Griffiths, The economics of multi-cutting and deformation region tools, Znt. J. Prod, Res., 22 (5) (1984) 727 - 745. 10 H. Optiz and W. Konig, On the wear of cutting tools, Proc. 8th Znt. Conf. on Mach. Tool Des. Res., University of Manchester, September f967, pp. 173 - 190. 11 E. J. A. Armarego and R. H. Brown, The Machining of Metals, Prentice-Hall, Englewood Cliffs, NJ, 1969. 12 G. C. Sen and A. Bhattacharyya, Principles of Metal Cutting, New Central Agency, India, 1969. 13 E. M. Trent, Metal Cutting, Butterworths, London, 2nd edn., 1984. 14 V. Arshinov and G. Alekseev, Metal Cutting Theory and Cutting Tool Design, MIR, Moscow, 1970.
(1988)
124
WEAR
33 .43
STUDIES
IN BORING
P. K. RAMAKRISHNA Department (Zndia)
33
TREPANNING
ASSOCIATION
DRILLING
RAO and M. S. SHUNMUGAM
of Mechanical Engineering,
Indian Institute of Technology,
Madras 600036
(Received July 22, 1987; revised November 13, 1987; accepted November 26, 1987)
Summary
Boring Trepanning Association (BTA) drills are capable of machining holes having a large length-to-diameter ratio in a single pass. This is a unique self-piloted operation in which cutting and burnishing are both carried out at the same time. Wear studies on BTA drills reveal quite useful information regarding their life and performance. This paper deals with wear studies on 20 mm diameter BTA drills (solid boring) of the Heller design using En9 steel work samples. The studies include examination of the flank and crater wear on the cutting edge, circular land wear and the supporting pad wear. The variation in wear with respect to cutting time for different speeds and feeds is reported.
1. Introduction
When the length-to-diameter ratio of a hole exceeds 10, gun drilling and deep hole boring become more economical [l]. The Boring Trepanning Association (BTA) method of deep hole machining basically utilizes a singlecutting-edge tool with an internal chip removal system. The drilling oil is fed to the cutting region under pressure into the annular space between the drill and the hole. It flows over the cutting edge and through the duct above the edge into the centre of the tubular boring bar, carrying the swarf with it. The BTA drilling head presents an asymmetrical cutting edge balanced by supporting pads at approximately 90” and 180” behind the cutting edge to provide guidance after the tool has entered the workpiece as shown in Fig. 1. Like any other cutting tool, tool wear is a major index of the performance of a BTA drill since it limits the useful life. Acceptable surface quality and integrity, dimensional accuracy and the overall economics of the process are directly influenced by tool wear. For a single-point tool, the most important forms of wear are flank and crater wear. On a BTA drill, in addition to the wear at the cutting edge, wear also occurs on the supporting pads and circular land. Figure 1 shows three distinct zones of wear on a BTA 0043-1648/88/$3.50
0 Elsevier Sequoia/Printed
in The Netherlands
34
i---
-.- Inner
cutting
edge
ntermediote
cutting
Outer cutting
edge
\
Cutting
edge
edge
Crater
Fig. 1. BTA drill showing typical areas of wear.
drill: (a) cutting edge wear on the rake and clearance faces; (b) supporting pad wear on the 90” pad and 180” pad; (c) circular land wear. Comey and Griffiths have studied the self-guiding action of BTA drills and concluded that the maximum burnishing pressure occurs in front of the pad [2]. Osman and Latmovic have emphasized the need to maintain a hydrodynamic lubricating film between the bore and the supporting pad in order to reduce pad wear [ 31. Fink has considered a tool life criterion based on flank wear or cutting edge chipping, whichever occurred earlier, to arrive at the optimum cutting conditions in deep hole mac~n~g [4]. Syrett and Bult have considered drill failure which corresponds to a maximum flank wear of 1 mm together with chip formation and surface finish in their investigation to evaluate the relative performance of cutting fluids [ 51. Griffiths has used a drill life criterion based on 0.5 mm mean flank wear at the cutting edge. He has pointed out that the choice of cutting data for deep drilling operations depends on tool wear, cutting forces and power, chip forma~on, hole quality and surface finish [6]. In an attempt to evalwte different tool materials, Sakuma et al. have considered the growth of flank wear at each cutting edge together with the cutting torque, thrust and surface finish [ 71. These studies were carried out under constant drilling conditions. Osman et al. have compared the wear occurring at the cutting edge, pad and circular land and formulated a relative performance index which would be useful in evaluating different cutting fluids [S]. Grieve and Griffiths have brought out the significance of wear on the cutting edge and the supporting pads and incorporated this wear into a cost model to carry out economic analyses [9].
35
From the literature, it is seen that wear occurring at all three zones, namely cutting edge, supporting pad and circular land, is important. Excessive wear at any one zone may limit the life of BTA drills, necessitating regrinding. It may be necessary to study variables affecting the wear at these zones and to adjust the variables to avoid excessive wear at any one zone. In this paper, a systematic study of wear at the cutting edge, supporting pad and circular land is reported. The machining trials have been carried out on En9 steel work samples using a 20 mm diameter BTA drill under different machining conditions. The wear time curves with respect to speed and feed have been included.
2. Experimental details 2.1. Machining trials BTA machining trials were carried out using 20 mm diameter BTA drills (solid boring) of the Heller design. As shown in Fig. 1, the cutting tip is divided into three sections. The outer and intermediate cutting edges separated by a step are at 18” and 12” respectively. The inner edge is reversed so that an edge rather than a point is at the centre of rotation. The point offset is approximately 20% of the radius and the outer and intermediate cutting edges are equal in length. The rake angle for the inner cutting edge is -30” compared with 0” for outer and intermediate edges. The types of carbides used for the cutting tip and supporting pads, in the present work, were P20 and PlO carbides respectively. The work material was En9 steel of 42 mm diameter and 450 mm length. The BTA machine used for the trials was based on a rotating tool system. The cutting fluid used was a mineral oil with extreme pressure additives having the trade name TRIPONOL-H (Bharath Petroleum Corporation). It contains reactive sulphur (0.8% by weight) and chlorine (0.1% by weight). The kinematic viscosity of the cutting fluid is (10 - 15) X 10m6 m2 s-’ at 40 “C and the flash point is 160 “C. The flow rate was maintained at 150 1 min-’ at a pressure of 34.5 bar. The machining trials were carried out over a range of feed and speed combinations obtained by maintaining a feed of 0.1 mm rev-’ and varying the speed and maintaining a speed of 64 m min-’ with different feed rates. A fresh tool was used for each cutting condition. 2.2. Measurement and characterization of wear The wear measurements were carried out at hole depths of 400, 1200 and 2000 mm for different machining conditions. Figure 2(a) shows crater formation on the intermediate and outer cutting edges. The depth of the crater was measured by taking profile traces on the rake face with a Perth-OMeter. Figure 2(b) shows a typical trace of a crater on the intermediate cutting edge. The flank wear at the cutting edge is shown in Fig. 3(a). A flank wear plot developed with measurements obtained using a tool-maker’s microscope is shown in Fig. 3(b). Figures 4(a) and 4(b) show the wear
36
(a)
----I-loop ZSpm
/-
r
Fig. 2. Crater wear (drilling depth, 2000 mm; speed, 64 m min-‘; feed, 0.1 mm rev-‘): (a) photograph showing crater wear; (b) profile trace of a crater on the intermediate cutting edge.
l.O@I
Fig. 3. Flank wear (drilling depth, 2000 mm; speed, 64 m min-‘; (a) photograph showing flank wear; (b) wear contours on flank.
feed, 0.1 mm rev-‘):
Fig. 4. Supporting pad wear (drilling depth, 2000 mm; speed, 64 m min-I; feed, 0.1 mm rev-‘): (a) SO” pad wear;(b) 180” pad wear; (e) profile trace on the 90” pad.
on 90” and 180” pads respectively. As it was difficult to characterize the pad wear on the basis of such an irregular pattern, it was decided to measure the depth of pad wear using profile traces. Figure 4(c) shows a typical trace taken on the 90” pad using a Perth-O-Meter. A trace was also obtained on the circular land as shown in Fig. 5(b). Figure 5(a) shows the nature of wear on the circular land. Figures 2 - 5 are for a speed of 64 m min-’ and a feed of 0.1 mm rev-’ after machining a depth of 2000 mm. For s~gle-post tools, various parameters are used to characterize the crater and flank wear occurring at the cutting edge [lo]. The BTA drill under investigation also exhibits wear on the rake and flank surfaces at the cutting tip. The depth KT of the crater on the rake face and the average flank wear VB shown in Figs. 6(a) and 6(b) are used in the present work to study the progress of wear on the cutting edge of the BTA drill with time. In addition to the cutting edge wear, both pad and circular land wear occur at the front end of the BTA drill. The depths Wp and WL of these wear zones are taken to characterize the wear pattern as shown in Fig. 6(c).
contour
3. Results and discussion At the cutting edge, wear was observed on the rake face and flank surface. Wear on the rake face is characterized by the formation of a crater
10pm
Groove
Fig. 5. Circular land wear (drilling depth, 2000 mm; speed, 64 m min-‘; feed, 0.1 mm rev-‘): (a) photograph showing land wear; (b) profile trace on circular land.
(c)
(a)
Crater
(b)
Flank
Pad/circular
wear
wear
land
wear
Fig. 6. Characterization of tool wear in BTA machining: (a) crater wear; (b) flank wear; (c) pad-circular land wear.
and it results from the action of the chip flowing along the face. Wear on the flank is caused by the rubbing action of the freshly cut surface. The crater wear on the intermediate and outer cutting edges was measured for various machining conditions. It was observed that the crater wear was predominant on the intermediate edge. Similarly, the flank wear on the clearance face of the intermediate cutting edge was also found to be more predominant than
39
that on the outer edge. This is due to the fact that, near the axis of a BTA drill, the cutting edge is subjected to unfavourable cutting conditions. Figures 2(b) and 3(b) show a typical crater profile of the intermediate cutting edge and the flank wear contour respectively. The variations in crater depth KT and the average flank wear VB on the intermediate cutting edge with cutting time are plotted in Fig. 7. The trends of these curves could be explained from the well-established wear character-
(a)
ib)
Cutting
Cutting
-
Flank
---
Crater
time,
wear depth
VS KT
min
-
Flank
---
Crater
wear depth
VS KT
time, min
Fig. 7. Wear-time curves for the intermediate cutting edge: (a) different speeds at a feed of 0.1 mm rev-l; (b) different feeds at a speed of 64 m min‘-l.
40
istics of a single-point carbide tool machining steel. For a given feed rate, crater wear increases with increase in speed (Fig. 7). As the speed increases, the temperature at the tool-chip interface increases. This leads to enhanced diffusion wear in which the metal and carbon atoms of the tool diffuse into the work material and are carried away in the chip [ 111. The effect of feed rate on the crater wear is shown in Fig. 7(b). The main effect of increased feed appears to be an increase in length of contact between the chip and the tool with increase in the size of the heated area from the edge and deeper below the rake face accompanied by an increase in the maximum temperature. However, at very high feed rates the cutting edge may be chipped owing to high stresses. The higher value of the flank wear at low speeds is mainly due to macro transfer of tool material involving mechanisms of abrasion and adhesion [ 121. Because of the slenderness of the BTA drill and the resulting chatter and vibration, the metal flow past the tool may be very uneven. This unevenly flowing metal imposes localized tensile stresses which break fragments away from the tool [13]. However, as the speed is increased the cutting temperature increases and softening occurs at the interface, reducing the adhesion and the intensity of wear [14]. The supporting pads balance the cutting forces generated at the cutting edge and burnish the machined surface by severe plastic deformation. The burnishing action over a period of time causes the pads to wear. The fact that both pads wear at their edges suggests that high pressures are present at the leading edge of the burnishing pads together with the high strain rate which gives rise to high temperatures. Figure 4(a) shows a typical contour of wear on a 90” pad. It may be noted that the wear is irregular. Therefore, the depth WP of wear is taken as a parameter to characterize pad wear (Fig. 4(c)). Figure 8 shows the trend of pad wear with respect to speed and feed for the 90” and 180” pads. The pad wear WP attains a minimum value at an intermediate speed and increases with increasing feed. As the feed increases, the amount of burnishing occurring at the front of the pads also increases. This results in a high pressure on the pad and the wear increases. The circular land wear is also characterized by its depth W, as shown in Fig. 6(c). A groove due to the previously cut surface which is a strainhardened layer is observed on the land. The variation in land wear with cutting speed and feed is shown in Fig. 9. Like flank wear, land wear also decreases with increasing cutting speed. However, higher feed rates increase the load on the land, thereby increasing the wear.
4. Conclusion A systematic approach to the measurement and characterization of tool wear in BTA machining has been presented in this paper. Three critical zones of wear have been identified: cutting edge, supporting pad and circular
41 18 14
-
180’ Pad wear __. _ 90 Pad wear
---
t
0
8
(a)
12 Cutting
16 time,
20
24
:
min
18_ E =
180. Pad wear
---
wear
90. Pad
-
I (431
I
G
I
I 8
I
I 12
Cutting
I
time,
I 16
I
I 20
I
II 24
min
Fig. 8. Wear-time curves for supporting pads: (a) different speeds at a feed of 0.1 mm rev-l; (b) different feeds at a speed of 64 m min-‘.
land. Study of the variation in wear at these zones with respect to speed and feed reveals the following. (1) The crater wear and flank wear are predominant on the intermediate cutting edge. (2) Crater wear increases with increasing speed and feed. (3) Flank and circular land wear decreases with increasing cutting speed. (4) Supporting pad wear is less at intermediate speeds. (5) Flank wear attains a minimum value at an intermediate feed whereas crater wear, circular land wear and supporting pad wear increase with increasing feed.
42
40
32 E x $
2L-
I 2
I L
I1 6
8
I 10
III
Cutting
(b)
Cutting
I time,
time,
III 20
I
I
min.
min
Fig. 9. Wear-time curves for circular land: (a) different speeds at a feed of 0.1 mm rev-‘; (b) different feeds at a speed of 64 m min-‘.
Acknowledgment The present work was conducted in the Design and Development Division of Widia (India) Ltd., Bangalore. The authors are grateful to the management of Widia and its Design and Development Division for their continuous support and encouragement.
43
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