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Reliability of a VMC and its improvement
Reliability Engineering and System Safety 72 (2001) 99±102
www.elsevier.com/locate/ress
Short communication
Reliability of a VMC and its improvement Yi Dai, Yazhou Jia* Mechanical Engineering Department, Jilin University of Technology, Changchun 130025, People's Republic of China Received 10 January 2000; accepted 29 October 2000
Abstract It is important to enhance the reliability of a vertical machining center (VMC). In this article, a large number of failure data of a type of VMC have been collected and based on an analysis of these failures, several de®ciencies have been found. Ways to improve the reliability of the VMC are proposed. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Reliability; Vertical machining center; Failure mode; Failure position
1. Introduction
3. Failure data of VMC
Market economy is playing an increasingly important role in China today. To cope with other competitors, many machine tool manufactories have begun to give importance to the reliability of CNC machine tools. Because CNC machine tools are expensive and complicated [1], analyzing their ®eld failure data becomes important [2]. We have taken efforts to do so [3,4]. Over a period of 2 years, we have investigated the failure data of 14 vertical machining centers (VMC) and have found the distribution of failure mode and that of failure position. In addition, we discuss the cause of failure and the way to enhance the reliability of the VMC.
Table 1 describes VMC failure phenomena, all of which come from VMC users. Though these records are not all regular and adequate, they are ®rsthand information.
2. Brief description of VMC The VMC discussed adopts the OSP-700 CNC system made by OKUMA company. Through pulse width modulation (PWM), which contains an insulated-gate bipolar transistor (IGBT), the AC induction motor drives the spindle directly. The speed of the spindle varies from 20 to 6000 rpm. Three feeding motions (named X, Y and Z) are driven by AC servomotors through a belt mechanism. There is no automatic tool changer in the VMC. When the VMC changes the cutting tool, the spindle moves in the vertical and horizontal directions till above the cutting tool drum in order to complete the operation. * Corresponding author. E-mail address: [email protected] (Y. Jia).
4. Distribution of failure mode of VMC We reviewed the failure phenomena in Table 1, and interviewed the repairer. The statistics of failure mode are shown in Table 2, based on which Fig. 1, i.e. the histogram of failure mode, was drawn. Fig. 1 demonstrates that the failure mode with highest frequency is machine element damage. Fig. 1 also shows that the sum of items 3 and 10 is 41.03%. Except for belt damage, which occurred seven times, other elements were damaged accidentally. Bearing damage occurred three times. Two-time damages were those to the spring, lubricating motor, magnetic transducer (positioning equipment), guard, clutch, encoder, and electric wire. One-time damage occurred in the pull rod, steel ball, hydraulic meter, motor gear, oil tube, electric box, spring axis, feeding sensor, servo unit, and so on. Fig. 1 indicates that the failure mode with the second highest frequency is the leaking of oil and water. Though leaking has been a tiresome problem confronting many factories, general leaking is not the actual problem; rather, this is a problem of choice of lubricating method because the main cause of leaking is lubrication. When changing the cutting tool, many instances such as the dislocation of the tool drum, a resistance in the motion of the tool drum door and a positioning fault of the spindle can
0951-8320/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0951-832 0(00)00104-6
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Y. Dai, Y. Jia / Reliability Engineering and System Safety 72 (2001) 99±102
Table 1 Failure phenomenon of VMC
Table 2 Failure mode distribution of VMC
Order
Failure phenomenon
Order
Failure mode
Frequency (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39
Damage of shelter for tool drum Abnormal sound when removing the tool from the spindle Z axis verticality not good Damage of pull rod Damage of ball in pull rod Oil leaks from pipe Alarm for feeding out of range Tool drum cannot be located Abnormal sound when spindle rotates Sharp sound when exchanging tool Oil leaks from within the rail Cooling water leaks to tool drum Scanning fault of software Tool drum distortion by bumping Damage of motor gear Tool drum fails to rotate Noise of ball screw Lubricating motor damage Hydraulic press meter short of damper Encoder wire wreck Changing tool time too long Wrong assembly of gas valve of guard door for tool drum X, Y belt wheel damage Abnormal sound in Z feeding motion Cooling water leaks from base case Spring of spindle spoilage Transducer wreck Spindle bumps the guard of the tool drum Hydraulic meter short of ªOº sealing washer Oil leaks from base case of VMC Cooling water pipe rupture Initial position of tool drum wrong Electric box too hot VMC guard net damage Tool drum door moves improperly Z belt break by pulling Encoder damage Spindle initial position too low While freeing tool, spindle clatters and bears higher load
1 2 3 4 5 6 7 8 9 10 11
Demanding adjusting of tool Feeding sensor dislocation Machine element damage Software failure Abnormal sound Spindle bumps tool drum Demanding adjusting of ball Wrong operation Leaking of oil and water Electronic component failure Demanding adjusting of spindle
8.97 2.56 29.49 1.28 8.97 5.13 2.56 1.28 27.6 11.54 1.28
drum induced this failure mode. It was also brought about by the following circumstances. In the process of changing tools, the spindle is sometimes required to turn through a tiny angle because of an error in the tool drum. As the cutting tool is freed from the pull rod, the piston of the cylinder moves down and pushes the pull rod at the same time. The transducer may conduct a feedback signal to the CNC system to make the spindle stop at that position. On condition that the pull rod is free, the spindle can turn back through a tiny angle as required. However, when the pull rod is pushed dead, the spindle can by no means turn. It was at such a time that the phenomena listed as items 2 and 39 in Table 1 took place. 5. Distribution of failure position of VMC
cause failures. Items 1 and 11 in Table 2 are these failure modes. When the VMC changed the tool in such cases, the failure mode of bumping, i.e. item 6 in Table 2, took place. In Fig. 1, item 5 is regarded as being on the verge of damage. The swing of the spindle and the slant of the tool
In a similar manner, we analyzed the failure phenomena in Table 1 and obtained the statistics of failure position as shown in Table 3. On the basis of Table 3, Fig. 2 was drawn to illustrate the distribution of the failure position. In order to research the failure conveniently, the changing tool system, which contains the tool drum and the major part of the spindle system, is listed as item 1 in Table 3. On the other hand, in order to analyze the problems clearly, the failure positions in Table 3 are classi®ed in more detail. For example, the belt mechanism, ®xed in the Z feeding motion system has been listed alone as item 8 in Table 3.
Fig. 1. Failure mode distribution of VMC.
Fig. 2. Failure position distribution of VMC.
Y. Dai, Y. Jia / Reliability Engineering and System Safety 72 (2001) 99±102
101
Table 3 Failure position distribution of VMC Order
Failure position
Frequency (%)
1 2 3 4 5 6 7 8 9 10 11 12
Changing tool system Z feeding motion system X feeding motion system Y feeding motion system Work table Electronic system Guard Belt mechanism Lubricating system Numerical control system Hydraulic system Spindle
37.17 5.13 3.85 1.28 1.28 5.13 1.28 8.97 24.36 1.28 7.69 2.56
As shown in Table 3 and Fig. 2, the highest frequency of 37.17% is for the changing tool system. The second highest frequency of 24.36% is for the lubricating system. The belt mechanism ranks third in frequency. From what has been mentioned above, the failure of the changing tool system is a large hindrance to the elevation of the reliability of the machining center. Effectively, with the development of speed variations for the AC motor and the adoption of the linear roller guide-way, the spindle and feeding system have been simpli®ed greatly. Their reliability has been raised remarkably. Compared with these, the changing tool system acquired little improvement in reliability. Secondly, the fact that the failure frequency of lubrication is far higher than that of the hydraulic system implies that lubrication failure is the main cause of leaking. Some failures such as item 11 in Table 1 are lubrication failures, not general ªleaking problemsº. Thirdly, in Table 3 and Fig. 2, the failure frequency of the CNC and electronic system is fewer than that of other positions. This shows that the reliability of the CNC and electronic system is much higher than that of the mechanical position in the VMC. 6. The failure cause and improvement of VMC The majority of VMC failure may be solved by repairing the damaged part carefully or substituting it with a new one. However, the sophisticated failures demand improvements in VMC structure and principle, which are discussed as follows: 1. Lubrication. The ball screws and rails of the VMC are lubricated using a lubricant whose rate of ¯ow is regulated by the control board of the VMC. If the rate of ¯ow is lower than required, the ball screws and rails work under bad conditions. Since the rate of ¯ow is higher than that needed, leaking occurs. Because the VMC works under various circumstances, the choice of rate of ¯ow is far from easy. So lubricating failure occurs
Fig. 3. Pull rod moves down to free tool.
frequently and the failures caused by adjusting the lubrication counter are by no means few. Thus, under present conditions where the operators cannot adjust the lubricant very well and all cooling water as well as lubricant are ®nally abandoned to the base case of the VMC, a better way to reduce the failure frequency of this VMC may be to adopt the grease lubricating style. 2. Positioning of spindle. When changing the cutting tool, the VMC uses the magnet transducer to inspect the spindle. This requires that the distance between the magnet transducer and the body emitting magnetism ®xed in the spindle be maintained strictly. Thus the radial jump of the spindle may affect the spindle positioning. The main causes determining the radial jump are the precision of the bearing, spindle and hole and the assembling precision of the spindle. In addition, the failure of dynamic equilibrium of the spindle system may also lead to a radial jump. Therefore, elevating the quality of production and installation of the spindle is one way of reducing failures when changing tools. On the other hand, the radial jump at the middle of the spindle is usually larger than that near the bearings. So arranging the positioning instrument at the upper end near the bearing is bene®cial to reduce failures. 3. Spindle clatters and bears higher load. We have discussed failure phenomena 2 and 39 in Table 1, which are included in the failure mode of item 5 in Table 2. As shown in Fig. 3, torque and rotation are transmitted from the motor gear to the spindle through the inner gear of the hollow axis. The pull rod is attached to the hollow axis. The principal cause of this failure is that the piston contacts the surface of the hollow axis, which conducts higher friction. When the pull rod is pushed dead, it cannot turn at all. Thus we add an axial ball bearing under the piston, which makes
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Y. Dai, Y. Jia / Reliability Engineering and System Safety 72 (2001) 99±102
a slight turn possible. It was proved to be effective in practice. 4. Feeding motion mechanism. The X, Y and Z feeding motions are driven through belts. These give rise to frequent damage of the belt and degradation of VMC reliability. So we think that it is best to eliminate the belt and join the servomotor to the ball screw directly on condition that the quali®ed coupler is chosen and careful assembly is done.
References [1] Thyer GE. Computer numerical control of machine tools. Industrial Press, 1988. [2] Keller AZ, Kamth ARR, Perea UD. Reliability analysis of CNC machine tools. Reliab Engng 1982;3:449±73. [3] Jia YZ. Probability distribution of machining center failures. Reliab Engng System Safety 1995;50:121±5. [4] Wang YQ. Failure probabilistic model of CNC lathes. Reliab Engng System Safety 1999;65:307±14.
www.elsevier.com/locate/ress
Short communication
Reliability of a VMC and its improvement Yi Dai, Yazhou Jia* Mechanical Engineering Department, Jilin University of Technology, Changchun 130025, People's Republic of China Received 10 January 2000; accepted 29 October 2000
Abstract It is important to enhance the reliability of a vertical machining center (VMC). In this article, a large number of failure data of a type of VMC have been collected and based on an analysis of these failures, several de®ciencies have been found. Ways to improve the reliability of the VMC are proposed. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Reliability; Vertical machining center; Failure mode; Failure position
1. Introduction
3. Failure data of VMC
Market economy is playing an increasingly important role in China today. To cope with other competitors, many machine tool manufactories have begun to give importance to the reliability of CNC machine tools. Because CNC machine tools are expensive and complicated [1], analyzing their ®eld failure data becomes important [2]. We have taken efforts to do so [3,4]. Over a period of 2 years, we have investigated the failure data of 14 vertical machining centers (VMC) and have found the distribution of failure mode and that of failure position. In addition, we discuss the cause of failure and the way to enhance the reliability of the VMC.
Table 1 describes VMC failure phenomena, all of which come from VMC users. Though these records are not all regular and adequate, they are ®rsthand information.
2. Brief description of VMC The VMC discussed adopts the OSP-700 CNC system made by OKUMA company. Through pulse width modulation (PWM), which contains an insulated-gate bipolar transistor (IGBT), the AC induction motor drives the spindle directly. The speed of the spindle varies from 20 to 6000 rpm. Three feeding motions (named X, Y and Z) are driven by AC servomotors through a belt mechanism. There is no automatic tool changer in the VMC. When the VMC changes the cutting tool, the spindle moves in the vertical and horizontal directions till above the cutting tool drum in order to complete the operation. * Corresponding author. E-mail address: [email protected] (Y. Jia).
4. Distribution of failure mode of VMC We reviewed the failure phenomena in Table 1, and interviewed the repairer. The statistics of failure mode are shown in Table 2, based on which Fig. 1, i.e. the histogram of failure mode, was drawn. Fig. 1 demonstrates that the failure mode with highest frequency is machine element damage. Fig. 1 also shows that the sum of items 3 and 10 is 41.03%. Except for belt damage, which occurred seven times, other elements were damaged accidentally. Bearing damage occurred three times. Two-time damages were those to the spring, lubricating motor, magnetic transducer (positioning equipment), guard, clutch, encoder, and electric wire. One-time damage occurred in the pull rod, steel ball, hydraulic meter, motor gear, oil tube, electric box, spring axis, feeding sensor, servo unit, and so on. Fig. 1 indicates that the failure mode with the second highest frequency is the leaking of oil and water. Though leaking has been a tiresome problem confronting many factories, general leaking is not the actual problem; rather, this is a problem of choice of lubricating method because the main cause of leaking is lubrication. When changing the cutting tool, many instances such as the dislocation of the tool drum, a resistance in the motion of the tool drum door and a positioning fault of the spindle can
0951-8320/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0951-832 0(00)00104-6
100
Y. Dai, Y. Jia / Reliability Engineering and System Safety 72 (2001) 99±102
Table 1 Failure phenomenon of VMC
Table 2 Failure mode distribution of VMC
Order
Failure phenomenon
Order
Failure mode
Frequency (%)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39
Damage of shelter for tool drum Abnormal sound when removing the tool from the spindle Z axis verticality not good Damage of pull rod Damage of ball in pull rod Oil leaks from pipe Alarm for feeding out of range Tool drum cannot be located Abnormal sound when spindle rotates Sharp sound when exchanging tool Oil leaks from within the rail Cooling water leaks to tool drum Scanning fault of software Tool drum distortion by bumping Damage of motor gear Tool drum fails to rotate Noise of ball screw Lubricating motor damage Hydraulic press meter short of damper Encoder wire wreck Changing tool time too long Wrong assembly of gas valve of guard door for tool drum X, Y belt wheel damage Abnormal sound in Z feeding motion Cooling water leaks from base case Spring of spindle spoilage Transducer wreck Spindle bumps the guard of the tool drum Hydraulic meter short of ªOº sealing washer Oil leaks from base case of VMC Cooling water pipe rupture Initial position of tool drum wrong Electric box too hot VMC guard net damage Tool drum door moves improperly Z belt break by pulling Encoder damage Spindle initial position too low While freeing tool, spindle clatters and bears higher load
1 2 3 4 5 6 7 8 9 10 11
Demanding adjusting of tool Feeding sensor dislocation Machine element damage Software failure Abnormal sound Spindle bumps tool drum Demanding adjusting of ball Wrong operation Leaking of oil and water Electronic component failure Demanding adjusting of spindle
8.97 2.56 29.49 1.28 8.97 5.13 2.56 1.28 27.6 11.54 1.28
drum induced this failure mode. It was also brought about by the following circumstances. In the process of changing tools, the spindle is sometimes required to turn through a tiny angle because of an error in the tool drum. As the cutting tool is freed from the pull rod, the piston of the cylinder moves down and pushes the pull rod at the same time. The transducer may conduct a feedback signal to the CNC system to make the spindle stop at that position. On condition that the pull rod is free, the spindle can turn back through a tiny angle as required. However, when the pull rod is pushed dead, the spindle can by no means turn. It was at such a time that the phenomena listed as items 2 and 39 in Table 1 took place. 5. Distribution of failure position of VMC
cause failures. Items 1 and 11 in Table 2 are these failure modes. When the VMC changed the tool in such cases, the failure mode of bumping, i.e. item 6 in Table 2, took place. In Fig. 1, item 5 is regarded as being on the verge of damage. The swing of the spindle and the slant of the tool
In a similar manner, we analyzed the failure phenomena in Table 1 and obtained the statistics of failure position as shown in Table 3. On the basis of Table 3, Fig. 2 was drawn to illustrate the distribution of the failure position. In order to research the failure conveniently, the changing tool system, which contains the tool drum and the major part of the spindle system, is listed as item 1 in Table 3. On the other hand, in order to analyze the problems clearly, the failure positions in Table 3 are classi®ed in more detail. For example, the belt mechanism, ®xed in the Z feeding motion system has been listed alone as item 8 in Table 3.
Fig. 1. Failure mode distribution of VMC.
Fig. 2. Failure position distribution of VMC.
Y. Dai, Y. Jia / Reliability Engineering and System Safety 72 (2001) 99±102
101
Table 3 Failure position distribution of VMC Order
Failure position
Frequency (%)
1 2 3 4 5 6 7 8 9 10 11 12
Changing tool system Z feeding motion system X feeding motion system Y feeding motion system Work table Electronic system Guard Belt mechanism Lubricating system Numerical control system Hydraulic system Spindle
37.17 5.13 3.85 1.28 1.28 5.13 1.28 8.97 24.36 1.28 7.69 2.56
As shown in Table 3 and Fig. 2, the highest frequency of 37.17% is for the changing tool system. The second highest frequency of 24.36% is for the lubricating system. The belt mechanism ranks third in frequency. From what has been mentioned above, the failure of the changing tool system is a large hindrance to the elevation of the reliability of the machining center. Effectively, with the development of speed variations for the AC motor and the adoption of the linear roller guide-way, the spindle and feeding system have been simpli®ed greatly. Their reliability has been raised remarkably. Compared with these, the changing tool system acquired little improvement in reliability. Secondly, the fact that the failure frequency of lubrication is far higher than that of the hydraulic system implies that lubrication failure is the main cause of leaking. Some failures such as item 11 in Table 1 are lubrication failures, not general ªleaking problemsº. Thirdly, in Table 3 and Fig. 2, the failure frequency of the CNC and electronic system is fewer than that of other positions. This shows that the reliability of the CNC and electronic system is much higher than that of the mechanical position in the VMC. 6. The failure cause and improvement of VMC The majority of VMC failure may be solved by repairing the damaged part carefully or substituting it with a new one. However, the sophisticated failures demand improvements in VMC structure and principle, which are discussed as follows: 1. Lubrication. The ball screws and rails of the VMC are lubricated using a lubricant whose rate of ¯ow is regulated by the control board of the VMC. If the rate of ¯ow is lower than required, the ball screws and rails work under bad conditions. Since the rate of ¯ow is higher than that needed, leaking occurs. Because the VMC works under various circumstances, the choice of rate of ¯ow is far from easy. So lubricating failure occurs
Fig. 3. Pull rod moves down to free tool.
frequently and the failures caused by adjusting the lubrication counter are by no means few. Thus, under present conditions where the operators cannot adjust the lubricant very well and all cooling water as well as lubricant are ®nally abandoned to the base case of the VMC, a better way to reduce the failure frequency of this VMC may be to adopt the grease lubricating style. 2. Positioning of spindle. When changing the cutting tool, the VMC uses the magnet transducer to inspect the spindle. This requires that the distance between the magnet transducer and the body emitting magnetism ®xed in the spindle be maintained strictly. Thus the radial jump of the spindle may affect the spindle positioning. The main causes determining the radial jump are the precision of the bearing, spindle and hole and the assembling precision of the spindle. In addition, the failure of dynamic equilibrium of the spindle system may also lead to a radial jump. Therefore, elevating the quality of production and installation of the spindle is one way of reducing failures when changing tools. On the other hand, the radial jump at the middle of the spindle is usually larger than that near the bearings. So arranging the positioning instrument at the upper end near the bearing is bene®cial to reduce failures. 3. Spindle clatters and bears higher load. We have discussed failure phenomena 2 and 39 in Table 1, which are included in the failure mode of item 5 in Table 2. As shown in Fig. 3, torque and rotation are transmitted from the motor gear to the spindle through the inner gear of the hollow axis. The pull rod is attached to the hollow axis. The principal cause of this failure is that the piston contacts the surface of the hollow axis, which conducts higher friction. When the pull rod is pushed dead, it cannot turn at all. Thus we add an axial ball bearing under the piston, which makes
102
Y. Dai, Y. Jia / Reliability Engineering and System Safety 72 (2001) 99±102
a slight turn possible. It was proved to be effective in practice. 4. Feeding motion mechanism. The X, Y and Z feeding motions are driven through belts. These give rise to frequent damage of the belt and degradation of VMC reliability. So we think that it is best to eliminate the belt and join the servomotor to the ball screw directly on condition that the quali®ed coupler is chosen and careful assembly is done.
References [1] Thyer GE. Computer numerical control of machine tools. Industrial Press, 1988. [2] Keller AZ, Kamth ARR, Perea UD. Reliability analysis of CNC machine tools. Reliab Engng 1982;3:449±73. [3] Jia YZ. Probability distribution of machining center failures. Reliab Engng System Safety 1995;50:121±5. [4] Wang YQ. Failure probabilistic model of CNC lathes. Reliab Engng System Safety 1999;65:307±14.