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  1. 1. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – INTERNATIONAL JOURNAL OF MECHANICAL ENGINEERING 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 5, September - October (2013) © IAEME AND TECHNOLOGY (IJMET) ISSN 0976 – 6340 (Print) ISSN 0976 – 6359 (Online) Volume 4, Issue 5, September - October (2013), pp. 191-199 © IAEME: www.iaeme.com/ijmet.asp Journal Impact Factor (2013): 5.7731 (Calculated by GISI) www.jifactor.com IJMET ©IAEME PROCESS FAILURE MODE AND EFFECT ANALYSIS ON END MILLING PROCESS- A CRITICAL STUDY Pravin Kumar .S1, Venkatakrishnan.R2, Vignesh Babu.S3 1 UG Graduate,Department of Mechanical Engineering, Government College of Technology, Coimbatore. 2 UG Graduate,Department of Mechanical Engineering, Government College of Technology, Coimbatore. 3 UG Graduate,Department of Mechanical Engineering, Kumaraguru College of Technology, Coimbatore. ABSTRACT An FMEA (Failure Mode and Effect Analysis) is a systematic method of identifying and preventing product and process problems before they occur. FMEAs are focused on preventing defects, enhancing safety, and increasing customer satisfaction. FMEAs are conducted in the product design or process development stages, although conducting an FMEA on existing products and processes can also yield substantial benefits. FMEA is precisely an analytical methodology used to ensure that potential problems have been considered and addressed throughout the product and process development cycle. It is essential to analyze the process before implementing and operating the machine. In this work, the process failure mode effect and analysis of End Milling process is done. A series of end milling process is done on several work pieces and the potential failure and defects in the work piece and the tool are studied. These are categorized based on FMEA, risk priority numbers are assigned to each one and by multiplying the ratings of occurrence, severity and detection. Finally the most risky failure according to the RPM numbers is found and the cause and effects along with the preventive measures are tabulated. This work serves as a failure prevention guide for those who perform the end milling operation towards an effective milling. KEYWORDS: Failure Modes, End Milling, Risk Priority Number, Depth of Cut, Cutting Speed. 1. INTRODUCTION In order to satisfy the increasing demands of the customers for high quality and reliable products, the manufacturers are forced to switch gears in their system so that they can deliver the product at the expected quality and reliability. The challenge is to design in quality and reliability 191
  2. 2. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 5, September - October (2013) © IAEME early in the development cycle. Failure Mode and Effect Analysis (FMEA) is used to identify potential failure modes, determine their effect on the operation of the product, and identify actions to mitigate the failures. With this the modes of failure and its effect on the component can be studied to a satisfactory extend. As anticipation of every failure mode is not possible, the working team has to strive to produce an extensive list covering major and most of the failure modes as possible. Effective use of FMEAs can have a positive impact on an organization’s bottom-line because of their preventive nature. FMEA enhances further improvisation of both the design and manufacturing processes in the future as it serves as a record of the current process in formations. With a strong and reliable FMEA, it is possible that we can engineer to design out failures and produce reliable, safe, and customer pleasing products. It is essential that such an effective analysis has to be carried out for improving various mechanical processes so that the demand of the customers can be satisfied. 1.1 FAILURE MODE & EFFECT ANALYSIS FMEA is an engineering technique used to identify, prioritize and alleviate potential problems from the system, design, or process before the problems are actualized (According to Omdahl, 1988). What does the term “Failure Modes” imply? Lots of definitions for this term can be obtained. According to the Automotive Industry Action Group (AIAG), a failure mode is “the way in which a product or process could fail to perform its desired function” (AIAG, 1995). Some sources define “failure mode” as a description of an undesired cause-effect chain of events (MIL-STD1629A, 1994). Others define “failure mode” as a link in the cause-effect chain (Stamatis, 1995: Humphries, 1994). To conclude with we consider the term failure mode as any errors or defects in a process, design, or item, especially those that affect the customer, and can be potential or actual. The term “Effect Analysis” also invites various definitions. The effect analysis is “The analysis of the outcome of the failure on the system, on the process and the service” (Stamatis, 1995: Humphries, 1994). To put it simply Effects analysis refers to studying the consequences of those failures. 1.2 ROLE OF FMEA IN MILLING The role of milling cutter or the mill is the most critical aspect that determines the out coming product’s finish, accuracy and also the life of milling cutter is a major factor determining the cost of the component. The dimensional accuracy and the type of finish are the expected parameters in the component and when this fails the whole process and the product becomes a scrap. The failure of the component is as specific as the failure of the process and failure of the cutter. The failure of the product may be because of the properties of the cutter or by design parameters of the machine or even by the properties of the metal used in the component and the cutter. The various modes of failure of the process may be like chipping or chip packing or breakage of cutter etc. These may be the failures caused as a result of improper milling but it is very important to analyze the failure modes, and effects of end milling processes. 2. IMPLEMENTATION OF FMEA In FMEA, failures are prioritized according to how serious their consequences are, how frequently they occur and how easily they can be detected. This FMEA conducted can be compiled and documented and this can be used in future to design an effective process cycle with an aim of avoiding the failures mentioned in the FMEA table. This is known as Design Failure Mode and effect analysis (DFMEA). Later it is used for process control, before and during ongoing operation of the process. Ideally, FMEA begins during the earliest conceptual stages of design and continues throughout the life of the product or service. FMEA helps select remedial actions that reduce cumulative impacts of life-cycle consequences (risks) from a systems failure (fault). The various steps in Process Failure and Effect analysis are as follows: 192
  3. 3. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 5, September - October (2013) © IAEME • Reviewing the process • List the potential effects and modes of failure • Assign a severity rating • Assign an occurrence rating • Assign a detection rating • Calculate the risk priority number (RPN) for each mode of failure • Take action to eliminate or reduce the high-risk failure modes • Calculate the resulting RPN as the failure modes are reduced or eliminated The FMEA in this work is done on End Milling by conducting several trails in vertical milling machine and assigning them severity, occurrence and detection ratings and calculating their RPN. 2.1 STEP 1: REVIEWING THE PROCESS The blueprint (or engineering drawing) of the product and a detailed flowchart of the operation are reviewed .The process parameters of the conducted tests are as follows: Machine : Vertical Milling Machine Make : M1TR HMT Tool : HSS Surface Table : 1700 x 300 mm Work Piece Material : Cast Iron Cutting Speed Used : 750 rpm Fig1 HMT Milling Machine Fig 2 End Mill Fig3 Work piece with failure 2.2 STEP 2: POTENTIAL EFFECTS AND MODES OF FAILURE Several trials were conducted with the above mentioned process parameters in the aforementioned machine and parameters. From the list of the reading obtained from the trials and the reading recorded in the previous failure charts the potential effects and failure modes are obtained. These failure modes and their effects are charted separately for the sake of calculating and assigning the ratings and risk priority numbers. With the failure modes listed on the FMEA Worksheet, each failure mode is reviewed and the potential effects of the failure should it occur are identified. For some of the failure modes, there are only one effect, while for other modes there are several effects. This step must be thorough because this information will feed into the assignment of risk rankings for each of the failures. It is helpful to think of this step as an if-then process: If the failure occurs, then what are the consequences. 2.3 STEP 3: OCCURANCE RATING In this step it is necessary to look at the number of times a failure occurs. This can be done by looking at similar products or processes and the failure modes that have been documented. A failure mode is given an occurrence ranking (O), again 1–10. If the occurrence is high (meaning > 4 for nonsafety failure modes and > 1 when the severity-number from step 1 is 1 or 0) actions are to be 193
  4. 4. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 5, September - October (2013) © IAEME determined. Occurrence also can be defined as %. If a non-safety issue happened less than 1%, we can give 1 to it. It is based on product and customer specification. The following table gives the values of the Occurrence Ratings. Table 1. Occurance Ratings Occurrence Rating Meaning 1 Failure eliminated or no know occurrence 2,3 Low or very few 4,5,6 Moderate or few occasional 7,8 High or repeated failure occurrence 9,10 Very high rate of failure or inevitable failures 2.4 STEP 4: SEVERITY RATING The severity ranking is an estimation of how serious the effects would be if a given failure did occur. In some cases it is clear, because of past experience, how serious the problem would be. In other cases, it is necessary to estimate the severity based on the knowledge of the process. There could be other factors to consider (contributors to the overall severity of the event being analyzed). Calculating the severity levels provides for a classification ranking that encompasses safety, production continuity, scrap loss, etc. user. Each effect is given a severity number (S) from 1 (no danger) to 10 (critical). These numbers help an engineer to prioritize the failure modes and their effects. If the sensitivity of an effect has a number 9 or 10, actions are considered to change the design by eliminating the failure mode, if possible, or protecting the user from the effect. A severity rating of 9 or 10 is generally reserved for those effects which would cause injury to a user or otherwise result in limitation. Severity Rating 1-2 3-5 6-7 8-9 10 Table 2. Severity Rating Description Failure is of such minor nature that the customer (internal or external) will probably not detect the failure. Failure will result in slight customer annoyance and/or slight deterioration of part or system performance Failure will result in customer dissatisfaction and annoyance and/or deterioration of part or system performance. Failure will result in high degree of customer dissatisfaction and cause non-functionality of system Failure will result in major customer dissatisfaction and cause non-system operation or non-compliance with regulations 194
  5. 5. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 5, September - October (2013) © IAEME 2.5 STEP 5: DETECTION RATING This section provides a ranking based on an assessment of the probability that the failure mode will be detected given the controls that are in place. The proper inspection methods need to be chosen. First, we should look at the current controls of the system, that prevent failure modes from occurring or which detect the failure before it reaches the customer. Hereafter one should identify testing, analysis, monitoring and other techniques that can be or have been used on similar systems to detect failures. Based on these studies one can effectively understand about the detection of the failure. Based on these detection ratings are given. This ranks the ability of planned tests and inspections to remove defects or detect failure modes in time. The assigned detection number measures the risk that the failure will escape detection. Here the rating is given in reverse order ie when the rating is lower, the probability of identifying the failure is high and when the rating is high the probability of identifying the failure is very less. So the assigned Detection rating gives the understanding of how easily the failure can escape the detection of customer. The following table gives the detection rating. Table 3 Detection Rating Detection Rating Description 1 Very certain that the failure will be detected 2-4 High probability that the defect will be detected 5-6 Moderate probability that the failure will be detected 7-8 Low probability that the failure will be detected 9 Very Low probability that the defect will be detected. 10 Fault will be passed to customer undetected 2.6 STEP 6: RISK PRIORITY NUMBER The risk priority number (RPN) is simply calculated by multiplying the severity ranking times the occurrence ranking times the detection ranking for each item. Risk Priority Number = Severity × Occurrence × Detection The total risk priority number should be calculated by adding all of the risk priority numbers. This number alone is meaningless because each FMEA has a different number of failure modes and effects. However, it can serve as a gauge to compare the revised total RPN once the recommended actions have been instituted. RPN play an important part in the choice of an action against failure modes. They are threshold values in the evaluation of these actions. The failure with highest RPN requires the highest priority for corrective action. This means it is not always the failure modes with the highest severity numbers that should be treated first. There could be less severe failures, but which occur more often and are less detectable. These actions can include specific inspection, testing or quality procedures, redesign (such as selection of new components), adding more redundancy and limiting environmental stresses or operating range. After these values are allocated, recommended actions with targets, responsibility and dates of implementation are noted. Once the actions have 195
  6. 6. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 5, September - October (2013) © IAEME been implemented in the design/process, the new RPN should be checked, to confirm the improvements. Whenever a design or a process changes, the FMEA should be updated. Table 4: FMEA for End Milling S No 1 Problem Effect Chip packing Poor chip dispersion Tool wear Severity Rating 7 Occurrence Rating 8 Detection Rating 7 Causes Solutions RPN Too great cutting depth Not enough chip room Adjust feed or speed Use end mill fewer flutes Apply more coolant. Use air pressure Slow down to correct feed Use higher speed Regrind earlier stage Cut less amount per pass Add margin (touch primary with oilstone) Regrind sooner 392 7 6 Dimensional inaccuracies 4 Defected job 2 Chip biting 2 No end tooth concavity 8 4 8 Too much wear on primary relief Incorrect condition 3 Improper cutting angle 3 5 Too much wear 5 Too tough condition 3 Increases tool wear 4 9 9 4 Burr Slow speed 3 3 7 6 Change in tolerance Feed too fast 4 4 7 6 Degradation of standards 8 8 Rough surface finish Not enough coolant 8 2 8 8 Change in tolerance and finishing 5 3 Lack of accuracy (machine & holder) Not enough rigidity (machine & holder) Not sufficient number of flutes 8 8 8 No dimensional accuracy 5 Dimensional inaccuracies No perpendicula 4 196 8 Change machine or holder or condition Use end mill with greater number of flutes 336 224 224 128 32 64 160 240 90 120 216 216 96 256 Feed too fast 6 Correct milling condition Change the cutting angle Change to easier condition Repair machine or holder 245 Too great a cutting Slow down to correct feed Reduce cutting 192
  7. 7. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 5, September - October (2013) © IAEME r side High tool wear amount Too long a flute length or long overall length Not sufficient number of flutes 5 3 6 8 Feed too fast 7 High Tool wear 5 2 6 2 8 Feed too fast on first cut Not enough rigidity of machine tool & holder 3 8 Chipping Dimensional inaccuracies 9 Lack of rigidity (tool) 6 Teeth too sharp 6 8 Speed too fast 7 Hard material 3 Dimensional inaccuracies Loose holder (workpiece) 3 6 9 4 Reduced tool life Loose holder 3 Wear 9 3 7 3 2 Biting chips 4 8 Improper feed speed (too slow) 4 5 Improper cutting angle 4 5 Too low a primary relief angle 197 amount Use proper length tool. Hold shank deeper Use end mill with greater number of flutes Slow down to proper feed Slow down on first bite Change rigid machine tool or holder Tighten tool holder Tighten workpiece fixture Use shortest end mill available. Hold shank deeper. Try down cut Change to lower cutting angle, primary relief Slow down, use more coolant Use higher grade, tool material, add surface treatment Change feed speed to change chip size or clear chips with coolant or air pressure Increase feed speed. Try down cut Change to correct cutting angle Change to larger relief angle 40 24 240 280 120 135 135 135 120 288 126 36 192 120 120
  8. 8. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 5, September - October (2013) © IAEME 8 3 3 Too long flute length or long overall length Too much wear 7 8 Feed and speed too fast 8 Not enough rigidity (machine & holder) 4 4 Too much relief angle Deviation in tolerance limits 3 8 Loose holder (workpiece) 7 7 Cutting too deep 3 5 Too long flute length or long overall length 7 6 5 6 Too much cutting friction Tough work material 3 Side wall taper in Workpiece Too large cutting amount Disturbing noises 11 7 3 Short tool life (dull teeth) Feed too fast 4 10 8 6 Chattering 6 6 9 Breakage Reduced tool life 6 Improper cutting angle 6 8 4 6 Feed Rate is heavy High Overhang of Tool 3 4 Reduced tool life High cost High tool wear Dimensional inaccuracies 8 6 4 6 198 Too few Flutes Slow down feed Adjust to smaller cutting amount per teeth Hold shank deeper, use shorter end mill Regrind at earlier stage Correct feed and speed Use better machine tool or holder or change condition Change to smaller relief angle. Add margin (touch primary with oil stone) Hold workpiece tighter Correct to smaller cutting depth Hold shank deeper, use shorter end mill or try down cut Regrind at earlier stage Select premium tool Change cutting angle & primary Reduced feed rates Use short end mill and hold the shank deeper Use endmill with multi flute 384 336 144 96 336 144 96 144 294 90 168 120 72 288 144 72
  9. 9. International Journal of Mechanical Engineering and Technology (IJMET), ISSN 0976 – 6340(Print), ISSN 0976 – 6359(Online) Volume 4, Issue 5, September - October (2013) © IAEME 3. RESULTS & DISCUSSION From the analysis, it has been found that the chip packing due to excess cutting depth has the highest risk priority number. This can be reduced by varying the feed rate and speed of spindle. To reduce tool breakage due to excess feed rate and very high spindle speed, we have to perform the process in rated speed and acceptable feed rates. To reduce chattering better tool holding and work holding devices are to be used and also we have to follow the rated speed and feed rates. To reduce chipping, initial speed has to be minimum and proper cutting speed is to be followed. To reduce tool wear proper lubrication and parameter perfection has to be achieved. 4. CONCLUSION Thus the End Milling process has been analyzed and the expected failure modes have been noted. From the results of the analysis the defects with greater risk priority number have been selected. The causes, effects and the possible alternatives are given along with the ratings and priorities. The Risk Priority numbers of the defects are given which indicates the necessity of care for defect free end milling process. Thus this analysis will be helpful as a reference guide to the end milling process failures. These corrective actions should be considered before end milling process to achieve an effective end milling process. REFERENCE 1. V Janarthanan, D Rajenthira Kumar. Root Cause analysis & process failure mode and effect analysis of tig welding on ss 3041 material(Proceeding of NC MISAA 2013, copyright 2013 PSGCT) 2. Aravind.P, Rooban Babu.R, Arun Dhakshinamoorthy, Venkat Prabhu.N, Subramanian .¸ An integrated approach for prediction of failures by process failure mode and effect analysis (pfmea) in mig welding-a predictive analysis (ISBN-978-93-82208-00-6) 3. D.H.Stamatis. failure mode and effect analysis : FMEA from theory to execution(Book 2nd Edition(1995) 4. Robin E. McDermott, Raymond J. Mikulak, Michael R. Beauregard. The basics of FMEAProductivity press(1996) 5. Aravind.P, Subramanian.SP, SriVishnu.G, Vignesh.P. Process failure mode and effect analysis on tig welding process - a criticality study(ISSN-223-1963) 6. “Failure modes and effects analysis (fmea)”- Copyright © 2004 Institute for Healthcare Improvement. 7. Carbide depot inc. A troubleshooting manual- 1474 Pettyjohn Road Bessemer, AL 35022 USA. 8. Kalpakjian. Manufacturing process for engineering materials (Pearson Education India, 1992) 9. S. Madhava Reddy, “Optimization of Surface Roughness in High-Speed End Milling Operation using Taguchi’s Method”, International Journal of Mechanical Engineering & Technology (IJMET), Volume 4, Issue 4, 2013, pp. 249 - 258, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359. 10. M.Chithirai Pon Selvan and Dr.N.Mohanasundara Raju, “Influence of Abrasive Waterjet Cutting Conditions on Depth of Cut of Mild Steel”, International Journal of Design and Manufacturing Technology (IJDMT), Volume 3, Issue 1, 2012, pp. 48 - 57, ISSN Print: 0976 – 6995, ISSN Online: 0976 – 7002. 11. A.Mariajayaprakash, Dr.T. SenthilVelan and K.P.Vivekananthan, “Optimisation of Shock Absorber Parameters using Failure Mode and Effect Analysis and Taguchi Method”, International Journal of Mechanical Engineering & Technology (IJMET), Volume 3, Issue 2, 2012, pp. 328 - 345, ISSN Print: 0976 – 6340, ISSN Online: 0976 – 6359. 199