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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
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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
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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:
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• 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
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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
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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
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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
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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
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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
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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.
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