# Fault Tree Analysis of Fly-Outs in Metal Lathe Machine Operations

```UDC 623.4.017:621.941
DOI: 10.7562/SE2015.5.01.02
Original article
www.safety.ni.ac.rs
AKINYEMI OLASUNKANMI
ORIOLA1
GIWA SOLOMON
OLANREWAJU2
OLUWOLE3
AKINTAN
1-5
Olabisi Onabanjo University, AgoIwoye, Nigeria,
Faculty of Engineering,
Department of Agricultural and
Mechanical Engineering
1
[email protected]
[email protected]
3
[email protected]
4
[email protected],
5
[email protected]
2
FAULT TREE ANALYSIS OF FLY-OUTS IN
METAL LATHE MACHINE OPERATIONS
Abstract: The most probable accident in lathe machining has been
identified to be fly-outs. This study aim at determining the causal
factors leading to fly-out accidents during lathe machining operations
and subsequently determine the probability of occurrence of fly-out
accident. Fault tree analysis (FTA) was used to identify risk factors.
Boolean algebra equations were used to analyse the probability of
fault occurrence. Monte Carlo simulation was carried out using
OpenFTA software and the output of 1000 iterations was compared
with the output of Boolean algebra. Safety intervention alternatives
were evaluated by comparative analysis of before and after
implementation of safety measures. Twenty four (24) minimum cut sets
comprising of 21 basic events and 3 undeveloped events were
identified. The top event has probability of 0.748 signifying high
likelihood for fly-out. Monte Carlo simulation gave lower and upper
bounds probabilities of 0.725 and 0.773, respectively. The event of the
chuck key not pulled out of the chuck before machining begins was
however noted to have the highest contribution to the occurrence of
fly-out accident. The result of safety intervention alternatives revealed
that the probability of fly-out becomes 0.192 with a safety benefit of
N27, 800 after the first tier implementation. Other tiers of safety
interventions will see the probability of fly-out go further down. By
this, safety engineer has a scale for effectiveness of respective safety
intervention programmes.
Key words: fly-out, accident, safety, intervention, lathe, machine,
operation.
INTRODUCTION
Over the last three decades there were development in
the maintenance and servicing industries, of a
distinctive approach to hazards and failures that cause
loss of life and property. This approach is commonly
called `loss prevention'. It involves putting much
greater emphasis on technological measures to control
hazards, accidents and on trying to get things right first
time. The rapid development of new technology has
essentially changed the nature of work and has
increased the complexity of systems within many
industries. Hence, the world becomes increasingly
complicated. These complex systems require a
combination between technical and human subsystems
(Kletz, 1999). In this sense, the failure of a subsystem
can often cause the failure of the entire system.
Moreover, catastrophic breakdowns of these systems
create serious threats, not only for those within the
organization, but also for the surrounding public.
Simultaneously, the accidents that occur in workplaces
have also become more complex and in some cases
more frequent.
In fact, increased technological dependence has led to
bigger accidents, involving more people, and greater
damage to property and the environment. It has become
clear that such vulnerability does not originate from
just human error, technological failures, or
environmental factors alone. Rather, it is the fixed
organizational policies and standards which have
repeatedly been shown to predate the catastrophe.
Therefore, safety practitioners in recent years have
begun to focus on the organizational values that might
enhance risk and crisis management and safe
performance in industries complex conditions. Some
scholars (Simon and Leik 1999) believed that culture
and technology actually go hand in hand. Culture
consists of attitudes, perceptions, beliefs, and values,
which need to be set in context. In the face of new
mandates, it is believed that culture can play a vital role
in helping organizations respond to the many safety
challenges.
Most accidents in Nigerian industries are a direct result
of not adhering to their established safety procedures,
as well as lack of strong safety culture, safe working
conditions, and employees’ safe work attitudes and
actions (Oyesola and Kola, 2014). Thus, the
participation of all employees including managers and
non-managers is vital in policymaking, establishing,
and implementing a feedback system that drives
continuously toward safety improvement in industrial
companies to achieve a successful safety program. It
must be mentioned that safety culture has an important
role in reducing occupational accidents in industry. The
identification of areas of vulnerability and of specific
hazards is of fundamental importance in loss
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SAFETY ENGINEERING - INŽENJERSTVO ZAŠTITE
prevention and safety. There is now available a whole
battery of hazard identification methods which may be
used to solve these problems (DOSH, 2008).
Human by default are susceptible to making errors and
infact neglecting certain safety rules and regulations, a
consequence of which could be so deadly both to
themselves, their co-workers, machineries and the
environment resulting in a possible loss of lives,
property and revocation of their operating licenses.
However, human error is just an aspect of safety as
environment, hardware and other factors also serve as
links to safe machining operations. With noise,
numerous machines and a handful of people on the
plant floor, one mistake can result in a serious incident
that can cause personal injury and wreak havoc on
production. Each year, millions of workers suffer from
non-fatal workplace injuries, resulting in an annual cost
of billions of dollars (EASHW, 2004). Outside the
primary objective of reducing injuries to people or
property, proving the value of a safety system is an
ongoing challenge for safety professionals and risk
managers. Many find it difficult to financially justify
discretionary investments in safety-related trainings
intended to reduce work-related injuries.
Safety investments greatly reduce cost of repairs. With
an up-front investment in safety programs and
safeguarding systems, the financial and employee
impact of incidents that occur in the facility can be
significantly diminished. Having realized this huge
capital investment on safety, evaluation and reevaluation to justify this huge spending are necessary
as well as analyzing historical accidents of ranging
proportions from fatal, minor to near-misses with a
view to tailor the existing safety policy to achieve the
ultimate goal for which the entire concept of safety is
based; to preserve lives and properties (SESR, 2012).
This paper seeks to conduct a hazard/causal factor
identification analysis capable of leading to fly-outs on
lathe machine operations and to evaluate in quantitative
terms using Fault Tree Analysis (FTA). Also to
determine the probability of failure by considering
elemental failures that can lead to Fly-outs and
recommend safety interventions, and to evaluate the
effectiveness of such interventions. It will also examine
how the probability of failure is affected by various
safety interventions.
MATERIALS AND METHOD
Having consulted and reviewed series of safety reports
associated to lathe operations of a case study workshop;
this research seeks to consider fly-outs during
machining operations. These fly-outs envisage the
possibility of tool fly out during a machining process,
work piece fly out as well as the effect of discontinuous
chips (swarf) removal during operations that ranges
from turning, shaping etc. to achieve the objectives
using the tools described in sections 2.1, 2.2 and 2.3.
Fault Tree Analysis
Fault tree analysis (FTA) is used to investigate
potential faults, its modes and causes and to quantify
their contribution to system unreliability in the course
of product design. FTA is a technique by which
conditions and factors that can contribute to a specified
undesired event are identified and organized in a
logical manner and represented pictorially (Jane, 2012).
FTA has been widely successfully used in various
fields. Tetlow and Jenkins (2005) used it to visualise
the importance of human factors for safe diving with
closed-circuit rebreathers. Kumar and Sneh, (2011)
applied it to analyse the reliability of piston
manufacturing system while Hu et al., (2011) used FTA
for hierarchical diagnosis model and sequential control
of manufacturing system to mention a few.
Boolean Algebra Equations
With human Experts judgments, Boolean algebra
equations were used to analyse the probability of fault
occurrence. Boolean algebra is a devise for dealing
mathematically with philosophical propositions which
have only two possible values of TRUE or FALSE
represented by the digits “0” and “1”. It deals with the
rules which govern various operations between the
binary variables. “AND” operation describes events
which can occur IF and only IF two (2) or more other
events are TRUE. “OR” Operation describes events
which can occur IF at least one (1) of the other events
are TRUE (Ovidiu, 2003).
Monte Carlo Simulation
Monte Carlo simulation of the fault tree was conducted
using the commercial software called “OpenFTA”.
1000 Iterations were carried out and the output
compared with the Boolean algebra equations. Monte
Carlo simulation, also called probability simulation, is
a technique widely used to understand the impact of
risk and uncertainty in forecasting models. It can tell
based on how the ranges of estimates are created, how
likely the resulting outcomes are. Monte Carlo
techniques are often the only practical way to evaluate
difficult integrals or to sample random variables
governed by complicated probability density functions
(Cowan, 2011). OpenFTA is an advanced tool for FTA.
With OpenFTA, superior graphical user interface, fault
trees can be constructed and modified with ease (FSCL,
2005).
Safety Intervention Measure
The safety intervention alternatives were evaluated by
comparative
analysis
of
before
and
after
implementation of safety measures. Safety intervention
for the respective faults was examined to evaluate how
well and how much the measure can bring about a
reduction in the probability of the top-event. This
tailors the research into the subject matter of
identifying hazard conditions, sequence of accident,
qualitative and quantitative evaluation, and finally, an
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A. O. Oriola, G. S. Olanrewaju, A. H. Oluwole, A. A. Lawal, M. Oladapo, Vol 5, No1 (2015) 9-19
evaluation of the case-study’s safety intervention
programme to see how the intervention would reduce
the probability of accident occurrence. The overall
evaluation of safety in line with the subject matter of
fly-out incorporates quantitative and qualitative
evaluations to channel a course for safety intervention.
This can be viewed as a case of sensitivity analysis
whereby the effect of safety evaluation is examined on
the probability of fly-out accidents to see how
respective intervention reduces the probability of topevent occurrence.
To ascertain the effectiveness of a safety intervention
program, an appraisal of the case-study safety
intervention programme was carried out by firstly
identifying areas that require intervention and by
making appropriate recommendation.
Figure 1. Hazard Zones of Metal Turning Lathe Machine
RESULTS
Lathe Hazard Identification and
Consequences Analysis
Safety concerns on lathe operations were considered
under various headings of major lathe hazards and the
commonest causes of death and injury from metal
lathes were evaluated. These include:
 Entanglement of clothing in moving parts such as
drive gears, chucks, lead and feed screws, and the work
piece;
 Being hit by loose objects on the lathe such as
chuck keys, tools or swarf;
 Entanglement from inappropriate tooling and
polishing techniques;
 Being struck by a workpiece that has not been
adequately secured in the lathe or is oversized.
Figure 1 shows the zones of metal turning lathe
hazards. Six hazard zones have been identified. Each
zone was analyzed to include the possible consequence
(e.g. entanglement) of the hazard and their
recommended controls. Table 1 contains a
comprehensive hazard identification and consequences
analysis of identifiable hazards during lathe operations
Qualitative Safety Evaluation:
Fault Tree Construction
The child root for a tool fly-out is as represented Figure
2. Seven causal factors capable of triggering a tool flyout during machining operations on a lathe were
identified as chuck fault, workpiece holding fault, tool
post fault, coolant fault, improper operating speed,
safety guards fault, swarf guard and chuck guard and
Improper mounting.
Further analysis of root/intermediate events into
minimum cut sets i.e. basic events that could lead to the
child node event; twenty four (24) basic events are
identified and presented in fault tree in Figure 3.
Probabilities for the identified failures are presented in
Table 2.
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SAFETY ENGINEERING - INŽENJERSTVO ZAŠTITE
Figure 2: Hazards/causal Factors Capable of Triggering a Tool Fly-out During Machining Operations on a Lathe.
Figure 3: Fault Tree of Fly-out Accident during Metal Lathe Machining Operation
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A. O. Oriola, G. S. Olanrewaju, A. H. Oluwole, A. A. Lawal, M. Oladapo, Vol 5, No1 (2015) 9-19
Table 1. Lathe Operations Hazards and Consequences
Hazards
Zone 1
Workpiece beyond the
Zone 2
Exposed
drive
mechanisms
(pulley,
belts, gears).
Lathe controls can only
be reached by passing
hand through working
zone.
Lack
of
function
markings on controls.
Placements of controls
machining process.
Unsecured tools and
objects stored or placed
Zone 3
Exposed chuck.
Chuck key left in chuck.
Jaws of chuck unable to
clamp
workpiece
securely.
Chuck has not been
the spindle.
Mounting and removing
heavy chucks and face
plates.
Use of a chuck that is
not compatible with
lathe
and/or
specifications.
Chucks and face plates
used on the lathe are
damaged or have catch
points.
Oversized workpiece in
self-centering
chuck
(three-jaw chuck)
Zone 4
Objects (e.g. cutting
tools) unsecured on
carriage (including tool
post) or swarf
Worn or damaged tools
being used on the lathe.
Possible consequence
During spindle rotation, bar can
bend and strike machinists
nearby.
Machinists can become entangled
in pulleys, belts or gears when
lathe is in operation.
Machinists can become entangled
in unguarded drive mechanisms,
chuck, chuck assembly or
workpiece when the lathe is in
operation.
Machinists can activate incorrect
controls resulting in an unplanned
function.
Machinists can activate incorrect
control resulting in an unplanned
function.
Stored objects can fall onto the
spinning chuck and be propelled
at the operator or nearby
machinists
Machinists can become entangled
on uneven surface of chuck or
workpiece when spinning.
Machinists near lathe can be
struck by key when projected
from the lathe.
Machinists can be struck by
workpiece not securely held in
the chuck.
Machinists can be struck by
chuck not securely held in the
spindle.
Machinists
can
sustain
musculoskeletal or crushing
injuries when changing heavy
chucks and faceplates.
Use of incorrect chucks can result
in the chuck or workpiece
becoming loose and striking
machinists
Machinists can become caught on
chucks and faceplates that are
poorly maintained or have
protrusions.
Chuck jaws in full extension to
allow for oversized workpieces
can be propelled from the lathe
when operated.
Unsecured objects can become
projectiles when the lathe is
started,
possibly
striking
machinists.
Use of worn or damaged tools can
result in tool failure and can
become projectiles or create
irregular or long cuttings that can
screws (assessment of
risk will need to include
the speed at which the
travel).
Zone 6
Unguarded protrusions
on the workpiece.
Coupling and clamps
used on the lathe are
damaged or have catch
points.
Unsupported
workpieces.
Machining
process
produces continuous or
unraveled cuttings.
Removing
metal
shavings, cuttings and
swarf from machining
area with hands.
Neighboring
workspaces are exposed
to swarf, cuttings or
workpieces during the
machining process.
Frequent traffic (human
and machinery) passing
through the work area
near the operator.
Incorrect methods used
for polishing workpieces
with emery cloth.
Others
Lack of or poorly placed
emergency
stop
button/pedal that results
in immediate standstill
of lathe operation.
Loose clothing, cuffed
or rolled back sleeves,
neckties,
jewelry
(including watches) and
long hair.
Environment
Inappropriate type and
position of lighting.
Untidy and unorganized
working
Environment.
Zone 5
13 | Safety Engineering
Machinists can become entangled
in exposed lead and feed screws
when the lathe is in operation,
particularly if the lathe is being
used by a number of users with
various levels of experience.
Machinists can become entangled
on protrusions on the workpiece
being turned.
Machinists can become caught on
coupling and clamps that are
poorly maintained or have
protrusions.
Unsupported workpieces can
become
loose,
striking
machinists.
Machinists can become entangled
in turning cuttings.
Unprotected
handling
of
shavings, cutting and swarf can
result in lacerations.
Swarf, cuttings or workpieces can
become projectiles and strike
nearby
machinists,
causing
injuries such as lacerations and
fractures.
While operating the lathe, the
operator can be bumped or
startled by passing traffic, causing
the operator to come into contact
with the lathe.
Machinist can become entangled
in the lathe.
Operator is unable to stop the
lathe in case of an emergency.
Loose clothing, accessories and
hair can become entangled in
moving parts of the lathe, chuck
assembly or workpiece.
The flashing effect of fluorescent
light can make a spinning lathe
appear to have stopped. This can
Lighting placed over the lathe can
be struck by projectiles from the
machining process. Machinists
nearby can be injured by the light
shattering.
Machinists can slip or trip on
cutting oils, swarf or cuttings that
are not cleaned from the floor.
Machinists can also trip over
lathe parts or workpieces that are
not returned to storage areas.
SAFETY ENGINEERING - INŽENJERSTVO ZAŠTITE
Probability of failure P (F) =
1 - (1 - V1) x (1 - V2) x (1 – V3) x (1 – V4) x (1 – V6) x
(1 – V10) (1 – V12) x (1 – V13) x (1 – V15) x (1 - V17) x
(1 – V19) x (1 – V20) x (1 – V21) x (1 – V22) x (1 – V23)
x (1 – V24) x (1 – V8 * V14 * V18) x (1 – V11* V16) x (1
– V5 * V7 * V9)
P (F)
= 1 – (0.251561 * 0.999849)
= 1 - 0.251523
= 7.484772E-001
= 0.748
Table 2 presents the faults and the respective
probabilities of faults. The following faults namely:
Chuck Associated Fault, Work holding Fault, Tool Post
Fault, Coolant Fault, Wrong Machining Speed, Safety
Guards and Improper Feed rate Fault respectively, are
the faults having the capability to initiate the
occurrence of fly-out. The probabilities of these faults
are 0.720001, 0.050, 0.035, 0.0000005, 0.015, 0.020
and 0.01515, respectively. The probabilities of these
faults reveal that chuck associated failure has the
highest likelihood/probability of initiating the top event
having a probability of failure of 0.720001, followed by
work holding faults with a probability of 0.050, tool
post failure with a probability of 0.035, safety guards
failure with a probability standing at 0.20 followed by
the fault from improper feed rate with a probability of
0.01515 and FINALLY faults from wrong machining
speed and coolant fault having probabilities of 0.015
and 0.0000005, respectively. It is noteworthy that
coolant failure has the least probability and hence, it
has the least capacity of initiating a fly-out during lathe
operations
Further presented in Table 3 is the result of basic event
analysis and their respective importance represented as
a percentage of the overall probability of top-event. A
graphical representation is also provided in Figure 4.
The result here reveals that V4 (event of chuck not
being pulled out before machining operation begins)
has the highest importance (93.52%) and if any safety
intervention is to be justified, it must be centralized on
the primary event with the highest importance. V4
represents the first tier of safety intervention.
Furthermore, V2, V3, V6, V15, V17 and V20 having an
importance of 2% apiece are the next areas of priority
(second tier of intervention) in terms of safety
intervention. However, V1, V9, V11, V13, V21, V22, V23
and V24 have lesser importance, with their importance
standing at 1.34% apiece. V13 with an importance of
0.67% can also be merged with the events of 2%. These
events are hence assigned for third tier intervention.
Other events have 0% importance and no major
intervention is needed.
Table 2. Probabilities of Failure of Basic Events
`
Event ID
V1
V2
V3
V4
V5
V6
V7
V8
V9
V10
V11
V12
V13
V14
V15
V16
V17
V18
V19
V20
V21
V22
V23
V24
CCT
CHG
CHT
CPO
DP
DSWARF
PUMP
LB
LK
LSF
LSNESS
OH
SCKF
SND
SWG
TFEEDING
TM
UN
WK
WNS
WS
WT1
WT2
WT3
Type
Basic Of Event
Underdeveloped
Basic
Basic
Basic
Basic
Underdeveloped
Basic
Basic
Basic
Basic
Underdeveloped
Basic
Basic
Underdeveloped
Basic
Basic
Basic
Basic
Basic
Basic
Basic
Basic
Basic
Description
Circuitry Fault
Chuck Guard Fault
Centre Height Fault
Chuck Not Pulled Out
Dislocation of Pipe
Discontinuous Swarf
Faulty Pump
Spindle Nose Looseness
Leakage
Loose Joints and Unsecured Fitting
Overhang
Speed Control Knob Fault
Spindle Nose Damage
Swarf Guard
Excessive Tool Feeding
Tool Mounting
Unclean Media
Wrong Key Size
Work Not Well Secured
Wrong Operating Speed
Wear And Tear of Chuck Keyway
Wear And Tear of Holding Device
Wear And Tear of Tool Post Clamps
14 | Safety Engineering
Failure
rate/Unit
time.
0.010
0.015
0.015
0.700
0.010
0.015
0.010
0.010
0.005
0.010
0.015
0.010
0.005
0.010
0.015
0.010
0.015
0.010
0.010
0.015
0.010
0.010
0.010
0.010
A. O. Oriola, G. S. Olanrewaju, A. H. Oluwole, A. A. Lawal, M. Oladapo, Vol 5, No1 (2015) 9-19
Safety Intervention Benefits
Table 4 is a tabulation of the basic events, their
respective description and safety intervention
recommendation. In this present study, safety
interventions were categorised as follow:
PERCENTAGE IMPORATANCE 100
90
80
PERCENTAGE IMPORTANCE 70
60
50
40
30
20
10
0
V1 V2 V3 V4 V5 V6 V7 V8 V9 V10 V11 V12 V13 V14 V15 V16 V17 V18 V19 V20 V21 V22 V23 V24
BASIC EVENTS Figure 4. Probabilities of Basic Events
 First tier safety intervention; training, supervision,
inspection and procurement and installation (selfejecting chuck key procurement etc.);
 Second tier safety intervention; preventive
maintenance and quality of maintenance;
 Third tier safety intervention; intermittent checklisting and supervision
Assuming the expected cost of fly-outs injury ranges
from simple laceration to complete facial surgery is
N50, 000, the resulting citicatility C of a lathe
machining fly-outs injury is:
Table 3. Primary Event Analysis
Event
V4
V2
V3
V6
V15
V17
V20
V1
V9
V11
V19
V21
V22
V23
V24
V13
V10
V16
V5
V7
V8
V12
V14
V18
Description
Chuck Not Pulled
Out
Chuck Guard
Fault
Centre Height
Fault
Discontinuous
Swarf
Swarf Guard
Tool Mounting
Work Not Well
Secured
Circuitry Fault
Leakage
Loose Joints and
Unsecured Fitting
Wrong Key Size
Wrong Operating
Speed
Wear And Tear of
Chuck Keyway
Wear And Tear of
Holding Device
Wear And Tear of
Tool Post Clamps
Speed Control
Knob Fault
Excessive Tool
Feeding
Dislocation of
Pipe
Faulty Pump
Spindle Nose
Looseness
Overhang
Spindle Nose
Damage
Unclean Media
Importance
(%)
93.52
Failure contribution
7.000000E-001
2.00
1.500000E-002
2.00
1.500000E-002
2.00
1.500000E-002
2.00
2.00
2.00
1.500000E-002
1.500000E-002
1.500000E-002
1.34
1.34
1.34
1.000000E-002
1.000000E-002
1.000000E-002
1.34
1.34
1.000000E-002
1.000000E-002
1.34
1.000000E-002
1.34
1.000000E-002
1.34
1.000000E-002
0.67
5.000000E-003
0.02
0.02
1.500000E-004
1.500000E-004
0.00
5.000000E-007
0.00
0.00
1.000000E-006
5.000000E-007
0.00
0.00
5.000000E-007
1.000000E-006
0.00
1.000000E-006
The event that machinist are expertly trained,
supervised and monitored that chuck keys are not left
in the chuck before machining starts would reduce the
probability of “chuck not pulled out” from 0.7 to 0.21.
However, the other failure modes could still occur and
the probability of fly-outs reduces to 0.192 with a new
criticality of N9, 600. The benefits or savings of the
implementation of the safety intervention is the
decrease in the criticalities i.e.
Monte Carlo Simulation
Using “OpenFTA”, the simulation results are as
presented below:
1. Number of primary events = 24;
2. Number of tests (iterations) = 1000;
3. Number of system failures = 976;
4. Probability of at least one component failure = 0.768
(exact) and
5. Probability of top event = 0.749 (+/- 0.024) i.e.
0.725 and 0.773.
The events, descriptions and failure contributions are
shown in Table 5.
The Boolean algebra analysis reveals that the top-event
has a probability of 0.748; however, the Monte Carlo
analysis offered a range of probability in which the top
event can happen (0.725 and 0.773). It is noteworthy at
this stage that the Boolean algebra result is within the
range of probabilities obtained using Monte Carlo
simulation. However, the percentage importance as
well as the fault contribution of some cut-sets suffered
a reduction while some remained constant after 1000
simulations.
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SAFETY ENGINEERING - INŽENJERSTVO ZAŠTITE
Table 4. Basic Events and Nature of Safety Intervention and Recommendation
Event
V1
Description
Circuitry Fault
V2
Chuck Guard Fault
V3
Centre Height Fault
V4
Chuck Not Pulled Out
V5
Dislocation of Pipe
V6
Discontinuous Swarf
V7
V8
Faulty Pump
Spindle Nose Looseness
V9
Leakage
V10
V11
Loose Joints and Unsecured Fitting
V12
Overhang
V13
Speed Control Knob Fault
V14
Spindle Nose Damage
V15
Swarf Guard
V16
Excessive Tool Feeding
V17
Tool Mounting
V18
V19
V20
Unclean Media
Wrong Key Size
Work Not Well Secured
V21
Wrong Operating Speed
V22
V23
V24
Wear And Tear of Chuck Keyway
Wear And Tear of Holding Device
Wear And Tear of Tool Post Clamps
Safety Intervention and Recommendation
Pro-active preventive maintenance of the machine, electrical component
inspection and check-listing.
Installation of chuck guards
Employers must ensure guarding does not stop workers using the lathe
in a safe manner or block the view of the task.
Where multiple chucks are used, guarding should cover the swing of the
lathe, not the size of a chuck.
Use a bar feed tube to hold workpiece that extends beyond the
Guard bar feed weights with hinged covers extending to the floor.
Modify the lathe speeds (RPM) to ensure bar will not bend when
machined.
Install barriers to stop workers entering space around headstock.
Adequate training of machinists and proper supervision of machining
operations.
Use of self-ejecting chuck key.
Use of extended key design that stops interlocked guard being lowered
when inserted in chuck.
Intermittent checklist should be drafted to monitor the position of the
pipe per time during machining operations.
Manufacturer specified federates should be adhered to and swarf should
be cleared timely.
Preventive maintenance.
Pro-active preventive maintenance and specific level inspection for
vibration.
Use of retaining nut with left-hand thread.
Training to ensure machinists pay absolute concentration on the task
before them so they can notice leakages on time.
Where appropriate, ensure lead and feed screws are guarded
Preventive maintenance and proper inspection practices.
Retightening of bolts, couplings and replacement of worn out parts.
Use workpieces of minimum length to reduce the amount of bar
Use of fixed or travelling steadies to support long, slender workpieces
between centres or to support outer end of long piece held in chuck for
drilling or boring.
Preventive maintenance.
Ensure control functions are clearly displayed.
Ensure operators are adequately trained in what order to use controls.
Use of retaining nut with left-hand thread and tightened with a torque
wrench to manufacturers specification.
Ensure swarf guards are installed and made operatable so as not to
hinder machining operations. Also, ensure swarf handles and buckets
are used when cleaning swarf, shaving and cuttings from lathe
An excessive tool feeding set up vibration and transmits the impulse to
the tool post, the chuck and the spindle thereby loosening the couplings.
Machinists should be trained to feed at optimal levels to avoid the
impulse transfer.
Ensure worn or damaged tools are removed and not used.
Ensure the tool is properly secured on the tool post.
Proper maintenance work and housekeeping.
Use of manufacturer specified key size for respective chucks.
Training; Chuck type and size selection should be given priority in line
with the machining operation to be carried out. Rightful selection of
chuck key to ensure the chuck jaws fully grip the work piece.
Training of machinists to adhere to RPMs as stipulated in the
manufacturer’s manual.
Ensure worn or damaged tools are removed and not used.
Ensure worn or damaged tools are removed and not used.
Ensure worn or damaged tools are removed and not used.
16 | Safety Engineering
A. O. Oriola, G. S. Olanrewaju, A. H. Oluwole, A. A. Lawal, M. Oladapo, Vol 5, No1 (2015) 9-19
Table 5. Monte Carlo Simulation Results
Event
V4
V15
V2
V6
V11
V19
V20
V23
V1
V17
V9
V3
V24
V22
V13
V5
V7
V8
V10
V12
V14
V16
V18
V21
Description
Importance
(%)
Failure
contribution
93.34
7.00E-01
2.56
2.46
1.92E-02
1.84E-02
V14
V15
V16
V17
V18
V19
V20
V21
2.36
1.77E-02
V22
2.25
1.69E-02
V23
1.54
1.15E-02
1.43
1.08E-02
1.43
1.08E-02
DISCUSSION
1.34
1.33
1.23
1.15E-02
9.98E-03
9.21E-03
1.13
8.45E-03
1.13
8.45E-03
1.02
7.68E-03
0.61
4.61E-03
0.00
0.00
0.00E+00
0.00E+00
0.00
0.00E+00
0.00
0.00
0.00E+00
0.00E+00
0.00
0.00E+00
0.00
0.00E+00
0.00
0.00E+00
0.00
0.00E+00
Accident and safety upheavals offer a great risk to
organizational life in terms of preserving its much
revered assets such as lives and properties. Machining
operations can be entirely safe. However, humans may
not adhere strictly to instructions and procedures. The
idea of safety engineering hence, is not to make a
vulnerable machinist pay the dare price of life
threatening injury, rather, keeping him safe despite his
shortfalls and from surrounding hazards. The nature of
accidents has been discovered to be a chain reaction,
with each basic event setting off a bigger fault,
transmitting the fault over and in that manner causing
an undesirable event - accident.
The hazard analysis conducted by this study on lathe
operations using root cause analysis of accidents
recorded for a case study of machine shop (Etherton et
al., 2015), and expert narrations to determine the
pattern and mode of accidents incumbent on lathe
operations revealed that fly-outs and entanglements are
the most widely occurring accidents. With FTA of the
causal factor; basic and intermediate events that could
lead to a Fly-out, twenty four (24) basic events were
identified. They include: Circuitry fault, Chuck Guard
fault Centre-Height fault, Chuck not pulled out,
Dislocation of pipe, Discontinuous swarf, Faulty pump,
Spindle nose looseness, Leakage, Leadscrew fault,
Loose joints and unsecured fittings, Overhang, Speed
control knob fault, Spindle nose damage, Swarf guard,
Excessive tool feeding, Tool mounting, Unclean media,
Wrong key size, Work not well secured, Wrong
operating speed, Wear and Tear of Chuck keyway,
Wear and Tear of Holding device, Wear and Tear of
tool post clamps.
In this present study, the use of Boolean algebra
showed that the top event has probability of 0.748 for
occurrence. A Monte Carlo simulation was equally
carried out in furtherance to this cause, the top event
was observed to have an lower bound and upper bound
of 0.725 and 0.773 respectively. This therefore
captured the probability obtained using Boolean
algebra. Evidently, the value obtained from the use of
Boolean algebra is well in within the results obtained
via the use of Monte Carlo simulation. The event of the
Chuck Not Pulled
Out
Swarf Guard
Chuck Guard Fault
Discontinuous
Swarf
Loose Joints and
Unsecured Fitting
Wrong Key Size
Work Not Well
Secured
Wear And Tear of
Holding Device
Circuitry Fault
Tool Mounting
Leakage
Centre Height
Fault
Wear And Tear of
Tool Post Clamps
Wear And Tear of
Chuck Keyway
Speed Control
Knob Fault
Dislocation of Pipe
Faulty Pump
Spindle Nose
Looseness
Overhang
Spindle Nose
Damage
Excessive Tool
Feeding
Unclean Media
Wrong Operating
Speed
Table 6 provides the difference between Boolean
algebra result and Monte Carlo result for 1000
iterations.
Table 6. Deviation of Boolean Algebra Result and
Monte Carlo Simulation Results
Event
V1
V2
V3
V4
V5
V6
V7
V8
V9
V10
V11
V12
V13
Description
Circuitry Fault
Chuck Guard Fault
Centre Height Fault
Chuck Not Pulled Out
Dislocation of Pipe
Discontinuous Swarf
Faulty Pump
Spindle Nose Looseness
Leakage
Loose Joints and Unsecured
Fitting
Overhang
Speed Control Knob Fault
Importan
ce (%)
0
0.46
-0.87
-0.18
0
0.36
0
0
-0.11
-0.02
Failure
contribution
0
0.23
-0.435
-0.00192
0
0.18
0
0
-0.08209
-1
0.91
0.679104
0
-0.06
0
-0.08955
V24
Spindle Nose Damage
Swarf Guard
Excessive Tool Feeding
Tool Mounting
Unclean Media
Wrong Key Size
Work Not Well Secured
Wrong Operating Speed
Wear And Tear of Chuck
Keyway
Wear And Tear of Holding
Device
Wear And Tear of Tool Post
Clamps
17 | Safety Engineering
0
0.56
-0.02
-0.67
0
0.2
-0.57
-1.34
0
0.28
-1
-0.335
0
0.149254
-0.285
-1
-0.32
-0.23881
0.09
0.067164
-0.21
-0.15672
SAFETY ENGINEERING - INŽENJERSTVO ZAŠTITE
chuck key not being pulled out of the chuck before the
commencement of machining was noted to have
highest probability of occurrence (0.7) hence, it has the
highest contribution to the top-event. Work-holding
and loose fitting are other faults having high
contribution to the occurrence of top-event. Percentage
importance of respective faults was used as the basis
for the application of safety intervention. Safety
interventions identified were training, safety equipment
procurement, guards, condition monitoring, inspection
and preventive maintenance, intermittent check-listing
and machining operation supervision. With the
implementation of the first tier of safety intervention
(training), the FTA revealed that if chuck-fly out can be
entirely eliminated by training machinists to use the
right size of chuck and chuck key, and remove the
chuck key before the commencement of machining
operation, the probability of chuck not pulled out is
0.21, then the probability of top event occurring will be
considerably lesser and will only amount to 0.192.
Consequently, the criticality of lathe machining fly-out
injury decreases from N37, 400 to N9, 600 with a
safety benefit of N27, 800.
CONCLUSION
The study conducted a Fault Tree Analysis in metal
lathe machining operation. Twenty four basic events
leading to the occurrence of fly-out accident were
identified. This includes Circuitry fault, Chuck Guard
fault Centre-Height fault, Chuck not pulled out,
Dislocation of pipe, Discontinuous swarf, Faulty pump,
Spindle nose looseness, Leakage, Leadscrew fault,
Loose joints and unsecured fittings, Overhang, Speed
control knob fault, Spindle nose damage, Swarf guard,
Excessive tool feeding, Tool mounting, Unclean media,
Wrong key size, Work not well secured, Wrong
operating speed, Wear and Tear of Chuck keyway,
Wear and Tear of Holding device, Wear and Tear of
tool post clamps.
The result of FTA revealed that fly-outs are the most
widely occurring accidents during metal lathe machine
operations with a probability of 0.748. Monte Carlo
analysis of the FTA shows the probability of fly-outs
having lower and upper bounds of 0.725 and 0.773,
respectively. The event of the chuck key not being
pulled out of the chuck before the commencement of
machining was noted as the event with the highest
probability of occurrence contributing to the top-event.
Safety intervention alternatives were implemented and
the result revealed that the probability of fly-out
becomes 0.192 with a safety benefit of N27, 800.
Increased safety benefits can be achieved if other safety
intervention alternatives are further implemented.
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[1] Amit K. and Sneh L. (2011). Reliability Analysis of
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Statistical Studies Vol. 4(3) pp. 43-55.
[2] Cowan G. (2011). Monte Carlo Techniques. Available at
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Retrieved March7, 2015.
[3] Department of Occupation Safety and Health (DOSH).
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www.fbme.utm.my. Retrieved January 12, 2012.
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(EASHW). (2004). Building in Safety.
European
Construction Safety Summit. Bilbao, Spain. Available at
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Fault Tree Analysis for the most demanding studies.
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[7] Hu, W., Starr, A.G. and Leung, A.Y.T. (2011).
Operational Fault Diagnosis for Manufacturing system.
Manchester School of Engineering, University of
Manchester, Manchester, M13 9Pl, UK.
[8] Jane, M. (2012). An Introduction to Fault Tree Analysis
(FTA). The University of Warwick. pp. 1-18.
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Prevention. Science Direct. Vol. 77(3) pp. 109-116.
[10] Oyesola, A. and Kola, O.O. (2014). Industrial Accident
and Safety Hazards at the Workplace: A Spatio-Physical
Workplace Approach. Mediterranean Journal of Social
Sciences. Vol 5(20) pp. 2949-2953.
[11] Ovidiu G. (2003). Digital Electronics. Available at
www.eeng.dcu.ie. Retrieved February 2015.
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(2012).
Available
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alcontrib. Retrieved August 5, 2012.
[13] Simon S.I. and Leik M. (1999). Implementing culture
change. Professional Safety, Vol. 44:3.
[14] Tetlow, S. and Jenkins S. (2005). The Use of Fault Tree
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18 | Safety Engineering
A. O. Oriola, G. S. Olanrewaju, A. H. Oluwole, A. A. Lawal, M. Oladapo, Vol 5, No1 (2015) 9-19
ANALIZA STABLA GREŠAKA ZBOG LETEĆIH ČESTICA
Akinyemi Olasunkanmi Oriola, Giwa Solomon Olanrewaju, Adeyemi Hezekiah Oluwole, Akintan
Sažetak: Najverovatniji uzrok nesreće pri radu sa strugom za obradu metala su leteće čestice strugotine. Cilj ovog
istraživanja je utvrđivanje uzročnih faktora koji dovode do nezgoda zbog letećih čestica strugotine pri mašinskoj
obradi, i određivanje verovatnoće nastanka ovog tipa nesreće. Analiza stabla grešaka (eng. Fault Tree Analzysis FTA) se koristi za identifikaciju faktora rizika. Jednačine Bulove algebre su korišćene za analizu verovatnoće
nastanka greške. Izvedena je simulacija Monte Karlo korišćenjem softvera OpenFTA a rezultat nakon 1000
ponavljanja je poređen sa rezultatima Bulove algebre. Alternativn bezbedonosne intervencija su procenjene
uporednom analizom pre i posle sprovođenja mera zaštite. Identifikovana su dvadeset četiri (24) minimalna
rezanja u kojima može nastati 21 osnovni događaj i 3 nerazvijena događaja. Najbitniji događaj ima verovatnoću
0.748 i označava veliku verovatnoću da će se doći do letenja strugotine. Simulacija Monte Karlo je dala donje i
gornje granice verovatnoće od 0,725 i 0,773, respektivno. Ukoliko se odrvrtač glave struga ne izvuče pre početka
mašinske obrade, ovaj događaj najviše doprinosi nastanku nesreće koja je prouzrokavana letećom strugotinom.
Rezultat alternativnih zaštitinih intervencija je pokazao da verovatnoća raste na 0,192 a korist zbog primenjenjih
mera bezbednosti postaje N27, 800 nakon prvog nivoa implementacije. Ostali nivoi bezbednosnih intervencija
pokazuju da verovatnoća nezgode usled leteće strugotine opada. Imajuću ovo u vidu, inženjer zaštite može izabrati
neki od odgovarajućih programa zaštite na osnovu skale efektivnosti.
Ključne reči: čestice leteće strugotine, nesreća, bezbednost, intervencija, strug, mašina, operacija.
19 | Safety Engineering
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