Failure Mode and Effect Analysis
Lecture 3-3
Error Proofing
Red Flag Conditions.
FMEA
A condition in the
manufacturing process
which commonly
provokes errors
1
Error Proofing: Ten Causes of Errors
FMEA
There are ten common causes of errors which
Error Proofing is designed correct or eliminate.
1. Processing omissions: Leaving out one or more process
steps.
2. Processing errors: Process operation not performed
according to the standard work procedures.
3. Error in setting up the workpiece: Using the wrong tooling
or setting machine adjustments in correctly for the current
product.
4. Missing parts: Not all parts included in the assembly,
welding, or other processes.
5. Improper part/item: Wrong part installed in assembly.
6. Processing wrong workpiece: Wrong part machined.
3
Error Proofing: Ten Causes of Errors
FMEA
7.
Operations errors: Carrying out an operation
incorrectly; having the incorrect revision of a standard
process or specification sheet.
8. Adjustment, measurement, dimension errors: Errors
in machine adjustments, testing measurements or
dimensions of a part coming in from a supplier.
9. Errors in equipment maintenance or repair: Defects
caused by incorrect repairs or component replacement.
10. Error in preparation of blades, jigs, or tools:
Damaged blades, poorly designed jigs, or wrong tools.
4
2
Error Proofing: Red Flag Conditions (cont’d)
FMEA
Each of the common Red Flag conditions may lead to production errors.
1. Adjustments
Red Flag:
Workers having to make adjustments to parts or
equipment to complete a process step.
2. Tooling and tooling changes
The use of perishable tools in production and/or tools
that are changed between production runs.
3. Dimensions/ specifications/ critical conditions
Operations which require the use of measurements to
position a part in operations, or situations which require
operations to be performed within designated critical
conditions. (e.g.., temperature, pressure, speed, etc.)
5
Error Proofing: Red Flag Conditions (cont’d)
FMEA
4. Many / mixed parts
Red Flag:
A process which involves a wide range of parts in varying
quantities and mix.
5. Multiple steps
A process that requires many small operations or sub-steps
to de done in a strict preset order.
6. Infrequent production
An operation or task which is not performed regularly.
7. Lack of an effective standard
Standard operating procedures (SOP’s) that are vague or
do not fully describe the correct and proven way to perform
a production process.
6
3
Error Proofing: Red Flag Conditions (cont’d)
FMEA
8. Symmetry
Machining or assembly operations which use an object
whose opposite sides are similar or identical.
9. Asymmetry
Operations which use a part, tool or fixture whose opposite
sides may look identical but are different in size, shape or
relative position.
10. Rapid Repetition
A process which requires quickly performing the same
operation over and over again.
7
Error Proofing: Red Flag Conditions (cont’d)
FMEA
11. High/Extremely High Volume
A process which requires quickly and repeatedly
performing a task with time pressure.
12. Environmental Conditions
Physical circumstances within and around the workplace
that can influence quality and workmanship.
8
4
Error Proofing: Types of Error Proofing Devices
FMEA
The following is a list of Error Proofing devices which can be used to
respond to Red Flag conditions.
• Guide / reference /
interference rod or pin.
• Template
• Limit switch / microswitch
• Counter
• Odd-part-out method
• Sequence restriction
• Standardize and solve
• Critical condition indicator
•
•
•
•
Detect delivery chute
Stopper / gate
Sensor
Mistake proof your Error
Proof device
• Eliminate the condition
• Redesign for symmetry
• Redesign for asymmetry
These devices are described in more detail on the following pages.
9
Error Proofing: Types of Error Proofing Devices (cont’d)
Guide / reference/ interference rod or pin
A guide or reference rod is a solid piece
of material like a stem or peg that
positions or orients a part, tool or fixture
and guarantees its correct placement. An
interference pin refers to a peg that
blocks, obstructs, or prevents the
incorrect positioning of a part, tool or
fixture. This pin or “boss”, as it is
sometimes called, can be fixed onto the
part itself, or on a tool or fixture.
small plate
reference holes
in plate
pins
jig
Example :
Mis-aliging plates in setup was causing
defects. Locator pins built into the jig
correspond to double holes drilled in the
center of every plate so that all sizes of
plates are automatically positioned
correctly by merely setting them on the jig.
Processing errors due to misalignment in
setup are eliminated.
medium
plate
small
plate
large plate
5
Error Proofing: Types of Error Proofing Devices
Template
A template is a pattern used to
represent an accurate copy of an
object used to guarantee accurate
positioning. Templates are frequently
used in inspection procedures and are
often made of thin metal, plastic, or
paper. An existing jig or fixture may be
modified to serve as a template.
button
positioning jig
button
Example:
Buttons were not being sown in the
correct position and spacing. A
positioning jig was developed for
sewing buttons which positions
buttons by putting the cuff end against
the jig mounted on the sewing
machine. This positions the cuff
accurately for the required number of
buttons and they come out neatly in a
row and evenly spaced.
needle
button
positioning jig
sleeve position
for 1st button position for
2nd button
position for
3rd button
Error Proofing: Types of Error Proofing Devices
Limit switch / microswitch
A limit switch or microswitch is an electrical device or instrument that, with a light contact on
its antenna section, can confirm the presence, position, dimension, breakage or degree of use
(wear) of a part, tool, or fixture. They are also called proximity switches, photoelectric
switches, and touch switches.
Example:
Holes were not being
drilled to the appropriate
depth. Two limit switches
were mounted on the drill
press. Faulty drilling is
indicated if limit switch 1
is released before limit
limit switch 1
switch 2 has been tripped
(indicating the start of
drilling without
limit switch 2
penetration). A buzzer is
sounded to alert the
operator.
buzzer
switch 1 confirms
beginning of drilling
switch 2 confirms
penetration
6
Error Proofing: Types of Error Proofing Devices
Counter
A counter is an indicator that keeps track of a number - the number of parts, turns, strokes,
output, or abnormalities of a given machine or operation.
Example:
In a process where parts are manufactured for
several different models, ten holes are tapped on
each work piece, using a single-spindle drill press.
Before Improvement: The operator had to
visually check and count the number of holes they
had tapped. This method of control relied strictly
on the workers’ vigilance and tapping was omitted
now and then.
After Improvement: A counter was added to the
tapping machine. The operator clears the counter
for each work piece and checks that the number
of taps is correct for the current model. Although
this amounts only to a method for assisting the
vigilance of the operator, it almost completely
eliminates omissions of tapping.
clear button
counter
Error Proofing: Types of Error Proofing Devices
Odd-part-out method
The odd-part-out method is a form of counting that does not rely on a counting device.
Instead, it isolates the pre-counted correct number of parts visually, and the visual display
tells us if all the parts are not used.
Example :
The parts needed for a given run of
products are counted out in advance
and given to the worker. If some
parts remain after the planned
number of products have been
assembled or if there are not
enough parts, it is immediately clear
that there is an abnormality. This
method of checking prevents units
with missing parts from being sent
out into the market.
part 1
part 2
part 3
The worker is given exactly the right
number of parts for the number of
products to be made
7
Error Proofing: Types of Error Proofing Devices
Sequence Restriction
A sequence restriction is useful when order is so important that any change or omission in
the order can result in costly errors. Look for concrete ways to restrict the sequence so it can
only follow the pre-set order. Sequence restriction devices guarantee that operations will
happen only in the pre-determined order. Order is often a key factor in bending, packing,
assembly, and inspection operations.
Example :
Tapes for testing were being use in an
incorrect order. A new “first-in, first-out” rack
was developed that dispenses tapes only in
the proper order for testing. When one tape is
removed for use, the next tape slides down,
ready for use. When a tape has been used,
the inspector places it in the top of the rack,
where it remains in the correct order. Errors
in the testing sequence were completely
eliminated.
Error Proofing: Types of Error Proofing Devices
Standardize Elements
Standard elements for an operation help
us identify non-standard occurrences or
errors. Standard elements such as
weight, dimension, or shape can hold the
key to the development of error proofing
devices.
Sometimes existing standardized
elements are not easily converted into
Error Proofing devices. In these cases,
we try to identify a special characteristic
and establish it as a new standard
element, so that it can help us isolate any
non-standard elements.
A
B
Example B:
The containers are
packed on a scale.
If there are too few
gears in a box, it
will not weigh
enough, and the
omission is
detected.
Example A:
compressed
air stream
Because empty boxes
are light, they are blown
off the conveyor belt
with compressed air
directed at the boxes
from the side.
8
Error Proofing: Types of Error Proofing Devices
Critical condition indicator
A critical condition detector is a device that detects two types of conditions:
1. The presence or absence of a specific, visible, pre-set quantity such as
the correct number of parts, correct weight, height, volume or depth.
2. Fluctuations in a non-visible conditions such as pressure, temperature,
current and non-visible fluids (air).
Example:
Two pressure gauges are
installed on the same
outlet at each measuring
site. The operator can
quickly determine the
reliability of the readings
on the gauges by
comparing them.
Is this gauge
reading reliable?
Error Proofing: Types of Error Proofing Devices
Detect delivery chute
A delivery chute is a passageway down which a unit (piece, part, or volume) is slid, sent,
transported or dropped on its way to some pre-set destination - to the next operation for
example. Because each unit passes through the chute, the chute itself can be used to
“inspect” the unit on route, detecting or sorting out errors before the unit reaches the next
operation. We call this type of chute a detect chute.
Example :
Items were arriving to a process
upside-down. A checkpoint is
installed in a delivery chute that
automatically removes upside down
items. The checkpoint has a notch
that causes the upside down items
to drop into the delivery box below.
Those items that are right side up
are allowed to pass through freely.
As a result, all the work pieces are
delivered to the next process in the
proper positions.
upside
down
right
side up
notch
9
Error Proofing: Types of Error Proofing Devices
Stopper / gate
A stopper or gate is a solid piece of
material that guarantees that a certain
operation is not performed. Gates and
stoppers are often used in combination with
limit switches and sensors to move when
certain conditions are met. A gate or
stopper will be set up to trip a switch,
causing a given operation to either halt or
begin.
For example, a wire motor thread passes
through a hole in a rubber stopper on its
way to a winding machine; if there is a
dimensional variation (thickness) in the wire
or if foreign matter adheres to the wire, the
stopper is pushed against a limit switch and
the operation shuts down.
stopper
stopper is
pulled away
tension
box
to machine
spring limit switch
limit switch
Example:
If there is any foreign matter or a shape or dimensional variation on the
wire, a stopper on the feed device catches on the wire at that place and
moves along with the wire. The machine stops automatically when the
stopper strikes a limit switch inside the machine.
Error Proofing: Types of Error Proofing Devices
Sensor
A sensor is an electrical device or instrument that detects and responds to fluctuations in
characteristics related to quality, safety or productivity. A sensor can confirm with a high
degree of precision the presence and position of a part, tool, or fixture and / or detect a
break, damage or wear.
B
A
light passes
through
light does
not pass
through
motor
Example:
Parts in a process were not being notched. A photoelectric detector was installed to
determine if each part had been notched successfully. The parts are rotated, and if
any light is detected the notching was successful. If no light is detected, the part is
identified as un-notched.
10
Error Proofing: Types of Error Proofing Devices
Mistake proof your Error Proof device
Lack of good maintenance can render the
most brilliant Error Proofing device useless–
or even dangerous. Dangerous, because if
precautions are not taken you can assume
that the device is watching your quality or
safety, when it is not.
pinwheel indicates
air stream
To avoid this, you must regularly maintain
every non-mechanical Error Proofing
device. This includes sensors, limit
switches, counters, gates and stoppers, and
any other device that relies on electricity,
temperature and pressure gauging, or
tolerances. Your device is reliable only to
the extent that these pre-determined
specifications are precisely maintained.
Example:
compressed
air stream
From the upstream position, operators rely on the
air stream Error Proofing device to detect empty
boxes. If the air stream does not flow steadily,
however, empty boxes will escape detection.
Since air is invisible, the team installed a child’s
pinwheel to show that the air stream is
functioning. They error-proofed their Error
Proofing device.
Error Proofing: Types of Error Proofing Devices
Eliminate the condition
The eliminate-condition method isolates a specific error characteristic and removes it.
If, for example, certain non-functional holes cause us to get confused about where (and
where not) to insert parts during assembly, we simply plug up the non-functional holes and eliminate the possibility of error.
INCORRECT
Example:
Assembly Part ‘B’
was being mis-placed
when inserted onto
Plate ‘A’. The extra
holes in Plate ‘A’
were removed,
eliminating the
improper insertion of
Part ‘B’.
B
A
extra holes
CORRECT
B
A
Eliminated extra holes so improper insertion
of assembly Part ‘B’ is impossible.
11
Error Proofing: Types of Error Proofing Devices
Redesign for symmetry
To redesign for symmetry means to revise or modify the appearance or function of a part,
tool or fixture so that its opposite sides are identical or nearly identical. In this way they
become interchangeable, eliminating errors resulting from improper placement or
orientation.
Example:
ram
Asymmetrical part
frequently assembled
improperly
chassis
new shape –
two grooves
The wrong end of a
shaft was being
incorrectly staked to
the chassis. After
error proofing, both
ends of the shaft are
grooved for an E-ring,
so either end can be
staked to the chassis
without creating an
error. The E-ring can
always be mounted
and it is impossible to
create a defect.
Error Proofing: Types of Error Proofing Devices
Redesign for asymmetry
To redesign for asymmetry means to revise or modify the appearance or function
of a part, tool or fixture so that is opposite sides are no longer identical. In this way
they can no longer be confused and mistakenly interchanged, eliminating errors
resulting from placement or orientation.
Example :
A plaque was being mounted upsidedown. The axis of the mounting pin of the
plaque was moved away from the center,
making it impossible to mount upside
down. Defects due to upside-down
mounting are completely eliminated.
Off-center pin prevents
incorrect insertion
12
Six Steps to Error Proofing
FMEA
These steps provide a framework for implementing Error Proofing in the factory.
These steps should be implemented:
• Reactively, in response to the observation of defects
• Proactively, after identifying Red Flag conditions which exist in a process.
Step 1: Identify and describe
Identify and describe the defect /red flag condition in detail. In the case of a
defect, examine the history of the defect. In order to establish accountability,
a team member should be identified to follow up on the defect/red flag.
Step 2: Determine the root cause
Conduct cause and effect diagramming to assess the root cause. This
determination is critical for applying error proofing techniques to
eliminate the defect/ red flag.
Step 3: Review the current standard procedure.
Document each element / step in the operation where the
defect occurs. Error proofing opportunities will be based
upon this careful procedure identification.
25
Six Steps to Error Proofing, cont.
FMEA
Step 4: Identify deviations from standards.
Observe the actual process and identify areas where the methods being
applied deviate from the standard operating procedure. These deviations will
point to areas where procedural improvements are needed.
Step 5: Identify the type of error-proofing device type required.
Identify the type of error-proofing device most likely to effectively
eliminate the defect.
Step 6: Create device(s) and test for effectiveness
Create and test the device for effectiveness. Modifications
are made until the device proves effective in preventing the
defect.
13
FMEA Homework Exercise 3
FMEA
• Complete a process FMEA for the operation
shown on the next pages
• Use the forms provided on the web page, or
create forms of your own. If your company
already has FMEA forms, you may use them
for this exercise
• Submit the FMEA to me attached to e-mail.
• Assignment 3 is due 4/9
27
FMEA Homework 3 (Spring 2004)
1. Evaluate the process of attaching a
pressure-sensitive adhesive label to an
instrument case.
LABELS ON
TAPE STRIP
2. The case contains sensitive electronic
components that cannot be exposed to
contamination from particles or vapors.
ADHESIVE
3. The hole is used for final adjustment ADHESIVE-FREE
AREA
and is covered with a pressure-sensitive
label. To keep the inside of the case from
being exposed to possible contamination
from the adhesive, the label was designed
with a no-adhesive area.
4. The label is attached by an operator
using tweezers to remove the label from
the strip and to place it over the hole. The
operator uses his or her finger to press
the label onto the case. (the operators
wear latex gloves to prevent
contamination.
5. The next operator inspects the label for
proper alignment and coverage.
TOP VIEW SHOWING LABEL IN PLACE
See next page
14
FMEA Homework 3
FMEA
PROCESS INSTRUCTIONS
1. Operator must wear latex gloves
2. Use pointed tweezers to remove label from strip and
place it in the center of the recessed area.
3. Press label with glove finger to secure it to cover
4. Inside of case must not be exposed to adhesive
5. Label must make an air-tight seal to prevent
particulate contamination
29
15