Error Proofing
Module Objectives
To explain the value of error proofing
To describe situations where error proofing is needed
To be able to implement error- proofed systems
What is Error Proofing?
. Fool proofing, mistake-proofing
Strives for zero defects
Leads to Quality Inspection Elimination
Respects the intelligence of workers
Takes over repetitive tasks/actions that depend on one’s memory
Frees an operator’s time and mind to pursue more creative and value added activities
Quality and Stability is essential in a lean manufacturing environment
Human error is a natural occurrence; all humans make errors
Machines are not defect-free
Why do Error Proofing?
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What are the Benefits of Error Proofing?
Enforces operational procedures or sequences
Signals or stops a process if an error occurs or a defect is created
Eliminates choices leading to incorrect actions
Prevents product damage
Prevents machine damage
Prevents personal injury
Eliminates inadvertent mistakes
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Definition of an Error
An error is any deviation from an intended process
Occurs when any condition necessary for successful processing is improper or absent
All defects are created by errors
Not all errors result in defects
Process
Flow
ERROR
To Next Process
POSSIBLE
DEFECT
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Forgetfulness
Errors due to misunderstanding
Errors in identification
Errors made by amateurs
Willful errors
Inadvertent errors
Errors due to slowness
Errors due to lack of standards
Surprise errors
Intentional errors
Different Kinds of Human Errors
Almost all defects are caused by human errors. However, there are at least ten kinds of human errors.
Reference Appendix A for error definitions and potential safeguards
Error Proofing: Ten Causes of Errors
There are ten common causes of errors which Error Proofing is designed to 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 machine settings 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
7. Operations errors: Carrying out an operation incorrectly; having the incorrect revision of a standard process or specification sheet
Error Proofing: Ten Causes of Errors (cont)
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 tooling:
Damaged blades, poorly designed jigs, or wrong tools
Error Proofing: Ten Causes of Errors (cont)
Red Flag Conditions
A condition in the manufacturing process which commonly provokes errors
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Error Proofing: Red Flag Conditions (cont)
Each of the common Red Flag conditions may lead to production errors.
1. Adjustments
2. Tooling and tooling changes
Red Flag:
Workers having to make adjustments to parts or equipment to complete a process step.
3. Dimensions/ specifications/ critical conditions
The use of perishable tools in production and/or tools that are changed between production runs.
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. (.., temperature, pressure, speed, etc.)
4. Many / mixed parts
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
Red Flag:
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.
Error Proofing: Red Flag Conditions (cont)
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.
Error Proofing: Red Flag Conditions (cont)
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.
Error Proofing: Red Flag Conditions (cont)
Methods and Tools for Error Proofing
Guide / reference / interference rod or pin
Template
Limit switch / microswitch
Counter
Odd-part-out method
Sequence restriction
Standardize and solve
Critical condition indicator
Automate
Detect delivery chute
Stopper / gate
Sensor
Mistake proof your Error Proof device
Eliminate the condition
Redesign for symmetry
Redesign for asymmetry
Automated lock outs
Error Proofing: Types of Error Proofing Devices
The following is a list of Error Proofing devices which can be used to respond to Red Flag conditions
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small
plate
Error Proofing: Types of Error Proofing Devices
Guide / reference/ interference rod or pin
Example :
Mis-aligning 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.
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
pins
jig
large plate
medium
plate
reference holes
in plate
button
button positioning jig
sleeve position
for 1st button
needle
button
positioning jig
position for
2nd button
position for
3rd button
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.
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.
Error Proofing: Types of Error Proofing Devices
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buzzer
limit switch 1
limit switch 2
switch 1 confirms
beginning of drilling
switch 2 confirms
penetration
Limit switch / microswitch
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 switch 2 has been tripped (indicating the start of drilling without penetration). A buzzer is sounded to alert the operator.
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.
Error Proofing: Types of Error Proofing Devices
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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 method for assisting the vigilance of the operator, it almost completely eliminates omissions of tapping.
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.
Error Proofing: Types of Error Proofing Devices
counter
clear button
part 1
part 2
part 3
The worker is given exactly the right number of parts for the number of products to be made
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.
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
compressed air stream
A
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.
Example A:
Because empty boxes are light, they are blown off the conveyor belt with compressed air directed at the boxes from the side.
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.
B
Error Proofing: Types of Error Proofing Devices
Is this gauge reading reliable?
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.
Error Proofing: Types of Error Proofing Devices
right side up
upside down
notch
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.
Error Proofing: Types of Error Proofing Devices
to machine
tension
box
spring
stopper is
pulled away
stopper
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.
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.
limit switch
limit switch
Error Proofing: Types of Error Proofing Devices
A
B
light passes through
motor
light does not pass through
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.
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.
Error Proofing: Types of Error Proofing Devices
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.
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.
compressed air stream
pinwheel indicates air stream
Mistake proof your Error Proof device
Example:
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
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.
extra holes
Eliminated extra holes so improper insertion of assembly Part ‘B’ is impossible.
CORRECT
Eliminate the condition
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’.
A
B
A
B
INCORRECT
Error Proofing: Types of Error Proofing Devices
chassis
ram
new shape –
two grooves
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:
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
Asymmetrical part
frequently
assembled
improperly
Off-center pin prevents incorrect insertion
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 upside-down. 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.
Error Proofing: Types of Error Proofing Devices
Six Steps to Error Proofing
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.
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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.
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.
Six Steps to Error Proofing (cont)
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.
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.
Six Steps to Error Proofing (cont)
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Error Proofing Exercise
Step 1: Error proof the process of consistently shooting the catapult ball to a fixed distance.
Step 2: Develop error proofing strategies for key leverage points and determine tests for effectiveness.
Module Objectives
To explain the value of error proofing
To describe situations where error proofing is needed
To be able to implement error- proofed systems
Appendix A
Error Definitions
and Safeguards
There are Different Kinds of Errors
Almost all defects are caused by human errors. However, there are at least ten kinds of human errors.
1. Forgetfulness: Sometimes we forget things when we are not concentrating. For example, the stationmaster forgets to lower the crossing gate. Safeguards: Alerting operator in advance or checking at regular intervals.
2. Errors due to misunderstanding: Sometimes we make mistakes when we jump to the wrong conclusion before we’re familiar with the situation. For example, a person not used to a car with automatic transmission steps on the brake, thinking it is the clutch. Safeguards: Training, checking in advance, standardizing work procedures.
3. Errors in identification: Sometimes we misjudge a situation because we view it too quickly or are too far away to see it clearly. For example, a $1 bill is mistaken for a $10 bill. Safeguards: Training, attentiveness, vigilance.
4. Errors made by amateurs: Sometimes we make mistakes through lack of experience. For example, a new worker does not know the operation or is just barely familiar with it. Safeguards: Skill building, work standardization.
There are Different Kinds of Errors (cont)
5. Willful errors: Sometimes errors occur when we decide that we can ignore rules under certain circumstances. For example, crossing a street against a red light because there are no cars in sight at the moment. Safeguards: Basic education and experience.
6. Inadvertent errors: Sometimes we are absentminded and make mistakes without knowing how they happened. For example, someone lost in thought tries to cross the street without even noticing that the light is red. Safeguards: Attentiveness, discipline, work standardization.
There are Different Kinds of Errors (cont)
7. Errors due to slowness: Sometimes we make mistakes when our actions are slowed down by delays in judgement. For example, a person learning to drive is slow to step on the brake. Safeguards: Skill building, work standardization.
8. Errors due to lack of standards: Some errors occur when there are no suitable instructions or work standards. For example, a measurement may be left to an individual worker’s discretion. Safeguards: Work standardization, work instructions.
There are Different Kinds of Errors (cont)
9. Surprise errors: Errors sometimes occur when
equipment runs differently than expected. For
example, a machine might malfunction without
warning. Safeguards: Total productive
maintenance, work standardization.
10. Intentional errors: Some people make mistakes deliberately. Crimes and sabotage are examples. Safeguards: Fundamental education, discipline.
There are Different Kinds of Errors (cont)
