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Poka yoke
- 1. April 2010 Agricultural and Construction Equipment CNH - World Class Manufacturing Team Poka Yoke 06. QC Pillar Training Material NH AG NH CE
- 2. April 2010 Zero Quality Control Poka-Yoke System Application Examples Contents
- 3. April 2010 Zero Quality Control is a quality control (QC) approach to achieve Zero defects on products. The Basic Principle Defects are prevented by controlling the performance of a process so that it cannot produce defects – even when a mistake is made by a machine or man. ZQC is a method for Mistake-proofing What is ZQC?
- 4. April 2010 Maintain customer satisfaction and loyalty. Defect always cost something, in terms of scrap or rework. Zero defect is a key factor to keep smaller inventory and lean production. Zero defects make work go smoother and keep higher employee morale. I’ll never buy this @#$%* brand again, also I’ll recommend my friends not to buy!!! Why focus on Zero Defects?
- 5. April 2010 4 M MAN Lack of training or motivation, overtime MATERIAL Supplier defects, improper handling, excessive variability MANAGEMENT Excessive product variability, poor production planning METHOD Inappropriate procedures, lack of process standards MACHINE Worn machine parts, damaged tools What causes Defects?
- 6. April 2010 Plan Do Check Plan Do- Check Traditional approach ZQC approach • Catch defects after they occur • Take improvement action • Inspection occurs at the source • Check is integrated with Do Quality Improvement Cycle
- 7. April 2010 Judgment Inspection Informative Inspection Source Inspection • Discovers defects but does not reduce them • Does not give quick feedback to improve the process for not making defects • Generally done at the end of the process • Giving constant feedback to the process that produce defects • Focus is on addressing the cause to solve it as quickly as possible • Example: SQC, self certification • Cannot eliminate defects completely • Catches errors and gives feedback before processing • Every single product is inspected • Quick feedback to correct the errors right away • This is the basic concept of ZQC 3 Approaches to Product Inspection
- 8. April 2010 Poka-Yoke systems use sensors or other devices installed in processing equipment to detect errors that might slip by the operator and prevent defects by one of the following: • A control system stops the equipment when irregularity happens or, • A warning system signals the operator to stop the machine Both the systems use three basic methods for using Poka-Yoke: 1. Contact method 2. Fixed Value method 3. Motion step method Using Poka-Yoke Systems
- 9. April 2010 Works by detecting whether a product makes physical or energy contact with a sensing device. Example Contact method
- 10. April 2010 A device counts the number of times something is done and signals or releases the product only when the required number is reached. This method can be used when a fixed number of repeated operations needs to be done at a process station. Fixed Value method Example
- 11. April 2010 This method is used to sense whether a motion or step in the process has been carried out within a certain expected time, such as a machine cycle time. It can also be used to ensure that things happen according to a certain sequence, which avoid errors. Motion-step method Example
- 12. April 2010 The sensing devices used in Poka-Yoke can be divided in 3 broad categories: • Physical contact sensing devices • Energy sensing devices • Sensors to detect physical condition changes Types of Sensing devices Example
- 13. April 2010 These devices work by physically touching the product or machine part. An electrical signal is generated by the physical contact to stop or start a machine. Examples Physical contact sensing devices
- 14. April 2010 These devices use energy rather than physical contact to determine whether an error is occurring. Examples Energy sensing devices
- 15. April 2010 More Examples Energy sensing devices…contd.
- 16. April 2010 These type of poka-yoke sensors detects change in physical conditions like pressure, temperature, electrical currents etc. Pressure change sensors Temperature change sensors Electrical current change sensors • Used to detect problems like oil supply interruptions • Example: Pressure gauges and pressure- sensitive switches. • Used to check surface temperatures of dies electronic parts and motors. • Example: thermometers, thermostats, thermistors etc. • Used to control the causes of defects by detecting the occurrence of electrical current. • Example: meter relays, current eye etc. Physical condition change sensing devices
- 17. April 2010 EXAMPLES OF POKA-YOKE APPLICATIONS
- 18. April 2010 Before After Example 1. Catching an incorrectly positioned work piece
- 19. April 2010 Before After 1st process 2nd process New drilled holes Correctly mounted Incorrectly mounted TOP VIEW CROSS-SECTION VIEW Correctly mounted Pin prevents incorrect mounting Pins for these positions Example 2. Preventing incorrect mounting of work piece
- 20. April 2010 Before After SIDE VIEW CROSS-SECTION VIEW unmilled part milled part chute TOP VIEW blocks chute unmilled part cannot pass blocks milled part passes through Example 3. Detecting unprocessed parts in a chute
- 21. April 2010 Before After Example 4. Ensuring proper positioning of blanks for stamping
- 22. April 2010 Before After Example 5. Detecting wrong positions of bonded metal parts
- 23. April 2010 Before After press Work- p/c jig correct press Work- p/c jig incorrect press Work- p/c jig only correct mounting possible to operating circuit Limit switch detects correct positioning Example 6. Catching mispositioned parts in a press
- 24. April 2010 Before After Example 7. Preventing omitted welds
- 25. April 2010 Before After Example 8. Reminding operator to change welding tips
- 26. April 2010 Before After Example 9. Making sure all bolts are installed
- 27. April 2010 Before After Model 6 use parts D & E? Example 10. Avoiding assembly with wrong parts
- 28. April 2010 Before After Example 11. Preventing omission or doubling of washers
- 29. April 2010 Before After Example 12. Catching uncut bolts before further processing
- 30. April 2010 Before After Example 13. Ensuring adequate testing of solder joints
- 31. April 2010 Before After Example 14. Testing for uncut grooves in a product
- 32. April 2010 Before After Example 15. Inspecting for missing staples on cases
- 33. April 2010 Before After Example 16. Inspecting for missing staples on cases
Editor's Notes
- Zero quality control is the basis for Poka-Yoke or mistake proofing. Introduce the poka-yoke concepts. Originally described as Baka-yoke, but as this means "fool-proofing" (or "idiot proofing") the name was changed to the milder Poka-yoke. Various application examples to help generating ideas in your work environment. Most of the examples are implemented by shop-floor people including engineering and maintenance. Frontline associates can also help designers to incorporate mistake proofing features in a new product.
- ZQC is an approach that does not point fingers at people. It recognizes that people and machines can do mistakes and it finds ways to keep those errors from turning into defects. No matter how good you are at finding defects, no matter how thoroughly you follow-up with corrective action, improving your defect inspection and feedback process will not prevent defects from happening in the first place. To prevent defects you must look at the process to determine what condition lead to that defect, then control that condition.
- Even one defective product can cause damage a lot about company’s image and eventually a lot of business also. If you buy a car and it breaks down in the middle of the road not only you will curse the car manufacturer for that moment, you will talk badly about them to your friends and relatives and probably recommend them not to buy the same. All the cost of rework or repair reduce productivity and reduces the product margin. When defects are accepted as a way of life, companies tend to keep buffer to cover them, and thus carry higher inventory cost. ZQC can bring the inventory cost down. It allows you to produce exactly the number customer wants. Also not pointing fingers to people help people not to feel ‘guilty’ or ‘foolish’ and thus keeps their morale higher.
- Some of these cause of defects are predictable and can be solved by better planning, but sometimes simple human mistakes are made which is totally unpredictable. So the goal of ZQC approach is to prevent these simple mistakes from happening. It does this by using a control function that ensures the necessary conditions to make good products.
- In the traditional cycle, optimal processing conditions are established in the Plan stage. The planned actions are carried out in the Do phase. Finally quality monitoring is performed in the Check phase. If a defect is found, the information is fed back to take corrective actions in the next Plan stage. This way defects get reduced but do not get eliminated. In the ZQC approach inspection takes place right at the point where error occurs, which is called source inspection. It has got short feedback loop, 100% check and mistake proofing.
- Many companies use the first 2 types, which are traditional. But the last one actually eliminates defects. In a judgment inspection, a person or a machine compares the products with standard and discovers the non conformity to reject them. In the informative inspection, actually feedback is given to the system to correct them in the next cycle. For example in Statistical Quality Control charts show the process errors, but by the time the feedback is taken into improvement actions already some defects are done. Same with the self-certification process, it guarantees that the next process is not fed with a defective part but this process does not guarantee that the operator makes no mistake! Instead in source inspection the errors are caught before processing, so they don’t turn into defects.
- Each of the three methods may be used with either control system or warning systems. It is good to understand what method suits better in your work environment.
- In the example, 2 air cylinder hold the product against a fixture and 4 screws are fastened on each corner fittings. If one screw is missed, the product would be defective! To avoid this 4 limit switches are connected to the cylinders that hold the product until all the switches are pressed. If one screw is left out, the product will not be released to the next operation.
- In the example a limit switch is tripped with each movement of the welding head, sending a signal to a counter that detects when the right number of movements has happened.
- The label dispensing machine uses a photo-electric switch connected to a timer. It would stop the line if it does not detect removal of a label within the machine’s cycle time.
- Each type of sensors include a wide range of devices that can be used in different situations.
- Limit switches and micro-switches are most common physical contact sensing devices. They are used to confirm the presence and position of an object that touch the small lever on the switch. Touch switch has antenna instead of lever. Trimetron is needle type gauge that sends signal to sound an alarm or stop equipment when the condition it measures are not within the acceptable limits.
- Energy sensing devices are used to inspect/detect proper sizes or color, passage of object on conveyor, feeding of parts etc. They use light energy, magnetic energy, electrical energy even physical force.
- Pressure change sensors are used to detect oil supply interruptions. Temperature change sensors are used to check surface temperature of dies, electronic parts etc. Meter relay control the causes of defects by detecting the occurrence of electrical currents, current eye monitors weld strength by detecting secondary voltage changes in the weld points.
- Most of the examples are quite simple and inexpensive. There may be several other solutions to the same problems. These will help you to think error proofing your own work areas.
- This operation attaches a brake wire to a clamp. The assembly person places the clamp into the jig to hold it while the wire is inserted. The problem is there are two kinds of clamps, left and right which are not interchangeable and both fit into the jig. What can you do to prevent mounting wrong side clamp? Two separate jigs are created by placing a block on different sides to prevent wrong sided clamps from being inserted. This is an example of contact method.
- Incorrect mounting of the work-piece would cause holes being drilled in wrong positions. What can be done to prevent this? Placing two pins on the jig in the specified hole positions, would allow the piece to be mounted only in the correct fashion, it does not fit in the other way. Contact method applied.
- Molded parts are milled in an automatic machine, then fed into the next process by a chute. Sometimes due to processing errors if a part is not milled properly it goes to the next process it can damage the next machine. What can be done to prevent this? Placing two small blocks in such a way that the thin “stem” of the milled parts only can pass through. The unmilled part gets caught in the block and stops the flow, operator can remove it to restore the flow. Another contact method.
- This example involves an automatic line for stamping numbers on metal plates. The blank plates sometimes catch on the edge stops that are supposed to position them. When stamping is done in this improper position defects are created. How this can be prevented? Proximity switches are mounted on the edge stops on the 2 sides where the blanks sometimes catch. Both switches trip only when the blank is properly positioned. When a blank catches on an edge stop, it does not trip the switch, and machine will not start. Contact method applied.
- Parts made of bonded steel and aluminum are fed into a machine for forming. In this process the steel is supposed to face upward. If by mistake the part goes into the forming machine with the aluminum side up, can damage the tools in later process. What can be done to prevent this from happen? A proximity switch is installed above the parts that are fed into the machine. This switch detects the distance of the steel surface from itself. So, when the aluminum side is up, the steel side is further away, so the switch doesn’t operate, which gives a warning signal to the operator. Contact method applied.
- The part with the neck on wrong side would create defect when processed. What can be done to prevent this? A limit switch is installed on the press jig which operates only when the neck touches it. The press does not operate when this limit switch is off. Contact method applied.
- A portable spot welder is used to make ten different welds on an automotive part. Sometimes some of the welds are missed accidentally. What would you do to prevent this mistake? The part to be welded is clamped in place by an air cylinder which is connected to a control board equipped with a counter. This system does not allow to unclamp the part unless the desired 10 weld spots are done. Fixed value method applied.
- The tip of the welding machine wears out with use. It needs to be changed after a certain number of welds so that the diameter of the welded area stays within the specifications. Sometimes people forget to change the tips when they are supposed to, which results defects. To prevent this a control board is used with a counter that tracks the number of welds done by the machine, when the critical number is made the control board signals the operator to change the tip. Fixed value method applied.
- In assembly of an automobile spoiler, nine mounting bolts are supposed to be installed after taking the bolts from a bin. Sometimes the assembler may forget some of the bolts. Can this be prevented? A part feeder is designed which counts 9 bolts upon pressing a lever. So it’s difficult to forget for the operator as he would have all the 9 bolts in his hand. Fixed value method applied.
- Auto parts assembly operation involves installing several components which vary from model to model. The parts are taken from bins according to a list of parts for each model. Sometimes parts are forgotten or wrong part is assembled by mistake. What is needed to be done to prevent this? Doors and lights are installed on the parts bins and wired to a model on the indicator board, the doors open for the bins holding the proper parts, and the lights above the bins make sure that the assembler sees all of the parts needed for that model. Motion step method applied.
- In an automotive assembly operation, a washer is attached to another part. The washers are supplied in a box and taken out by the assembler. Although this is done carefully, sometimes washers are omitted or doubled. To prevent this a device is made that feeds a single washer when the assembly machine is turned on. The washer stops on a small platform and is picked up and installed by the assembler. If the washer is forgotten, it will be pushed onto a catch tray by the next washer that is pushed out, altering the assembler. Washers cannot be doubled because the device will not feed another washer until the assembly machine is turned on. Motion step method applied.
- An automated machine is used to cut grooves under the heads of certain bolts. Sometimes a machine error results in bolts that do not have the grooves cut. Ungrooved bolts sent to next process might damage the machine or tool. To prevent this, the chute through which the bolts leave the machine is modified with a pair of plates that act as a passive poka-yoke device. If the bolt is properly processed it passes through, but it gets stopped if the groove is not cut properly. Which triggers an alarm so that the operator can pull out the unprocessed piece. This is using contact method.
- The strength of a solder joint on a lighting fixture part is tested afterward by pulling on the lead wires. This test is not reliable and different people have different pulling strength so this does not have any consistency in the process. Sometimes even they may forget to do the test. Both the situations allow weakly soldered parts to go to the next process. A jig is designed which hold the part in place using springs that press steel balls firmly against the part. Testing is accomplished simply by pulling on the lead wires to remove the part from the jig. This prevents both forgetting the test and provide consistency in the pulling strength also. This is using contact method.
- The process involves cutting oil grooves in a particular product. Due to tool changes and damaged bits, products without grooves are missed sometimes in inspection and are sent to the customer who returns them with a compliant. A device is installed at the next process to test for the presence of the oil grooves. The device uses two springs-loaded pins that are linked above by a simple interlocking circuit mechanism. One pin is set for the height of the ungrooved area of the product and the other pin is set for the height of the grooved area. When the groove is present, both pins move together and the equipment keeps moving. When no groove is present, the groove-detecting pin is pushed up, making a gap in the interlocking circuit and shutting down the machine. This is using contact method.
- A packaging machine puts together corrugated cases in which products are shipped. The cases are fastened with heavy duty staples on the top and the bottom flaps. Sometimes the machine runs out of the staple or jams and omits staples on the package. It is easy to inspect the top flaps but very difficult on the bottom side, where they are needed most. To prevent a package not stapled completely from being transferred to the next process, a metal detecting sensor is placed at the exit of the conveyor. This sensor is equipped with a counter that detects the number of staples passing through it. This sensor is attached to a stopper which goes up when the desired number of staples are not counted. This device uses fixed value method.
- A particular assembly operation on a washing machine involves tightening two kinds of screws, using two different air powered drivers that hang from the ceiling. Sometimes people forget to use the second air driver which means products move to the next process without being properly tightened. To prevent this the cords for the two air drivers are now linked by a horizontal rod with a heavy metal ring threaded on it. When the driver A is pulled down and used, the ring slides to one end of the rod. When the assembler pulls down driver B, the ring slides to the other end and hits a limit switch to signal that the operation is complete. The system uses a thinner; if the limit switch is not hit within the usual cycle, a warning sound is made. This device is using motion-step method.