I was just asked what is the difference between absolute and realative accuracies, and I didn't have an answer. I was hoping someone here could give a good definition of each. Thanks in advance........
On our On-Line SPC system, we are having facility to calculate Tool Offset & send it to CNC controller for Auto correction. Will this feature eliminates my In-Process Gage on Grinding Machine? What are the Pro's & Cons of this feature vis-a-vis In-Process Gage?
Closed Loop systems are the highest order of process control. The only real downside is knowing and understanding what the algorithm is the system uses for calculating the control limits and how the system handles special causes (does it just reset more?) If you are doing precision machining, and all sources of variation are negligible except tool wear, then the control limits need to be based on the rectangular distribution - not the normal distribution. This distribution allows for wider control limits, which minimizes the offsetting and therefore reduces overcontrol. It also allows wider control limits for acceptable Cpk versus the normal curve calculations.
Many grinding machines have "jump on" gauges that monitor the parts while they are being ground. These are relatively simple gauges that typically are measuring single features such as a diameter or face location. Sometimes they can store max and min information to allow the machines to stop in the event predetermined thresholds are exceeded. The signals from such gauges are used to control machine functions such as feedrate changes, spark out, end to end taper compensation, etc. If your in-process gauges are this type, they do not typically use spc, as they are a closed loop control system used to control machine function for each part. They are not appropriate for verifying product conformance to specifications or for process control, due to the limited information they provide. The target points of these gauges can be adjusted to agree with your offline gauges that are used to monitor the part quality. You should not eliminate the offline gauges used for process control.
If your spc gauge is a post process gauge that feeds back to the machine tool after the part is machined, and it is collecting data that will be used to compensate the tool to offset tool wear, it is important to select the proper algorithms. If there is piece to piece or common cause variation in the process, you may need to set it up to compensate using a 5 part moving average. This will allow the gauge to monitor the trend or slope of the tool wear, and compensate when it reaches a specified limit. If a single part is used, you will overcompensate making your performance worse. Special causes can also be problematic, many of the suppliers of such equipment have rules that can be used to stop the machine if undesired patterns show up (they assume that if a large change occurs unexpectedly, you should investigate the cause, perhaps a tool has chipped or broken) One of the most experienced companies that does this type of work is Marposs.
Another type of gauge that is becoming popular is gauge probes included in CNC machines as a part of the complement of tools. These can be used to establish datums, provide machining offsets, and detect tool problems. Be careful how these are used, as any correction made from a single measurement runs the risk of overcontrolling the process and increasing variation.
This is one of the most difficult things to justify due to todays management focus on tangible return on investment, usually with very high TARR requirements. Many of the benefits of SPC are not easily quantified, as they could include increased customer satsfaction, reduced warranty cost, reduced scrap rates, improved facility utilization, etc. Many of these metrics cannot be specifically linked 100% to the use of spc in a given process, although if used they will all improve. If your current operation does not use spc at all and is relying on lot control or 100% inspection, there may also be some labor or facility cost savings as well by removing some inspection requirements.
The need for an online system also depends on the scope and complexity of your operations. Automation of spc does not insure success, the real key is setting up the correct sampling plan for the proper process characteristics and reacting to the signals that spc will give you. This can be done just as effectively with manual charting for smaller operations.
Fundamentally, this is one area that should be driven by your companies' quality strategy. In order to be able to continually improve your processes and products, you must have the ability to measure the targeting, variability, and stability of your processes. This is what SPC allows you to do. If you cannot do this, it will be difficult to compete with those who do.
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vidyadhar 2/6/2004
On our On-Line SPC system, we are having facility to calculate Tool Offset & send it to CNC controller for Auto correction. Will this feature eliminates my In-Process Gage on Grinding Machine? What are the Pro's & Cons of this feature vis-a-vis In-Process Gage?
bobdoering 2/7/2004
Closed Loop systems are the highest order of process control. The only real downside is knowing and understanding what the algorithm is the system uses for calculating the control limits and how the system handles special causes (does it just reset more?) If you are doing precision machining, and all sources of variation are negligible except tool wear, then the control limits need to be based on the rectangular distribution - not the normal distribution. This distribution allows for wider control limits, which minimizes the offsetting and therefore reduces overcontrol. It also allows wider control limits for acceptable Cpk versus the normal curve calculations.
-Bob Doering
mkomarmy 2/19/2004
Many grinding machines have "jump on" gauges that monitor the parts while they are being ground. These are relatively simple gauges that typically are measuring single features such as a diameter or face location. Sometimes they can store max and min information to allow the machines to stop in the event predetermined thresholds are exceeded. The signals from such gauges are used to control machine functions such as feedrate changes, spark out, end to end taper compensation, etc. If your in-process gauges are this type, they do not typically use spc, as they are a closed loop control system used to control machine function for each part. They are not appropriate for verifying product conformance to specifications or for process control, due to the limited information they provide. The target points of these gauges can be adjusted to agree with your offline gauges that are used to monitor the part quality. You should not eliminate the offline gauges used for process control.
If your spc gauge is a post process gauge that feeds back to the machine tool after the part is machined, and it is collecting data that will be used to compensate the tool to offset tool wear, it is important to select the proper algorithms. If there is piece to piece or common cause variation in the process, you may need to set it up to compensate using a 5 part moving average. This will allow the gauge to monitor the trend or slope of the tool wear, and compensate when it reaches a specified limit. If a single part is used, you will overcompensate making your performance worse. Special causes can also be problematic, many of the suppliers of such equipment have rules that can be used to stop the machine if undesired patterns show up (they assume that if a large change occurs unexpectedly, you should investigate the cause, perhaps a tool has chipped or broken) One of the most experienced companies that does this type of work is Marposs.
Another type of gauge that is becoming popular is gauge probes included in CNC machines as a part of the complement of tools. These can be used to establish datums, provide machining offsets, and detect tool problems. Be careful how these are used, as any correction made from a single measurement runs the risk of overcontrolling the process and increasing variation.
mkomarmy 1/29/2004
This is one of the most difficult things to justify due to todays management focus on tangible return on investment, usually with very high TARR requirements. Many of the benefits of SPC are not easily quantified, as they could include increased customer satsfaction, reduced warranty cost, reduced scrap rates, improved facility utilization, etc. Many of these metrics cannot be specifically linked 100% to the use of spc in a given process, although if used they will all improve. If your current operation does not use spc at all and is relying on lot control or 100% inspection, there may also be some labor or facility cost savings as well by removing some inspection requirements.
The need for an online system also depends on the scope and complexity of your operations. Automation of spc does not insure success, the real key is setting up the correct sampling plan for the proper process characteristics and reacting to the signals that spc will give you. This can be done just as effectively with manual charting for smaller operations.
Fundamentally, this is one area that should be driven by your companies' quality strategy. In order to be able to continually improve your processes and products, you must have the ability to measure the targeting, variability, and stability of your processes. This is what SPC allows you to do. If you cannot do this, it will be difficult to compete with those who do.