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by Tom Groff , Optical Gaging Products (OGP).

Shop Technology Magazine discusses the merits of off-line measurement with Optical Gaging Products' Tom Goff, and how manufacturers can benefit from the different systems available on the market today.

Q: Many machine shops use in-line measurement systems to monitor specific dimensions, or use hard gauge set-ups to do complete part layouts. How can an off-line measurement system benefit such shops?

A: Off-line measurement systems in general offer better accuracy, repeatability and process stability compared to in-line systems. The systems are designed for precision measurement and have all the features of a stable and repeatable gauge designed-in.    

Systems such as video coordinate measuring systems are also very versatile. A single system can replace many, many hard gage set-ups and free up the time of the highly skilled layout personnel for other tasks.   A multisensor CMM system can do even more, since calibrated datapoints can be gathered using an array of contact and non-contact sensors and analyzed in common. The name of the game is confirming that parts are made to print. A good multisensor CMM can do that faster, smarter and more comprehensively than a simple in-line measurement system, or hard gage set-ups.

Q: What are the key considerations a machine shop should take into account if it is considering an automated off-line measurement system?

A: Three things top the list: First of all, the system must gather datapoints with uncertainty that is acceptable for the tolerances of the parts being measured. It’s not helpful if 80 per cent of the error budget is used by the measuring system. Second, the system must have the measurement envelope and capabilities to make the required measurements. Third, the measuring system’s software must be robust and sufficiently easy to use that the productivity gains offered by such a system are realized.  An automated measuring system is of little value if it isn’t being used because it’s difficult or inconvenient to program.

Q: Is it critical to have both in-line and off-line or can a manufacturer choose only one?

A: The choice depends on the nature of the processes being controlled. First article inspections for process qualification need to be comprehensive. Typically, in-line measurement systems are not set-up to measure all dimensions, only a few critical ones, so some more complete method of confirming the process qualification is required. Most OEMs rely on a combination of parts made in-house and ones made by outside suppliers. For these cases, some method of confirming the conformance of the supplied parts is required, and it’s helpful when that data can be correlated to that of parts made in house. 


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OGP's micro probe used in a multi-sensor measuring system.

Q: What cost considerations are involved in setting up an off-line measurement system?

A: The key cost drivers are the capital cost and depreciation of the equipment itself, and its installation. Oftentimes, facilities upgrades are needed to provide the required environment for a high precision CMM system. Other types of CMMs are more robust and suited to factory floor installation. The costs of training, fixturing, software and calibration can also be significant. All these factors should be considered in the ROI model. The most important cost consideration is this:  how many good parts are being thrown away because the metrology to determine their conformance is lacking?      

Q: How have technologies for off-line measurement changed in recent years?

A: There are a number of key trends. One is the advent of multi-sensor CMM systems that combine optics, tactile probes, lasers and other data acquisition tools in a single CMM with software that enables these sensors to be used interchangeably and randomly. A multi-sensor system can make more measurements more quickly and more accurately because the ideal sensor can be deployed for each specific measurement. Another key trend is the use of Geometric Dimensioning and Tolerancing (GD&T) as opposed to the older style of +/- or "square” tolerance zones on most machined parts. To properly apply the ASME Y.14.5 standards for GD&T requires sophisticated measurement and analysis software. Many CMM softwares do not handle all the requirements of the Y.14.5 standard, or handles them superficially, making it difficult to compare results from one measurement system to another. Software such as Smartprofile from Kotem that does comply fully with Y.14.5 can be used with any type of CMM system. 

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OGP's discrete laser sensor used in multi-sensor measuring systems.

Q: Are there new off-line systems that make such a system more efficient and increase productivity flow?

A: The greatest productivity boost comes from the multi-sensor systems that enable one CMM to perform many different kinds of measurements on many types of parts. A single multi-sensor system can do the work of multiple video, tactile and laser systems. With a single system, the users only need to be trained to use one metrology software, which is a further productivity boost. Part fixturing that automatically indexes or flips parts can be used to minimize load/un-load time and speed processing, as can tools such as bar-code readers or OCR systems that identify parts and run the measurement routine automatically. Robust systems can be used to automatically load and un-load pallets of parts from off-line systems, giving them near on-line speeds with the benefits of accuracy and gauge R&R that are unique to stand-alone measurement systems.

Tom Groff is North American sales manager, Optical Gaging Products (OGP).

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