Q: When inspecting diameters with tolerances of .0005 and below, are there any studies relating to the accuracy of different inspection methods, such as a coordinate measuring machine (CMM) versus a digital bore gage with setting ring combination?
A: The answer to this question can often be one of opinion and/or personal preference. What I will present are my opinions, along with some known facts.
Non-contact measurement systems such as optical and laser equipment are bulky, expensive and impractical. With these systems, the part must be taken to the system. This is not much good in a production environment.
While a CMM is without a doubt very accurate, they are also slow. Like the optical or laser equipment, the parts must be taken to the system. In many production situations it is more practical to check the part in the machine. Also, even though CMMs come with reticulated heads, measuring at abstract angles or various depths is not always an option. It is also wise to keep in mind that deeper bores would require longer stylus probes. This is a situation that can introduce concerns of error and rapid movement can generate false contact readings with longer styli simply due to the motion.
A final thing to keep in mind is the high initial price of a CMM, as well as the maintenance costs.
Two and three point contact measurement is readily available. Popular digital bore gages are calibrated to a master ring. The rings themselves can be verified with a CMM or sent out for certification traceable to national standards. Most digital bore gages can be set up to interface with a statistical process control system. This is important when process control is vital.
Cylinder bore gages (generally two point contact) can sometimes have problems with linear accuracy. Analog versions can be more prone to operator error.
While two point systems will more readily detect ovality, where this is not a major concern, three point digital systems are, in this quality technician’s opinion, the best all-around option.
When I am inspecting parts in which ovality could be an issue, if the parts are readily portable, I will check a percentage with a CMM to verify their roundness. However, for speed, accuracy, practicality, and price, a three point digital bore gage would be the way I would go to verify product with tight tolerances.
A final note: If parts are relatively small and can be in contact with other materials, robotics is often used with air gage instruments. This is another expense but can be introduced in high volume manufacturing.
I hope this will help.
ASQ Senior Member, CQT,CQI
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