Single point measurements made by CMM must be highly accurate and repeatable in order for the very limited measurements made by them to be useful in determining conformance of an entire surface to specification. But there are other ways to achieve this same result, using advanced methods and taking advantage of the computer’s ability to quickly analyze large amounts of data.
Traditionally, CMM measurements on a few points are used to gage a part. For example, gear teeth for engine drive trains, are typically analyzed by using only 45 points to completely determine the entire surface of a gear tooth. These are single point measurements and each point takes time to assure the machine is making a stable and accurate measurement. Alternatively, making many measurements in a single, small region can result mathematically in similar accuracy and repeatability. So by taking a large number of such samplings, a surface can be very well determined without the need for the mechanical complexities of the highly accurate CMM. Moreover, lots of data can provide small spacing in measurement position, thereby giving much better lateral resolution over an entire surface. How does this work?
CMM manufacturers claim 2 micron accuracy/precision. Some users of CMMs need only single point measurements and so CMMs are ideally suited to thir needs. Others, however, believe they need this accuracy and precision because of the very limited number of points they gather to attempt forming a faithful and/or (more importantly) repeatable representation of a three dimensional surface. This includes gear tooth faces, artificial knees and other such productss where surface shape is important. Even such high accuracy cannot guarantee that entire surfaces are within tolerance, but only that certain points on a surface meet specifications. Moreover, the process is slow, sometimes requiring parts to be sent out for inspection.
The scanning LVR CMM-1 provides 1,000 to 100,000 points in just a few seconds over an entire surface. In low resolution mode, viewing the raw data, adjacent points appear to vary significantly. But these variations are of known origin. Some points are just above the true surface representation, and some below. But the sheer number of points is equivalent to each small area being sampled hundreds of times, and this, in turn, constrains the “average” surface to much tighter limits than any small group of points. This means that a fast scan can provide results equivalent to micron level accuracy for surfaces.
Simply put, a point contact Coordinate Measuring Machine must measure each point very accurately because so few points are measured. By contrast, the scanning LVR CMM-1 measures a sufficient number of points that even in low resolution mode, a fast scan can give very accurate surface measurements by producing a surface that is a best fit to all the points. Alternatively, in high resolution mode, the LVR CMM-1 can slowly scan a part, and produce better than 10 micron resolution for many applications. And, finally, LVR encourages its use in a production environments, where immediate results can save money.
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