What can take low-volume, precision inspection to the next level? For many years now, the method of choice for low-volume, general-purpose inspection has been surface plate work using test indicators and height gages. Recently, electronic height gages have made the layout inspection process a little more accurate. Speed of measurement also has improved, since electronic height gages give direct measurements and allow for storing data and programming repetitive measurement processes.
Are there even more advanced electronic height gages on the horizon that will allow us to achieve greater precision while still maintaining our ability to do a wide variety of measurement tasks? Probably not. The problem is not the height gage, but the reference surface it rests on. No matter how precisely we build the height gage, the accuracy of the measurement still depends on the flatness of the surface plate. Surface plates probably cannot measure up to the increasing demands for higher tolerance measurements.
So where do we go from here? Consider the universal measurement machine or universal length gage as ways to perform a wide variety of measurements and inspections with speed and improved accuracy. While there may be no all-purpose machine, a universal length gage can do quite a lot. Applications include internal and external measurements of diameters and lengths; internal and external thread measurements; calibration inspection of mechanical indicators and gages; location of points, lines, holes and surfaces and internal and external tapers.
Universal measurement machines were developed to speed up the inspection process and reduce the potential for measurement error. They differ from typical comparative- style gaging because they have a much larger measuring range but still can obtain resolution and accuracy approaching some comparators. In order to achieve high-measurement performance, the machines have built-in reference standards of either glass scales or an interferometer system.
When equipped with various contact accessories, universal measurement machines can easily be used to check length, diameter, pitch diameter, roundness, straightness, parallelism and taper. They typically will measure parts from 5 to 40 inches long, but machines are available with even larger capacities.
These systems are frequently referred to as machines because they are built according to the same design criteria as machine tools. Critically important measuring head and tail stock slide bearings are mounted on a strong and rigid base. The reference system is mounted as close as possible to the machine’s line of measurement to avoid errors. In addition, various computer techniques are used to map and correct slide errors; average multiple measurements and compensate for temperature variation.
No, it’s not a surface plate and height gage, but a machine that is extremely fast, versatile and accurate. However, measurements made with the machine are only as good as the measurement process. Therefore, it is important to keep all the components of the process the same, for example, measuring the exact same location on the part. Machines do this with computer-aided systems that help set up a measurement process and then lead subsequent users through it.
The height gage and surface plate have been a mainstay for highly productive, low-part-volume, general-purpose measurements. The universal measuring machine incorporates the spirit of this process while relying on a highly accurate internal reference to improve measurement precision and repeatability.