HEIGHT GAUGES FROM THE TOP
HEIGHT GAUGES FROM THE TOP
The height gauge is a conceptual extension of the handheld caliper gauge, except that it rests on a heavy base that keeps the scale square to the surface. Originally, height gauges had a bevelled pointer on the moveable jaw that used to either mark or scribe the part when doing layout work, or to find a height characteristic on a part and display it on the gauge’s readout.
Today height gauges are not only designed to do what their name implies – measure height – but also diameters, distances and even bolt circle patterns. Therefore, instead of a beveled pointer, a family of interchangeable contact points is available with a vast array of diameters and shapes and even offsets to get into virtually any characteristic on a part.
Once you have the height gauge set in place there are two critical references which needs to be established. The first is the zero-reference for the measuring system with automated height gauges. Done automatically whenever the gauge is on; the gauge will automatically move down to touch the surface to set its reference point. It is a recommended practice to initiate this zeroing routine a second time, just to make sure that no dirt or other anomaly has introduced an incorrect reference.
The other important reference is the correction for probe ball diameter. If a height gauge is use only for length measurements taken with the probe moving down, then probe diameter is not important. The contact point of the probe will be the same as in zeroing. However, if groove’s diameters or whole locations is being measure, or if any measurements taken with the probe moving upwards, then the probe ball diameter must be known and taken into account.
On height gauges that have even the most basic electronic control, probe diameter measure as part of a setup routine and is automatically included in all measurements. The automated process uses a fixture provided with the gauge that sets up a plane that measure by the gauge from both directions. The gauge then looks at the difference between the two measurements and calculates this as the ball diameter.
Failing to recheck for ball diameter when a probe tip is changed can be a deadly pitfall. Going from a 10-mm to a 5-mm ball tip would be disastrous if not recalculated.
MAKING A MEASUREMENT:
With the new motorized digital versions, measurement is an easy keystroke function. When ready to make a measurement, say a height in reference to the zero point, slide the measuring carriage up over the part and press the height measurement button, approaching from the top. The motorized drive will bring the contact to the surface and the measurement is completed and displayed.
However, with the modern height gauge this only begins the measuring capabilities. As measurements made they are stored, and from the measurement data, heights, midpoints, diameters and relationships are only a keystroke away.
SOURCES OF ERROR:
Regardless of type, all height gauges have a similar inherent problem: they measure height. In addition, the larger the height gauge, the bigger the potential problem. The problem is not the actual height. It is the relationship of the height to the base.
A major error in the design of a basic height gauge is taking a design that meant to measure 12″ and simply extending the post to measure 36″, without changing the base design or the cross-area of the measuring post. What happens that particular gauge will tend to wobble and flex. Although you may not be able to see the 0.001″ wobble, it can become a significant part of the part tolerance and certainly influence the measurement.
Beefing up the column to reduce the flexture of the post is only a partial improvement; as such, a gauge may still tend to be top heavy. What needs is; to make the base longer, wider and to build in some mass. Decreasing the ratio of the post to the base will significantly improve performance.
Since the height gauge is use with a surface plate, it is only as good as the plate, which provides the reference for the part and the gauge. Many surface plates are clean and well maintained, but others may not be as clean as they look. A small imperfection, metal chip or even a hair, while almost impossible to see, could throw off the measurement by 0.020″ at a height of only 10″.
Some height gauge models also incorporate a quick-adjusting release that allows the move-able measuring point to move directly to the location of the check before the measuring system takes over.
George Schuetz, Mahr.