Metrology
Metrology
Height Gages from the Top
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Height Gages From the Top
George Schuetz, Mahr Inc.

 

 The height gage is a conceptual extension of the handheld caliper gage, except that it rests on a heavy base that keeps the scale square to the surface. Originally height gages had a beveled pointer on the moveable jaw that was used either to mark or scribe the part when doing layout work, or to find a height characteristic on a part and display it on the gage's readout. 

 Today height gages are not only designed to do what their name implies – measure height – but also diameters, distances and even bolt circle patterns. So 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.

Measurement Setup
 Once you have the height gage set in place, there are two critical references that need to be established. The first is the zero-reference for the measuring system. With automated height gages, this is done automatically whenever the gage is turned on; the gage will automatically move down to touch the surface to set its reference point. It's 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 gage is to be used 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. But, if grooves, diameters, or hole locations are being measured, or if any measurements are taken with the probe moving upwards, then the probe ball diameter must be known and taken into account.

 On height gages that have even the most basic electronic control, probe diameter can be measured as part of a setup routine and is automatically included in all measurements. The automated process uses a fixture provided with the gage that sets up a plane that is measured by the gage from both directions. The gage 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 gage this only begins the measuring capabilities. As measurements are 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 gages have a similar inherent problem: they measure height. And the larger the height gage, the bigger the potential problem. It's not the actual height that's the problem. It's the relationship of the height to the base. 

 A major error in the design of a basic height gage is taking a design that was 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 then is that the gage 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 flexure of the post is only a partial improvement, as such a gage may still tend to be top heavy. What's needed is to make the base longer and wider, and to build in some mass. Decreasing the ratio of the post to the base will significantly improve performance.

 Since the height gage is used with a surface plate, it is only as good as the plate, which provides the reference for the part and the gage. 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 gage models also incorporate a quick-adjusting release that allows the moveable measuring point to be moved directly to the location of the check before the measuring system takes over.