As with any measurement, the quality of the result depends on the measurement instrument and the care with which the operator handles the measurement procedure. Many gages are designed to make this as easy as possible. A snap gage, for example, has the reference anvil, frame and measuring instrument built in. The same is true with a bench stand. To obtain a good measurement, all the operator really has to do is correctly apply the gage to the part.
With an electronic height gage, this is not the case. Electronic height gages are not as self-contained as other gages. They have the precision scale and the sensing probe but no integral reference, which is the most critical part. Most electronic height gages are used on a granite surface plate, and the plate provides the reference for both the height gage and the part that is being measured. The quality of this base plate directly influences the measuring result. Thus, it’s important to keep the plate free from dust, chips and dirt.
Most height gages are direct-reading instruments that generally operate in ranges up to 36 inches. As such, they are especially susceptible to variations in temperature. Because the body heat of the operator (98.6°F) is clearly above room temperature (68°F), any heat conveyed to elements of the measuring circuit (base plate, test piece, height measuring instrument, stylus) can cause local heat expansion and induce measurement errors. Operators should be very careful in observing the following rules:
- Avoid touching the test piece with your bare hands directly before taking the measurement. Use gloves.
- Do not touch other elements of the measuring circuit.
- Only touch the height gage at points provided for this purpose; handles are usually provided to move the gage or engage the air bearings for positioning.
- Avoid drafts.
- Avoid direct sunlight on the instrument, test piece or base plate.
- Don’t set up the measuring station in proximity to radiators or in the path of air ducts.
- Do not check test pieces that were transported through very hot/cold rooms shortly before measurement.
- For high precision measurements, put the test piece on the base plate and let it adjust to ambient temperature (this takes approximately 15 minutes to 8 hours, depending on the size of the part).
Once the measuring loop is verified, there are two other 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 when the gage is turned on. A manually driven gage must be zeroed on the granite plate before it can be used. With a motor driven unit, the gage will automatically move down to touch the surface to set its reference point. It’s not a bad practice to initiate this zeroing routine a second time to make sure that no dirt or other anomaly has introduced an incorrect reference. Because setting this reference is critical to all the measurements you will make, it’s worth the time and effort.
The other important reference is the correction for the 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.
Ball diameter is specified for the probe, but there is always some degree of variation. Actual ball diameter should be added to any dimension that is probed in the upward direction. On height gages that have even the most basic electronic control, this dimension 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, or the test can be simulated with gage blocks. The fixture 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 uses this as the ball diameter.
The same gage block check can be done by hand on purely manual machines, or the ball diameter may be measured off-line with a micrometer. Just as with setting the zero reference, this check should be repeated a number of times. A lot of gages will provide this repeat check automatically and reject the ball diameter reference if it does not repeat to within a preset limit.
Failing to recheck for ball diameter can be a deadly pitfall when a probe tip is changed. Going from a 10- to 5-mm ball tip would be disastrous if not recalculated.