Geometry Gaging Tips
We have all run across cases where matching parts that measure okay don't seem to quite fit together. This often happens with ID and OD parts that tend to have very tight clearances, typically within
It is generally understood that the results of precision measurements – such as from a form measuring instrument –
Like any precision measurement, the goal of Circular Form metrology is to provide a measured value that is as accurate as possible.
Roundness is far from the only circular geometry specification that machinists may be required to meet and, therefore, to inspect.
Most gages are accurate as delivered from the manufacturer, but every gage embraces certain limitations and assumptions. When selecting a gage or a gaging method, it is essential to establish a clear objective.
We have introduced the subject of circular geometry gaging by looking at the instrumentation, and we noted that one reason for the recent proliferation of geometry gages is the use of personal computers as gage controllers.
With the implementation of touch screen operation, canned measurement routines and automatic ranging, form measuring machines have become nearly foolproof and have successfully migrated from the lab to the shop floor.
Last issue we looked at some of the setup and analysis issues that can influence the results of your form measurement, including: the way the parts are staged; the influence of the stylus tip; using the wrong filter; confusion over the look of the graph; measuring the wrong parameter; and the influence of poor part tilt/centering.
In previous columns we discussed the relationship between part geometry (i.e., roundness) and dimensional tolerance. Circular geometry gages, with their precision spindles, are the best and the standard method for measuring out-of-roundness.
Form or geometry gages are generally divided into two categories depending on what form parameters they are designed to measure. While there is no formal designation to differentiate these two categories, sometimes they are simply referred to as roundness gages or cylindricity gages. Their difference is primarily based on their ability to make self-referenced or datum-referenced measurements.
What do you do when you're doing everything right and it's still coming out all wrong?