Dial Bore Gage Setting and Usage
Master Rings or Parallel Flats?
A dial bore gage must be set to a reference master with a known dimension. Ideally, this is the dimension of your work piece to be inspected. The master may be a setting ring or two flat surfaces which are accurately spaced to the given dimension.
Setting a dial bore gage to a master setting ring is the preferred method because the ring duplicates the geometry of the workpiece being checked. It is the fastest and most economical method for high-production gaging. It is also the most accurate, especially for bore sizes under one inch, or bore sizes that require higher accuracy. Master rings also have the advantage of being more directly traceable to NIST. Master rings can be gotten with the exact dimensions of your test piece.
Parallel Flat Surfaces
Using flat parallel surfaces is a practice generally associated with dial bore gaging of larger tolerances, or on short-run job-shop applications that do not justify the cost of a special master ring gage. Mastering to flat parallel surfaces requires considerable operator skill in establishing accurate gaging contact in two planes at once (up and down, and left to right). A micrometer with flat and parallel anvils can be used for this purpose.
Errors when the gaging contact is worn
Any dial bore gage which is set between parallel flat surfaces (such as gage blocks, bore gage setting master, or micrometer spindles) must have rounded gaging contacts. They can not have a worn, flat surface. Any flats on the gaging contact will cause setting errors. Take a close look at this illustration (courtesy of Dorsey Metrology):
The error is rougly equivalent to twice the height of the chordal segment that is produced by the radius of the bore and the width of the flat on the gaging surface.
If the bore gage is set using a master ring of the same diameter as the test piece, then there will be no error.
The use of maximum wear-resistance material substantially reduces the possibility of worn contact point errors. Tungsten carbide or diamond tipped contact points are examples of the materials that should be used.