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Indicating bore gages come in two basic varieties: adjustable-capacity gages with interchangeable contacts or extensions and fixed-size gages with plug-type bodies. While indicating plug gages can measure closer tolerances with higher repeatability than adjustable ones, these are only two of several factors to consider when selecting a bore gage. A wrong decision can mean unnecessary expense, low throughput and even inaccurate data.
There is still a place in many shops for adjustable bore gages. Where tolerances are medium to broad or production runs are low or involve many different bore sizes to be measured, adjustable gages can be a bargain. Their range is typically two to three times greater than that of plug gages (0.010 inch vs. 0.003 inch to 0.006 inch), so they are more practical to use with broader tolerances. Because an adjustable gage can measure a range of hole sizes, some shops can get away with just three units, with capacities of 0.500 to 1.00 inch, 1 to 2 inches and 2 to 8 inches. With indicating plug gages, on the other hand, a separate size plug is required for every different bore size to be measured.
The two types of gages are comparable in price (generally $500 to $600), but for a broad-tolerance operation, the smaller number of adjustable gages required will create a substantial savings. When you figure in the cost of masters, the savings can be multiplied.
For large ID applications, adjustable bore gages are again the economical choice. Over about 4.5 inches, most plug-type gages are "specials" and, consequently, expensive (likewise for masters). Adjustable gages and masters are available from stock with capacities up to 24 inches.
The greatest benefit of fixed-size plug gages is the elimination of "rocking" to center the gage in the bore. The self-centering plug gage virtually eliminates operator influences and requires very little training. Rocking an adjustable gage is a refined skill that must be learned and performed conscientiously. A poorly trained operator, or one who is tired or hurried, is likely to produce incorrect measurements. Adjustable gages are also more subject to intentional operator influences, also known as the "close-enough syndrome."
The elimination of rocking speeds measurement taking; mastering is likewise simplified and accelerated. In any production run where volumes are high and/or tolerances are tight, plug gages create time savings that amortize the higher price.
I am not endorsing indicating plug gages over adjustable ones, but they do offer more benefits overall. Plug gages have larger bearing surfaces that make them less subject to wear. They are capable of better repeatability and discrimination, and they are the logical choice for use with an electronic data collection system. It is nearly impossible to hold an adjustable gage steady on the true diameter of the bore and, at the same time, push a button to record the reading. Use adjustable bore gages where production runs are low and/or tolerances are medium to broad. Use self-centering plug-type gages where quantities are high and/or tolerances are medium to tight.
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