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The "center gages" are not really used to check completed threads to be in spec.They're used for single-point threading on a lathe, and have a few uses. One is to assure the tool you are using is of the correct shape. (Back in the day we would grind our tool bit from a blank to fit this gage....not you simply and inexpensively purchase the insert.)Another of it's uses is to assure that the tool bit is at the correct angle to the work (material being threaded). The work will be chucked in the lathe, gage will be placed against the side of the work, and the tool bit is brought in until it mates perfectly to the v-notch on the gage, tool holder is then locked in place, begin threading process.Yes, certain thread pitches can quickly be compared on the scale, but probably most machinist prefer to use a thread gage to check pitch or TPI (threads per inch).