There is no requirement in the gage standards for the Taperlock shank to be concentric with the applied gage. To state it more clearly; if the Taperlock shank is concentric it occurred by pure chance.
Basically,the plug gage blank is made in a screw machine. The basic details and shape of the blank are machined and the blank is parted off the rod. After it is parted off the rod, centers are machined in the ends of the blank. Because the addition of the centers is accomplished in a separate operation, the centers are not concentric, or on-center, with the other machined features of the gage blank. They are close to on-center, but can still be significantly divergent from the existing blank's machined details. Gage makers do not try to fix this off-center condition because if they did the Taperlock shank would be too small to fit into the standard Taperlock gage handle.
When it is time for the blank to be made into a gage, the gage maker mounts the blank in the grinding machine on the centers. The Taperlock shank is where the drive dog is attached. Once the machine is ready, all the grinding will be concentric with the centers. The Taperlock shank is not touched by the grinding machine and will remain off center.
The Taperlock shank could be made concentric to the gage, but this would add many dollars to the cost of the gage for the minimal benefit received by the investment. Thus it is never done; except on special request, and then for a special price.
Below is a video which was made by a gage maker. He has taken a raw Taperlock gage blank, and mounted it on centers in his grinding machine with the drive dog attached to the gage surface. He has used black marker and colored the Taperlock surface. As he approaches the Taperlock surface with his grinding wheel you can see how the black is removed illustrating that the Taperlock surface is off-center.
Original Posted: 11/8/2011
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