dlane you connect your dial test indicator in that gadget then chuck it in your mill to dial things in on center, tram a mill, etc. It telescopes in/out for different diameters. I have tried a number of different types of indicator holders, the ones that clamp on the outside of the spindle for example, this is the only one that's rigid, adjusts easy, etc. $73.84 on sale at MSC. https://www.mscdirect.com/product/details/03994746?rItem=03994746