Abstract: An apparatus for performing various machining operations on the interior surface areas of a large steam valve or the like which includes a value seat ring with a valve seat thereon secured to a valve seat ring step interiorly of the valve body. The apparatus includes a support plate fixedly secured to the open end of the valve body, a drive tube rotatably and axially supported from the support plate, a spider assembly mounted on the inner end of the drive tube and including radially expandable and retractable jaws or centering members for engaging the interior of the valve body to stabilize and support the rotatable drive tube and orient the drive tube concentric with the valve seat or other interior surface areas to be machined. Axial movement of the drive tube is obtained by a lockable rack gear and pinion gear assembly while rotation of the drive tube is accomplished by a worm gear drive arrangement mounted on the support plate and engaged with a bushing connected to the drive tube.