A special-purpose grinding tool and fixtures have been developed for reworking an edge of a seal surface on a nozzle-throat-support housing in a rocket engine (see Figure 1) to remove defects caused by corrosion. Although the design of the tooling is specific to the original rocket-engine application, some aspects of the design may stimulate thinking about solutions to other special machining problems that cannot be solved readily by use of standard tooling alone.
According to the engineering specification that governs the rework of the seal surface, the radial depth of edge material removed must not exceed 0.032 in. (0.81 mm). Previously, the rework involved the manual filing of the edge. The filing procedure was difficult to control; the maximum allowable depth was sometimes exceeded, making it necessary to scrap the housing. The present tooling was developed to provide greater accuracy and reliability in reworking the seal surface and to prevent removal of edge material beyond the maximum allowable radial depth.
The tooling (see Figure 2) includes a base fixture equipped with indexing bearings that ride along datum surfaces of the throat-support housing. The base is secured to the housing by spring-loaded pincher bearings on the inside and a spring-loaded pivot bearing on the outside. An adjustable outrigger bearing adds stability and provides for alignment of the index bearings. Two precise linear shafts that have been press-fit into the base fixture serve as supports and guides for the grinding tool.
The grinding tool is an automotive valve-seat grinding stone on the shaft of a low-speed air motor. A cradle that is positioned between the two shafts and that rides along the shafts on bushing bearings holds the motor at the angle needed to enforce the correct alignment of the grinding stone with the surface of the workpiece.
A micrometer on the base fixture is used to adjust the position of a hard stop that limits the depth of grinding; this adjustment is performed before grinding. During grinding, a technician applies gentle, radially outward force to the cradle while moving the base fixture circumferentially to make a smooth transition to the workpiece surface adjacent to the defect to be removed.
In addition to providing better control over the material-removal process, this tooling saves time and relieves technicians of the tedious and fatiguing filing task. Unlike in manual filing, the technician need not exert major force while stroking. For a typical housing, manual filing takes about ten hours. With the present grinding tool and fixtures, the process can be completed in less than one hour.
This work was done by Ronald B. Montgomery, M. Bryan Ream, and Brent A. Mecham of Thiokol Corp. for Marshall Space Flight Center.