GIMBAL FRAME - stainless type 303

Problem

Attempts to make this 12 pound, stainless steel type 303 gimbal frame as anything but an investment casting ended in failure. But just any investment casting wouldn't satisfy production requirements. The investment casting had to meet these requirements:

1. The application and mission of the part demanded consistent conformance to the x-ray and fluorescent penetrant standards.
2. Importance of the program required that shipping schedules once agreed upon, had to be met.
3. The casting had to meet drawing tolerances if numerical control equipment for finish machining operations was to be used successfully.

Solution

A hictol investment casting, checked 100% with the inspection fixture described below, was the answer, providing a happy ending to the problem.

This view shows the fixture with a casting in place. Two slots in the base of the fixture accurately locate each pair of legs and connecting bar. The casting legs must fit these slots without forcing and -

1. The casting drops down on three pins similar to the one visible, see #2, which support the casting and establish basic plane number 1.
2. One drive screw, see #10, moves the casting firmly against two fixed pins, see #4, which establish the second basic Plane.
3. A second drive screw, see #5, moves the casting against a third fixed pin, see #6, which establishes the third basic plane.
4. After the casting is locked in position, a removable scribe pin, see #7, is set into the top of the center post and scribes a circle representing the machined hole size. Insufficient stock and eccentricity, if present, are quickly spotted.
5. Next, the flush pins, see #8, are pressed against each side of the casting to establish accuracy of each side wall location. Tolerance for dimensions to the sides of the castings as well as any machining stock allowance is accounted for in the depth of the stop in the flush pins.
6. The sweep pointer, see #9, rotates about, and up and down, the center shaft. Inside clearance is checked by swinging the pointer around the shaft within its control range.
7. The final step is to check location of four tapped holes, rwo in each connecting bar at the bottom of the legs, see #10. The scribed lines show whether or not the tap will break thru the edge of the casting.

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