Alignment Fixture

This is a tool to align wheels that support a 36 foot diameter observatory dome. It is the dome for the Gueymard .9 meter telescope at Houston Museum of Natural Science's George Observatory in Needville Texas. This heavy steel dome is supported by twenty 8" diameter by 2-1/2" wide urethane tired cast iron wheels. To minimize rotating resistance of the dome, these wheels should be accurately aligned tangent to the circle of the dome. This is done the easiest by having all the support wheel axles point within a couple inches of the dome physical center.

This tool consists of a fixture that mounts to the wheel and a removable laser to point toward a target at the center of the dome. The laser is removable to protect it from bumps or jars while the fixture is being mounted. The laser is calibrated in a lathe to inscibe a circle less than 1/2" in diameter at 18 feet while being rotated through 360 degrees.

The fixture is located on the cast iron wheel rim by a combination of rollpins and setscrews. Three 1/4"-28 setscrews are adjusted to protrude on the inside of the fixture by .093" +-.001". These three setscrews are locked in place with an 8-32 setscrew and a plastic insert to prevent marring the threads. These three points provide a plane that locates against the side of the cast iron wheel rim. Two 1/4" rollpins protrude 1/4" and hook against the inner rim of the wheel. These two locater pins help hold the fixture so the three setscrews are properly located on the side of the wheel rim. The fixture is then held in place against the back of the wheel rim by a clamp screw on the front of the fixture. Once the fixture is mounted properly on the wheel, the laser can be installed and the wheel alignment checked.