Warning: Disconnect all electrical power to the comparator before you begin.
Remove the optical comparator screen to allow access to the mirror. To do this, insert a hex wrench or other object into the center of the brass wheel at the top of the screen and lift the wheel up above the screen edge. While holding up on the wheel press in on the bottom of the optical comparator screen. This will allow the top to move out past the brass wheel. You can now gently release the brass wheel spring pressure and lift out the screen.
Lift out the three black hole covers in the rear of the comparator. This will allow you to access the optical comparator mirror mounting screws. Wrap tissue paper around the 12” mirror so it does not get damaged while removing it. Hold the mirror inside the optical comparator while removing the three screws in the rear of the machine (two people are recommended for this procedure). The mirror is glued to a mirror frame. There are three springs between the mirror frame and the mirror supports inside the machine. Each of the mounting screws runs through a spring and into the mirror frame. When you remove the screws and the mirror, the springs will fall out. You should remove the screws and springs one at a time to prevent them from falling and damaging the lower mirror. You should protect the lower mirror from damage by wrapping the lower mirror with tissue paper and placing a sheet of cardboard in the machine between the two mirrors.
Installing the new optical comparator mirror
Hold a spring against the back of the mirror frame. Hold the mirror in place inside the machine while someone starts the screw from behind the comparator. It might be necessary to compress the spring slightly by hand to get the screw to start in the threads of the mirror frame. Start with the top screw then do the same for the bottom two. After starting all of the screws, tighten the screws evenly using a plastic gage (provided) or gage blocks (approximately .570”). Insert the gage between the mirror frame and the mirror mounting rails inside the optical comparator. Insert the gage near each of the springs. With the gage in place tighten each of the screws so that some resistance can be felt when the gage is removed. You want to get the mirror even on all three points. This will provide you with a good starting point when re-setting the magnification.
Install the screen by first setting it in the screen drive wheel (bottom right) while pressing back the small encoder wheel (lower right) with the edge of the screen. Set the bottom left of the screen in the left lower screen roller. Then while holding the screen close to the top brass roller lift the roller as you did for removal and push the screen into place. Let the roller down gently. Making sure the edge of the glass is seated in the roller groove. You can press in on the bottom of the screen to move the top of the screen out. This will help you when setting the roller down on the edge of the screen.
Caution: If the screen is not seated in the brass roller groove the screen might break when it is rotated and the roller snaps into place.
Note: Changing the optical comparator mirror will affect the magnification and the position of the X-axis stage horizon shown on the screen.
You should now be able to regain the correct magnification and horizon position by only adjusting the one mirror that you replaced. Set the horizontal line of the optical comparator screen to its proper position using the vernier scale and screen protractor. With the screen in the proper orientation bring the standard into correct magnification by adjusting the three screws in the rear of the machine. When resetting the magnification be sure to bring the horizon into its correct orientation on the screen. Adjusting the lower screws of the 12” mirror unevenly will affect the position of the X-axis horizon relative to the optical comparator screen.
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