Optical Comparators

To check the magnification you must be sure the optical comparator stage is squared up. To do this you can place a banking fixture in one of the stage vees and bank a square against it. Mount a magnetic base dial indicator (.0001”) to the face of the optical comparator near the chart ring and set up the indicator to run along the face of the square that is parallel to the focus travel. Move the focus in and out. If the indicator does not read 0 loosen the two helix clamps under the table and helix the table until the indicator reads 0 when focusing in and out. Make sure the table does not move when the locks are tightened.

With the table now square to the focus travel, optical comparator lens and projected image check the image horizon in relation to the screen vernier 0. To do this place a pointy object (like the corner of a razor blade) on the center of the stage. Have it stick straight up from the stage. Bring the object into focus in the center of the optical comparator screen. Turn the screen until the 0 protractor mark on the glass screen is lined up with the 0 mark on the vernier under the screen. Now (without moving the screen) move the point of the image on the screen to just touch the bottom of the horizontal (horizon) line. Now without moving the screen move the X axis left and right the pointer (image) should stay just touching the bottom of the optical comparator screen horizon line as it is moved. If the image moves diagonally on the screen the image horizon is off. This can be corrected two ways. Adjusting the vernier on the chart ring to match the image horizon (for a slight adjustment) or by adjusting the mirrors (for a gross adjustment). Twisting one mirror left or right will dramatically affect the horizon. The lower mirror has more affect on this than the upper one. The idea is to keep an eye on the horizon while you are adjusting the optical comparator magnification this way the horizon is good when the magnification is finished.

If the magnification is not correct adjust the optical comparator mirrors as needed to optimize the magnification. If when you are done with the magnification the horizon is slightly off you can correct it by loosening the three chart ring screws and rotating the ring slightly. Zeros on the calibration sheet indicate the magnification in that quadrant of the screen is within the acceptable limit listed on the sheet. Once the magnification is set for one lens, other lenses can be easily checked for proper magnification.

This can be done by placing another lens in the comparator and looking at the image from the Acu-Rite Measurement Master on the screen. For instance the one inch scale on the master should measure ten inches on the screen if you are using a 10X lens. If it does not measure exactly ten inches loosen the three screws on the back side of the lens and screw the back part of the lens in or out until the correct magnification is achieved. Use this technique for any additional lenses. The optical comparator mirrors only need to be adjusted for one lens. The remaining lenses are adjusted to set their magnification. The circle around the lens magnification on the inspection sheet indicates the final inspection was performed using that lens.

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.