Optical Comparators

I guess we neglected this blog for a while. But we’re back and we are ready to write more. More about our machines, perhaps more about quality control in general. It’s been a busy month, or two, our official website was redesigned, new functionality was included.

We are getting ready to start producing video podcasts too. Keep your fingers crossed.

In the meantime we will try to keep this blog constantly updated with more information about optical comparators.

..unless something drastic is done.

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Dial test indicators are sometimes found in optical comparators as an additional sensors in 3 axis measurement. Dial test indicators made by Dorsey Metrology combine accuracy of pantograph gage with versatility of traditional test indicators.

Dial test indicator shown above is Dorsey Metrology model TI4-005D. Indicator can be purchased as a standalone dial test indicator or as a kit with additional contacts. For more information visit Dorsey’s dial test indicator page.

Dorsey Metrology recently released a new model of optical comparator with 14 inch screen. 14HE optical comparator is designed to take less amount of space, reduced overall weight while still attaining high quality build among other optical comparators.

14HE optical comparator made by Dorsey Metrology.

Tables below show available styles for grid  charts for optical comparators made by Dorsey Metrology.

Dorsey Metrology offers many styles for its optical comparator charts. Tables below show available styles for grid and radius charts.

Illustration shows Dorsey’s grid and radius chart. Please contact our sales department at +1-845-454-3111 for ordering information.

When ordering optical comparator chart, please specify the following:

• style of chart (more available – images coming soon)
• type and model of optical comparator
• lens magnification
• chart material – glass or plastic
• additional information required for particular chart or scale

Squareness of X axis to Y axis

Squareness of X axis to Y axis is checked by placing a square on the center of the optical comparator table banked up against one of the stage vees. Face it to the right or left side. Place a magnetic base indicator on the face of the comparator near the chart ring. Lower the Y-axis to the bottom and place the tip of the indicator on the side of the square. Raise the Y-axis and check the squareness of the X axis in relation to the Y axis travel. It should be .0001” or less per inch (≤ .0006” for 6” travel). If adjustment is needed the knee bracket must be loosened and twisted to correct.

Squareness of Y axis to focus plane

Squareness of Y axis to focus plane is checked in the same manner as squareness of X axis to Y axis only with the face of the square facing forward or back. This should be .0005” per inch or less (≤ .003” for 6” travel). If this is not correct you would need to shim under the lower focus bearings to bring into spec.

X axis stage linear traveling accuracy

For this specification we use the formula ± (150+L/.02)µin. For a 10” travel optical comparator stage we would divide 10” by .02 to get 500 then add to it 150 for a ± total of 650 micro inches over 10” travel (±.00065”). The optical comparator stage is inspected using the 6” scale on the Acu-Rite master in the center of the stage. The deviation from the expected reading is recorded on the inspection sheet in micro inches (.000001”), a minus sign before the value indicates the measurement was under size. If corrections are needed a new comp. value can be entered into the optical comparator digital readout for the axis you are working on. Errors that are linear in nature are easily compensated for. If your readings are very non-linear it could indicate damage or looseness. The remarks section of the inspection sheet shows any error compensation set in the digital readout for that axis.

Y axis stage linear traveling accuracy is checked the same as X only with the master standing vertical on the stage.

ERP calibration counts

This value indicates the number of pulses recorded by the rotary encoder during calibration. The idea is to have a point of reference to use in the future if any errors or changes are needed to make the digital angle display more accurate. To re-calibrate the rotary encoder follow the instruction in the optical comparator manual and record the new value on the inspection sheet after a satisfactory value is achieved on the display.

ERP Repeatability

A check in this box indicates that the angle display is within ± 30 minutes of angle over a 360° rotation (±0.5°).

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.

Guide below describe how to set up 38mm condenser on Dorsey Metrology 16H optical comparator.

Insert a 2mm hex wrench down into the top of the current optical comparator condenser ring and loosen the setscrew holding the 51mm condenser. You can access the setscrew by removing the optical comparator lamp house cover and inserting the hex wrench through the small notch in the front of the lamp house.

Remove the 51mm condenser and install the 38mm condenser (with adapter ring) in its place. The convex side of all Dorsey condensers face towards the comparator. Once in place you will need to adjust the optical comparator condenser for best focus. To set the condenser focus hold a small mirror about 3 inches in front of the condenser. Turn the lamp on and slide the condenser in or out until you can see a sharp image of the bulb filament focused on a wall or ceiling about 25 feet away (further away is ok). Once the bulb filament is in focus tighten the set screw. The condenser is now ready for use.

You can use this focusing procedure to re adjust the 51mm condenser if it is switched back.

First free the two scale wires that run to the digital readout and coil them up and secure with a rubber band or wire twist tie.

Move the stage to the extreme left and right to enable removal of the four stage mounting screws.

Open the two lead screw quick releases and lift the stage off of the comparator holding it horizontal. CAUTION: If it is tilted it will slide quickly to one side or front to back and might fall or get damaged.

Take note: If there are any shims or washers under the stage at the mounting location. If shims or washers are present they will have to be replaced the same way they are located now.

Place the new stage on the comparator and then remove the shipping locks. CAUTION: If you tilt the stage during installation it will slide to the side and will damage the teeth in the quick release that engages the lead screw or will damage the stage.

Secure the stage to the optical comparator with the four mounting screws. Leave three of the screws loose and one just snug so that the stage can pivot around it. Turn the screen so that the vernier scale is at 0 or the screen pointer indicates that the horizontal line is set to its horizontal position. Place a pointer on the optical comparator stage so that it appears on the screen along the horizontal line. Now using the lowest magnification lens move the stage left and right using the quick release. If the pointer does not follow the screen horizontal line twist the stage so that the left and right stage travel matches the horizontal line. Then tighten all stage mounting screws and re-attach the scale cables.

Use the locking hardware removed from the new optical comparator stage to lock the old stage for return shipping.

There are several bulbs available for the optical comparators. The older units used a 12 Volt 100 Watt bulb for the projection and the same for the surface. These bulbs are built in reflector type of bulbs (cone shaped). Both optical comparator bulbs would be located inside the case of the machine with fiber optic cables bringing the light to the lamp house that points toward the machine for profile lighting and another fiber optic cable that splits into two and brings light to either side of the lens for surface lighting. These point toward the operator.

On the newer optical comparators there is one 24 volt 150 watt bulb (no reflector, shaped like your finger) in the top of the lamp house (no fiber optic cable) and one 24 Volt 250 watt bulb (reflector type, cone shaped) in the bottom of the lamp house. This fiber optic cable splits into two and brings light to either side of the lens for surface lighting. These point toward the operator.

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.

Illustration below shows one of our optical comparators general purpose glass chart, style 12.

To see larger version follow the link glass chart. Illustration is a svg file and can be zoomed in without any loss in quality.

Optical comparator surface illumination

Picture above shows optical comparator with surface illumination. Optical comparator is made by Dorsey Metrology, model 16H.