Optical Comparators

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.

Standard optical comparator charts

Dorsey Metrology uses satin ground glass of superior quality for all our optical comparator glass charts. The charts are very durable and will last for many years with proper care. Glass charts should be cleaned only with mild soap and water solution using soft cotton cloth. Window cleaners, thinners, and alcohol should never be used to clean your glass chart. All optical comparator glass charts are traceable to the N.I.S.T. and are available with certification at no extra cost.

Glass charts are typically required when:

• The existing chart has been severely chipped or broken
• The lines on the existing screen have faded or worn off

The new chart will remain in the comparator for an extended period of time.

Optical comparator overlay charts

Overlay charts are made of plastic and are placed over the existing glass screen. They are typically required when charts are interchanged on a frequent basis. Dorsey Metrology offers 4 types of plastic overlay material:

• .007″ thick clear mylar
• .007″ thick frosted mylar
• .050″ thick clear vinyl
• .050″ thick frosted vinyl

Optical comparator scales

Dorsey Metrology also offers three types of optical comparator scales:

• inch/decimal
• decimal/metric
• multi-magnification

The inch/decimal and decimal/metric scales are available in glass and are useful for sight verification. The multi-magnification scales are useful for direct screen measurements of projected images on a comparator that is not equip with either digital readouts or micrometer readers. These scales are available in glass or 050″ thick vinyl. All optical comparator scales are traceable to the N.I.S.T. Certification sheets specifying traceability are available at no extra cost. Glass scales are available with serialized numbers. All scales are intended for sight verification and not for comparator calibration.

How to order your optical comparator chart?

When ordering your optical comparator chart, please have the following information available:

• style of chart
• type of optical comparator and model number
• lens magnification
• chart material
• any additional information required for a particular chart or scale

To adjust the optical comparator internal edge sensor so that it is exactly in the center of the screen cross lines use the following procedure.

Using only the optical comparator profile light, place an object on the optical comparator stage that will cast a straight image on the screen in a vertical and horizontal direction. A gage block would work fine. The image should cover all or at least half of the screen diameter.

Rotate the screen so the protractor is at zero. This will make the cross lines vertical and horizontal.

“Teach” the edge detector (refer to the optical comparator digital readout manual if necessary) and make sure that the readout beeps when you cross an edge. Note: You might have to move the image far from the center of the screen to test for the beep.

Bring the top of the image to the horizontal cross line. Making sure some of the image is left and right of the vertical cross line.

The adjustment for the optical comparator edge detector is located on the top of the comparator case. There are three button head screws in a triangle pattern approximately in the center on the case. Two screws on one side and one screw on the other. Turning either one of the screws on the side with two screws will move the location of the sensor up and down  (only a small amount of adjustment should be needed).

Adjust the up and down until the beep noise flutters. That indicates you are right on the line. Test the location by moving the image up and down. Re-adjust if necessary.

After the horizontal is adjusted align the right side of the image along the vertical cross line. Making sure some of the image is above and below the horizontal line. Adjust the left and right sensor using the single screw until the beep noise flutters. That indicates you are right on the line. Test the location by moving the image left and right. Re-adjust if necessary.

How internal edge detection works.

Light from the objective lens (main optical comparator lens) exits the back of the lens and passes through a “hot” mirror. This mirror reflects the infrared light and some visible light and lets most of the visible light pass through to form an image on the optical comparator screen. The light reflected by the hot mirror is directed up to a small mirror located inside the top of the comparator. From there it is directed down and onto a fiber optic cable located in the left front section of the optical comparator. In effect the fiber optic cable sees in infrared light anything that passes over the center cross lines of the screen. You will see the image on the screen in visible light.

As the edge of the part you are measuring passes over the cross lines located in the center of the screen the edge sensor (if turned on) will detect that the cross line area has moved from light to dark and will enter a point on the readout. Then as you continue moving the part the sensor will detect that the cross-hair area has changed from dark to light and enter another point.

By using internal edge detection you can speed up the inspection process and achieve greater accuracy of measured parts by eliminating some of the human error associated with lining up the part on the cross lines by eye.

Danger : Disconnect all power to optical comparator and accessories before starting to work.

This repair should be done by a qualified service person. Please read through all instructions before beginning work.

Before removing the optical comparator fan note the direction of the airflow or if the fan is not working. Look on the fan for an arrow either stamped in the metal or a label indicating the direction of flow. During this replacement you will need to orient the new fan so the airflow is in the same direction. Disconnect the three wires going to the profile fan (there should be three round connectors about 6” from the fan, the wires can be separated at this point). Remove the screws holding the old profile fan in place and mount the new fan in place with the airflow in the correct direction. The new optical comparator fan has 5-millimeter threads for mounting or 4-millimeter screws can pass through the threads. Depending on the mounting style used for your optical comparator. Connect the two power wires and green ground wire of the new fan (it does not make a difference which of the black fan wires goes to which of the black and white supply wires). After the fan installation is complete reconnect the power to the optical comparator and accessories. The new fan should run when the machine power switch is turned on.

Optical comparator thread inspection

Picture above shows typical application of optical comparators – thread inspections. Optical comparator is made by Dorsey Metrology, model 16H with optional surface illumination (not switched on in photo) and precision fixture with 2.12″ jaw opening.

Before doing any work on electric parts of optical comparator make sure that power is off or power cord is disconnected.

Optical comparator profile lamp

To change the optical comparator lamp first use the two thumbscrews located on top of the lamp house to remove the lamp house cover. Remove the bulb by grasping and pulling the bulb directly up. Remove the new optical comparator replacement bulb from its box, and slide the new bulb in place. Switch main power on, remove the objective lens, and set lamp intensity to low. Verify that the lamp filament projected on the optical comparator screen is centered over the cross-hairs. Re-seat lamp and or adjust holder if required.

If the glass lamp is accidentally marked, remove the marks immediately by swabbing with methanol. The optical comparator profile lamp operates at a high temperature and any finger marks will be “burned” into the surface of the quartz reducing the output and life of the lamp.

Optical comparator surface illumination lamp

To change the surface illumination lamp first remove the lamp house cover.

To remove the bulb, grasp and rotate the bulb ejection wire to eject bulb. Remove the new replacement bulb from its box, flip the bulb ejector wire down and slide the new bulb in place. Switch main power on and set optical comparator surface illumination lamp intensity to low. Verify that the projected illumination is centered on the end of the fiber optic cable. Re-seat lamp and or adjust holder ( by bending face ) if required.

Same caution steps apply to optical comparator surface illumination lamp as to profile lamp.