Optical Comparators

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.

Optical comparators can be very complex machines. However every optical comparator has similar design features and follows the same working principles. Optical comparators are used to magnify inspected part and cast its shadow onto a screen with chart to check for imperfections. Every optical comparator has a light source, which emits light beam. That light beam travels through prism and projects the shadow of inspected part. The shadow is then reflected through series of mirrors and finally ends up on chart screen. The operator can then compare the part against preset chart. Optical comparators have multiple lenses available, with different magnification steps. Zoom options vary from 5X to 500X. Screen sizes are also available in different sizes, from 10” to astonishing 80”. Combining screen size with optical zoom operator can inspect really big parts for really small anomalies. Another features found in modern optical comparators are digital readouts. These can vary from very simple, 1 or 2 axes digital readouts to more complicated 3 axes, programmable readouts, with geometric displays. More sophisticated digital readouts feature serial or parallel interfaces to interact with personal computers. In the nearest future those readouts can be expected to be equipped with more modern ports, such as usb, firewire and possibly Ethernet or lightpeak. Digital readouts with computer connection interfaces allow to transfer inspection results and to further export to CAD applications to manipulate extracted data.

Last important element found in optical comparators is stage. Optical comparator stages can be fully motorized or manual. Stages allow adjusting the position of inspected part. Inspected elements mounted on stage are resistant to external vibration providing more accurate results.

To summarize, the principle of optical comparator is to magnify inspected part. Basic elements found in most, if not every optical comparator are:

• Light source
• Mirrors
• Screen with chart ring
• Stage, motorized or manual
• Digital readout

Optical comparators must receive proper care to ensure longer lifespan and retain measurement accuracy. There are few parts that require regular cleaning:

• lenses
• mirrors
• screens

Any accumulated dust must be removed from the surface of the lenses with a photographic lens brush. The surface of the inner optical comparator lens (projection or condenser) must not be touched. The element of the projection lens facing the mirror is most critical in this respect. A single finger mark on this surface will noticeably degrade the quality of the projected image. It should be noted that the projection lens element facing the work piece is far less critical.

Any grease or finger marks may be removed with a new photographic cleaning tissue. If necessary, the tissue may be moistened with methanol or lens cleaning fluid to assist cleaning.

Access to the optical comparator mirrors is gained by removing the screen. To remove grease or finger marks from the mirror surfaces use a surgical cotton wool swab soaked with methanol. Apply the swab very lightly to the mirror surface finally giving a light polish with another clean dry swab. Do not use force. All mirror surfaces should be cleaned as infrequently as possible. Never use polishes, window cleaning aerosols or liquids when cleaning the mirrors or lenses.

All optical comparator screens are made out of ground glass with printed cross lines. The cross lines can be removed if the screen is not cleaned properly. We recommend that the screens be cleaned only with soap (a mild non-lotion dish-washing solution) and plenty of water. Use a lint free soft cotton cloth to gently clean. Let the screen air dry.

If your environment is extremely dirty, please contact us about optional encapsulated image screens which cover the markings with a protective membrane.

Our sysadmin has recently deployed google browser in our network. With the availability of extensions it should be a decent replacement for firefox. However we were unable to install any additional extensions. None of the tutorials available on the web was helpful. We tried developers build with no luck. We tried adding “−−enable-extensions” to Chrome shortcut without any results. Google Chrome would simply download .crx file and download it again when opening. That became very frustrating, for our sysadmin and for end users. The solution lied in installation profile. Simply remove or rename Chrome profile folder and extensions will install without any glitch. The profile folder, in windows Vista and 7 is usually located under c:\users\yourusername\appdata\local\google\chrome\user data\default. Just rename or delete default folder. Remember, all your settings are inside so you would loose all bookmarks, history and so on. So make sure you back up all you personal settings first.

Surface Illumination

Optical comparators illumination of surface features on solid objects is achieved through the use of high intensity fiber optic light guides. These guides are adjustable and can be extended by pulling on the cable ends protruding from either side of the lens mounts. They may be secured in place with the clamping screws on the side of each of the guide blocks.

For the brightest reflected image, place the light guides as close as possible to the face of the component to be inspected. Where components have a directional surface finish, e.g. ground faces, the surface texture should be placed at right angles to the table, this will give the brightest reflected image on the screen.

As the fiber optic cable is a “cold light” source, the heat radiated to the component under test is minimal.

Manual Table Adjustment

The X axis of the stage has a quick release for coarse motion. This feature should not be used to move the stage during a measuring routine. Also, when the quick release is engaged, the control knob of the X axis is disabled.

To engage the quick release, rotate the lever to the furthest position to the right. To re-engage the control knob rotate the lever to the furthest left position.

Stage Helix Adjustment

The top plate of the optical comparator stage has a helix adjustment, which allows the top plate of the stage to be rotated. To do this the 2 locking clamps located beneath the stage must first be loosened. The stage plate can now be rotated ±15 degrees. The helix indicator shows the amount of rotation in 5’ increments. Once the desired helix is adjusted tighten the locking clamps to hold this position.
To restore the helix to the zero (0) position, loosen the locking clamp rotate the helix adjustment to zero.

Vertical Stage Adjustment

Vertical adjustment of the optical comparator work stage is by means of the large hand wheel on the left below the work stage knee.

Screen Angular Measurement

The electronic venire protractor screen, consists of a frosted glass screen divided into four equal quadrants by precision cross lines. The screen is rotated by means of a small hand wheel on the right of the screen. The standard electronic venire protractor can then be used to determine angles with 1 minute resolution.

Electronic Rotary Protractor

16H optical comparators are equipped with a digital electronic rotary screen protractor. These compact units display the current rotary position of the screen and utilizes non-volatile memory to permanently store the calibration data regardless of power loss. Optical comparators display rotary position in either degrees and minutes or degrees and 1/100 of a degree.

The guide below applies to Dorsey Metrology 16H Benchmark optical comparator. It might not reflect actual features found on other products.

First step in optical comparator operation is of course switching the power on. The projector lamps are usually controlled by rocker switches on the lower front panel of the projector body. Separate switches control the profile and surface illumination lamps. The profile lamp uses a 2 position switch: down is low intensity, while up is high intensity.

Before using optical comparator to inspect part some pre-run checks might be required. First ensure that the projector is secure on its stand or on a substantial bench. Having selected a lens of the required magnification and locked it securely in position, locate the component to be inspected in a suitable work holding fixture which must be securely clamped to the table, the optical comparator is now ready for use.

Focusing

Focus is achieved by rotating the small hand wheel on the right hand side of the stage knee. The lens focal plane is approximately mid-way along the focus traverse, central to the dovetail slot in the table.

Profile Projection Intensity

There are two profile light intensity levels, high or low, selected from the rocker switch on the panel below the screen. Select the level that gives the most comfortable viewing, according to the object projected and the external lighting conditions prevailing. Profile lighting is provided by a 24V 150W tungsten halogen lamp. An iris diaphragm and or green filter is available as an optional extra which may be fitted direct onto the lamp house to reduce the level of reflection from brightly polished surfaces e.g. ground threads, etc.

Optical comparators are designed and manufactured so far as is reasonably practical to allow its safe operation when used in accordance with the safety instructions. Optical comparator must be used in position that does not constitute a hazard, where the operator and maintenance staff have free access to the control and maintenance of the equipment, and are not subject to any external hazards. Services should be conducted by an authorized person, to approved safety standards. The electrical supply should be taken from a correctly rated source.

Power requirements

The standard optical comparator is usually supplied to accept 110-120V AC, 50-60 Hz main power. It may also be configured to accept either 220-240V AC, 50-60HZ or 100V AC, 50-60Hz. Unless clearly labeled (next to the power input plug) optical comparators are configured for 110-120V AC, 50-60Hz. Listed below are wire color codes:

Note optical comparators must be plugged into an outlet rated at a minimum of 15A and must not be operated without an adequate ground connection.

Optical comparators are designed to work in a normal workshop environment but it is important to locate the unit as far as possible from sources of grinding grit, oil mist, and vibrations. Where possible the projector should be located with the screen facing away from direct sunlight and overhead lighting.

Bench-top optical comparators should be placed on a strong sturdy bench capable of securely supporting a minimum of 550lbs/250Kg with a point pressure capability minimum of 165psi. Dorsey Metrology optionally supplies a cabinet/stand designed for this application. Canopy and curtains are not provided as standard, but are available as an optional extra fitted at the factory on the projector stand. These items are strongly recommended when using lenses of high magnification or with surface projection of difficult components.

In part 2 of this guide we will focus on selecting right lenses, digital readouts and additional options and accessories for optical comparators.

When selecting proper lens follow the chart below to decide what lens will match the tolerances required. A basic rule of thumb is that
a typical attentive operator can repeatedly discriminate .004″ on the comparator screen. Dividing the “discernible resolution” by the lens magnification determines the minimum resolution attainable for each lens.

Another thing to consider when selection optical comparator is digital readout. Select a basic XY digital readout if only positions and distances are required. If measurement of circles, angles, and parametric distance is required, then select a readout with geometric capability. Repetitive part measurement may encourage the selection of a CNC capable readout. Automatic edge sensing should be considered to eliminate operator subjectivity and increase repeatability and accuracy.

Additional options are available when choosing optical comparator. Repeatability and accuracy will suffer if the workpiece is not properly and securely held. Careful consideration should be given to tooling and to the surface on which you place your optical comparator.