Optical Comparators

As you no doubt already know, the International Manufacturing Technology Show (IMTS) is just a short month away.  Dorsey Metrology will be there from Sept. 13 – 18 in booth E-5350 with our new comparator, the 14HE, and a selection of digital readouts.

Dorsey Metrology, one of the world’s most respected gage makers manufacturing optical comparators in the US, is proud to announce the latest addition to their expanding range of products.  This new comparator has the same high quality features as the current product line, but in a smaller package and at a lower price point.

The New 14″ Industry Standard The 14HE has a 14 inch/350 mm diameter screen and an 8″ x 4′ X andY travel.  The nickel plated cast iron work-stage has both coarse and fine adjustment control together with a massive 50lb working capacity.  As always, cross roller bearings are on all three axes.  Dorsey is known for their unique internal edge detection, which is optional on this model.  A variety of digital readouts from different manufacturers are offered to compliment this. Features:

• 14″ (350mm) screen for optimum viewing
• High resolution ground glass screen with 90 degree cross lines and chart clips
• Machined chart ring with vernier protractor, 1 minute graduation
• Output for electronic rotary screen protractor
• Coated telecentric par focal optics
• Available optional internal edge sensing
• Quick change single lens mount
• Optional Fiber optic surface illumination
• Nickel plated, completely cast iron workstage with single universal dovetail
• Triple axes solid crossed roller stage system
• 16″ x 4″ overall stage size
• 8″ X axis travel with “glide release”
• 4″ Y axis travel (rise & fall)
• 50 lbs. capacity
• ± 5 degree lightsource helix adjustment
• NIST traceable calibration certificate
• 2 year limited warranty

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.

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.

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.

Before buying optical comparator some  items that should be taken into consideration.

Light path is probably the first thing one would like to look into. Optical comparators comes in two configurations, with vertical light path and horizontal light path. Vertical light path instruments have a beam of light traveling vertically. Parts being inspected are placed on a plate of glass that the light beam travels through. Vertical optical comparators are ideal for flat parts like gaskets, O rings, stamped parts and electronics. Dorsey’s line of vertical optical comparators feature quick release mechanisms on both axes making measurements on our vertical machines much faster than on horizontal machines which feature quick release on only 1 axis.

Horizontal light path instruments have a beam of light traveling horizontally across a workstage. This type of machine is ideal for large heavy parts and shafts to be held on V blocks or between centers. Typical applications include castings, transmission shafts, thread form measurement and machined components.

Another important thing when selecting optical comparator is screen size. Screen sizes from 12″ to 80″ are available. Before choosing a screen size, determine how much of the part really must be viewed at one time. When using a digital readout system it is not necessary to view the entire part to measure it. Calculations can be made by dividing the screen diameter by the lens magnification. For example using 10X lens on a 16″ optical comparator would enable viewing 1.6″ of the part on the screen (16″÷10=1.6″). Verify that the workstage size, travel and weight capacity will accommodate all of the parts that are intended to be inspected. In general, optical comparators with screen sizes 16″ and smaller are benchtop units with weight capacities up to 110 pounds.

Please feel free to contact us with any questions regarding sales, applications, or competitive products.

An optical comparator, sometimes referred as profile projector is a inspection instrument that utilize traditional optics to magnify and project the image of  inspected part onto a glass screen. The optical comparator cast a beam of high-intensity light on inspected part. Light travels through prism and projects the shadow of an object. The shadow is then compared with a chart showing tolerance levels for the part. Optical comparators are available in a wide variety of styles and configurations. Typical options are – projection screen sizes from 10″ to 80″, magnification range from 5x to 250x, horizontal or vertical light path, various stages, digital readouts and surface illumination systems.