Mechanical Comparator:
It is self controlled and no power or any other form of energy is required. It
employs mechanical means for magnifying the small movement of the measuring
stylus. The movement is due to the difference between the standard and the actual
dimension being checked.
The method for magnifying the small stylus movement in all the mechanical comparators is
by means of levers, gear trains or combination of these. They are available of different make
and each has it's own characteristic. The various types of mechanical comparators are dial
indicator, rack and pinion, sigma comparator, Johansson mikrokator
Sigma Comparator:
The plunger is attached to a bar which is supported between the bending plates at
the top and bottom portion as shown in Figure (a)
The bar is restricted to move in the vertical direction. A knife edge is fixed to the bar. The knife edge is attached to the sapphire plate which is attached to the moving block. The knife edge extorts a force on the moving block through sapphire plate. Moving block is attached to the fixed block with the help of crossed strips as shown in Figure (b). When the force is applied on the moving block, it will give an angular deflection. A Y-arm which is attached to the moving block transmits the rotary motion
to the driving drum of radius r.
Optical comparator
What is an optical comparator?
An optical comparator is a measuring device used to analyze and measure the dimensional variations of the workpiece. The optical comparator was invented by James Hartness in 1922.
Later this comparator was commercially introduced by J&L Machines. co. The optical comparator uses the reflection of light as its basic working principle.
The optical comparator projects the magnified image of the workpiece in the display screen to compare with the standard parameter. This device is contactless with a workpiece while projecting its image on the screen for magnification.
Based on the image projected by the optical comparator it is broadly classified into two variants as inverted and erect. The inverted image variant produces upside-down and left to right images.
The erect variant producers the same orientation of the workpiece as an image. While comparing each variant, the erect variant comes with some advanced internal features.
Parts & construction of an optical comparator
The optical comparator is constructed using the following parts as listed below.
- Plunger.
- Pivoted lever.
- Mirror.
- Condenser lens.
- Projection lens.
- Table.
- Base.
- Light source.
- Scale & Screen.
- Plunger
A plunger is a metal component that acts as a sensing parameter to deduct the dimensional variations in the workpiece being measured. It reciprocates, according to the irregularities present in the workpiece.
Pivoted lever
A lever is fixed at a pivot point and connects a plunger and mirror at both ends. The pivot point is placed close to the plunger. The movement of the plunger is enhanced by the pivoted lever mechanism.
Mirror
In an optical comparator, a mirror acts as a reflecting medium that reflects the incoming light rays from the light source. The mirror is pivoted at a center point and hinged at one end of the pivot lever.
Condenser lens
Every optical instrument will have a condenser lens in it. Its main purpose is to convert the divergent light rays from the light source into parallel light rays. Condenser lenses are also named objective lenses.
Projection lens
The projection lens is placed next to the condenser lens and it projects the condensed parallel light rays to the reflective mirror.
Table
The workpiece to be inspected is placed at a flat table and the plunger will be in contact with the workpiece.
Base
The entire setup, including the table, is mounted on the base.
Optical comparator working principle
The working principle of the optical comparator is mainly based on two basic principles such as the law of reflection and refraction of light. The tilting mirror deflects the incoming light rays from the source and acts as an optical lever.
The magnification of the optical comparator depends on the optical lever principle. By using the optical lever principle, an automatic magnification is achieved in such a way that the reflected beam deflects twice the angle of the tilting mirror or the total angle of deflection is increased by 2dθ.
Construction & Working of an optical comparator
Now let’s see how an optical comparator works.
Initially, the instrument is calibrated using a standard parameter or workpiece, and the scale value is set to zero. The workpiece to be inspected is kept on the table and made contact with the plunger. The irregularities in the workpiece are sensed by the plunger and it is vertically displaced.
The vertical displacement of the plunger is amplified due to the pivoted lever fitted at its one end. The pivotal lever mechanism tilts the mirror about its hinge. The light rays from the light source are allowed to pass through the condenser lens. It converts the divergent light rays into parallel light rays.
The condensed light rays pass through the projection lens and then finally project on the reflecting mirror. Simultaneously, the mirror is tilted through the angle (α) according to the vertical displacement of the plunger.
After the mirror is tilted, the normal plane moves through an angle α and the angle between the incident ray and normal becomes θ+α. Similarly, the angle between the reflected ray and the normal also becomes α+θ. Thus, the total angle will be 2θ+2α.
Types of optical comparator
There are different types of optical comparators available and some of them are listed below.
Zeiss ultra-optimeter
Zeiss optimeter
This comparator is constructed by various parts such as mirror, objective lens, condenser lens, green filter, graticule, eyepiece, and a light source (lamp). It consists of two mirrors one is fixed and the other is movable.
The monochromatic light from the light source passes through the green filter and the condenser lens. The green filter is used to filter the green light from the light source and the condenser lens converts the divergent light to a parallel beam of light.
A plunger is attached at one end of the movable mirror and it moves in both horizontal and vertical directions with respect to irregularities present in the workpiece. A fixed mirror is placed parallel to the movable mirror. This mirror is used for achieving better magnification.
The light beam from the source is reflected on the movable mirror (A) and again it is reflected on the fixed mirror (B). From fixed mirror (B), it again reflects movable mirror (A). And finally, it projects to the eyepiece.
This optical comparator gives a higher degree of magnification because it involves double reflection of light. It is very sensitive and accurate.
Zeiss Optimeter
In Zeiss optimeter, their optical system is constructed using a tilting mirror, condenser lens, prism, reflector, eyepiece, light source, and plunger as a mechanical component. The light from the light source is reflected by an illuminating mirror.
After the light gets reflected on the tilted mirror, the image can be seen in the scale. Due to the irregularities present in the workpiece, there will be some movement in the plunger. And this displacement of the plunger results in the movement of the scale image.
The displacement of the scale image is analyzed using the scale reading presented on the focal plane of the scale image. The Zeiss optimeter has 1μm as the scale value and ± 100μm as the measuring range.
This optical comparator can measure up to 180mm and force about 200g. This comparator is also used widely in many applications.
Applications of an optical comparator
These are the following applications of an optical comparator.
- Optical comparators are very useful in inspection of gears, screws, thread, cams, etc.
- Location of pitch circle diameter in a mechanical component can be easily analysed using an optical comparator.
- It is very helpful in analysing tool wear in many mechanical machine elements.
- They are mostly used for inspection purpose in tool room and shop floor.
- It is used for quality control in production line. It is used in most of the metrology laboratories for research and inspection purpose.
- It is also used by many products manufacturing industries to analyse their Precision of their product.
Advantages of an optical comparator
The optical comparator has some advantages. The following advantages are
- It doesn’t have many mechanical parts, so there will be no wear and tear due to friction.
- It is lightweight.
- These optical comparators can give very high magnification and accurate results.
- It is easy to use and very sensitive.
- There will be no parallax error while using this comparator.
- Readings can be taken without influence of room lightings due to the illuminated scale used in their construction.
Disadvantages of an optical comparator
The optical comparator also has certain disadvantages. The following disadvantages are
- It is very costly, especially the optical system setup.
- To make the measuring process easier, it is essential to use these comparators in a darkroom.
- Some of these optical comparators are usually larger in size.
- There must be an external power source which is required to operate light source continuously.
- The eyepiece of microscope is used to view the scale so it is not convenient for continuous usage.
- The continuous heat from the light source make cause the arrangements to drift.
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