Comparator Notes Metrology
The general principle of comparator is to indicate the differences in size between the standard and the work being measured by means of some pointer on a scale with sufficient magnification
It thus does not measure the actual dimension but indicates how much it differs from the basic dimension
• All comparators consist of three basic features
1)A sensing device
• which faithfully senses the input signal
2) A Magnifying or amplifying system
• to increase the signal to suitable magnitude. Mechanical, Optical, Pneumatic,hydraulic and electronic methods are used for this purpose.
3) A display system (usually a scale and pointer)
Which utilizes the amplified signal to provide a suitable readout.
• Need for a comparator
A comparator is used in mass production to inspect the components to close tolerances with high degree of precision and speed. Ex: Piston
• Use of line standards such as vernier and micrometer calipers require considerable skill
• Many dimensions can be checked in a very short time
Comparators can be classified as
1. Mechanical 2. Optical 3. Electrical &Electronic
4. Pneumatic
5. Fluid displacement comparators
6. Mechanical optical comparator
7. Electro-mechanical comparator
8. Multi-check comparator.
• Design Consideration
1.Ability to record variation of at least 0.0025 mm
2.Linear recording scale
3.Variance free (Instrument readings should not vary when repeated measurements of given quantity are made
4.Scale indicators clear and free from oscillations
5.Free from backlash, unnecessary friction &clearances inertia should be low.
6.Measuring pressure should be low and constant
7.Supporting frame to be rigid and withstand reasonable ill usage without permanent damage
8.Capable of measuring wide range.
• Basic operation
- The comparator is first adjusted to zero on its dial with a gauge block in position
- The work piece to be checked is then placed in position
- The comparator gives the difference in dimension in relation to the gauge block.
• Uses of comparator
- Laboratory Standards:
to set the working or inspection gauges
2.Working Gauges
To prevent the work spoilage and to maintain required tolerances
- Final Inspection Gauges
- Receiving Inspection Gauges
- For checking newly purchased gauges
• Essential characteristics of a comparator
- Robust design and construction
- Linear characteristics of scale
- High magnification
- Quick in Results
- Versatility
- Minimum Wear of contact Point
- Free from Oscillations
- Free from backlash
- Quick insertion of work piece
- Adjustable table
- Compensation from temperature effects
- Means to prevent damage
• Working principle of a Mechanical Comparator
A mechanical comparator employees mechanical means for magnifying the small movement of the stylus
the magnification of the small stylus movement is obtained by means of levers, gear trains, rack and pinion or a combination
The usual magnification obtained by these comparators ranges from about 250 to 1000
• Types of Mechanical Comparators
- Dial indicator (Dial Gauge)
- Read Type
- Johansson Mikrokator
- Sigma Comparator
• Dial indicator/ Dial Gauge
• The simplest type of mechanical comparator
• It consists of a base with a rigid column rising from its rear
• An arm mounted on this column and it carries a dial gauge at its outer end
• The indicator is set at zero by the use of slip gauges
• The part to be checked is placed below the plunger
• The linear movement of the plunger is magnified by means of mechanical means to a sizable rotation of the pointer
This type is generally used for inspection of small precision machined parts
The dial gauge with various attachments are commonly used
This comparator is ideal for the checking of components with a tolerance of + 0.005 mm
• Johansson Mikrokator
• Working of the comparator
A very thin metal strip at the centre carries a light pointer made up of glass .
One end of the strip is connected to the adjustable cantilever strip and the other end is to the spring elbow, in turn connected to the plunger.
The slight movements of the plunger will make the bell crank lever to rotate.
This rotation will create tension in the strip and causes the strip to rotate thereby the strip start to untwist resulting in the movement of the point
The spring ensures that the plunger returns when the contact is removed
The length of the cantilever can be varied to adjust the magnification
The mechanical magnification is obtained by “Abramson Movement”
• Reed Type Comparator
The gauging head is usually a sensitive, high quality, dial indicator mounted on a base supported by a sturdy column.
The reed mechanism is frictionless device for magnifying small motions of spindle.
It consists of a fixed block A which is rigidly fastened to the gauge head case, and floating block B, which carries the gauging spindle and is connected horizontally to the fixed block by reeds C
Vertical reed is attached to each block with upper ends joined together. These vertical reeds are shown in the figure by letter D. Beyond this joint extends a pointer or target.
A linear motion of the spindle moves the free block vertically causing the vertical reed on the floating block to slide past the vertical reed on the fixed block.
How- ever, as these vertical reeds are joined at the upper end, instead of slipping, the movement causes both reeds swing through an arc and as the target is merely an extension of the vertical reeds, it swings through a much wider arc.
The amount of target swing is proportional
to the distance the floating block has
moved but of course very much magnified.
The scale may be calibrated by means of gauge block (slip gauges) to indicate
any deviation from an initial setting.
Sensitivities of the order of 0.25 micron
per scale
division.
The mechanical amplification is usually less than 100, but it is multiplied by
the optical lens system. It is available in amplifications ranging from x 500
to x 1000.
• Sigma Comparator
The plunger is mounted on a pair of slit diaphragms in order to have
frictionless linear movement. A knife edge is mounted on it and bears upon the
face of the
moving member of a cross strip hinge.
The cross strip hinge consists of the moving component and a fixed member which are connected by thin flexible strips alternately at right angle to each other.
Thus if an external force is
applied to the moving member ; it will pivot, as would a hinge, about the line
of intersection of the strips.
To the moving member an arm of Y shape and having effective length I is
attached.
• Advantages of mechanical comparators
1.Cheaper than all the other type of comparators
2.Does not require any external source of power or air supply
3.These comparators use a linear scale that can be easily understood.
4.Usually these comparators are robust and compact but are very easy to handle
5.These are small in size and can are portable from one place to other very easily without much difficulty
• Disadvantages
1.Contains more number of moving parts so there develops friction which in turn reduces the accuracy
2.Slackness in the moving parts reduces accuracy very drastically
3.These have more inertia so the instrument is prone to vibrational effects
4. Limited range of the instrument is another drawback as the pointer moves over a fixed scale
5. Parallax error may also arise when proper scale is used
• Mechanical optical comparators
In mechanical optical comparators small displacements of the measuring plunger are amplified first by a mechanical system consisting of pivoted levers.
The amplified mechanical movement is further amplified by a simple optical system involving the projection of an image.
The usual arrangement employed is such that the mechanical system causes a plane reflector to tilt about an axis and the image of an index is projected on a scale on the inner surface of a ground-glass screen.
Optical magnification provides high degree of measuring precision due to reduction of moving members and better wear resistance qualities.
Optical magnification is also free from friction, bending, wear etc.
• The whole system could be explained diagrammatically by Fig. below, which gives very simple arrangement and explains the principle of above comparator.
In this system,
• Mechanical amplification= l2/l1
• and Optical amplification = l4/l3 * 2.
• It is multiplied by 2, because if mirror is tilted by an angle δθ , then image will be tilted by 2 * δθ .
• Thus overall magnification of this system=2*l2/l1 * l4/l3.
• Zeiss ultra optimeter
• The optical system of this instrument involves double reflection of light and thus gives higher degree of magnification.
• A lamp sends light rays to green filter, which filters all but green light, which is less fatiguing to the eye. The green light then passes to a condenser which via an index mark projects it on to a movable mirror M1, whence it is reflected to another fixed mirror M2, and then back again to the first moveable mirror.
• The second objective lens brings the reflected beam from the first mirror to a focus at a transparent graticule containing a precise scale which is viewed by the eye-piece.
The projected image of index line on the graticule can be adjusted by means of screw in order to set the zero.
When correctly adjusted, the image of the index line is seen against that of the graticule scale.
The special end of the contact plunger rests against the outer end of the first movable mirror so that any vertical movement of the plunger will tilt the mirror.
The extreme sensitivity of this instrument necessitates special precautions in its operation to avoid temperature effects.
• Zeiss optotest comparator
This is the commercial measuring instrument. This utilises a plunger titled mirror, objective lens, prism and observing eye-piece to provide a high degree of magnification.
The mirror is mounted on a knife-edge and it can be tilted about the fulcrum by any linear vertical movement of the contact plunger.
A beam of light passes through a graticule suitably engraved with a linear scale and the movement of mirror causes this scale to move up or down past a translucent screen inside the observing hood of the instrument.
The eye placed near the eye-piece views the image of a small scale engraved on glass after reflection from the plunger actuated mirror and the prism in the plan view in Fig
In the focal plane of the eye-piece, a fine reference line (Index) is provided and the system of lenses is so arranged that the image of the scale is projected in the same focal plane.
Thus with movement of scale the image can be measured with reference to the fixed line. The division of the scale image opposite the index line indicates the amount of movement of contact plunger.
The image of the scale and the index line could also be viewed through a projection system.
The overall magnification of the comparator
is given by 2f/d * Eye-piece magnification, where f
is the focal length of the lens and d is the distance between
the knife-edge and the plunger.
• Advantages of optical comparators
- It has small number of moving parts and hence a higher accuracy.
- In the optical comparators, the scale can be made to move past a datum line and thus have high range and no parallax errors.
- It has very high magnification.
- Optical lever is weightless
• Disadvantages
- As the instrument has high magnification, heat from the lamp, transformer etc. may cause the setting to drift.
- An electrical supply is necessary.
- The apparatus is usually large and expensive.
- When the scale is projected on a screen, then it is essential to use the instrument to a dark room in order to take the readings easily.
- The instruments in which the scale is viewed through the eyepiece of a microscope are not convenient for continuous use.
• Pneumatic comparator
• charactrestics
(a) Very high amplifications are possible.
(b) As no physical contact is made either with the setting gauge or the part being measured, there is no loss of accuracy because of gauge wear.
(c) Internal dimensions can be readily measured not only with respect to tolerance boundaries but also geometric form.
(d) It is independent of operator skill.
(e) High pressure air gauging can be done with cleansing of the parts which helps to eliminate errors due to dirt and foreign matter.
f) Gauging pressures can be kept sufficiently low to prevent part deflection.
g) Dimensional variations throughout the length of shaft or cylinder bore can be explored
h) Not only it measures the actual size, but it can also be used to salvage oversized pieces
i)The total life cost of the gauging heads in much less.
(j) It is accurate, flexible, reliable, universal and speedy device for inspecting parts in mass production.
(k) It is best suited for checking multiple dimensions and conditions on a part simultaneously in least possible time.
• Advantages of Pneumatic Comparators:
i. The gauging member does not come into contact with the part to be measured and hence practically no wear takes place on the gauging member.
ii. It has usually very small number of moving parts and in some cases none. Thus the accuracy is more due to less friction and less inertia.
iii. Measuring pressure is very small and the jet of air helps in cleaning the dust, if any, from the part to be measured.
iv. It is possible to have very high magnification.
v. The indicating instrument can be remote from the measuring unit.
vi. It is very suitable device for measuring diameter of holes where the diameter is small compared with the length.
vii. It is probably the best method for determining the ovality and taperness of the circular bores.
• Disadvantages:
i. It requires elaborate auxiliary equipment such as accurate pressure regulator.
ii. The scale is generally not uniform.
iii. When indicating device is the glass tube, then high magnification is necessary in order to avoid the meniscus errors.
iv. The apparatus is not easily portable and is rather elaborate for many industrial applications.
v. Different gauging heads are required for different dimensions.