Mechanical Comparators
Introduction
All measurements require the unknown quantity to be compared with a known quantity, called the standard. A measurement is generally made with respect to time, mass, and length. In each of these cases, three elements are involved: the unknown, the standard, and a system for comparing them. we came across linear measurement instruments, such as verniers and micrometers, in which standards are in-built and calibrated. Hence, these instruments enable us to directly measure a linear dimension up to the given degree of accuracy.
On the other hand, in certain devices the standards are separated from the instrument. It compares the unknown length with the standard. Such measurement is known as comparison measurement and the instrument, which provides such a comparison, is called a comparator. In other words, a comparator works on relative measurement. It gives only dimensional differences in relation to a basic dimension or master setting. Comparators are generally used for linear measurements, and the various comparators currently available basically differ in their methods of amplifying and recording the variations measured.
We can classify comparators into mechanical device and electrical device on the basis of the means used for comparison. In recent times, engineers prefer to classify comparators as low and high-amplification comparators, which also reflect the sophistication of the technology that is behind these devices. Accordingly, we can draw the following classification. With respect to the principle used for amplifying and recording measurements, comparators are classified as follows:
1. Mechanical comparators
2. Mechanical–optical comparators
3. Electrical and electronic comparators
4. Pneumatic comparators
5. Other types such as projection comparators and multi-check comparators
Each of these types of comparators has many variants, which provide flexibility to the user to make an appropriate and economical selection for a particular metrological application.
Mechanical comparators have a long history and have been used for many centuries. They provide simple and cost-effective solutions. The skills for fabricating and using them can be learnt relatively easily compared to other types of comparators. The following are some of the important comparators in metrology.
Systems of Displacement Amplification used in Mechanical Comparators
(i) Rack and Pinion. In it the measuring spindle integral with a rack, engages a pinion which amplifies the movement of the plunger through a gear train. (Refer Fig.1)
(ii) Cam and Gear train. In this case the measuring spindle acts on a cam which transmits the motion to the amplifying gear train. (Refer Fig. 2)
(iii) Lever with Toothed sector. In this case a lever with a toothed sector at its end engages a pinion in the hub of a crown gear sector which further meshes with a final pinion to produce indication. (Refer Fig. 3)
(iv) Compound Levers. Here levers forming a couple with compound action are connected through segments and pinion to produce final pointer movement.
(v) Twisted Taut Strip. The movement of the measuring spindle tilts the knee causing straining which further causes the twisted taut band to rotate proportionally. The motion of the strip is displayed by the attached pointer.
(vi) Lever combined with band wound around drum. In this case, the movement of the measuring spindle tilts the hinged block, causing swing of the fork which induces rotation of the drum.
(vii) Reeds combined with optical display. In this case parallelogram reeds are used which transfer measuring spindle movement to a deflecting reed whose extension carries a target utilized in an optical path.
Dial Indicator
The dial indicator or the dial gauge is one of the simplest and the most widely used comparators. It is primarily used to compare workpieces against a master. The basic features of a dial gauge consist of a body with a circular graduated dial, a contact point connected to a gear train, and an indicating hand that directly indicates the linear displacement of the contact point.
The contact point is first set against the master, and the dial scale is set to zero by rotating the bezel. Now, the master is removed and the workpiece is set below the contact point; the difference in dimensions between the master and the workpiece can be directly read on the dial scale. Dial gauges are used along with V-blocks in a metrology laboratory to check the roundness of components. A dial gauge is also part of standard measuring devices such as bore gauges, depth gauges, and vibrometers. The contact point in a dial indicator is of an interchangeable type and provides versatility to the instrument. It is available as a mounting and in a variety of hard, wear-resistant materials. Heat-treated steel, boron carbide, sapphire, and diamond are some of the preferred materials. Although flat and round contact points are commonly used, tapered and button-type contact points are also used in some applications. The stem holds the contact point and provides the required length and rigidity for ease of measurement. The bezel clamp enables locking of the dial after setting the scale to zero. The scale of the dial indicator, usually referred to as dial, provides the required least count for measurement, which normally varies from 0.01 to 0.05 mm. The scale has a limited range of linear measurements, varying from 5 to 25 mm. In order to meet close least count, the dial has to be large enough to improve readability.
The dials are of two types: continuous and balanced. A continuous dial has graduations starting from zero and extends to the end of the recommended range. It can be either clockwise or anti-clockwise. The dial corresponds to the unilateral tolerance of dimensions. On the other hand, a balanced dial has graduations marked both ways of zero. This dial corresponds to the use of bilateral tolerance. Figure 8 illustrates the difference between the two types of dials. Metrological features of a dial indicator differ entirely from measuring instruments such as slide calipers or micrometers. It measures neither the actual dimension nor does it have a reference point. It measures the amount of deviation with respect to a standard. In other words, we measure not length, but change in length. In a way, this comparison measurement is dynamic, unlike direct measurement, which is static. Obviously, the ability to detect and measure the change is the sensitivity of the instrument.
Lever Comparators
A Lever Comparator is a simple and important type of mechanical comparator. It employs a ‘lever’ to obtain magnification of movement or displacement.
Principle of Operation:
The Principle of operation of lever type comparator is shown in Fig. 9, First of all, a pile of slip gauges of standard dimension is placed to anvil surface, below the plunger and the pointer set to zero.Now, place the component to be measured on the anvil surface below plunger by removing the pile of slip gauges.If there is any difference in size, the plunger moves up and down. These plunger movements are magnified, by lever and deflect the pointer on a graduated scale.A compression spring limits the measuring pressure. The magnification achieved depends upon the length of lever on both sides of the pivot.
Reed Type Mechanical Comparator
In these types of Mechanical comparator is used for magnifying the small motions of the spindle, where the amplification obtained is less than 100.
Construction:
It consists of a fixed block A which is rigidly fastened between floating block B and the gauge head case which carries the gauging spindle and is connected horizontally to the fixed blocks by reeds C. Vertical needs are attached to each block with top ends joined together. It contains a pointer, which is the most important item.
This setup is shown in the figure:
Working:
The reed mechanism is considered to be the friction-less device. Here the linear motion of the comparator spindle moves the floating block upward which raises the vertical reeds, resulting in the deflector of the pointer. The amount of deflection is directly proportional to the distance of the spindle displacement.
Johansson Mikrokator
The basic element in this type of comparator is a light pointer made of glass fixed to a thin twisted metal strip. Most of us, during childhood, would be familiar with a simple toy having a button spinning on a loop of string. Whenever the loop is pulled outwards, the string unwinds, thereby spinning the button at high speed. This type of comparator, which was developed by the Johansson Ltd Company of USA, uses this principle in an ingenious manner to obtain high mechanical magnification. The basic principle is also referred to as the ‘Abramson movement’ after H. Abramson who developed the comparator.
The two halves of the thin metal strip, which carries the light pointer, are twisted in opposite directions. Therefore, any pull on the strip will cause the pointer to rotate. While one end of the strip is fixed to an adjustable cantilever link, the other end is anchored to a bell crank lever, as shown in Fig. The other end of the bell crank lever is fixed to a plunger. Any linear motion of the plunger will result in a movement of the bell crank lever, which exerts either a push or a pull force on the metal strip. Accordingly, the glass pointer will rotate either clockwise or anticlockwise, depending on the direction of plunger movement. The comparator is designed in such a fashion that even a minute movement of the plunger will cause a perceptible rotation of the glass pointer. A calibrated scale is employed with the pointer so that any axial movement of the plunger can be recorded conveniently. We can easily see the relationship of the length and width of the strip with the degree of amplification. Thus, dq /dl ∝ l/nw2, where dθ/dl is the amplification of the mikrokator, l is the length of the metal strip measured along the neutral axis, n is the number of turns on the metal strip, and w is the width of the metal strip. It is clear from the preceding equation that magnification varies inversely with the number of turns and width of the metal strip. The lesser the number of turns and thinner the strip, the higher is the magnification.
On the other hand, magnification varies directly with the length of the metal strip. These three parameters are varied optimally to get a compact but robust instrument. A pull on the metal strip subjects it to tensile force. In order to prevent excessive stress on the central portion of the metal strip, perforations are made in the strip, which can be noticed in Fig. 11 slit washer is provided to arrest the rotation of the plunger along its axis.
Sigma Comparator
Different parts of this comparator are:
(i) Base: It consists of a cast iron base, for mounting all the parts of the comparator along with the work component to be measured.
(ii) Column: It consists of a threaded vertical column, mounted on the base to hold the measuring head.
(iii) Measuring Head: It consists of a Measuring Head, mounted on the vertical threaded column. Measuring Head provided with pointer, scale, tolerance pointer setting control knobs, trigger, measuring contract tip.
(iv) Work Table: A work table is provided at bottom of the column, below the measuring head, having perfectly plain horizontal surface for placement of component to be measured or checked.
(v) Vertical Spindle: Measuring head carries a vertical spindle which is mounted on two flat steel springs. The spindle works inside fixed guides attached to the back plate of the head. This arrangement provides a frictionless movement of the spindle. The springs provide a resistant pressure on the spindle.
(vi) Measuring Contact Tip: A measuring contact tip is fitted with a shank and shank is fitted to spindle.
(vii) Stop: A stop is suitably provided in the assembly to restrict the spindle movement at the lowest position of the scale.
(viii) Trigger: A trigger lever projects outside the measuring head. This is incorporated in the mechanism for elevating the measuring contact when required.
Procedure:
For checking the size of a component, the dial pointer is first set to zero reading by means of a combination of slip gauges of standard dimensions, resting on the work table. This combination of slip gauges then replaced by the work piece and difference in dimensions is noted from the movement of a pointer on graduated scale.
Advantages of Mechanical Comparators:
(i) Low Cost: These instruments are usually cheaper than other types of comparators.
(ii) Need no Electricity: These instruments do not required any external source of power supply or air as in the case of pneumatic or electrical comparators. Hence outside sources do not affect the accuracy of the comparator.
(iii) Linear Scale: These installments usually have linear scale, which is easy to read.
(iv) Easy to Handle: These installments are usually robust and compact so easy to handle.
(v) Suitable For Workshop: These instruments are portable and can be issued from the store keeper in the workshop.
Disadvantages of Mechanical Comparators:
(i) Friction is More: These instruments usually have many moving linkages as compared to the other type of comparators. Due to more moving parts, the friction is more.
(ii) Inertia is More: These instruments usually have more inertia. Hence these instruments are very sensitive to vibrations.
(iii) Accuracy is Less: These instruments usually have low accuracy due to more friction and high inertia.
(iv) Wear, Play, Backlash: Any wear, play, backlash or dimensional inaccuracy in the device used will also be magnified. This increases the error in the measurement.
(v) Range is Limited: These instruments usually have limited range of measurement, as the pointer moves over a fixed scale.
(vi) Parallax Error: These instruments are usually affected with error due to parallax as the pointer moves over a fixed scale.
Written by:
Swaraj Kothekar
Onkar Kotmire
Kshitij Dwivedi
Prathmesh Kudale
Yohaan Kudtarkar
