ASSIGNMENT 1

1. List at least 5 characteristics of the following measurement standards

A) Line Standards:

(i)Scale can be accurately emblemed, but the engraved lines posses thickness and it is not possible to accurately measure

(ii)Scale is used over a wide range

(iii)Scale markings are subjected to wear. However the ends are subjected to wear and this leads to undersize measurements

(iv)Scale does not posses built in datum. Therefore it is not possible to the scale with the axis of measurement

(v)Scales are subjected to parallax errors

(vi)Assistance of magnifying glass or microscope is required.

B) End Standards

(i)Highly accurate and used for measurement of closed tolerances in precision engineering as well as standard laboratories, tool rooms, inspection departments

(ii)They require more time for measurement and measure only one dimension.

(iii)They wear at their measuring faces

(iv)They are not subjected to parallax error.

C) Wavelength standards

(i)Can’t be destroyed

(ii)Is subjected to destruction by wear and tear

(iii)Can be used for making comparative measurements of very high accuracy

(iv)It gives the unit of length which can be produced consistently at all times

(v)The standard facility can be easily available in all standard laboratories and industries.

2. Give 3 design requirements of a sine bar which must be applied to maintain high accuracy in angular measurements.

i. The roller or plugs must be of equal diameter and true geometric cylindersiiThe distance between the roller axes must be precise and known, and these axes must be mutually paralleliii. The edge of the bar used for measuring must be flat and parallel with the roller axes, and equidistant with each

3.

CAT 1

A) Essential requirements of Comparators

i) Robust design and construction – enables it to withstand the effects of ordinary use without affecting its accuracy

ii) Linear scale – Recording/measuring scale should be linear and uniform and its indications should be clear

iii) High magnification –should be such that a small deviation can be easily detected

iv) Versatility – Should be so designed that it can be used for a wide range of measurements

v) Quick in results – Indicating system should be such that readings are obtained in less time

vi) Indicator should come to rest rapidly and should be free from oscillations

B) Give 4 advantages achieved in use of Comparators over direct measurement using work standards

i) The smallest increment measurable using gauge blocks to measure, say, a gap gauge directly is the smallest increment in the gauge blocks.A comparator subdivides these increments

ii) The measuring pressure can be better controlled using a comparator,the pressure being the same for all cases including the setting& reading from the instruments

iii)The range over which measurements are made and the chance of errors due to cumulative errors in the instrument movement is reduced

iv) There is rapid measurement in mass production since direct measurement is time consuming

v)There is high accuracy since the accuracyis independent of correct feel or operator skill compared to direct measurement

C) Briefly describe with the aid of sketches the mechanisms of movement of;

I)Twisted-strip or Johansson Mikrokator

It has the pointer mounted at the centre of a thin strip of metal permanently twisted in opposite directions on either side of the pointer. As the plunger is raised its movement is transmitted to the strip by the bell-crank lever arrangement, causing the strip to stretch and unwind, and the pointer to move across the scale.

DIAGRAM

II)Sigma Comparator

Its amplification is obtained by means of a compound lever arrangement. As the plunger is raised, the moving block follows the knife edge and the Y-shaped arm moves about the cross-strip pivot. A phosphor-bronze driving band is carried from the ends of the Y-arm about a

driving drum of radius r to which is attached a pointer of length R.Thus as the Y-arm moves it causes the drum to rotate, carrying the pointer with it.

DIAGRAM

III)Optical Comparator

All optical comparators involve some system of magnification, generally through tilting of a mirror which provides an optical lever by reflecting a beam of light .The Cooke comparator works on this principle.

  Cooke’s Optical Comparator

 A plunger working in a head consists of a mechanical lever carrying two pivots at its ends. On one end a plunger actuates it and the other end actuates a mirror. A circular scale is provided. The mirror onto the scale accordingly reflects a beam of light coming through an electric bulb.

Optical comparators are used in metrology labs and standard room, but not in routine production checking .The optical system offers the advantage of lightness & simplicity in its indicating unit.

IV)Pneumatic Comparator

A pneumatic gauge consists of 2 important Units:

•An air controller to regulate the pressure and the amount of airflow from the supply. The unit incorporates a manometer

•A gauging head designed for the work to be checked.

Air supply from the supply is fed into the instrument at pressure higher than the cons tan t p ressure requ i red in the manomete r . Ai r en te r s the tube ex tend ing downwards into a tank of liquid. Initially the tube is filled with liquid to the same level as that in the tank. Entry of air into the top of the tube exerts pressure on the liquid to completely empty it. Any excess pressure than that necessary to clear the tube will escape into the tank as air bubbles. The pressure between the valve V and the control jet G is therefore always the same, irrespective of any variation in the air supply pressure.The air will now pass through the control jet at the full controlled pressure and will reach the measuring jet S. If this jet S cannot pass the full volume of the air from the control jet, then a pressure will tend to develop between them. The backpressure i s ins tan t ly re leased th rough the open ing in to the manomete r tube where it will change the height of the liquid, which indicates the amount of backpressure built up. The back pressure is the result of restriction at the measuring  jet due to the effect of variations in the dimension of the work being checked so that the variations in the height of the liquid of the manometer are a measure of the dimension variations.

The pneumatic method is easily adaptable for the examination of bores, since the machining element can be housed inside the plug used for accommodating the component. This method is

very simple and minimum wear of working parts takes place, but it requires a supply of air to provide the motive force.

V) Electrical Comparator

Electrical and electronic comparators depend on Wheatstone bridge circuit for their operations. We know that for the bridge is to balance electrically the ratio of the resistance’s in each pair must be equal. Fig 3.2

The principle of electrical comparator (electrical limit gauge) is explained with reference to the above figure. If alternating current is applied to the bridge, the inductance and capacitance of the arms must also be accounted for along with resistance. The pair of coils forms a pair of inductance. The movement of the plunger displaces an armature thus causing a variation in the inductance in the coils. The amount of unbalance caused by movement of measuring plunger is amplified and shown on a linear scale magnifications of about 30,000 are possible with this system. Zero setting arrangement is provided. The degree of magnification is adjustable and other examples of electrical comparators are electricator, electric gage and sigma electronic comparator.

VI)Fluid Displacement Comparator

A fine bore capillary tube is arranged so that its lower end is placed in a chamber of relatively large cross-section area and containing a fluid of low viscosity. At the bottom of the chamber, a diaphragm is fitted, which gets deflected by pressure transmitted to it by the measuring plunger. Due to deflection of diaphragm, a small quantity of liquid rises up in the capillary tube. A scale is arranged at the side of tube and limit pointers are set relative to this to indicate the high and low limits of the dimensions being measured.

CAT 2.

1. Explain using sketches how you can make measurements of tapers on the following;

i) Taper plug gauges

The angle of taper of a taper plug can be usually determined by using a sinebar on which are mounted centres for work holding.The measurements are normally made over calibrated precision rollersof equal diameter,the separation of individual measurements being controlled by gauge blocks.

Diagram

In triangle ABC, h is the height of the gauge blocks and BC=(M2 – M1)/2 in which M1 and M2

are the measurements over the rollers.

Tan 0= BC/AB = (M2 –M1)/2h

Thus, semi-angle of taper can be determined by direct measurement.

DMAX =DS + 2H Tan 0

DMIN =2(H-S) Tan 0

ii) Taper ring gauges

The procedure for measuring taper ring gauges is similar to that used for taper plug gauges,the measurements being made with precision calibrated balls and gauge blocks.

DIAGRAM

Tan 0 =(M1 – M2 )/2h

But in this case;

DL =M1 + d [1 + Cot (90 – 0)/2 ]

Where d = ball diameter

DMAX =DL – 2(H-S)Tan 0

DMIN =DL -2H Tan 0

iii) Taper bores (smaller diameter taper)

For smaller diameter tapers, angular and diametric measurements can be made by the arrangement shown, in which the heights at which balls of different diameters seat directly on the taper are measured.

The centre LM =h2 – h1 – r1 + r2

(Sin O)/2 =NL/LM = (r1 – r2)/ (h2 - h1 – r1 + r2)

The semi-angle of the taper having been found, its uniformity may be determined by taking further measurement on a ball of such diameter that it rests approximately midway along the length of the taper, and repeating the calculations for the new measured values.

diagram

ASSIGNMENT 2

1a) Explain with the aid of an optical system the principle of operation of an Angle Dekkor

A glass screen in the focal plane of a collimating lens has engraved on its underside a scale of minutes. It is illuminated via a prism and is projected by the collimating lens as a parallel beam of light, strikes a plane reflector and is refocused by the lens to appear as a reflected image on the screen. The image falls, not across a simple datum line but across a similar fixed scale at right angles to the illuminated image. Thus the reading on the illuminated scale measures angular deviations from one axis at 90 degrees to the optical axis, and the reading on the fixed scale gives the deviation about mutually at right angles to the other.

diagram

b) Using sketches explain the procedure of determining the accuracy of the included 37 degrees 10mins 18secs wedges using an angle dekkor.

c)Describe with the aid of sketches two other applications of an angle dekkor

2a) Explain 3 reasons why it is necessary to control the surface texture of a machined component

i)The better the surface finish, the longer the fatigue life. Surface irregularity causes failure due to fatigue

ii) Smooth surface finish reduces wear. Rate of wear depends on surface area in contact. A rough surface with large peaks & valleys will have less contact than a smooth one and its rate of wear will be greater.

iii) Surface texture makes it possible to measure the surface quality of a part and give its numerical value. The texture allows surfaces to be compared on the basis of an opinion as to which is rougher.

b) Describe the principle of operation of the Tomlinson’s surface meter

It uses mechano optical magnification methods. The body of the instrument is drawn slowly across the surface by a screw turned at 1 rev/min by a synchronous motor, the glass remaining stationary. Irregularities in the surface cause vertical movements of the stylus which cause the cross-roller to pivot about point A and thus produce a magnified motion at the marking diamond on the arm. This motion coupled with the horizontal movement produces a trace on the glass magnified in the vertical direction at 100 times, there being no horizontal magnification. The smoked glass is transferred to an optical projector and magnified further at 50 times. The overall vertical magnification thus becomes 5000 times and 50 times horizontally. The trace is then taken and analyzed.

DIAGRAM

c) Using sketches explain 2 methods of mathematically analyzing the traces obtained from the above instrument

3a) Explain the principle of an Auto-Collimator

Collimation and de-collimation are carried out by the same lens. There exists a point source of light ‘O’ in the focal plane of a collimating lens. It is projected as a parallel beam of light but instead of passing through a de-collimating lens it strikes a plane reflector set at 90 degrees to the optical axis. It is therefore reflected back along its own path and refocused at the source ‘O’ by the same lens through which it was projected; this is called auto-collimation.

If the reflector is now tilted through an angle ‘d’,the reflected beam will turn through ‘2d’ and will be refocused to produce an image at the source at O1,a distance x from the source O.Thus the distance between the image and the source is a measure of the angular displacement of the reflector.

diagram

b) How are the following tests performed using an Auto-Collimator

I. Testing the squareness of a horizontal machine table in relation to the movement of the vertical slide

2. The accuracy of rotation of a dividing head

c) Explain using a sketch the operating principle of a profile projector