VJCET
MECHANICAL MEASUREMENTS LABORATORY
STRAIGHTNESS MEASUREMENT USING AUTOCOLLIMATOR AIM:
To test the straightness of milling machine table using Autocollimator. INSTRUMENTS REQUIRED: Autocollimator, Reflector. PRINCIPLE OF AUTOCOLLIMATOR: AUTOCOLLIMATOR:
The Autoco Autocolli llimat mator or is a precess precession ion optica opticall instru instrumen mentt for measuri measuring ng very very small small angular displacement over a significant distance. They can be used for evaluating alignment of machine surface, surface plate flatness, square ness of one surface to another, straightness, and a variety of other orientation measurements. A bright light source is condensed through a lens. The light then goes through a graticule with an etched image. This is then projected through a beam splitter, which projects the the imag imagee on the the grat gratic icul ulee thro throug ugh h the the coll collim imati ating ng lenses lenses of the the auto autoco coll llim imat ator or.. The The collimated image goes to a precision mirror at the surface to be evaluated. The mirror reflects the image back through the collimating lenses of the interferometer. If the mirror is precisely perpendicular to the collimated light path, the image will be perfectly aligned with the viewing optic graticule. Otherwise, it will appear as an offset image. image. The distan distance ce (or angle) angle) of, deviation deviation,, can be determ determine ined d by gradua graduatio tions ns on the graticule or by adjusting the cross hairs of the graticule to coincide with the image. In the latter type, the amount of movement is tracked by graduations on the adjusting screws that move the cross hairs.
Fig. 1: Autocollimator
An Autoco Autocolli llimato matorr is basical basically ly a telesco telescope, pe, perman permanent ently ly focuse focused d at infini infinity. ty. The Autocollimator tube is mounted horizontally on a stand provided with leveling screws. It consist consistss of an achrom achromati aticc objecti objective ve lens, lens, Beam Beam Splitt Splitter er & Eyepie Eyepiece ce Gratic Graticule. ule. It also incorporates an illuminated target Graticule. The image of the transmitted type target cross line line Grat Gratic icul ulee is proj projec ected ted alon along g line line of sight sight with with the the help help of a 6V, 6V, 2W, 2W, bulb bulb,, and and condensing condensing lens from the side of the tube with the help of the beam Splitter. Every point point of the target Graticule produces a parallel beam of light from the Objective lens. A plane reflector is positioned with its plane normal to the axis of the Autocollimator. The instrument STRAIGHTNESS MEASUREMENT USING AUTOCOLLIMATOR
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VJCET
MECHANICAL MEASUREMENTS LABORATORY
uses one of the basic principles of reflection that a parallel beam of light, projected on to a plane reflector square to the beam, is reflected along the same path. The reflected parallel beam of light entering the lens is brought to the focus in a focal plane perpendicular to the plane of the target Graticule by the beam splitter. At this focal plane position the Eyepiece Graticule is mounted. The eyepiece Graticule and the reflected image of the target Graticule are both viewed simultaneously through the eyepiece. Whenever reflector is tilted with respect to the incident beam, the return angle of the reflected beam is twice the angular displacement of the reflector. The amount of this displacement can be read off directly through the instrument eyepiece or by micrometer. PROCEDURE:
1. Place the Autocollimator with the stand on a surface to be inspected. 2. Place the reflector along the axis of the Auto collimator. 3. Connect the Auto collimator illumination leads to the power supply. Switch on and turn the brightness to the maximum. 4. Adjust the stand so that the Auto collimator appears to be pointing straight at the flat reflective surface. 5. View through the Eyepiece and try to locate the cross line image of the target Graticule. 6. Check that the returned cross line image is in the field of the view. 7. Observe through the eyepiece and adjust the micrometer so that the eyepiece Graticule horizontal line is made to coincide with the horizontal line of image of the target Graticule. 8. Note the readings on the micrometer drum. This reading is the reference for all subsequent observations. (Position A). 9. Move the reflector to Position B, 100 mm away along the same axis from the earlier position A. 10. Observe through the eyepiece, you will observe a gap between the two horizontal lines if there is a level difference between the position A and B. 11. By means of the micrometer move the horizontal line to coincide again. Note the micrometer reading-Position B. 12. The difference in the readings for Position A and B refers to the angular tilt at position B. 13. The angular tilt is always referred to a length of 100 mm being the length of the reflector. The angular tilt is read directly in seconds (difference in micrometer readings). The angles are then converted in microns as given below. tan θ =X/100.
X = (100*Tan θ) *1000 in microns X represents the level at position B with respect to position A. 14. Once again move the position of the reflector to position C, 100 mm away from position B and repeat steps 10 to 11. 15. The difference in the readings for Position A and C refers to the angular tilt at position C. 16. Calculate X as explained in step 13 for position C with reference to Position A. 17. Repeat the above observation for various positions along a straight line. 18. Tabulate the readings and plot a graph to know the straightness of the surface plate. 19. Find the Straightness error.
STRAIGHTNESS MEASUREMENT USING AUTOCOLLIMATOR
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VJCET
MECHANICAL MEASUREMENTS LABORATORY
OBSERVATIONS: Angular Angular Level Cumulative Error of Position Position in Level ,X Adjustment 0 difference level Straightness tilt, θ tilt θ No. mm (microns) required (microns) difference (microns) (sec) (degrees)
SAMPLE CALCULATION:
1. Level in microns, ‘X’
= (100*tan θ) *1000 microns
2. Level difference
= Level in microns – Level in microns of initial position A
3. Adjustment required
= (Cumulative level difference of final position) X .(
Pr esent . position Final . position
)
4. Error of straightness
= Cumulative level difference – Adjustment required
5. Straightness error
= Max Error of straightness – Min Error of straightness
SAMPLE GRAPH: 5
Fig. 2: Graph showing error Vs distance RESULT:
4 r o 3 r r E s 2 s e n 1 t h g 0 i a r t -1 S
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The straightness -2 error is -3 calculated for Distance the given surface using Autocollimator. The straightness graph is plotted.
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Straightness error = INFERENCE:
STRAIGHTNESS MEASUREMENT USING AUTOCOLLIMATOR
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