PROBLEMS
Grade 00 This set is kept in the standards room and is used for inspection/calibration of high precision only. It is also used to check the accuracy of the workshop and grade 1 slip gauges
5.6 OPTICAL INSTRUMENTS FOR ANGULAR MEASUREMENT
5.6.3 Angle Dekkor
An angle dekkor is a small variation of the autocollimator. This instrument is essentially used as a comparator and measures the change in angular position of the reflector in two planes.
It has an illuminated scale, which receives light directed through a prism. The light beam carrying the image of the illuminated scale passes through the collimating lens, as shown in Fig. 5.26, and falls onto the reflecting surface of the workpiece. After getting reflected from the workpiece, it is refocused by the lens in field view of the eyepiece. While doing so, the image of the illuminated scale would have undergone a rotation of 90° with respect to the optical axis. Now, the light beam will pass through the datum scale fixed across the path of the light beam, as shown in Fig. 5.26. When viewed through the eyepiece, the reading on the illuminated scale measures angular deviations from one axis at 90° to the optical axis, and the reading on the fixed datum scale measures the deviation about an axis mutually perpendicular to this.
The view through the eyepiece, which gives the point of intersection of the two scales, is shown in Fig. 5.27. The scales usually measure up to an accuracy of 1'. This reading actually indicates changes in angular position of the reflector in two planes. In other words, the initial
Eyepiece
Workpiece Light
source
Datum scale Illuminated
scale
Collimating lens Prism
Fig. 5.26 Angle dekkor
0 10 30 40
10 20 40 50 60
Fig. 5.27 Intersection of two scales
reading of the angle dekkor corresponds to the reading on the two scales before shifting the position of the reflector. After the reflector undergoes an angular tilt, the second reading is noted down by recording the point of intersection on both scales. The difference in readings on the two scales indicates the tilt of the reflector in two planes at 90° to each other.
The optical system in an angle dekkor is enclosed in a tube, which is mounted on an adjustable bracket. It has a wide range of applications, as angular variations can be directly read through the eyepiece of the instrument. Some of the typical applications are as follows:
1. Measurement of sloping angle of V-blocks 2. Calibration of taper gauges
3. Measurement of angles of conical parts
4. Measurement of angles of work part surfaces, which are simultaneously inclined in two planes
5. Determination of a precise angular setting for machining operations, for example, milling a slot at some precise angle to a previously machined datum surface.
A QUICK OVERVIEW
• The precise measurement of angles is an imp- ortant requirement in workshops and tool rooms.
We need to measure angles of interchangeable parts, gears, jigs, fixtures, etc. There are a wide range of instruments, starting from simple scaled instruments to sophisticated types, which use laser interferometry techniques that can be used for angle measurement.
• The universal bevel protractor with a 5' accuracy is commonly found in all tool rooms and metrology laboratories. Since the universal bevel protractor can measure both acute and obtuse angles, care should be exercised to clearly differentiate between the angle being indicated on the scale and its supplement.
• The sine bar is used to measure angles based on the sine principle. Simple variants of the sine bar such as sine block, sine plate, sine table, and sine centre have many applications in metrology.
• Angle gauges, which are made of high-grade wear-resistant steel, work in a manner similar to slip gauges. While slip gauges can be built to give linear dimensions, angle gauges can be built
to give the required angle. The gauges come in a standard set of angle blocks that can be wrung together in a suitable combination to build an angle.
• The main use of spirit level-based instruments like clinometers is not for measuring angles per se. They are used for measuring alignment of machine parts and determination of flatness and straightness.
• Optical instruments enable very precise mea- surement of angles. The four principles that govern the application of optics in metrology are magnification, accuracy, alignment, and interferometry. While the autocollimator can measure small angles with very high sensitivity, an angle dekkor can be used as a comparator to measure the change in angular position of the object in two planes. An autocollimator is used for the measurement of straightness and flatness of machine parts and accessories such as guide- ways, machine tables, and surface plates, among others.
ANGULAR MEASUREMENT 139
MULTIPLE-CHOICE QUESTIONS
1. While measuring the angle of a workpiece using a universal bevel protractor in clockwise direction, in which quadrants can the angle be read directly from the scale?
(a) Quadrants I and II (b) Quadrants I and III (c) Quadrant I only (d) All the quadrants
2. Which of the following statements is false with respect to a bevel protractor?
(a) If the angle of a work part is being measured in II quadrant, the actual angle is given by the supplement.
(b) If the angle of a work part is being measured in IV quadrant, the actual angle is given by the supplement.
(c) The angle measured in II quadrant is always an obtuse angle.
(d) The angle measured in IV quadrant is always an acute angle.
3. Which type of bevel protractor has a vernier scale as well as an acute angle attachment?
(a) Type A (c) Type C (b) Type B (d) Type D
4. The purpose of providing relief holes in sine bars is to
(a) improve accuracy (c) reduce weight (b) improve precision (d) reduce wear
5. The maximum angle that can be set using a sine bar is limited to
(a) 15° (c) 45°
(b) 30° (d) 60°
6. Which of the following statements is true?
(a) The longer the sine bar, the better the accuracy.
(b) The shorter the sine bar, the better the accuracy.
(c) Accuracy of a sine bar does not depend on an ambient temperature.
(d) A sine bar cannot measure unknown angles.
7. What is the minimum number of angle gauges required to set any angle between 0° and 180° in increments of 5'?
(a) 8 (c) 12
(b) 10 (d) 15
8. Which of the following statements is true?
(a) An angle dekkor is a small variation of a clinometer.
(b) An angle dekkor is a small variation of an autocollimator.
(c) An angle dekkor is a small variation of a sine bar.
(d) None of these
9. Which of the following instruments is capable of measuring compound angles?
(a) Sine centre
(b) Compound sine plate (c) Compound surface plate (d) All of these
10. Which of the following is correct if the 5° angle block is reversed and combined with the 30°
angle block?
(a) The resulting angle becomes 25°.
(b) The resulting angle becomes 35°.
(c) The resulting angle remains 30°.
(d) Such a combination is not possible.
11. It is much easier to calibrate angle gauge blocks compared to slip gauges, because
(a) angle gauges are made of superior material (b) there are better instruments to calibrate
angles rather than linear dimensions (c) the angle being measured is always a portion
of a triangle comprising 180°
(d) the angle being measured is a portion of a full circle and is, therefore, self-proving 12. Which of the following can extend the range of
the angle block set to 360°?
(a) True square (c) Combination set (b) Try square (d) Sine plate
13. The performance of the spirit level is governed by the geometrical relationship between the bubble and
(a) a single datum (b) its top plate (c) two references
(d) three references along mutually per- pendicular directions
14. The sensitivity of a spirit level depends on
(a) the width of the bubble (b) accuracy of base plate (c) both (a) and (b)
(d) the radius of curvature of the bubble tube 15. Precision measurement of angles of non-mirror-
quality surfaces is possible only with (a) a laser autocollimator
(b) a visual autocollimator (c) an angle dekkor (d) any of these
REVIEW QUESTIONS
1. What is the chief difference between length standard and angle standard?
2. What is the working principle of a universal bevel protractor? What are the precautions to be taken while using it?
3. The vernier scale in a bevel protractor is read in the same direction as the dial. Why?
4. How does an optical bevel protractor differ from a mechanical type?
5. What is the basic difference between sine bars, sine plates, and sine tables?
6. In a sine bar, when should the set-up be made for the complement of an angle?
7. Discuss the essential requirements for main- taining accuracy in the construction of a sine bar.
8. How is a sine bar specified?
9. What is the maximum recommended angle to which a sine bar can be set? Discuss the relationship between the angle being set and the error of measurement for a sine bar.
10. Explain how conical workpieces are inspected on a sine centre.
11. Why are angle gauges also referred to as
‘Tomlinson gauges’?
12. Using the set of 16 angle gauges, how can we set the angle 20°40'10''?
13. The compound angle of a forging die needs to be inspected. What is the procedure of inspection using angle gauges? Discuss with a simple sketch.
14. The main aim of clinometers is not to measure angles, but to measure alignment, straightness, and parallelism. Clarify.
15. Describe with a sketch the principle behind the working of an autocollimator.
16. Discuss any two important uses of an auto- collimator in the industry.
17. Give the differences among the following:
visual collimator, digital collimator, and laser collimator.
18. Explain with the help of a plot how straightness of a machine guideway is assessed using an autocollimator.
19. How does an angle dekkor differ from an autocollimator?
20. Discuss the applications of an angle dekkor in metrology.
Answers to Multiple-choice Questions
1. (b) 2. (d) 3. (a) 4. (c) 5. (c) 6. (a) 7. (b) 8. (b) 9. (b) 10. (a) 11. (d) 12. (a) 13. (c) 14. (d) 15. (a)