Measuring Instruments

1. Measuring Instruments

2. Metrology • science of weights and measures

3. Discrimination • degree to which a measuring instrument divides a basic unit of length

4. 10 to one rule for discrimination • a measuring instrument should discriminate 10 times finer than the smallest unit that it will be used to measure • instrument lands between two lines - do not use to measure

5. Parallax error • the apparent shift of an object when viewed from different angles

6. Calibration • Process by which a measuring instrument is compared to a known standard • The international prototype kilogram is a platinum iridium alloy, created in 1879, that is the standard by which all the world’s weight measurements are calibrated. It is lock in a Paris vault and is only brought out on very rare occasions • The length of a meter is defined by the distance light travels in a vacuum in 1/299,792,458 seconds

8. Handle precision measuring instruments with care

9. Use of precision measuring instruments • keep calibrated • when possible, take readings while the instrument is in contact with the work piece • on cylindrical work pieces, check twice - 90 degrees apart • best temperature is 68 degrees F.

10. Direct reading instruments

11. 1. Steel Rules

12. 2. Calipers (Can be Vernier, Dial, or Digital Reading) • Outside • Inside • Depth

13. 3. Micrometers (Can be Vernier, Dial, or Digital Reading) • Outside • frame, anvil, spindle, sleeve, thimble • Inside • Depth

14. Comparison measuring instruments • comparing to a known standard • represents a known distance • used in conjunction with an instrument that has the capability of showing measurement • sometimes referred to as transfer measurement • direct reading instruments should be used whenever possible

15. Common types • 1. spring calipers • 2. telescoping gages • 3. small hole gages • 4. adjustable parallels • 5. radius gages • 6. thickness gages • 7. planer gages (similar to adjustable parallels)

16. 8. Squares • a. machinists combination square set • protractor • centering head • b. solid beam square • c. cylindrical square

17. 9. Indicators • balanced vs. continuous readings • mounting must be rigid • magnetic bases • flexible link holding arms • height gage applications (test)

18. 2 types of indicators • a. dial • some can be used to make linear measurement • discriminations range from .00005 to .001” • ranges are from .003 to 2.0” • b. test • used for comparisons only (no direct measurements) • discrimination approx .0005” • range of about .030”

19. Angle Terminology • acute angles = < 90 degrees • right angles = 90 degrees • obtuse angles = > degrees • full circle = 360 degrees • 1 degree = 60 minutes of an arc • 1 minute = 60 seconds of an arc • radian = metric unit of angular measurement

20. Angle measuring instruments (not for measuring angularity)

21. 1. Plate protractor • discrimination of 1 degree

22. 2. Bevel protractor • part of the machinists combination set - discrimination of 1 degree

23. 3. Universal bevel protractor • includes a vernier scale • measure to accurately to within 5 minutes of a degree (remember there are 60 minutes in one degree

24. 4. Sine bar • 5” and 10” center spacing of cylinders

25. To find angles with the sine bar: • elevate the sine bar with adjustable parallels until the indicator reads zero at each end of the workpiece • parallel is removed and measured with a micrometer • transpose the sine bar elevation formula and solve for the angle • sin of the angle = elevation / sin bar length (5 or 10”)

26. Gage blocks • permit comparison between working measurement and instruments of mfg.

27. Uses: • calibration of measuring instruments • establishment of precise angles (with sine bars) • positioning of machine tools and cutting tools • used to set snap gages

28. Common set has 81 to 88 blocks

29. Range in thickness from .050 to 4.0”

30. 3 Grades • grade 1 = lab • grade 2 = inspection • grade 3 = shop

31. Grade on tolerance = +/- .000002”

32. Tolerance on a stack of 30 gage blocks assembled to represent 20”?

33. Use as few combinations as possible when stacking

34. Care • avoid unnecessary contact with hands • clean with proper solvents • handle and lay on lint free cloth • don’t leave blocks rung for extended periods of time • spray with a preservative

35. Wringing gage blocks

36. Main cause of gage block wear is wringing poorly cleaned blocks

37. Checking with optical flats

38. Wear blocks • used in applications where direct contact is made • usually .050” or .100”

39. Accessories • scribes • bases • screw sets

40. Other machines • Optical comparator • Coordinate measuring machine (CMM)