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. Variables (Errors)

11. 1. Observational errors
misreading the gage
parallax

12. 2. Manipulative errors holding the gage incorrectly
not locating datums properly
mounting the part on the wrong datums
not aligning the gage properly
measuring pressure /ratchet stops

13. 3. Bias errors
rounding off

14. 4. Gage errors precision loss (sluggish or sticking indicator)
accuracy loss (incorrect gage block stack, bent micrometer frame, out of calibration error

15. 5. Part errors Within the piece variation (taper, roundness, etc.)
dirty parts
poor surface finish
flaws/ burrs

16. Direct reading instruments

17. 1. Steel Rules

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

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

20. 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

21. Common types 1. spring calipers 2. telescoping gages
3. small hole gages

22. 4. adjustable parallels
5. planer gages

23. 6. thickness gages
7. radius gages

24. Attribute Gages
Go - No Go gages that only tell if the part is good or bad. They don’t provide a measurement of how good or bad.

25. 8. Squares
Machinists Combination Square Set

26. solid beam square
Cylindrical Square

27. Indicators – 2 Types a. dial
some can be used to make linear measurement
discriminations range from to .001”
ranges are from .003 to 2.0”

28. b. test used for comparisons only (no direct measurements)
discrimination approx .0005”
range of about .030”

29. Indicator Bases mounting must be rigid magnetic bases
flexible link holding arms
height gage applications

30. Surface Plates

31. Primary measurement is height
measurements are from the plate surface, up

32. Ideal plane for dimensional measurement should be perfectly flat
nothing is perfect

33. Surface plate materials
1. Cast iron
magnetic
if damaged, raised material is produced on surface
easily rust
2. Granite
far superior
produce no raised material
lower in price
retain flatness longer than cast iron
do not rust

34. Care of surface plates no unnecessary objects on the surface
clean before and after use
keep it covered
placing and removing objects from the plate

35. Accessories Gage blocks (rect. and square) Angle plate (Knee) V-blocks
Fixed parallels
Planer gage
1,2,3 Blocks

38. 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

39. Angle measuring instruments (not for measuring angularity)

40. 1. Plate protractor
discrimination of 1 degree

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

42. 3. Universal bevel protractor - vernier
measure to accurately to within 5 minutes of a degree (remember there are 60 minutes in one degree

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

46. 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”)

47. Gage blocks
permit comparison between working measurement & instruments of mfg.

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

49. Common set has 81 to 88 blocks

50. Range in thickness from .050 to 4.0”

51. 3 Grades
grade 1 = lab
grade 2 = inspection
grade 3 = shop

52. Grade on tolerance = +/- .000002”

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

54. Use as few combinations as possible when stacking

55. 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

56. Wringing gage blocks

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

58. Checking with optical flats

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

60. Accessories
scribes
bases
screw sets

61. Other machines
Optical comparator
Coordinate measuring machine (CMM)