Metrology is the measurement of dimensions to ensure parts are manufactured consistently and accurately. Inspection provides feedback on part size compared to specifications. Measurements are traditionally taken after manufacturing but new trends involve in-process inspection. Instruments are calibrated at 20°C for accuracy. Allowance is the intentional difference between mating parts, while tolerance is the permissible deviation from a desired dimension. Different measurement instruments like micrometers, calipers, and CMMs are used depending on the type of measurement and required precision. Optical instruments can measure delicate surfaces.

1. Manufacturing
Processes
Chap. 35 - Metrology

2. Metrology
• Definition:
– “The measurement of dimensions”.
• Relevance:
– Dimensions are measured to ensure that a part is
manufactured consistently and within the specified
range of accuracy.

3. Inspection
Objective:
• To provide feedback information on the actual
size of the part, with respect to (wrt) the specified
size in engineering drawing.
• Traditionally measurements are taken after a part
or component has been manufactured.
• New trend is for “in-process” inspection, taking
measurements while the part is being produced.

4. Standard Measuring Temperature
• Instruments are typically calibrated at 20 deg C / 68
deg F.
• One should try to take measurements at this
temperature to favor accuracy.
• This is the standard for precision measuring work. If
you are measuring accuracy greater that 0.0001”, you
usually measure in a controlled environment (ISO
regulated).

5. Allowance vs Tolerance
• Needed when you want to fit two mating parts.
•Allowance:Intentional difference in dimensions between mating
parts.
(~ Smallest exterior fitting part – Largest interior fitting part)
(smallest hole – largest shaft)
- Determines the tightest fit between the parts.
Allowance can be specified as:
- A clearance: largest shaft is smaller than smallest hole.
- An interference: hole is smaller that shaft.

6. Allowance vs Tolerance
• Tolerance: Undesirable but permissible deviation from a desired
dimension.
•Reason: No part can be made exactly to a specified dimension,
except by chance.
• Such level of exactness is not necessary or economical.
• It is necessary to allow deviation from theoretical or nominal
value.
• Deviation must be controlled so parts will function well together.

7. Allowance vs Tolerance
• Relevance:
Tolerances impact the proper functioning and manufacturing
cost of a part. The smaller the tolerance, the higher the
manufacturing cost.
Important only when a part is to be assembled or mated with
another part. Free/non-functional surfaces do not need close
tolerances.

8. Dimensional Tolerances
Can specify bilateral, lateral or limits.
• Bilateral: 2.000”  0.002”
• Unilateral: 2.000” + 0.000”
- 0.004”
• Limits: 2.002” 2.004”
1.998” 2.000”

9. Fits
• Fits are categorized in classes, 1 through 8, and are
specified according to the application.
– Loose, Free, Medium, Snug, Wringing, Tight, Medium, Heavy
Force and Shrink Fit
• For example:
– loose implies a large allowance where accuracy is not
essential.
– snug means zero allowance, no motion desired, tightest
achievable manual fit.
– shrink fit: large negative allowance, used for permanent
shrink on steel members.

10. Geometric Tolerances
• Maximum allowable deviation of a form or a position from a
perfect geometry (as implied by an engineering drawing).
• Tolerance represents the diameter or width of a zone required for
part accuracy.
– Form tolerances: Flatness, straightness, roundness, cylindricity.
– Profile: Line, Surface
– Orientation: Angularity, Perpendicularity, Parallelism
– Location: Position, Concentricity

11. Datums
• Reference entities from which tolerances are specified
or stated. They can be a point, an axis, a plane or
surface.
• Up to 3 datum surfaces can be used to specify
tolerance.
• Ex.

12. Datum Reference examples
• Straightness: indicates the limits of how much a surface or axis can bow wrt a
straight line.
• Flatness: The planar surface must lies between two parallel planes 0.50” apart.
• Perpendicularity: (Vertical) plane must be perp. To the reference within 0.5”
• Circularity Cylindricity:
– circular feature must be within a tolerance defined by two concentric
circles cylinders.
• Profile: acceptable deviation of an outline of an object from that specified.
0.20 - A -
0.50 - A -
0.010 - A -

13. Key Terms:
Accuracy: degree of agreement between measured
dimension and its true magnitude.
Precision: degree to which an instrument gives a repeated
measurement.
Resolution: smallest dimension that can be read on an
instrument.
Rule of 10: “An instrument should be 10 times more precise
than the dimensional tolerances of the part being
measured”: ~ gage capability.

14. Factors for selecting a proper measuring
instrument:
• Gage capability:
– (the gage must be 10 times more precise than the tolerance being
measured).
• Linearity:
– is calibration accurate over entire measuring range?
• Repeatability:
– can I take the same reading over and over over a standard?
• Stability:
– is calibration stable over time? How sensitive is it to temperature,
humidity?
• Sensitivity / resolution:
– the smallest difference in dimensions the instrument can detect.
• Magnification:
– the more accurate the device, the greater the magnif. factor it should have.

15. Factors for selecting a proper measuring
instrument:
• Size and type of part or features to be measured
• Environmental conditions
• Required Operator Skills
• Cost of Equipment
• Speed

16. Factors that contribute to deviation of
dimensions:
• Static/Dynamic deflections due to vibrations and
fluctuating forces.
• Variations in properties and dimensions of incoming
material.
• Distortion due to temperature changes.
• Tool wear.
• Human error.

17. Length Standards in Industry
• Gage Blocks
– Provide industry with linear standards of high accuracy.
– Used in everyday manufacturing.
– Conceived by Carl Johansson in 1900.
– Are highly precise, individual square, round or
rectangular blocks of various sizes.
– Can be assembled to achieve different lengths.
– Flat surfaces are ground to a mirror finish.

18. Length Standards in Industry
• Gage Blocks
– Have two very flat and parallel surfaces at a specified
distance apart. Flatness / parallelism within .00002”.
– Commonly used as accurate reference lengths.
– Are heat treated to relieve internal stresses and
minimize dimensional change.
– Can assemble by sliding one past another with hand
pressure. Can build any desired dimension.

19. Gages

20. Line Graduated Instruments
Linear – Direct Reading
Steel Rules/Scales: for making linear measurements;
accuracy up to 0.040”.
Vernier Calipers: for measuring inside and outside lengths;
Accuracy up to 0.001”. Also come with
digital readouts: less subject to human
errors.
Micrometers: for measuring thickness, inside or
outside dimensions of parts. Accuracy
up to 0.0001”. Digital mikes can be
hooked up to a PC for statistical process
control.

21. Line Graduated Instruments

22. Line Graduated Instruments
Linear – Indirect Reading
• Calipers/ Dividers: used to
transfer the measured size
to a direct –reading instrument
like a rule. Has limited accuracy.

23. Angle Measurement
• Surface Plate:
– A horizontal slab usually made of cast iron or
natural stone (granite).
– Used for its low thermal expansion, resistance
to corrosion and being non-magnetic.
• Angle Gage Blocks:
– can be assembled in various combinations and
are used similarly to a sine bar.

24. Angle Measurement

25. Bevel Protractor: Place blades of protractor
against part.
Combination Square: Used for 45 and 90 degree
angles.
Sine Bar: Place part on an inclined bar
and adjust angle via gage
blacks on a surface plate. Use dial indicator to scan
the surface of the part.
Angle Measurement

26. Angle Measurement

27. Comparative Length
- Dial Indicators: mechanical device that converts
linear displacement of a pointer to a rotation of an
indicator on a circular dial. Accuracy up to
.00004”.
- Electronic Gages: senses motion of contact pointer
through changes in resistance of a strain gage.
- Advantages: ease of operation, rapid response, digital
readout, reduced possibility of human error.

28. Comparative Length

29. Non-Contact Instruments
- Laser Scan Micrometer:
Used for rotating, vibrating, high temperature or
delicate parts. Resolution up to .000005”.

30. Measurement of Geometric Features
• Straightness:
– can use a straightedge, dial indicators, transits, or laser
beams.
• Flatness and Perpendicularity:
– can measured via a surface plate and a dial indicator.
• Roundness:
– the deviation from true roundness (a perfect circle).
Critical for proper functioning of rotating shafts,
pistons, etc.

31. Measurement of Geometric Features
- Full Indicator Movement Method:
- Place round part on V-Block, rotate the part while an indicator
touches the surface. Rotate the part 1 full turn. Difference between
max and min reading is the TIR (total indicator reading).
• Profile:
- Measure via a profile gage or template or dial indicators.
• Threads:
- Measure with thread plug gages, screw-pitch gages, snap gages.
• Contours:
- Optical Comparators are used to check profiles on a screen to
which an image is projected.

32. Coordinate Measuring Machines
• Consists of a surface plate and a
bridge to which a ram is attached. A
probe is fixed at the end of the ram.
• Machine can be programmed or
taught to move to different locations
and take specific measurements.
• It is a high-speed measurement
instrument, with accuracy up to
.00001”.
• Machine at MRC has accuracy of
0.00015”.

33. Gages
• Fixed Gages: indicate whether a part is too large or to small compared
to an established dimension. Do not measure actual
dimensions.
• Plug Gages: typically used for holes. Has two sides: a GO
and a NO-GO side. GO side is smaller. GO side slides
into a hole smaller that the gage diam. NO-GO side will
not go into the hole.
• Ring gages: used for shafts or similar round parts.
• Snap gages: used to measure external dimensions. Have
adjustable gaging surfaces that can be set to create a GO
NO-GO gage.

34. Optical Instruments
• Used to measure surfaces that are too delicate or small for contact
inspection instruments.
- Microscopes: used to measure very fine details on small workpieces.
Toolmaker’s Microscope can read up to .0001”.
- Fiberscopes and Boroscopes: used when surfaces are inaccessible to
the instrument. Used to inspect engine turbine blades without
disassembly.
- SEM: Magnification up to 100,000X. Excellent detail can be seen.

35. Optical Instruments