Engineering student notes

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