1. Manufacturing Processes
Chap Metrology

2. Metrology Definition: Relevance: “The measurement of dimensions”.
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 ”, 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: ”  0.002”
Unilateral: ” ” ”
Limits: ” 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.

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 ”.
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 ”.
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 ”. 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. Angle Measurement
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.

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 ”.
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 ”.

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 ”.
Machine at MRC has accuracy of ”.

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