Introduction to Precision Metrology

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Introduction to Precision Metrology
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General Comments

• Precision tools delicate treat them with
  respect
• Set them down carefully preferably something
  soft
• Wood, linoleum avoid metals, ceramics, glass
• Check gauging surfaces clean off and set to
  zero
• When finished wipe clean and return to storage
  box
• Use rag with a drop or two of oil
• With care these tools will last for years
• If mis-treated they become worthless in days

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General Comments contd

• Using precision tools is somewhat complex
• Result concentration on details not bigger
  picture
• Keep plastic ruler handy double check
  measurements
• More mixture of English and metric units
• Measure twice cut once

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What you will do in lab

• Use some precision tools to measure sample parts
• Gain a feel for use of the tool and its
  capabilities
• Use some tools in order of increasing resolution
• Gain a feeling for tool features that increase
  resolution
• Get an understanding of the types of tools
  available
• Tools used are generally available for mechanical
  inspection
• Learn about ISO 5460, techniques to measure
  features
• Set ups to measure specific features called out
  on drawings
• Examples, flatness, straightness, roundness

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Calipers from ISO 3599
Moderately accurate (.02 mm) multi-purpose length
measuring tool Typically 150 mm capacity
available to 1 m Measures outside, inside and
depth dimensions Close jaws on piece of paper,
pull out and check zero Digital version much
easier to use than dial or vernier Many digital
devices have a serial interface to a computer
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Outside mircometers ISO 3611
Most common are 25 mm capacity available to 500
mm check w gage Accurate to .01 mm with
resolution to .002 mm on better grades Use a
micrometer stand with larger sizes if
practical Commonly used for measuring diameters
use ball tips for plane surfaces Ratchet gives
uniform squeeze between anvils Digital versions
help avoid reading errors measure twice
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Dial indicators test and long travel
Test indicators short range high sensitivity
used as null devices Long range needle makes
many revolutions has counter dial Used for
rough centering, thickness measurement Mounted in
a stand either stationary while part moves or
indicator moves along a straightedge with part
still Generally looking for minimum indicator
movement End result given as TIR Total
Indicator Reading or Runout
Also attached to bore depth gages Digital
dial versions available Can be fitted with
various tips
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Gauge or Jo Blocks ISO 3650
Come in sets to 150 mm with 88 blocks Minimum
step 1 µm Make up any dimension by wringing
blocks together Sets come in several grades,
typical for shop use is grade AS-1 with thinner
blocks good to sub-microns
Sets include two wear blocks to be mounted to
either end of stack to protect the measurement
blocks from wear
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Gauge blocks cont
Measuring surfaces are specular and optically
flat Faces have matt finish Nominal length values
are etched into the face
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Angle gauge blocks
16 blocks 1 second to 90 in 1 second
increments Useful for measuring optical
wedges Surfaces are optically flat and specular
can use with an autocollimator Same is true of
gauge blocks
Sides are not finished to same specs as angle
faces but are perpendicular to angle faces so
angle readings are correct
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Inside micrometers
Uses 12.5 or 25 mm travel hollow micrometer head
and set of precision length rods Standard set can
measure inside dimensions from 50 to 600 mm In
side mics are harder to use accurately because
you are looking for the maximum extension that
just fits a hole diameter, that is, mic must be
parallel to the diameter to get a true reading
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Sine plate or bar
Typically 10 between hinge rod and gauge block
rod Calculate sine of angle, multiply by 10 and
put gauge block stack under gauge block rod Can
theoretically set angle to 2 seconds of arc with
.0001 blocks
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Inspection of mechanical parts

• Part is held against datums called out on drawing
• Indicator is slid along part for checking
  features with a spatial extent
• Indicator reading must remain inside tolerance
  band

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Use of V blocks
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Measuring parallelism