introduction to precision metrology. general comments precision tools – delicate – treat them...
TRANSCRIPT
Introduction to Precision Metrology
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
General Comments cont’d
• 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
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
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
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
Dial indicators – test and long travelTest 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
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
Gauge blocks con’tMeasuring surfaces are specular and optically flat
Faces have matt finish
Nominal length values are etched into the face
Angle gauge blocks16 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
Inside micrometersUses 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
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
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
Use of “V” blocks
Align V-Block with Straight-edge on Surface Plate
Use clamps to secure rod to V-block surface
Indicate along top surface while moving along straight edge - straightness
Indicate along side surface while rotating cylinder - circularity
Measuring parallelism