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