[SIX SIGMA GLOSSARY ALPHABETICAL INDEX] [SIX SIGMA GLOSSARY INDEX OF TOPICS]

• Calipers
• Gauge Blocks
• Micrometers
• Optical Comparators
• Tensile Strength
• Titration

Calipers are devices for measuring a linear dimensions, this may be the distance between two parallel faces or a diameter. Calipers are used for internal and external measurements.

Calipers come in many shapes and forms. The machinists' caliper contains no measuring scales but is used in conjunction with a reference part or rule. Calipers that incorporate a measuring scale include vernier, dial indicator and digital calipers:

The purpose of Gauge blocks is to provide a linear reference dimension to very close tolerance. Gauge blocks used for calibrating measuring instruments, such as micrometers and calipers.

Gauge blocks are rectangular or square steel blocks with a very accurate width. They are sold in sets. A typical set has 91 pieces with widths varying from 0.010" to 0.100" in 0.001"

In use several blocks are stacked by to give the required overall dimension.

The surfaces of the blocks are extremely flat and highly polished. The blocks are placed together, so that the faces overlap, and then slid into alignment. This creates a vacuum between the faces that makes them stick together, they can only be disassembled with a sliding action. This method of stacking the blocks is known as 'wringing'

There are four grades of blocks:

Micrometers are devices for measuring linear dimensions:

The conventional micrometer has a thimble that rotates on a barrel. The thimble rotates on a screw that opens or closes the micrometer:

One full turn of the thimble changes the measured dimension by 0.025". There is a scale along the barrel divided into intervals of 0.025", thus a complete turn of the barrel will cause a one increment movement. There are also 25 equi-spaced reference marks on the thimble, moving one increment of these marks changes the measurement by 0.001"

Optical comparators are devices that project a magnified image of a part profile onto a screen where it is compared to a standard overlay or scale.

Optical comparators are used for complex parts, and parts that are otherwise hard to measure. The profile of screw thread is a typical example of a feature that lends itself to this method.

In addition to dimensional readings optical comparators can detect defects (burs, scratches, incomplete features etc.)

Tensile testing involves making a test specimen. The specimen is then gripped at the ends by suitable apparatus in a tensile testing machine and an axial force exerted slowly until the specimen fails:

A graph of extension versus load is usually produced to better show the yield point, the ultimate strength and the stress at fracture.

The shape of the specimen is critical because the gripping forces can apply forces that can make the specimen fail prematurely. The specimen should be made according to the appropriate standard, for that type of material eg. ASTM E8 or D638.

Titration is used to find the concentration of a substance in solution. It involves adding a standard reagent of known concentration in carefully measured amounts until a clearly defined reaction is observed. this may be a color change, a change in electrical resistance etc.

For example the solution being tested may contain an unknown amount of acid. It would be titrated with a base of known concentration until he neutral point is reached. This point would be determined from the color change of an indicator.