Modern Machine Shop

SEP 2018

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MMS SEPTEMBER 2018 32 mmsonline.com QUALITY GAGING TIPS Measurement Tools Micrometers—Shifting into High Gear GEORGE SCHUETZ | COLUMNIST One of the most popular instruments on the shop floor continues to evolve to meet changing needs. The basic micrometer is one of the most popular and versatile handheld measuring tools on the shop f loor. It provides direct size information quickly, delivers high resolution and adapts easily to many different measurement applica- tions. The micrometer in its most basic form, with a f lat reference and a measuring anvil, has continued to improve since its conception 150 years ago. Numerous alternate contact point configu- rations have been developed over the years to satisfy changing measurement applications. For example, ball contacts are used to measure the wall thickness of tubes and other cylindrical components. A one ball/radius style may be used for inspection of wall thickness on tubing, while two ball/radius contacts can inspect thickness between holes. In some cases, the ball contacts can be supplied as attachments for use with a standard, f lat-tipped micrometer. Reduced spindle-style micrometers have a turned-down diameter on both the anvil and the spindle. These contacts are used to measure inside recesses where the normal diameter may be too wide to penetrate. Due to the very small contact areas of the anvil and spindle, these micrometers may take a little getting used to. Blade contacts are useful for measuring the outside diameter of a cylindrical part from inside a turned groove on its surface. These grooves can be so narrow that neither a standard nor reduced-face micrometer will fit completely into the groove. Blade contacts are slender and f lat enough to nest readily into narrow-bottomed grooves. The problem is that the spindle surface of most micrometers rotates as the micrometer barrel is turned. Eventually, a blade inside a groove would be constrained from rotating. To solve this problem, blade micrometers have a spindle that slides along the axis of movement instead of rotating. Therefore, this style of micrometer calls for greater care. On the same part, a disc microm- eter designed for thickness measurements on features with narrow clearances can measure the distance between grooves. The mea- surement contacts are relatively large, disc-like f lats that extend beyond the diameter spindle and anvil. However,

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