LO 5-3 Identify and describe common instruments and techniques to make linear precision measurements

Micrometers

To make even smaller, more accurate measurements than a vernier-type sliding caliper, a micrometer can be used. Like sliding calipers, micrometers can measure inside, outside, or depth FIGURE 14. This means that while the construction of external-, internal-, and depth-type micrometers changes considerably, the basic measuring mechanism remains the same. A standard mechanical-type micrometer has a scale capable of measuring linear distances with a resolution to the nearest one-thousandth of an inch or millimeters, which is written as 0.001" (0.002 mm) or smaller. The displays on many easier-to-use digital electronic micrometers, ranging up to 4" (100 mm), can provide resolution to 0.00005", with twice the accuracy of mechanical micrometers (according to manufacturers). Technicians should know how to write and express these numbers:

• One-thousandth of an inch or mm is expressed as 0.001
• One hundredth of either measure is 0.01
• One tenth is 0.1
• One ten thousandth is 0.00001
• Half a thousandth is 0.00005

FIGURE 05-14: Micrometers are used to make linear measurements of inside, outside, and depth dimensions.

FIGURE 05-15: Common terms used for the parts of an outside-type micrometer.

Micrometer Construction

A frequently used mechanical micrometer is an outside micrometer, which can measure the diameter of round objects, such as a valve stem or crankshaft journal, and the thickness of flat objects like shims FIGURE 15.

Construction of the outside micrometer consists of the following parts:

• The C-shaped frame, which is either forged, cast, or machined steel. A large open hole or multiple holes in the frame help minimize measurement error due to heat-related expansion of the frame.
• The spindle, which is the round, precision-machined critical component of the tool that moves toward the work piece when the thimble is turned. On the spindle are precision threads cut externally into the hardened shaft. The thread pitch, or the number of threads per inch on the micrometer, are the key to determining how far the spindle moves per rotation of the thimble. This distance is different for metric and USCM micrometers. A thread pitch of 40 threads to the inch will move the spindle one-fortieth (or 0.025) of an inch with each turn. A metric micrometer has a thread pitch 0.5 mm between each thread, which advances the spindle 0.5 mm per spindle rotation.
• The anvil, which is pressed into the frame opposite the spindle and serves as a stop for the spindle. It is usually adjustable to enable “zeroing,” or calibrating the micrometer.
• The sleeve, which is also called a barrel , is fixed to the frame and has a graduated scale to make linear measurements. Inside the sleeve is a nut with internal threads that engage the spindle threads.
• The thimble, which is the rotating component that is turned with the thumb. The thimble has graduated markings that are used with markings on the sleeve to measure an object.
• The ratchet-type torque-limiting stop, which is optional, limits the amount of torque applied to the thimble to prevent over-squeezing of an object in the spindle. Typically, the torque limiter will slip and not turn the thimble further when a calibrated torque is reached.
• A locking mechanism, which, after the object is measured, a lever or lock nut is tightened to hold the spindle measurement to allow it to be viewed more easily.

Microcontroller-based electronic tools are available that enable the transmission and storing of measurement data through a wireless data collection system.

Micrometer Range
A popular mechanical micrometer used by engine technicians measures a range up to 4" or approximately 100 mm; however, the selection of micrometer frame size is made so that the travel between the spindle and the anvil is no more than one inch or ∼25 mm. This travel distance or movement is called the micrometer range . For example, the range of a 1" micrometer is from 0–1". A 2" micrometer has a range from 1–2". Extensions can be added to the spindles of larger micrometer frames when measuring depth, inside, or outside dimensions.

To take measurements, the work is held between the spindle and the anvil with the left hand while the thimble is turned with the thumb and index finger of the right hand. A torque-limiting ratchet mechanism prevents distortion of measurements caused by over-tightening of the spindle. Before reading the final linear measurement, the micrometer adjustment is locked in place.