Measuring instruments

• Measuring


  Very important in Manufacturing. Most products are manufactured to standard sizes and shapes. Standardization is a necessity for interchangeable parts and is also important for economic reasons.
• Inspection of products/items


  Can be done in two ways: by attributes or by variables
• Inspection by attributes


  • In manufacturing, particularly in mass production it may not be necessary to know the exact dimensions of a part, only that it is within previously established limits. Limits can often be determined more easily than specific dimensions by the use of attributes type instruments called gauges.
• Gauges


  • Plug gauges, ring gauges, thread gauge, radius gauges
• Inspection by variables


  • Use of calibrated instruments to determine the actual dimensions of the product for comparison with the size desired.
• Variable inspection instruments


  • Vernier callipers, micrometer screw gauges
• Linear Measuring Instruments


  Direct reading: Object being measured can be read directly on this scale. Indirect reading: Used to transfer the size of the dimension being measured to a direct reading scale thus obtaining the desired size and information indirectly.
• Ruler


  • Simplest and most commonly used for making linear measurements
• Vernier Calliper


  • Consists a graduated beam and a sliding jaw with a vernier. Can be used to measure outside diameters, inside diameters and depth measurements.
• Least count of Vernier Calliper


  The least distance, which can be measured by vernier callipers.
• Zero error in Vernier Calliper


  If the vernier zero mark should not coincide with the zero mark of the main scale, is said to have a zero error. If the zero mark of the vernier scale is on the right of the zero mark of the main scale then the zero error is reduced from the observed reading to obtain the actual reading. If it is on the left of the zero mark of the main scale then the error is added to the observed reading.
• Micrometer screw gauge


  • Used to measure even smaller dimensions than the vernier callipers. It uses an auxiliary scale (measuring hundredths of a millimetre), which is marked on a rotary thimble.

  
• Least count of Micrometer screw gauge


  The amount by which the thimble moves forward or backward for one complete revolution.
• Depth gauge


  • An accurate and reliable tool to use for depth measurement. Measures from a reference plane to a point.

  
• Height gauge


  • Used for linear measurement
• Callipers


  • Used for indirect linear measurement
• Angular measurement


  Angles are measured in degrees and decimal subdivisions of a degree. The primary unit of angular measurement is the degree. Each degree is divided into 60 parts known as minutes. Each minute is also divided into 60 parts known as seconds.
• Plate Protractor


  • Capable of measuring to within 1-degree
• Bevel Protractor


  • Similar to common protractor except that it has a movable element. The two blades of the protractor are placed in contact with the part being measured and the angle is read directly on the vernier scale.

  
• Sine bar


  • A bar of known length. When gauge blocks are placed under one end, the sine bar will tilt to a specific angle.
• The height setting of a sine bar is limited with the available gauge block (often 0.0001"). This results in an error that may be negligible, or in some cases quite significant.