Engineering Metrology Linear measurements Dr. Belal Gharaibeh 13/10/2011 Dr. B. Gharaibeh Fall 2011 1
Introduction • modern manufacturing can produce features that are more accurate than we can measure by hand, therefore we need tools to assist us. • • These tools allow us to quantitatively evaluate physical properties of objects. • • EVERY industry uses these tools to some extent, for example, • - machine shops • - tailors • - dentists • - automotive manufacturers • - etc. Dr. B. Gharaibeh Fall 2011 2
Definitions • Assembly - the connection of two or more separate parts to make a new single part. • Basic Dimension - The target dimension for a part. This typically has an associated tolerance. • Dimension - A size of a feature, either measured, or specified. • Dimensional Metrology - The use of instruments to determine object sizes shapes, form, etc. • Limits - These typically define a dimensional range that a measurement can be expected to fall within. • Machine Tool - Generally used to refer to a machine that performs a manufacturing operation. This is sometimes confused with the actual cutting tools, such as a drill bit, that do the cutting. Dr. B. Gharaibeh Fall 2011 3
Definitions • Measurement - The determination of an unknown dimension. This requires that known standards be used directly, or indirectly for comparison. • Metric System - A measurement system that has been standardized globally, and is commonly used in all modern engineering projects. • Metrology - The science of measurement. The purpose of this discipline is to establish means of determining physical quantities, such as dimensions, temperature, force, etc. • Repeatability - Imperfections in mechanical systems can mean that during a Mechanical cycle, a process does not stop at the same location, or move through the same spot each time. The variation range is referred to as repeatability. • Standards - a known set of dimensions, or ideals to compare others against. • Standard Sizes - a component, or a dimension that is chosen from a table of standard sizes/forms. • Tolerance - The allowable variation in a basic dimension before a part is considered unacceptable Dr. B. Gharaibeh Fall 2011 4
Tolerance • Impossible to make perfect parts • Too small tolerance, cost is high • Boeing 747-400 has 6 million parts, measurement of 28 features, 150 million measurements • NIST (U.S. National Institute of Standard and technology); tolerance shrink by a factor of 3 every 10 years ultra-precision ion-beam machining 0.001 m m Dr. B. Gharaibeh Fall 2011 5
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Importance of tolerance • Parts from the same machine can be different – Speed of operation – Temperature – Lubrication – Variation of incoming material – Other factors • ISO system; definitions Dr. B. Gharaibeh Fall 2011 7
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Scales • The most common tool for crude measurements is the scale (also known as rules, or rulers) • These are limited by the human eye • Parallax error can be a factor when making measurements with a scale-line of sight not being normal to the scale of the instrument Dr. B. Gharaibeh Fall 2011 9
Errors 1- Instrument limitations. 2-Geometric errors (flatness and parallelism). 3-Thickness of the grade line. 4-Least increment limitation. 5-Observation error. 6- Alignment error. 7- Parallax error (object not well aligned with scale). Dr. B. Gharaibeh Fall 2011 10
Calipers • A tool used to transfer measurements from a part to a scale, or other instrument. • Calipers may be difficult to use, and they require that the operator follow a few basic rules, – Do not force them, they will bend easily, and invalidate measurements made – Try to get a feel, or personal technique for using these instruments. – If measurements are made using calipers for comparison, one operator should make all of the measurements (this keeps the feel factor a minimal error source). • These instruments are very useful when dealing with hard to reach locations that normal measuring instruments cannot reach. • Obviously the added step in the measurement will significantly decrease the accuracy Dr. B. Gharaibeh Fall 2011 11
Construction & Use: • They consist of two legs hinged at the top with the ends of the legs span the parts to be inspected. Dr. B. Gharaibeh Fall 2011 12
Types of calipers. • Calipers can be classified as : Outside, Inside. • Calipers can be classified as : Spring, Firm joint, Lock joint , Transfer. Dr. B. Gharaibeh Fall 2011 13
firm joint calipers Outside firm joint caliper Inside firm joint caliper Dr. B. Gharaibeh Fall 2011 14
Spring caliper outside spring caliper Inside spring caliper Dr. B. Gharaibeh Fall 2011 15
• Operating principle: They are devices for comparing measurements against known dimensions. Dr. B. Gharaibeh Fall 2011 16
• Qualities: They should be free from cracks, seams, dirt, flaws and must have smooth bright finish. • Nominal Size is the distance between the center of the rolling end and the extreme working end of a leg. Dr. B. Gharaibeh Fall 2011 17
• Caliper’s Capacity is the maximum dimension that can be measured by the caliper. It should not be lesser than the nominal size. • The accuracy depends on the sense and feel of the operator. Therefore, caliper should be held gently and square to the work with slight gauging pressure applied. Dr. B. Gharaibeh Fall 2011 18
Instruments • Vernier scales • Like a normal scale with extra secondary scale subdividing major increments • Secondary scale is one increment shorter than a main scale therefore indicating relative distance between two offsets of the main scale • On imperial sliding vernier scales the main scale divisions are 0.050” apart, and on the vernier scale they are 0.049”, giving a reading of 0.001” per graduation. • On metric sliding vernier scales the main scale divisions are 1mm apart, and the vernier scale they are 0.98 mm, giving a reading of 0.02mm per graduation. Dr. B. Gharaibeh Fall 2011 19
Scalar vernier Dr. B. Gharaibeh Fall 2011 20
How to read a vernier caliper. • The reading on the main scale just before the zero of the vernier is noted. This is called Main scale reading (M.S.R). • The number of division on the vernier which coincides perfectly with any one of the main scale divisions is noted.T his is called vernier coincidence (V.C). • The vernier coincidence (V.C) is multiplied by least count to get the fraction of a main scale division. This is added to the main scale reading (M.S.R) to total reading. Dr. B. Gharaibeh Fall 2011 21
Micrometers Dr. B. Gharaibeh Fall 2011 22
Micrometer scales • This is a very common method for measuring instruments, and is based on the thread principle. • In effect, as a thread is turned, a large motion on the outside of the thread will result in a very small advance in the position of the thread. • On imperial micrometers the divisions are typically .025” on the sleeve, and 0.001” on the thimble. The thread used has 40 T.P.I. = a pitch of 0.025 • Metric micrometers typically have 1 and 0.5 mm divisions on the sleeve, and 0.01mm divisions on the thimble. The thread has a pitch of 0.5mm. • Depth micrometers have an anvil that protrudes, out the end, and as a result the scales are reversed to measure extension, instead of retraction. Dr. B. Gharaibeh Fall 2011 23
• The micrometers pictured below have major scales, as well as minor scales. The major scales are read first, and the micrometer scales are read second and the readings added on. • The metric micrometer below reads 13.5 = 13.5mm on the major scale, and 31 = .31mm on the thimble, for a total of 13.81mm • The Imperial scale below shows a micrometer reading of 4.5 = .45” on the main scale, and 9 =.009” on the thimble, for a total of .459 Dr. B. Gharaibeh Fall 2011 24
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Magnification The operation of micrometers is based on magnification using threads. • A large movement on the outside of the micrometer thimble will result in a small motion of the spindle • There are two factors in this magnification. First, the difference in radius between the thread, and the thimble will give a change in sensitivity relative to the difference in radii. Second, the pitch of the thread will provide a reduction in motion. M = magnification from the moving head to the hand motion C = measuring diameter of the instrument D = diameter of the thread pitch = the number of threads per unit length Dr. B. Gharaibeh Fall 2011 28
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