Chapter 4 Geometric Shape Measurement PDF

MEQ531 METROLOGY GEOMETRIC SHAPE MEASUREMENT Prepared by: Siti Mardini Hashim TECHNICAL DRAWING First-angle projection is the ISO standard and is primarily used in...

MEQ531 METROLOGY GEOMETRIC SHAPE MEASUREMENT Prepared by: Siti Mardini Hashim TECHNICAL DRAWING First-angle projection is the ISO standard and is primarily used in Europe TECHNICAL DRAWING Third-angle projection is primarily used in the United States and Canada, where it is the default projection system according to ASME standard ASME Y14.3-2003 and British Standard BS 8888 TOLERANCES 1. Dimensional tolerance - the total amount a specific dimension is permitted to vary, which is the difference between maximum and minimum permitted limits of size. 2. Geometric tolerance - the maximum or minimum variation from true geometric form or position that may be permitted in manufacturing. Geometric tolerance should be employed only for those requirements of a part critical to its functioning or interchangeability. These much more difficult to measure/verify as compared to dimensional tolerances. GD&T GD&T is a way of describing the dimensions and tolerances that’s different from traditional coordinate measurement plus/minus tolerancing. Fundamentally, engineers design a part with perfect geometry in CAD, but manufactured parts are never perfect. Proper use of GD&T can improve quality and reduce time and cost of delivery. It accomplishes this by providing a common language to accurately express design intent and focusing on functional interfaces to tolerance a part. These are the main benefits of using Geometric Dimensioning and Tolerancing (GD&T): Standardized design language Clear, precise and consistent communication between customers, suppliers, and production teams Method for calculating the worst-case mating limits Repeatable production and inspection processes Assembly is assured from qualified production parts ANSI Y14.5-1994 & ISO/TC 213 Standard TYPES OF GEOMETRIC TOLERANCES  Form tolerances: straightness, circularity, flatness, cylindricity  Orientation tolerances; perpendicularity, parallelism, angularity  Location tolerances: position, symmetry, concentricity. Symbols for Geometric Tolerances Feature Control Frame Datum In GD&T, a datum is a theoretically exact point, line, plane, or axis that serves as a reference for other features on a part. Datum Why are datums important? Clear Communication: Datums provide a standardized way to communicate design intent, ensuring that everyone involved in the manufacturing and inspection process understands the reference points for measurements. Functional Relationships: Datums establish the functional relationships between features, ensuring that parts assemble correctly and function as intended. Manufacturing Consistency: By providing a consistent reference system, datums help to ensure that parts are manufactured to the same specifications, regardless of the manufacturing process or equipment used. Inspection Accuracy: Datums provide a reliable basis for inspection, ensuring that measurements are taken from the correct reference points. Material Condition Modifiers The Modifiers (MMC,LMC,RFS) are used to clarify implied tolerances. There are three directly implied modifiers to the tolerance value. These are; 1-) Regardless of Feature Size (RFS) 2-) Maximum Material Condition (MMC) 3-) Least Material Condition (LMC) Regardless of Feature Size (RFS): RFS is the default modifier. so if there is no modifier symbol shown in feature control frame, it means RFS is the default modifier. RFS is used when the size feature does not affect the specified tolerance. Material Condition Modifiers Maximum Material Condition (MMC): MMC can be used to constrain the tolerance on given dimension/dimensions. MMC can be defined as the condition of a part feature where the maximum amount of material is required. MMC is also used to maintain clearance and fit between shafts and holes. With MMC, The given tolerance will be applied as Maximum shaft diameter and Minimum hole diameter. Least Material Condition (LMC): LMC condition where a size feature contains the least/minimum amount of material within the stated limits of size. LMC can be defined as the condition of a part feature where the least/minimum amount of material is required. With LMC, The given tolerance will be applied as Least/Minimum shaft diameter and Maximum hole diameter. STRAIGHTNESS MEASUREMENT  Straightness is a condition where one line element of a surface or an axis must lie in a straight line. It controls how much a feature of one line element of a surface can deviate from a straight line.  Straightness tolerance is applied in the view where the elements to be controlled.  Straightness tolerance specifies a tolerance zone within element or derived median line must lie. The feature control frame needs to be attached to the surface with an extension line or a leader when a surface needs to be controlled.  Tolerance zone for both cylindrical and flat surface is applied along the entire surface. STRAIGHTNESS MEASUREMENT STRAIGHTNESS OF SHAFT STRAIGHTNESS OF HOLE TOOLS 1. Straight Edges and Levels: Traditional Tools: These are simple, visual tools for basic straightness checks. Straight Edge: A precision-ground, flat bar used to compare against a surface or line. Deviations from straightness can be observed as gaps between the straight edge and the feature. Level: Uses a bubble in a liquid-filled vial to determine if a surface is horizontal or vertical. Can be used to assess straightness indirectly by checking for consistent levelness along a line. Limitations: These methods are subjective and lack the precision for tight tolerance requirements. TOOLS 2. Dial Indicators: Method: A dial indicator with a suitable stand can be moved along a line or surface, measuring deviations from a straight path. Suitable for: Shorter lines or surfaces, and for checking straightness during machining operations. Advantages: Provides numerical readings for more precise measurement. Limitations: Can be sensitive to variations in contact pressure and surface roughness. TOOLS 3. Coordinate Measuring Machines (CMMs): Method: A CMM uses a touch probe to measure the coordinates of points along a line or surface. Software then analyzes the data to determine straightness deviations. Suitable for: Complex shapes and high-precision measurements. Advantages: Highly accurate, versatile, and can measure multiple geometric features. Limitations: Can be expensive and requires skilled operators. TOOLS 4. Autocollimators: Method: Uses optical principles to measure small angular deviations. Can be used to assess straightness by measuring the angular changes along a line. Suitable for: High-precision straightness measurement, especially for flat surfaces and machine tool alignments. Advantages: Very high accuracy and can measure over long distances. Limitations: Can be sensitive to vibrations and requires careful setup.  FLATNESS  Flatness is a condition of a specified surface having all elements in one plane.  Flatness tolerance provides a tolerance zone of specified and defined by two parallel planes in where the specified surface must lie.  Flatness is applied to an individual surface, flatness tolerance does not need to be related to a datum. FLATNESS The Surface must lie between two planes 0.004 apart from each other and the specified surface must be within the specified limit of size tolerance. FLATNESS TOOLS 1. Feeler Gauges: Gap Measurement: Thin strips of metal with precisely known thicknesses used to measure the gap between the part and the surface plate. Method: Different thickness gauges are inserted into the gap until the largest gauge that fits without force is found. This indicates the maximum deviation from flatness. Suitable for: Quick checks and for assessing relatively large deviations. Advantages: Portable and easy to use. Limitations: Limited accuracy and resolution. TOOLS 2. Height Gauges: Point Measurement: A vertical measuring instrument with a scriber or indicator that can be used to measure the height of points on the surface relative to the surface plate. Method: The height gauge is moved across the surface, and readings are taken at various points. Deviations in the readings indicate flatness errors. Suitable for: Measuring flatness over a larger area. Advantages: Provides numerical readings and can be used to generate a profile of the surface. Limitations: Accuracy depends on the flatness of the surface plate and the operator's skill.  TOOLS 3. Coordinate Measuring Machines (CMMs): 3D Measurement: Uses a touch probe to measure the coordinates of points on the surface. Software then analyzes the data to determine flatness deviations. Suitable for: Complex shapes and high-precision measurements. Advantages: Highly accurate, versatile, and can measure multiple geometric features. Limitations: Can be expensive and requires skilled operators. TOOLS 4. Optical Flats: Interference Method: Uses a highly flat, transparent plate (optical flat) placed on the surface to be measured. Light interference patterns (fringes) are created, and the number and shape of the fringes indicate the flatness deviations. Suitable for: Extremely precise flatness measurement, often used for optical components and precision gauges. Advantages: Very high accuracy and can detect minute deviations. Limitations: Requires a monochromatic light source and careful interpretation of the fringe patterns. ROUNDNESS/CIRCULARITY  Circularity tolerance is used to control the roundness of circular parts or features.  Circular features can be defined by cylinders, spheres, and cones.  Circularity tolerance controls each circular element of a cylinder independent of each other.  Circularity tolerance is applied to an individual surface, Circularity tolerance does not need to be related to a DATUM. ROUNDNESS  TOOLS 1. V-Blocks and Dial Indicators: Simple Setup: The part is placed on two V-blocks, and a dial indicator is used to measure radial deviations as the part is rotated. Suitable for: Quick checks and for parts with relatively loose tolerances. Advantages: Simple to use and relatively inexpensive. Limitations: Lower accuracy compared to roundness testers or CMMs. Sensitive to the accuracy of the V-blocks and the operator's skill  TOOLS 2. Roundness Measuring Machines (Roundness Testers): Dedicated Instruments: These are specifically designed to measure circularity. Rotating Spindle: The part is mounted on a rotating spindle, and a precision stylus traces the surface as it rotates. Advantages: High accuracy, dedicated to roundness measurement, provides detailed analysis of the circular profile. Limitations: Can be expensive and may require specialized fixtures for holding the part.  TOOLS 3. Coordinate Measuring Machines (CMMs): Versatile Measurement: CMMs can measure various geometric features, including circularity. Touch Probe: A CMM uses a touch probe to measure the coordinates of points around the circumference of the feature. Software Analysis: The software then fits a circle to the measured points and calculates roundness errors. Advantages: Highly accurate, versatile, and can measure other features simultaneously. Limitations: May require more time to measure circularity compared to a dedicated roundness tester. CYLINDRICITY  Cylindricity is a condition of a manufacturing part surface of revolution in where all points of the circular surface are equi- distant from actual axis.  Cylindricity tolerance is applies where cylindrical part features must have good circularity, straightness and taper. Thus Cylindricity tolerance applies both longitudinal and circular element of the surface.  Cylindricity tolerance is applied to an individual surface, cylidricity tolerance does not need to be related to a datum.  Cylindricity tolerance controls the entire surface of a cylinder. CYLINDRICITY PARALLELISM o Parallelism tolerance zone is the condition of a surface or center plane equidistant at all points from a datum plane, or an axis. The distance between the parallel lines, or surfaces, is specified by the geometric tolerance zone. o Parallelism can be also specified a tolerance zone defined by two parallel planes or lines parallel to a datum plane or axis, respectively, within where the surface or axis of the feature must lie. o LMC or MMC can apply to feature of size. PARALLELISM TOOLS 1. Dial Indicators with Stands: Relative Measurement: A dial indicator mounted on a stand can be used to measure deviations from parallelism. The indicator is moved along the feature, and any variations in the readings indicate a lack of parallelism. Suitable for: Checking parallelism during machining operations or for relatively small parts. Advantages: Provides numerical readings for more precise measurement. Limitations: Accuracy depends on the flatness of the reference surface and the operator's skill.  TOOLS 2. Coordinate Measuring Machines (CMMs): 3D Measurement: A CMM uses a touch probe to measure the coordinates of points on the feature and the datum. Software then analyzes the data to determine the angle between them and calculate parallelism deviations. Suitable for: Complex shapes and high-precision measurements. Advantages: Highly accurate, versatile, and can measure multiple geometric features. Limitations: Can be expensive and requires skilled operators.  TOOLS 3. Autocollimators: Angular Measurement: Autocollimators measure small angular deviations. They can be used to assess parallelism by measuring the angular difference between the feature and the datum at multiple points. Suitable for: High-precision parallelism measurement, especially for flat surfaces and machine tool alignments. Advantages: Very high accuracy and can measure over long distances. Limitations: Can be sensitive to vibrations and requires careful setup. PERPENDICULARIT Y Perpendicularity tolerance is a three- dimensional geometric tolerance that controls how much a surface, axis, or plane can deviate from a 90-degree angle or it is can be defined as a condition of a surface, median plant, or axis at 90 degree to a datum plane or axis. TOOLS 1. Squares: Simple Tools: These are basic tools for visual checks of perpendicularity. Solid Square: A precision-machined tool with two surfaces at a perfect 90-degree angle. It's placed against the feature and datum to visually assess perpendicularity. Adjustable Square: Similar to a solid square, but with an adjustable blade for greater flexibility. Suitable for: Quick checks and for parts with relatively loose tolerances. Advantages: Simple to use and inexpensive. Limitations: Limited accuracy and subjective interpretation.  TOOLS 2. Coordinate Measuring Machines (CMMs): 3D Measurement: A CMM uses a touch probe to measure the coordinates of points on the feature and the datum. Software then analyzes the data to determine the angle between them and calculate perpendicularity deviations. Suitable for: Complex shapes and high-precision measurements. Advantages: Highly accurate, versatile, and can measure multiple geometric features. Limitations: Can be expensive and requires skilled operators.  TOOLS 3. Laser Scanners: Non-Contact Measurement: Laser scanners capture a point cloud representing the feature and the datum. Software then analyzes the point cloud data to determine the angle between them. Suitable for: Large parts and complex shapes where high accuracy is required. Advantages: Fast and efficient, non-contact measurement. Limitations: Can be expensive and requires careful setup and calibration. ANGULARITY Angularity tolerance specifies a tolerance zone defined by two parallel plane at the specified angle other than 90 degree from a datum plane or axis within where the surface or the axis of the feature must lie.  TOOLS 1. Angle Gauges (Sine Bars/Plates): Precision Measurement: These tools use trigonometry (sine function) to set precise angles. They are used in conjunction with a surface plate and gauge blocks to create a known angle, which is then compared to the angle of the feature. Suitable for: High-accuracy measurement of angles. Advantages: Very accurate and can be used to measure a wide range of angles. Limitations: Requires careful setup and can be time-consuming.  TOOLS 2. Bevel Protractors: Adjustable Angle Measurement: These protractors have an adjustable blade that can be set to the desired angle. They are then used to compare the angle of the feature to the set angle. Suitable for: Measuring angles on parts where a standard protractor is difficult to use. Advantages: More versatile than standard protractors. Limitations: Accuracy depends on the calibration of the protractor.  AUTOCOLLIMATOR Autocollimator is an optical instrument that is used to measure small angular differences. This device is very sensitive to change in angles and provides a very accurate measurement of angular differences. It is basically a combination of an infinity telescope and a collimator. Autocollimators are mainly used to align optical components and measure optical and mechanical deflections. WORKING PRINCIPLE  PROS & CONS OF AUTOCOLLIMATOR PROS CONS 1) It has very high accuracy. 1) Maintenance is required regularly. 2) It can measure wide range angle. 2) It is time consuming. 3) It is very easy to set up and operate. 3) It requires sample cutting and processing for 4) Calibration traceable to international tracing by the detector. standards. 5) It Can be used to see the result visually or electronically i.e in a computer screen. 6) Wide range of accessories and levels available. SUSTAINABLE DEVELOPMENT GOALS METROLOGY & SDGs COORDINATE MEASURING MACHINE (CMM)  A coordinate measuring machine (CMM) is a tool that uses discrete points on an item's surface to detect the geometry of the object.  While optical and white light sensors do exist, mechanical and laser sensors are the most often utilized types of probes in CMM’s.  The probe location may be managed manually by an operator or it may be managed by a computer, depending on the equipment.  A CMM machine is a device that uses coordinate technology to measure the dimensions of machine or tool components. Every points is expressed in terms of its x,y,z coordinates.  CMMs can be used for measurements ranging from the simple (such as diameter and length) to the extremely complex (such as free-form part measurements, measurements of automotive engines, measurements of the center of a cooling hole on a jet propulsion aerofoil, measurements of gears, measurements of metal die castings etc.). CMM COMPONENTS  Mechanical Setup with 3 axes moveme nt & the displacement transducer  Probe head to probe the work piece in a spatial direction  Control Unit  Computer with software to calculate & represent the results POTENTIAL SOURCES OF CMM ERROR