Surface geometry measurement

Introduction to Material Science and Engineering

  • Course: ME F315 - Advanced Manufacturing Process

  • Institution: BITS Pilani, K K Birla Goa Campus

  • Lecture 1: Introduction to Material Science

Surface Geometry Measurement Techniques

  • Instructor: Sudhy S. Panicker, Department of Mechanical Engineering, BITS Pilani-K K Birla Goa Campus

  • Email: sudhyp@goa.bits-pilani.ac.in

Flatness

  • Definition: Flatness is the minimum distance between two planes that confine a surface, indicating how even the surface is.

  • Importance: Determines whether all points on the surface lie within the same plane.

Flatness Measurements Using Optical Flat

  • ISO 1101 Definition: Defines flatness as the distance between two parallel planes confining the surface.

  • ASME Y14.5 Standard: Specifies a tolerance zone between two parallel planes.

  • Example: A tolerance of 0.25 units means the top surface must lie within two parallel planes that are at most 0.25 units apart.

Measurement Techniques Using Optical Flat

  • Procedure for Flatness Measurements:

    • Place the optical flat on the test surface.

    • Illuminate the surface with monochromatic light (laser, helium, or low-pressure sodium).

    • Observe interference fringes (light bands) and analyze their shape and spacing to determine flatness.

Flatness Measurements Using Gauges

  • Instruments: Dial gauge and height gauge.

Flatness Measurements Using Coordinate Measuring Machine (CMM)

  • Capabilities: Stylus takes accurate measurements and compares data automatically to produce flatness reports.

  • Key Features:

    • Effects of scanning speed and step increments examined by unique measuring subroutines.

    • Measures undulations on surfaces, providing quality indications through software.

Straightness

  • Definition: A line is straight if the deviation from two perpendicular reference planes remains within limits.

  • Tolerance: Defined as the maximum deviation of points from the reference line.

  • Measurement Techniques: Can range from spirit levels to laser-based devices.

Straightness Measurements

  • Methods:

    1. Using CMM to lightly touch the target with the stylus.

    2. Height gauge to ensure equal heights on both sides after securing the part.

    3. Laser alignment and precision level.

    4. Measures warpage in long objects.

Wedge Method for Straightness Measurements

  • Technique: Utilize a straight edge supported by slip gauges of known values to check for surface straightness.

  • Consideration: Slip gauges must not wring to maintain sensitivity.

Autocollimator Method for Straightness Measurements

  • Principle: Similar to level method; measures angular variations using a mirror and autocollimator for height differences.

Circularity (Roundness)

  • Definition: Circularity measures when a feature is designed to be round. It requires assessment of multiple sections to ensure compliance.

Cylindricity

  • Description: Evaluates a component's conformance to an ideal cylinder in 3-D, ensuring it is round and straight along its axis.

  • Identification Method: Typically defined using the minimum-zone method.

Cylindricity Measurements

  • Technique: Measures using a turntable and measuring instrument capturing the circular profile as the part rotates.

Concentricity

  • Definition: Indicates multiple objects sharing a common center, controlling the central axis of a referenced feature against a datum axis.

Concentricity Measurements

  • Methods:

    1. CMM for thin-walled elements as a common axis.

    2. Use of straightness to ensure concentricity.

    3. Maximum inscribed circle for aligning distant holes.

Runout

  • Definition: A measure of how true a rotating body runs around its axis, critical for components like bearings and spindles.

  • Types:

    • Radial runout – off-center rotation.

    • Axial runout – angle to the axis.

Runout Measurements

  • Method: Utilize a height gauge while rotating the part; deviations measure against runout tolerance to verify specifications.

Coordinate Measuring Machines (CMMs)

  • Types: Cantilever and bridge type; modes include manual, semi-automated, and computer-controlled operations.

Measurement Criteria Using CMM

  • Least Squares Criterion (LSC): Best-fitting line for data points.

  • Minimum Zone Criterion (MZC): Determines concentric circles enclosing measured points with minimum distance.

Surface Geometry and Topography

  • Surface Types: Normal, rough, wavy surfaces; affected by manufacturing irregularities.

  • Texture Evaluation: Short spatial wavelength components (roughness) and longer wavelengths (waviness).

Factors Influencing Surface Irregularities

  • Causes: Feed marks from cutting tools, chatter marks from vibrations, material ruptures during machining, and tool irregularities.

Importance of Surface Geometry

  • Affects wear/friction, conductivity, corrosion properties, and cost implications in manufacturing.

Key Factors Affecting Surface Roughness

  • Variables: Spindle speed, feed rate, depth of cut, tool diameter, number of inserts, and material type.

Roughness Measurement Techniques

  • Methods: Centre Line Average (C.L.A.) and Root Mean Square (R.M.S.) methods to quantify surface deviations.

Optical Profilometer

  • Function: Microscope that uses light interference to measure surface contours, useful in detecting surface defects and roughness.