Engineering Metrology: Dimensions and Tolerances

Engineering Metrology

• Metrology is the science of measurement that stems from the greek word 'metron'.
• It combines both the theoretical and application aspects of measurement

Importance of Metrology

• Metrology enabled the creation of interchangeable parts, which fuelled modern development in manufacturing
• Henry Ford made all the rear axles on the Model T to the same dimensions and tolerances
• Technicians on cars like Royal Royce will hand fit each part, which can take up to 30 minutes per part

Dimensions, Tolerances, and Surfaces

• Dimensions, tolerances, and the effect of manufacturing processes determine the performance of a manufactured product
• Determining the material is important, as it directly affect ease of machining, dimensions and tolerances

Dimensions

• "A dimension is a numerical value expressed in appropriate units of measure and indicated on a drawing and in other documents along with lines, symbols, and notes to define the size or geometric characteristic, or both, of a part or part feature" - (ANSI Y14.5M-1982)
• Dimensions represent nominal or basic sizes of the part and its features on part drawings
• The dimension is the part size desired if there were no errors in the fabrication process

Tolerances

• "A tolerance is the total amount by which a specific dimension is permitted to vary. The tolerance is the difference between the maximum and minimum limits" - (ANSI Y14.5M-1982)
• Variations always occur in manufacturing, which causes variations in part sizes
• Tolerances define the allowed limits for the allowed variation

Tolerance

• Tolerare means to put up with or endure
• It is not possible to make perfectly sized parts
• A small tolerance results in a high cost
• An aircraft such as a Boeing 747 consists of 6 million parts and as many as 150 million measurements

Importance of Tolerance

• Parts from the same machine can be slightly different due to speed of operation, temperature, lubrication, material variation, and other factors

Types of Tolerance

• Bilateral tolerance Variation is permitted in both the positive and negative directions from nominal dimension
  • A bilateral tolerance can be unbalanced
• Unilateral tolerance Variation from the specified dimension is permitted in only one direction
  • Only positive or negative, but not both
• Limit Dimensions Permissible part feature variation is between maximum and minimum dimensions allowed

Assigned Tolerance Methods

• A shaft will commonly be assigned tolerances, which can be bilateral, unilateral, and/or limit dimensions

Definitions

• Allowance The specific difference in dimensions between mating parts
• Basic size The dimension from which limits of size are derived
• Bilateral tolerance Deviation from the basic size (+ or -)
• Clearance The space between mating parts
• Clearance fit Allows rotation or sliding between mating parts
• Datum Theoretically exact axis, point, line or plane
• Feature Physically identifiable portion of a part (hole, slot, pin, chamfer, etc.)
• Fit Range of looseness or tightness
• Geometric tolerancing Tolerance of shape features of the part
• Interference Negative clearance
• Interference fit When 2 parts must be forced together
• Limit dimension Maximum and minimum dimension of a part
• MMC(maximum material condition) Feature of size contains the maximum amount of material within the stated limits of size
• Positional tolerancing Specifies the true position, size, and form of feature of the part
• Transition fit Small clearance or interference allows accurate location of parts

Surface Technology

• Surface Technology is concerned with;
  • Defining the characteristic of a surface
  • Surface texture
  • Surface Integrity
  • Relationship between manufacturing processes and characteristics of resulting surface

Surfaces

• Nominal surface is the designer's intended surface contour of parts defined by the engineering drawing
  • Straight lines, ideal circles, round holes, and edges are geometrically perfect nominal surfaces
• Actual surfaces are developed by the manufacturing process
  • Variance in process leads to wide variation in surface characterisics
• Surface properties and behavior differ from the bulk of the part
• Surfaces impact both the function and appearance of a manufactured part

Why are Surfaces Important?

• Surfaces contribute to the part's aesthetic - a smooth surface makes a favorable impression
• Surface integrity affects the part's safety and performance
• Friction and wear depend on surface characteristics
• Surface characteristics affect mechanical/physical properties and can serve as stress concentration points
• Subsequent operations depend on good surface quality - painting, coating, welding, soldering, etc.
• Smooth surfaces make better electrical contacts because of higher electrical and thermal conductivity

Metallic Part Surface

• Surface texture is the outermost layer
• Below surface texture is the altered layer
• Below the altered layer is the substrate

Surface Characteristics

• Microscopic scale reveals irregularities and imperfection
• There are various characteristics of the surface:
  • Substrate The bulk of the part, under the surface
  • Surface texture Includes the roughness, waviness, and flaws
  • Altered Layer May be affected by work hardening, heat, chemical or electrical energy
  • Surface Integrity Definition, specification, and control of surface layer-includes surface texture and altered layer

Surface Integrity

• Contamination of the film by Rust (Iron) or Al2O3 can affect appearance and inhibit soldering, plating, and use of adhesives

Surface Texture

• Repetitive and/or random deviations from the nominal surface of an object

Four Elements of Surface Texture

• Roughness Small, finely-spaced deviations from nominal surface
  • Determined by material characteristics and processes that formed the surface
• Waviness Deviations of much larger spacing
  • Waviness deviations occur due to work deflection, vibration, heat treatment, and similar factors
  • Roughness is superimposed on waviness
• Lay Predominant direction or pattern of surface texture
• Flaws Irregularities that occur occasionally on the surface
  • Includes cracks, scratches, inclusions, and similar defects in the surface

Roughness and Surface Finish

• Surface Roughness is related to the surface deviations based on roughness
• Surface Smoothness is a more subjective based on what is smooth to the touch or easy to clean

Measures of Roughness

• The average of vertical deviations from the nominal surface over a specified distance

Surface Roughness Equations

• Arithmetic average (AA) is based on absolute values of deviations and is also reffered to surface roughness
• There are alternative calculations for surface roughness equation approximation

Cutoff Length

• Waviness included in Rₐ computatition can skew results
• Cutoff length parameter is used is a filter to separate waviness from roughness deviation's
• Cutoff length is a sampling distance taken along the surface
• A sampling distance shorter than the waviness eliminates waviness deviations and only includes roughness deviations

Surface Integrity

• Surface texture alone does not completely describe a surface
• Metallurgical changes may take place beneath the surface and have a significant effect on a material's mechanical properties
• Surface Integrity is the study and control of this subsurface layer and the changes in it

Energy Forms in Surface Integrity

• Surface changes occur with the application of various forms of energy during processing
• Typical energy types that can alter the surface are;
  • Mechanical
  • Thermal
  • Chemical
  • Electrical
• Mechanical energy example is common in manufacturing with forging, extrusions, and machining

Surface Texture

• The metallurgy chart relates the tolerance to processes used and is accurate at the .05mm range.
• Sand castings are inaccurate with values of 10-20x more than that used for machined parts

Surfaces and Manufacturing Processes

• Some processes are inherently capable of producing smoother surfaces
• Processing cost increases with improvement in surface finish
• Honing, lapping, polishing, and superfinishing can provide superior finishes

Explore the science of measurement in engineering metrology, covering theoretical and practical aspects. Learn about the importance of metrology in enabling interchangeable parts and modern manufacturing. Understand how dimensions, tolerances, and materials influence the performance of manufactured products.