Introduction to Metrology

Definition of Metrology

• Metrology is from the Greek word 'metron' and refers to the science of measurement, encompassing all theoretical and practical aspects.

Importance of Metrology

• The concept of interchangeable parts has allowed for the development of modern manufacturing.
• Henry Ford required all rear axles on the Model T to have the same dimensions and tolerances.
• Rear axles made on different days would be interchangeable.
• Royal Royce hand-fits each part, and technicians spend up to 30 minutes on each part to ensure they fit correctly.
• Metrology is the science of measurement.
• Design engineers design parts to specific dimensions and tolerances so parts function together.
• Manufacturing engineers use metrology to check the parts.

Car Manufacturing Example

• Workers on a car assembly line might only have 15 seconds of cycle time at their station.
• If a part doesn't fit within this time, the worker stops the line.

Dimensions, Tolerances and Surfaces

• Dimensions, tolerances, and related attributes.
• Surfaces play a role
• Effects of various manufacturing processes

Critical Information

• If you ask a machinist to make a widget, they need to know:
  • What is the material? Which impacts ease of machining.
  • What are the dimensions?
  • What are the tolerances? Which will impact time and cost.
  • How many do you want?
  • When do you need them?
  • What's your budget?
  • Machinists cost between $50 to $80 an hour.

Dimensions and Tolerances

• Dimensions and tolerances determine the performance of a manufactured product.
• Dimensions refer to the linear or angular sizes of a component as specified on the part drawing.
• Tolerances are allowable variations from the specified part dimensions that are permitted in manufacturing.

Dimensions

• A dimension as specified by ANSI Y14.5M-1982, is a numerical value expressed in appropriate units.
• Dimensions are indicated on a drawing along with lines, symbols, and notes to define the size or geometric characteristic of a part.
• Dimensions on part drawings represent nominal or basic sizes of the part and its features.
• Dimensions indicate the part size desired by the designer, if the part could be made without any errors.

Tolerances

• The definition is the allowance, as specified by ANSI Y14.5M-1982, is "the total amount by which a specific dimension is permitted to vary."
• The tolerance is the difference between the maximum and minimum limits.
• Variations occur in any manufacturing process, leading to part size variations.
• Tolerances are needed to define the allowed variation limits.

Tolerance

• Tolerare is defined in the slides as put up with, endure.
• It is impossible to make parts perfectly.
• Too small a tolerance can lead to high costs.
• A Boeing 747-400 has 6 million parts and requires measurement of 28 features for an accumulated 150 million measurements.
• NIST, the U.S. National Institute of Standard and Technology is seeing tolerances shrink by a factor of 3 every 10 years due to ultraprecision ion-beam machining shrinking to 0.001µm.

Importance of Tolerance

• Parts from the same machine can be slightly different.
• Factors like speed, temperature, lubrication, incoming material variation, affects parts from the same machine.
• See other factors and or look at the ISO system for definitions.

Bilateral Tolerance

• Variation is permitted in both positive and negative directions from the nominal dimension.
• Bilateral tolerance can be unbalanced, with unequal positive and negative deviations.

Unilateral Tolerance

• Variation is permitted in only one direction from the specified dimension.
• This means deviation is only permitted in the positive or negative direction, but not both.

Limit Dimensions

• Permissible variation in a part feature size consists of specifying the maximum and minimum dimensions that is allowed.

Tolerance Control

• Basic size, deviation and tolerance on a shaft are defined according to the ISO system.

Methods of Assigning Tolerances

• Tolerances on a shaft can be assigned using:
  • Bilateral tolerance.
  • Unilateral tolerance.
  • Limit dimensions.

Geometric Tolerance

• Geometric tolerance involves tolerances related to shape features of a part.
• Geometric characteristic symbols are indicated on engineering drawings for manufactured parts.

Definitions

• Allowance is the specific difference in dimensions between mating parts.
• Basic size is the dimension from which limits of size are derived.
• Bilateral tolerance is the deviation from the basic size (+ or -).
• Clearance is the space between mating parts.
• A clearance fit is one that allows for rotation or sliding between mating parts.
• Datum is a theoretically exact axis, point, line, or plane.
• A feature is a physically identifiable portion of a part, such as a hole, slot, pin, or chamfer.
• Fit is the range of looseness or tightness between two mating parts.
• Geometric tolerancing involves tolerances related to shape features of a part.
• Interference is a negative clearance.
• An interference fit has negative clearance.
• Limit dimension indicates the maximum and minimum dimensions of a part.
• MMC is the maximum material condition, which is the condition where a feature of size contains the maximum amount of material within the stated limits of size.
• Positional tolerancing is a system that specifies the true position, size, and form of a part's features, along with allowable variation.
• A transition fit is a fit with small clearance or interference that allows for accurate location of mating parts.

Surface Technology

• Surface technology concerns defining surface characteristics.
• It also involves examining surface texture and surface integrity.
• The effect of manufacturing processes and characteristics of the resulting surface is included.

Surfaces

• Nominal surface corresponds to the designer's intended surface contour on an engineering drawing.
• Nominal surfaces appear as straight lines, ideal circles, and geometrically perfect features..
• Actual surfaces of a part are determined by manufacturing processes.
• Therefore, resulting in wide variations in surface characteristics.

Importance of Surfaces

• Surfaces have properties and behavior that differs from the bulk of the part.
• Surfaces impact the function and appearance of manufactured parts:
  • How surfaces feel
  • How surfaces look -How it behaves for coating or sealing
• Surfaces impact how surfaces behaves in contact, and therefore, the surface will wear
• Surfaces impact retainment, and therefore wear
• How well it will hold a load

Why Surfaces are Important

• Surfaces are important to the aesthetic value of the part.
• Customer has a favourable impression if the surface is smooth
• Surfaces affect safety.
• Friction and wear depend critically on surface characteristics.
• Surfaces impact mechanical and physical properties.
• Surface flaws impact stress concentration points and physical properties
• Assembly of parts are affected by their surfaces.
• Adhesively bonded joints will be stronger when the surface is slightly rough
• Surfaces affect subsequent operations (painting, coating, welding, soldering).
• Smooth surfaces make better electrical contacts.

Metallic Part Surface

• The magnified cross-section includes the altered layer, the substrate material, and may be covered with surface texture.

Surface Characteristics

• Surface characteristics are defined by examining microscopic irregularities and imperfections.
• Consider the Bulk of the Part which is defined as:
  • Rougness
  • Waviness
  • Flaws
• The Altered Layer may include:
  • Work hardening (Mechanical energy)
  • Heat (thermal energy)
  • Chemicals
  • Electrical energy
• Surface Integrity includes surface texture and other characteristics.
• Surface integrity includes definition, specification and control of surface layer.

Contamination

• Contamination may be in the form of:
  • Oxide Film e.g. Rust (Iron), Al2O3(Aluminum)
  • Dirt, Oil, Cutting Fluids, Lubricants and Absorbed Gases
• Contamination will affect appearance and inhibit joining, soldering, plating.

Surface Structure of Metals

• Surface texture
• Altere layer
• Substrate

Surface Texture

• Repetitive and/or random deviations from the nominal surface of an object.
• Has features of:
  • Crater
  • Lay direction
  • Waviness height
  • Roughness height -Roughness width

Surface Texture

• Topography and geometric features of a surface
• When highly magnified, a surface is usually not straight or smooth because it has roughness, waviness, and other flaws.
• A surface will possess a pattern or direction as is a result of the mechanical process that produced it.

Four Elements of Surface Texture

• Roughness - small, finely-spaced deviations from nominal surface
  • Roughness is influenced by material characteristics and the processes that formed the surface
• Waviness - deviations of much larger spacing -Waviness deviations occur due to work deflection, vibration, heat treatment
• Roughness is superimposed on waviness

Surface Roughness and Surface Finish

• Surface roughness is a measurable characteristic based on roughness deviations.
• Surface finish is a subjective term denoting smoothness and general quality of a surface. As such:
• Surface finish is often used as a synonym for surface roughness, both used to describe surface texture

Surface Roughness

• Measured as an average of vertical deviations from nominal surface over a specified area

Surface Roughness Equation

• Describes the Arithmetic average (AA) based on absolute values of deviations.
• Describes what is referred to as average roughness.

Surface Integrity

• Surface texture alone does not fully describe a surface.
• Metallurgical changes might exist in the altered subsurface layer that can impact:
  • The material's mechanical properties.

Energy Forms in Surface Integrity

• Surface changes caused by:
  • Mechanical energy
  • Thermal energy.
  • Chemical energy.
  • Electrical energy.
• Mechanical energy is the most common in manufacturing (forging, extrusion)

Surface Changes by Mechanical Energy

• Include residual stresses, micro/macroscopic cracks, voids/inclusions, and hardness fluctuations.
• Voids example centerbursting in extrusion
• Hardness example is strain machining plastic deformation

Surface Changes by Thermal Energy

• Includes heat-affected zones, metallurgical and hardness changes, and redeposited/ resolidified material.

Surface Changes by Chemical Energy

• Include corrosion, contamination, concentrate on certain components, alloying depletion and resulting hardness changes:.

Surface Changes by Electrical Energy

• Changes in conductivity, creation of craters from processing techniques, surface depression and short circuits are produced by electrical techniques.