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Dimensions, Tolerances, Surface

Dimensions

• Dimensions: linear or angular sizes of a component specified on the part drawing
• Dimentions on part drawings represent nominal or basic sizes of the part and its features
• The dimension indicates the part size desired by the designer

Tolerances

• Tolerances: allowable variations from the sepcified part dimensions that are permitted in manufacturing
• A tolerance is the total amount by which a specific dimension is permitted to vary.
• Tolerances are used to define the limits of the allowed variation

1. Bilateral Tolerance

• Variation is permitted in both positive and negative directions
• +-방향 모두 나타냄

2. Unilateral Tolerance

• Variation from the specified dimension is permitted in only one direction, either positive or negative, but not both
• 한 방향으로만 표기
• Bilateral과 Unilateral은 의미가 다름, 목표지점의 차이

3. Limit Dimensions

• Permissivle variation in a part feature size, consisting of the maximum and minimum dimensions allowed
• 값을 정해줌

Surface

• Nominal surface
  • intended surface contour of part, defined by lines in the engineering drawing
  • appear as absolutely straight lines, ideal circles, round holes and others that are geometrically perfect
• Actual surgaces
  • the variety of manufacturing processes result in wide variations in surface characteristics

Why Surfaces are Important

1. Aesthetic reasons
2. Surfaces affect safety
3. Friction and wear depend on surface characteristics
4. Surfaces affect mechanical and physical properties
5. Assembly of parts is affected by their surfaces
6. Smooth surfaces make better electrical contacts

Surface Texture

• The topography and geometric features of the surface
• When highly magnified, the surface is anything but straight and smooth.
• It has roughness, waviness and flaws

4 Elements of Surface Texture

1. Roughness: small, roughness is superimposed on waviness
2. Waviness: much larger spacing
3. Lay: predominant direction or pattern of the surface texture
4. Flaws: includes cracks, scratches

Surface Integrity

• Surface integrity includes surface texture as well as the altered layer beneath

Surface Changes Caused by Processing

• Surface changes are caused by the application of various forms of energy during processing

Tolerances and Manufacturing Processes

• Most mechining processes are quite accurate
• Sand castings are generally inaccurate

The Design Process

Design Process and Functional Requirment

1. Conceptualization(creativity)
2. Synthesis(creativity)
3. Analysis(engineering analysis)
4. Evaluation(simulation, cose, physical model)
5. Representation(formal drawing or modeling)

Design Representation

• Requirement of the representation mothod
  1. Precisely convey the design cocept
  2. Easy to use

Dimentioning

• Requirements
  1. Unambiguous
  2. Completeness
  3. No redundancy
  4. : 모든 정보를 줄 필요 없음, 4개의 치수 중 하나는 제거해야 함

Tolerance Stacking

• Dimension은 다른 치수들을 빼면 됨
• Tolerance는 다른 Tolerance들의 합

Tolerance Assignment

• Tolerance is Money
• Tolerance Value가 커질 수록 Function은 하락, Cost 상승

Reason of Having Tolerance

• No manufacturing process is perfect
• Nominal dimension can not be achieved exactly
• Without tolerance we lose the control and as a consequence cause functional or assembly failure

Effects of Tolerance

1. Functional Constraints

• 이 제품의 성능의 허용 범위에 따른 Tolerance 결정

1. Assembly Constraints

• Compounding fitting
• The dimension of each segment affects others

Geometric Tolerances

Straightness

• Tolerance zone between two straightness lines

Flatness

• Tolerance zone defined by tow parallel planes

Circularity(Roundness)

• Circle as a result of the intersection by any plane perpendicular to a common axis