Geometry assurance Flashcards
What is GD&T?
- Geometric Dimensioning and Tolerancing
- Is a system for defining and communicating engineering tolerances
- Its purpose is to describe the engineering intent of parts and assemblies
- Different types with or without references
- Ex. without references: flatness, circularity, surface profile
- Ex. which requires references: position, perpendicularity, parallelism
What is PMI?
- Product and Manufacturing Information
- Today there is a trend to replace 2D drawings by 3D models, this requires the possibility to add drawing information ex. tolerances. This info is called PMI
What informations does a PMI include?
- GD&T
- BOM (bill of materials
- Surface finish
- Weld symbols
- Material specifications
- Metadata & notes
- Other annotations
What are the advantages with having 3D models as PMI?
- Minimizes loss of any critical information due to human error
- Avoids updating the 2D drawings
- Improves communications between designers and manufacturers
- Steamlines the product development process
- Has positive time and cost implications
What affects the variation in product geometies?
Variation in subassembly geometries and assembly parameters (locators, equipment…)
What affects the variation in subassembly geometries?
Variation in part geometries and assembly parameters (locators, equipment…)
What affects the variation in part geometries?
Variation in process and material parameters
What are the two main parameters when talking about variation and robustness?
- Locating schemes: lock 6 DOFs in space, control robustness, rigid/non-rigid (controls robustness)
- Tolerances: define allowed variation, control cost (controls variation)
What is the purpose of a locating schema?
To lock the position of a part in space
What is a master location scheme?
It is the set of 6 locating points on a part that locates the part in the:
- manufacturing fixture (as far as possible)
- inspection fixture
- assembly fixture
- final assembly/product
To keep control over variation these 6 points shall be the same in all steps.
A master location scheme is physically realized by locating features like planes, holes, slots…
What is a 3-2-1 locating scheme?
- Primary points A1, A2 & A3 defines a plane and locks the geometry in space in two rotations and on translation: TZ, RX, RY
- Secondary points B1 & B2 defines a line and locks the geometry in space in one rotation and one translation: TY, RZ
-Tetriary point C1 locks the geometry in space in one translation: TX
By this 6 DOFs are locked by six points.
What type of locating schemes are used for non-rigid part and assemblies?
More than 6 locators are often used to secure the position in space. The additional locators are called supports.
What characterizes a robust design?
- The fact that its output characteristics are intensive to input variation
- The geometrically robust design is a design that allows manufacturing and assembly variation without jeopardizing function or aesthetics.
- The geometrical robustness is controlled by the position of the locators.
What does tolerance define?
- Defines allowed variation with respect to product functionality or aesthetics
- Proper definition of part tolerances is based on top down decomposition of product requirements
How does variation appear and how is it measured?
- Variation comes from the manufacturing processes
- Standard deviation is a measure of variation
