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
Why are tight tolerances (low variation) expensive to meet?
- Higher requirements on machine precision
- Tighter interval for changing tools
- Multiple process steps
- Higher requirement on inspection/selection
- Unnecessary tight tolerances may be avoided by robust locating schemes
Describe Monte Carlo variation simulation
- Calculates a statistical prediction of the variation in critical measures
- Statistical method - random data
- Tolerances on parts (inputs) are randomly generated within defined distribution, tolerances and Cp
- Distributions for critical measures (outputs) are generated from thousands of iterations
- All kinematical relations and sensitives are captured in a 3D assembly model
Describe contribution analysis
- Calculates a ranked list of how all input tolerances contributes to the variation in the critical measures
- All input parameters are varied (one at a time) within their tolerances on 3 levels
- Max output is registered
- Contribution from each parameter to the variation of one measure is calculated in percent as:
% contribution= 100*(delta output)^2/(summa från i=1 till n av (output^2))
(se bild!)
Describe stability analysis
- Evaluates geometrical robustness and degree of coupling
- Analyzes the influence of each part locating scheme on:
(1) Variation amplification, color-coding
(2) Position stability of parts
(3) Critical product dimensions (measures) - Is done by disturbing each locating point with a unit disturbance
- And summarizing their contribution with RSS
- Is often used to evaluate different positioning systems
Why is it important to optimize locating schemes?
Improves welding conditions by minimizing gap and flush in weld seam for example.
What does perceived geometrical quality depend on?
- Visual sensitivity: geometrical form, split-line placement
- Geometrical sensitivity: locator placement
- Tolerances/distributions
