Summary odd pages Erik Flashcards
What is boundary representation?
Parametrical solids
Hierarchical relations (assembly modelling)
Assembly -> sub-assembly -> part design
Positives/negatives of component based assembly modelling
Define positions of coordinate system.
Positives:
Simple, Does not require feature information
Negatives:
Changes in parts that doesn’t show on assembly level
Positives/negatives of feature based assembly modelling
Determine positions by restrictions/position between mating form features
Positives:
Assembly modelling done at higher level
Restrictions as relations between features possible, instead of surfaces
Allows design changes to spread between parts
Negatives:
Relation chain is hard to structure properly
What does higher order curves entail?
Increased precision, longer calculation times and higher risk of corrupt curves. 3rd order curves are most common
What types of curve types are there?
Bezier-curves
- Mathematical definition: P(u) = SUM(p_i * B_(i,n) * (u)), 0 < u ≤ 1, summan går från i=0 till n
- B_(i,n): how the control points affect the curve
- P_i: control points
- n = degree of polynomial
n + 1 = number of control points
B-Splines
- Developed from Bezier-curves
- Improved local control (+)
- Adding control points without increasing polynomial degree (+)
- Easier defined joined curve segments (+)
- Each segment is controlled by 4 closest control points (+)
Rational parametric curves
- Can represent conical and circular shapes exactly (Bezier & B-splines cannot)
How do you create a solid model using surface modeling? 5 steps
- Create wireframe elements (ex. points, planes, curves)
- Create surfaces
- Trim surfaces
- Join surfaces
- Fill surfaces
What are digital mock-ups (DMU) defined by and used for?
- Assemblies with over 1 000 parts
- Assemblies with parts from different CAD systems
- It is used for:
- Visualization
- Packaging studies
- Assembly simulations
- NOT used for mas calculations
Geometry representation: Triangulated surface model
What are the standards for geometry exchange?
- IGES - Mathematical
- STEP - Mathematical
- JT - Mathematical & triangulated
- VRML - Triangulated
- STL - Triangulated
What is off-line programming and what is it used for?
- Used to simulate robots, NC machines or CMM (Coordinate Measurement Machines)
- Is done before implementation in order to:
- Avoid expensive mistakes
- Not having to shut down production for testing
- Faster and more efficient programming
- Ability to test more versions of programs
What geometry models are there and what is the benefit of having geometry models?
- CAD-model - Defines geometry
- Mechanical model - Evaluate mechanical behaviour
- Visual model - Show the product, used for marketing
- Ergonomic model - View the ergonomics, used for assembly and serviceability
Benefits:
- Minimizing the need for costly physical prototypes
- The ability to find problems as early as possible in the development process (cheaper and easier to fix)
- Faster development process with efficient tools
What is a multi-body system?
It is a generic tool for analysis of forces and motion of a mechanical system. It must include the following:
- Rigid bodies
- Constraints - Joints & motions
- Forces - Gravity, loads, forces, friction
What are the requirements and the procedure for a Monte Carlo simulation?
The model consists of the following:
- 3D assembly model with defined locating schemes
- Input tolerances with range and type of distribution
- Critical measurements
The simulation is the following:
- Randomly assign one value for each input within its defined tolerances
- Assemble the model according to the defined locating schemes
- Calculate the critical measures and store the results for each iteration
- Repeat this at least 1000 times
- Calculate the distribution of the critical measures
What are the requirements and the procedure for a contribution analysis?
The model consists of the following:
- 3D assembly model with defined locating schemes
- Input tolerances with range and type of distribution
- Critical measurements
The simulation is the following:
- All input parameters are varied (one at a time) within their tolerances on 3 levels
- Max output is registered for all measures
- Contribution is calculated in percent as:
% contribution = 100 * ((delta_output_i)^2)/SUM(i-1,n, delta_output^2)
What is the procedure for a stability analysis?
The simulation is this:
- Each locating point is disturbed with a unit disturbance
- The amplification to the output, color-coding, part position or critical product dimension, is calculated
- The amplification for each individual locating point is summarizes with RSS to give a value for the locating scheme
It is used to identify sensitive areas and sensitivity factors and guides optimization of locator position
