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Chapter: Mechanical : Computer Aided Design : Assembly of Parts

Assembly of Parts

Assembly modeling is a technology and method used by computer-aided design and product visualization computer software systems to handle multiple files that represent components within a product. The components within an assembly are represented as solid or surface models.

ASSEMBLY OF PARTS

 

PRE-REQUISITE DISCUSSION

 

Assembly modeling is a technology and method used by computer-aided design and product visualization computer software systems to handle multiple files that represent components within a product. The components within an assembly are represented as solid or surface models.

 

 

ASSEMBLY MODELING OF PARTS

 

 

         Assembly modeling is a combination of two or more components using parametric relationships.

 

         Typically a designer would start with a base part

 

         Add other components to the base part using merge commands.

 

 

Assembly Tree


Exploded view

 

An exploded view consists of series of steps. One can create steps by selecting and dragging parts in graphical area.

Example - Assembly of Pulley block



Bottom-up assembly approach - :

 

         Allows the designer to use part drawings that already exist (off the shelf)

 

         Provides the designer w ith more control over individual parts

 

         Multiple copies (instan ces) of parts can be inserted into the assembly

 

Top-down assembly approach - :

 

         The approach is ideal for large assemblies consisting of thousands of parts.

 

         The approach is used to deal with large designs including multiple design teams.

 

         It lends itself well to t he conceptual design phase

 

         E.g. :

 

         Piping and fittings

 

         Welds

 

         Lock pins

 

Degrees of freedom -:


Translation – movement along X, Y, and Z axis

         Rotation – rotate about X, Y, and Z axis

 

Mating conditions -:


Assembly Constraints

 

         Constraints can be used to create permanent relationships between parts

 

         THEY use the same commands as 2D constraints

 

         Typical constraints:

 

–   two faces meet

 

–   axes coincident

 

–   two faces parallel at fixed distance


Assembly sequence affects

 

       difficulty of assembly steps

 

       need for fixture

 

       potential for parts damage during assembly and part mating

 

       ability to do in-process testing

 

       occurrence of the need for reworking

 

       time of assembly

 

       assembly skill level

 

       unit cost of assembly

 

Mating condition

 

Part coordinates MCS (modeling coord.)

 

Base part: Datum

 

Global CS

 

Local CS

 

Explicit position and direction vs mating conditions

 

4 x 4 homogeneous transformation matrix

 

Mating feature

 

Types: against, fits, contact, coplanar fits: center lines are concentric

 

       Mating condition = mating type + two faces

 

       Normal vector + one point on the face

 

       against: two normal vectors are in against directions

 

       fits: between two cylinders: center lines are concentric

 

       Against and fits allows rotation and translation between parts

 

Interference fit

 

       Fits is clearance fit

 

       tight fits is interference fit

 

       Coplanar: two normal vectors are parallel

 

       ‘Coplanar’ complements ‘against’

 

Example Pin and block


Assembly from instances


Exploded view of universal joint


Assembly view of universal joint


 

 

LAYOUT OF INTELLIGENT ASSEMBLY MODELING AND SIMULATION

 

The goal of IAMS is to avoid this expensive and time-consuming process by facilitating semblability checking in a virtual, simulated environment.

 

 

In addition to part-part interference checking, the IAMS tool will check for tool accessibility, stability, and ergonomics.

 

Intelligent Assembly Modeling and Simulation


 

 

PRECEDENCE DIAGRAM

 

Designed to show all the possible assembly sequences of a product.

 

Each individual assembly operation is assigned a number.

 

Diagram is usually organized into columns


 

PRODUCTION DRAWING LIMITS, FITS AND TOLERANCE

 

 

Limit system

 

There are three terms used in the limit system:

 

1.     Tolerance: Deviation from a basic value is defined as Tolerance. It can be obtained by taking the difference between the maximum and minimum permissible limits.

 

2.     Limits: Two extreme permissible sizes between which the actual size is contained are defined as limits.

 

3.     Deviation: The algebraic difference between a size and its corresponding basic size. There are two types of deviations: 1) Upper deviation 2) Lower deviation

 

The fundamental deviation is eith er the upper or lower deviation, depending on which is closer to the basic size.

 

Tolerances

 

Due to human erro rs, machine settings, etc., it is nearly impossib le to manufacture an absolute dimension as specified by the designer. Deviation in dimensi ons from the basic value always arises. Thi s deviation of dimensions from the basic v alue is known as Tolerance.

 

The figure shows mechanical tolerances which occur during operations.


 

Fits

 

The relation between two mating parts is called fit. Depending upon the actual lim its of the hole or shaft sizes, fits may be classified as clearance fit, transition fit and interference fit.

 

Clearance fit

 

Clearance fit is defined as a cleara nce between mating parts. In clearance fit, ther e is always a positive clearance between the hole and shaft.

 

Transition fit

 

Transition fit may result in either an interference or clearance, depending upon th e actual values of the tolerance of individual parts.

 

Interference fit

 

Interference fit is obtained if the difference between the hole and shaft sizes is negative before assembly. Interference fit generally ranges from mini mum to maximum interference. The two extr eme cases of interference are as follows:

 

Minimum interference

 

The magnitude of the difference (negative) between the maximum size of the hole and the minimum size of the shaft in an interference fit before assembly.

 

Maximum interference

 

The magnitude of the difference between the minimum size of the hole and the maximum size of the shaft in an interference or a transition fit before assembly.

 

Hole Basis and shaft basis system:

 

In identifying limit dimensions for the three classes of fit, two systems are in use:

 

1.   Hole basis system: The size of the shaft is obtained by subtracting the allowance from the basic size of the hole. Tolerances are then applied to each part separately. In this system, the lower deviation of the hole is zero. The letter symbol indication for this is 'H'.

 

2.   Shaft basis system: The upper deviation of the shaft is zero, and the size of the hole is obtained by adding the allowance to the basic size of the shaft. The letter symbol indication is 'h'.



 

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