1. UNIT IV
Syllabus
Assembly modelling, interference of positions and orientation, tolerance
analysis, mass property calculations, mechanism simulation and interference
checking.
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2. Assembly modelling
• An assembly is a collection of independent parts.
• In general, assemblies are simply groups of parts which are brought together
in some fashion so that it can perform intended purposes.
• This topic emphasis the physical assembly of a product such as manual
assembly vs automatic assembly, force and mass of parts, tool and
equipment involved in assembly, tolerance analysis and interference checking.
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10. Assembly planning
• Assembly planning is a key to creating successful assemblies.
• Before create an assembly, we should consider the following issues
• Identify the dependencies between the components of an assembly
• Identify the dependencies between the failures of each part
• Analyse the order of assembling the parts.
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18. Three assembly approaches
• Bottom – up assembly approach
• Top – down assembly approach
• Combination of both
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19. Bottom – up assembly approach
• In this approach, we create the individual parts independently,
insert them into an assembly, and use the mating conditions to
locate and orient them in the assembly as required by the
assembly design.
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21. Procedure for Bottom-UP Assembly
1. Log in and start the CAD/CAM system.
2. Select the assembly mode.
3. Open a new assembly file.
4. Use an Insert => Component => From File command (or its equivalent) to insert the block.
5. Repeat Step 4 for the plate.
6. Mate and align the plate with the block. Use a coincident mating condition between Face 1 and
Face 2. Also position the two instances close together using a Closest alignment command.
7. Repeat Step 6 for Face 3 and Face 4.
8. Mate the holes in the block and the plate. Use a concentric mating condition. The assembly state,
after Steps 6 to 8, is shown below.
9. Insert the pin into the assembly using an Insert command or its equivalent.
10. Mate the pin with its hole. Use a concentric mating condition between the two. Also, use a
coincident mating condition between Face 5 and Face 6 as shown below.
11. Save all the files and exit the CAD system.
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23. Top – down assembly approach
• The top down approach, while good for any size assembly, is ideal
for large assemblies consisting of ten of thousands of
components.
• In this approach begins with an assembly layout sketch. The
layout serves as the behind the scenes backbone of the assembly.
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24. 1. Log in and start the CAD/CAM system.
2. Select the assembly mode.
3. Open a new assembly file.
4. Create the assembly layout sketch shown below. Define the dimensions as shown in the layout. The lines connecting
the two pulleys are tangent to the pulleys.
5. Open a new part file, construct the large pulley part, and save it as pulleylprt and exit. Use 3.0 inches as the pulley
diameter and 0.5 inch as its thickness,
6. Open a new part file, construct the small pulley part, and save it as pulley2.prt and exit. Use 1.0 inch as the pulley
diameter and 0.5 inch as its thickness, as shown below.
7. Open a new part file, construct the belt part, and save it as belt.prt and exit. Use 5.0 0.5 in rectangular section and 0.1
ill as its thickness, as shown below.
8. Open the assembly file topliown.asm and insert the following components: pulley l.prt, pulleyl.prt, and two instances
of belt.prt.
9. Relate the reference dimensions of the individual components to the corresponding dimensions in the assembly
layout sketch.
10. Now you may change the parameters in the layout sketch to see the effect in the individual components and in the
assembly.
11. Save all the files and exit the CAD system.
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Procedure for Top-Down Assembly
27. Interference of position and orientation
• The interference of the position and orientation of a part in an assembly
from mating conditions requires computing its 4x4 homogeneous
transformation matrix from these conditions.
• This matrix relates the parts local coordinate system (part CS) to the
assembly’s global coordinate system (assembly CS).
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28. • The simplest method for specifying the location and orientation of each part
in an assembly is to provide the 4x4 homogeneous transformation matrix
[T].
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32. Tolerance analysis
• Tolerance is the permissible variation in the size of a dimension and is the
difference between the upper and lower acceptable limits
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33. Fits and Types of Fits
• The degree of tightness or looseness between mating parts is known as 'fit'.
The nature of fit is characterized by the presence and size of clearance or
interference. There are three types of fits as follows.
(a) Clearance fits
(b) Interference fits
(c) Transition fits.
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34. Clearance fits
• In clearance fits, the shaft is always smaller than the hole.
• A positive allowance exists between the largest possible shaft and the lowest
possible hole, i.e. at the maximum material condition.
• In this type of fit, the tolerance zone of shaft is always below the hole.
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35. Interference fits
• In interference fit, the shaft is always larger than hole.
• The tolerance zone of the shaft is entirely above that of the hole.
• Interference fits are used in fixed permanent joints.
• Examples are steel tyres on railway car wheels, pump impeller on shaft and
cylinder liner in cylinder block.
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36. Transition fits
• Transition fits are midway between clearance and interference fits.
• Main use of these fits is to ensure a proper location of mating parts which
are often disassembled.
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37. Types of tolerance system
• Based on the hole and shaft, the tolerance system is divided into two types.
Hole basis system
Shaft basis system
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38. Hole Basis System
• In this system, the hole is kept constant and the shaft diameter is changed to obtain various
types of fits. The basic size of the hole is taken as the low limit size of the hole. High limit
size of the hole and the two limits of size for shaft are then selected to give the desired fit.
Holes are denoted by ‘H’ and shafts get different letters according to requirement.
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39. Shaft Basis System
• In this, the shaft is kept constant and the hole is varied to get various fits. In this, basic size
of shaft is taken as one of the limits of size for the shaft. The other limit for the shaft and
other two sizes for hole is then selected. In this system, shaft is denoted by ‘h’ and holes get
different letters to give the desired fit.
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42. Geometric tolerance
• Geometric tolerance specifies the maximum variation of form or position by
defining a tolerance zone with in which the feature is to be constrained.
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50. Method of Tolerance analysis
• Worst-case arithmetic method
• Worst-case statistical method
• Monte Carlo simulation method
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51. Worst-case arithmetic method
• This method uses the limits of dimensions to carry out the tolerance
calculations. The expected or actual distribution of dimensions is not taken
into account. This method assumes that all dimensions in the tolerance stack
up may be at their worst-case maximum or minimum, regardless of the
improbability. The individual variables are placed at their tolerance limits in
order to make the measurement as large or as small as possible. This method
predicts the maximum expected variation of the measurement.
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52. Worst-Case statistical method
• The dimensions of parts of an assembly follow a probabilistic distribution
curve. Therefore, a similar distribution curve pattern is followed in the
frequency distribution curve of the dimensions of the final assembly.
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53. Monte Carlo simulation method
• Monte Carlo can be used in all situations in which the above two methods can be
used and they can yield more precise estimates. Monte Carlo Simulation is a
powerful tool for tolerance analysis of mechanical assemblies, both nonlinear
assembly functions and non-normal distributions. For this reason, Monte Carlo
technique is easily the most popular tool used in tolerancing problems.
• It follows binomial distribution
• It uses a random number generator –component distributions-assembly function
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54. Mass property calculations
• It is one of the major application involved in CAD/CAM systems.
• This calculation involve
• Mass
• Centre of gravity
• First moment of inertia
• Second moment of inertia
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55. Mass
• Mass is the amount of matter contained in an object.
• It depends on its volume and density of the material of the object.
• the mass of an object is calculated initially by considering a small element
and it is then expanded to whole object.
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56. Centroid or Centre of Gravity
• Centre of gravity is defined by its centriod.
• It is defined as the centre where the total mass of the body can be assumed
to be concentrated.
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57. First Moment of Inertia
• The first moment of inertia is defined as the moment of area, volume or
mass with respect to a given plane. It is the moment about a line or edge.
• In general, these planes or axes are the standard three planes (XY, XZ, and
YZ) or axis (X, Y, and Z).
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58. Second Moment of Inertia
• The second moment of inertia about a given axis is equal to the product of
the mass and the square of the perpendicular distance between the mass and
axis.
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59. Mechanism simulation
• A mechanism is a mechanical device which transfers the motion from source
to an output.
• It is a key technology for designing new products, developing new
manufacturing process and evaluating the performance of mechanism.
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62. advantages
• Identification and tuning of critical components
• Time and money are saved by removing faults before manufacture.
• Primary method used for evaluation of systems before manufacture.
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63. disadvantages
• Takes too long and cost too much while simulating.
• Building hardware prototypes is impractical for large systems.
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65. Interference checking
Interference checking is the process of checking if any parts of an assembly penetrate
or overlapped each other or not.
If an interference is detected between two parts, CAD system displays the interference
volume to allow the users to examine and rectify/eliminate it.
There are three types of interference in fixture design
• interference among fixture components
• interference between fixture component and a work piece
• Interference between fixture components and a machining envelope
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