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| Molded Flange Tutorial A |
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| Molded Flange Tutorial B |
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| Molded Flange Tutorial C |
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| Molded Flange Tutorial D |
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| Molded Flange Tutorial E |
A parametric model is defined as . . . a model which contains BOTH a geometric description (shape and orientation) AND specific numerical parameters (dimensions and number of entities).
As an example consider the model of a generic sphere. If the model is defined by shape AND also by a specific dimension such as a diameter or radius, then this would be a PARAMETRIC model.
As the title implies, a parametric model is a model that is defined by shape as well as being defined by numerical values. Any time a NUMBER is used to describe an object we naturally want to know how precise (how close) must we hold to the stated value of that number?
If the model is defined by shape as a cylinder and by dimension as four inches long with a radius of .5 inches for each "end" what is the tolerance of these numerical values?
Additionally - the number of occurrences of a particular entity are also considered part of a parametric model, for instance if a disc had 4 equally spaced holes on its surface, the note identifying FOUR holes in this model would be a characteristic that would qualify such a disc as a parametric model.
Throughout this lesson we should understand that by definition, a PARAMETRIC model must include the concept of TOLERANCE. It is not possible to dimension an object WITHOUT imposing or specifying some type of tolerance in regard to the dimensions specified.
If a feature size is TOLERANCED, it is allowed to vary with a range of values or limits. Tolerancing ENABLES an engineer to design interchangeable or replacement parts.
Interchangeability refers to the concept of manufacturing (mass producing) a part as a REPLACEMENT PART.
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| Range of Values |
A tolerance is the amount of size variation that is permitted in the manufacture of the part. A tolerance can be chosen that specifies a large or a small variation.
The AMOUNT of tolerance is dependent upon what is required to ensure the functionality of the part and/or assembly of the model/part. The reason that we produce parts with a variation from the stated design values is because IT IS IMPOSSIBLE to manufacture parts without some variation.
The stated limits are a form of QUALITY CONTROL. Some manufacturing processes are more stable than other processes allowing a TIGHTER manufacturing tolerance to be built into the part, however it may be more economical to choose a "worse" method of manufacture (that is cheaper) and simply sort the parts afterward to find the good parts to ship and scrap the bad ones or sell them at a discounted price.
The better a manufacturing process is designed - the least likely it is that "the process" will produce parts that are "out of tolerance". The "real life" problem is to weight the cost of implementing a "perfect process" that almost always produces good parts with the high cost of maintenance and updated equipment that are capable of this type of consistency.
A million dollar press may produce parts within tolerance 90% of the time while a press costing half as much may produce parts within tolerance 75% of the time. It is simply a matter of "doing the math" in regard to upfront costs, labor to inspect the parts, labor to fix the parts, customer satisfaction and things of this sort . . .
The bottom line is that it just NOT POSSIBLE to produce perfect parts, so the designer must determine how good GOOD ENOUGH really is and plan the manufacturing process and drawing tolerances accordingly.
How to choose a tolerance for your design: 1) Specify a tolerance with whatever DEGREE OF ACCURACY that is required for the design to work properly. 2) Choose a tolerance that is not UNNECESSARILY accurate or EXCESSIVELY inaccurate.
Choosing the correct tolerance for a particular application depends upon the following:
1) The design intent (end use) of the part
2) Cost
3) How it is manufactured (what process is used)
4) Experience (the better you know the part, the better qualified you will be to determine what is
an UNNECESSARY degree of accuracy and what is an acceptable degree of accuracy).
TOLERANCING STANDARDS:
Standards are needed to make it possible to manufacture parts at DIFFERENT TIMES and in DIFFERENT PLACES that still assemble properly.
Standards are needed to establish dimensional limits for parts that are to be interchangeable.
The TWO MOST COMMON TOLERANCING STANDARDS ARE:
American National Standards Institute (ANSI/ASME)
International Standards Organization (ISO)
TYPES of Tolerances:
1) Limit dimensions
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| Limit Dimensions |
still pass inspection as in the diameter dimension shown.
Notice THE ORDER - the high limit is placed ABOVE the
low limit and when both are placed on one line - the low limit is given BEFORE the high
limit.
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| Plus or Minus Tolerances |
Give a basic size and the variation that can occur
AROUND the basic size.
3) Page or Block Tolerances:
Is actually a GENERAL NOTE that applies to all
dimensions that are not covered by some other tolerancing
type.
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| Page or Block Tolerance |
GENERAL SHAFT/HOLE ASSEMBLY TOLERANCE:
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| Max - Min material condition |
This example is used to show that BOTH the shaft AND the hole are allowed to VARY between a maximum and a minimum diameter.
DEFINITIONS:
Limits - The maximum and the minimum size that the part is allowed to be.
Basic Size - The size from which the limits are calculated.
Tolerance - The total amount that a specific dimension is permitted to vary.
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| Shaft/Hole Tolerance |
START BY MAKING THE FOLLOWING SELECTIONS . . .
NEW PART - FRONT PLANE in the FeatureManager.
What are the LIMITS of the shaft dimension? .023 - .024
What are the BASIC dimensions of the shaft AND hole? .25
What is the TOLERANCE of the hole dimension? .020 inch
What is the Maximum Material Condition (MMC) of the shaft? This is the size when it consists of the MOST material. This would correspond to the LARGEST dimension of the shaft. .240 inches.
What is the Least Material Condition (LMC)? The size of the part when it consists of the LEAST
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| Fill in tables below with this information |
This would correspond to the LARGEST dimension of the hole .270 inches.
Maximum Clearance. The maximum amount of SPACE that can exist between the hole and the
shaft. = LMC hole - LMC shaft
Minimum Clearance (allowance). The minimum amount of SPACE that can exist between the hole
and the shaft. = MMC hole - MMC shaft
TYPES OF FITS:
1) Clearance fit - There is ALWAYS a space Min Clearance GREATER than 0
2) Interference fit - There is NEVER a space Max Clearance LESS THAN or EQUAL to 0
3) Transition fit - Depending upon the size of the shaft and hole there COULD BE a
space or no space at all.
Max Clearance GREATER than 0
Min Clearance LESS than 0
4) Line fit - There is a space or a contact (hole dia. = shaft dia.)
Max Clearance GREATER than 0
Type of fit EXAMPLES:
Given the following shaft and hole limits, can you fill in the correct information on the BASIC size and TYPE of fit without looking at the answers?
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| Basic Size and Type of Fit |
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