FASTENERS. SECURING THE ASSEMBLY There are three ways to join parts: welding adhesives fasteners.
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Transcript of FASTENERS. SECURING THE ASSEMBLY There are three ways to join parts: welding adhesives fasteners.
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FASTENERS
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SECURING THE ASSEMBLYThere are three ways to join parts:
welding
adhesives
fasteners
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Method of Assembly
Welding is used when similar materials need to be permanently joined
Adhesives are used for dissimilar materials to be permanently joined and for sealing.
Fasteners join components - whether similar or not - and provide the ability to take apart later
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Fasteners are Popular!
About 70% of all joints are connected using fasteners
Fasteners add 5 percent to the cost of assembly
More than 500,000 types of standard fasteners are available today.
Americans use nearly 200 billion fasteners annually -
26 billion in the automotive industry alone
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FASTENER SELECTION
Design for Manufacturability (DFM) suggests:
• Use off-the-shelf, readily available standard fasteners
• Use the minimal number of fasteners in the design
• Design for ease of assembly and ease of maintenance
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What do fasteners do?
Hold/fasten separate parts together
Transmit power, such as opening/closing a faucet
Used to measure or move a part
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Screw Threads
A thread is a helical or spiral groove formed on the outside or inside of a cylinder.
A die is used to cut external threads.
A tap is used to cut internal threads
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Thread FormsThe thread form is the profile of the thread (the shape of the thread - rectangular, triangular, etc.)
The type of thread form serves different purposes.
ACME or SQUARE threads transmit power, such as opening or closing a valve.
KNUCKLE threads are used for the base of a light bulb, bottle or jar tops.
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Some More Terms
LEAD - the distance the fastener travels in one complete turn.
PITCH - is the distance between corresponding points on adjacent threads
FIT - measure of tightness/looseness in mating parts
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LEAD - the distance traveled in one turn of the screw
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ANATOMY OF A FASTENER
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FIT - tightness/looseness between mating parts
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How to specify a fastener
•Thread length
•Thread form
•Thread Series
•Thread Class
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Thread Length
(D = diameter of fastener)
For steel - minimum length of engagement = D
For cast iron - minimum length of engagement = 1.50xD
For bronze/zinc - minimum length of engagement = 2.0xD
For aluminum - minimum length of engagement = 2.5xD
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Thread Form
3 Major choices:
•UNC
•UNF
•SI
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Thread Series
Unified National Coarse
Unified National Fine
8-pitch
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Thread Class
External thread classes:
1A, 2A, 3A
Internal thread classes:
1B, 2B, 3B
The higher the class, the tighter the fit…3 is the tightest
90% of all commercial and industrial fasteners are 2A and 2B
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How to annotate threads on a drawing
.250 - 20 UNC-2A
.250 = nominal diameter
20 = number of threads per inch
UNC = thread form
2 = class number
A = external thread
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Pipe thread annotation
6-8 NPT
6 = nominal pipe diameter in inches
8 = number of threads per inch
N = American Standard National thread
P = pipe
T = taper
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Let’s draw a fastener!
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HOW TO DRAW A THREAD IN DRAFTING
THREE METHODS -
• SCHEMATIC
• SIMPLIFIED
• DETAILED
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SCHEMATICThe schematic method is used only for external threads or sectioned internal threads, NOT for internal nonsectioned threads
INTERNAL
EXTERNAL
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SIMPLIFIED
This method shows internal threads and external threads
INTERNAL
EXTERNAL
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DETAILEDThe detailed is an approximation of the actual appearance of the screw threads. This is the hardest type of fastener representation and is usually reserved for more artistic types of applications.
EXTERNAL
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Set up construction lines
Set up a construction line set
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Add thread
Draw this shape by using mirror, copy and line
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Array to add threads
Command: _arraySelect objects: wSelect objects: Other corner: 8 foundSelect objects:Rectangular or Polar array (<R>/P):Number of rows (---) <1>:Number of columns (|||) <1>: 18Distance between columns (|||): -0.125
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Add head of fastener
You will need to do some editing to clean up the area between the head & the threads
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And, here’s your fastener!