Friday, 1 July 2016
Tuesday, 28 June 2016
Threads & Fasteners
THREADS & FASTENERS
Fasteners:
Fasteners includes:
a. bolts
and nuts (threaded)
b. set
screws (threaded)
c. washers
d. keys
e. Pins
FASTENERS
ARE NOT A PERMANENT MEANS OF ASSEMBLY SUCH AS WELDING OR ADHESIVES.
Fasteners
and threaded features are specified engineering drawing.
→ Threaded
features:
Threads are specified in a thread note.
→ General
Fasteners:
Purchasing information must be given to allow the fastener to be ordered
correctly.
Screw Thread: A
ridge of uniform section in the form of a helix.
External Thread: External
threads are on the outside of a member.
An external thread is cut using a die or
a lathe.
Internal Thread: Internal
threads are on the inside of a member. An internal thread is cut using a
tap.
Thread Definitions:
Major DIA (D): The
largest diameter (For both internal and external threads).
Minor DIA (d): The
smallest diameter.
Depth of thread: (D-d)/2
Pitch DIA (dP): The diameter at which a line cuts the spaces
and threads equally.
Crest: The
top surface.
Root: The bottom
Surface.
Side: The surface
between the crest and root.
Pitch (P): The
distance from a point on a screw thread to a corresponding point on the next
thread (in/Threads).
Angle of Thread (A): The
angle between the threads.
Screw Axis: The
longitudinal center line.
Lead: The distance a
screw thread advances axially in one turn.
Types of Thread:
There are many different types of thread
forms (shape) available. The most common are;
→ Unified
→ Metric
Thread form choice depends on;
→ what
it will be used for
→ length
of engagement
→ Load
etc.
Manufacturing of Threads:
Internal Threads:
Internal Threads:
a. First
a tap drill hole is cut with a twist drill.
b. Then
the threads are cut using a tap.
c. Chamfers
are sometimes cut to allow for easy engagement.
External Threads: Start
with a shaft the same size as the major diameter.The threads are then cut using
a die or on a lathe.
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