Threads and Threading
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Transcript Threads and Threading
Threads and Threading
Types
1. Translation threads
square
acme
buttress
2. American National thread
form
WWII
- US military equipment did not
interchange with equipment made in
Britain and Canada
3. Unified thread form
After
WWII countries agreed on need
for interchangeability
Essentially the same as the American
National thread form except Unified
has a rounded root and either a
rounded or flat crest
Is interchangeable with the American
National thread form
Has 60 deg thread angle
Two major thread series - UNC and
UNF
4. Metric threads
5. Right hand vs. left hand
Screw thread Nomenclature
(external)
Major Dia: largest dia
Minor Dia: smallest dia
Pitch Dia: imaginary point
where width of groove and
thread are equal
Root: bottom surface
connecting 2 sides of a
thread
Crest: top
Pitch: linear distance from
corresponding points on a
thread
Depth: Distance from crest
to root perpendicular to axis
of thread on one side
Flank: Sides of a thread that
connect crest to root
Helix Angle
Distance
of movement
compared to each revolution
The Lead of the helix
Lead: Distance of 1
revolution (lead = pitch on a
single lead thread)
Thread angle: included
angle between flanks of
thread
Unified Screw thread
designation (5 components)
1/2-13-UNC-2A
1/2
= major dia
13 = threads per inch
UNC = Unified National Course
2 = Class of fit
A = External Thread form (B =
internal)
Classes of fit
Class
1 - largest mfg tolerances,
used for ease of assembly
Class 2 - used on largest percentage
of threaded fasteners
Class 3 - smallest mfg tolerances,
threads will be tight when assembled
Methods of manufacturing
threads
taps
and dies
lathe
milling
grinding - used when material cannot
be machined
rolling - most common
Thread measuring
instruments-regardless of
method, pitch dia is always
measured or compared
1.
mating part - simplest, no
measurement involved
2. comparator micrometer - does not
measure pitch, only compares to a
known standard
3. thread micrometer - each
micrometer measures a range of TPI
– (8 - 13), (14 - 20), (22 - 30), (32 - 40)
Measuring instruments
(cont.)
4. three wire system (most accurate)
5.
go / no go thread gages - used in
production where quick gaging is
necessary
6. optical comparator - light beam
shows a profile of the thread for
checking thread form, helix angle,
and depth (external threads)
Taps and Dies
Taps
- create internal threads
Dies - create external threads
Usually made of high speed steel
Standard set consists of:
Taper
tap - used for starting a tapped
thread square with the hole
Plug tap - most common
Bottoming tap - to produce threads
almost to the bottom of a blind hole
Types of taps:
interrupted
thread taps - used for
tough materials - alternate teeth
reduce friction
spiral pointed (gun) - chips are
forced ahead of the tap
spiral fluted - helical flutes to draw
chips out of the tap
thread forming taps - fluteless taps
that do not cut, they displace the
material to form the threads - ductile
materials
Tapping procedures
by
hand with a tap wrench
by machine
Drilling the proper hole
diameter
tap
drill size
selected from a chart
hole should be reamed before
tapping
Tapping problems (Table B-3)
Types of dies
Round
split adjustable or (button) allow for small adjustments in size
2 piece split die. - blanks are placed
in cap with guide
Hand threading procedures
always
start the die on the leading
(throat) side
use lathe, drill press, or mill to start
the die squarely
use lubricant
chamfer the end of the rod
reverse the die (or tap) after each full
turn to clear chips