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