Threads and Threading Types 1 Translation threads u

Threads and Threading

Types

1. Translation threads u square u acme u buttress

2. American National thread form u WWII - US military equipment did not interchange with equipment made in Britain and Canada

3. Unified thread form u After WWII countries agreed on need for interchangeability u Essentially the same as the American National thread form except Unified has a rounded root and either a rounded or flat crest u Is interchangeable with the American National thread form u Has 60 deg thread angle u 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 u Distance of movement compared to each revolution u 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-2 A u 1/2 = major dia u 13 = threads per inch u UNC = Unified National Course u 2 = Class of fit u A = External Thread form (B = internal)

Classes of fit u Class 1 - largest mfg tolerances, used for ease of assembly u Class 2 - used on largest percentage of threaded fasteners u Class 3 - smallest mfg tolerances, threads will be tight when assembled

Methods of manufacturing threads u taps and dies u lathe u milling u grinding - used when material cannot be machined u rolling - most common

Thread measuring instruments-regardless of method, pitch dia is always measured or compared u 1. mating part - simplest, no measurement involved u 2. comparator micrometer - does not measure pitch, only compares to a known standard u 3. thread micrometer - each micrometer measures a range of TPI – (8 - 13), (14 - 20), (22 - 30), (32 - 40)

Measuring instruments (cont. ) u 4. three wire system (most accurate) u 5. go / no go thread gages - used in production where quick gaging is necessary u 6. optical comparator - light beam shows a profile of the thread for checking thread form, helix angle, and depth (external threads)

Taps and Dies u Taps - create internal threads u Dies - create external threads u Usually made of high speed steel

Standard set consists of: u Taper tap - used for starting a tapped thread square with the hole u Plug tap - most common u Bottoming tap - to produce threads almost to the bottom of a blind hole

Types of taps: u interrupted thread taps - used for tough materials - alternate teeth reduce friction u spiral pointed (gun) - chips are forced ahead of the tap u spiral fluted - helical flutes to draw chips out of the tap u thread forming taps - fluteless taps that do not cut, they displace the material to form the threads - ductile materials

Tapping procedures u by hand with a tap wrench u by machine

Drilling the proper hole diameter u tap drill size u selected from a chart u hole should be reamed before tapping u Tapping problems (Table B-3)

Types of dies u Round split adjustable or (button) allow for small adjustments in size u 2 piece split die. - blanks are placed in cap with guide

Hand threading procedures u always start the die on the leading (throat) side u use lathe, drill press, or mill to start the die squarely u use lubricant u chamfer the end of the rod u reverse the die (or tap) after each full turn to clear chips