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 Tapered pipe taps

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