Turning 3 NM27

Screw Threads  There are 4 basic Thread Forms  Vee Threads  Buttress  Square  Trapezoidal

Screw Threads  Vee Thread

Screw Threads  Buttress Thread

Screw Threads  Square Thread

Screw Threads  Trapezoidal Thread (30° Angle)

Screw Threads  Thread Terminology  Pitch  Distance from a point on one Thread Groove to the exact same point on the next Thread Groove

Screw Threads  Thread Terminology  Lead  Pitch x Number of Starts

Screw Threads  Thread Terminology  Starts  Grooves running side by side on the same cylinder

Screw Threads  Lead = 12 Lead = 12 Lead = 6  Pitch = 3 Pitch = 6 Pitch = 6  Starts = 4 Starts = 2 Starts = 1

Screwcutting  How many Starts? 21

Screwcutting 3 2

Screwcutting 3 4

Screwcutting  Job 1

Screwcutting  Alternative to Angling Compound Slide Compound Slide moves = ½ Cross Slide Movement

Screwcutting  Methods of Testing Threads  Test Nut

Screwcutting  Methods of Testing Threads  Go – No Go Thread Caliper Gauge

Screwcutting  Methods of Testing Threads  Three Wire Method

Screwcutting  Three Wire Method requires calculating: Best Wire Size = P 2 x Cos 30º (ie. ½ included angle)

Screwcutting  Three Wire Method requires calculating: Measurement Over Wires = M = D – x P + 3 x W

Screwcutting  External Threads

Screwcutting  Full Form Carbide Insert

Screwcutting  Internal Thread

Screwcutting  Tracking Ratio  Tracking Ratio =  Usually expressed as the lowest whole number ratio  Eg. =1 = Magic # Lead of Work Lead of Lead Screw 1236 (The Magic # is used to determine the Starwheel or Chaser Dial details)

Screwcutting  Tracking Ratio  Tracking Ratio =  Usually expressed as the lowest whole number ratio  Eg. =5 = Magic # Lead of Work Lead of Lead Screw

Screwcutting  Tracking the Thread  Tracking the Thread means to engage into the same Thread groove on successive pass of the tool.

Screwcutting Methods of Tracking the Thread  Starwheel  Chaser Dial  Reversing the Machine  Repeat Distance Method

Screwcutting Starwheel  A thin gear like disc with teeth cut of a size to engage with the Leadscrew of the Lathe

Screwcutting Chaser Dial  Sometimes used in conjunction with a Starwheel to assist correct engagement of the Leadscrew

Screwcutting Chaser Dial Details

Screwcutting Reversing the Machine  Reversing the Spindle is useful where a Thread with an unusual Lead needs to be produced  Usually achieved by reversing the direction of the Lathe spindle without disengaging any of the drive gears

Screwcutting Repeat Distance Method  Repeat Distance is calculated by multiplying the Tracking Ratio by the Lead of the Leadscrew

Screwcutting Threads  Metric Threads available by direct selection