Screw Threads Screw Cutting

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? 1 2

Screwcutting How many Starts? 2 3

Screwcutting How many Starts? 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 – 1.5155 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 3 6 1 2 (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 5 24 1.25 6

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

Screwcutting Change Gears - Simple Gear Train Standard Leads

Screwcutting Threads To produce an M40x4 select: Range II, Lever Position C6

Screwcutting Threads To produce an M24x3 select: Range I, Lever Position A1

Screwcutting Threads Sometimes Threads are NOT available by direct selection, eg. 2.5 mm Lead

Screwcutting Threads As 2.5 mm is not on the Chart of available Leads we must improvise

Screwcutting Threads We can cut Leads not normally available on the chart by using: Formula D D1 x D2 F F1 x F2

Screwcutting Compound Gear Train

Screwcutting Threads We can cut Leads not normally available on the chart by using: Formula D Dn x R F Fn x S

Screwcutting Threads To cut a 2.5 mm Lead: Formula D 30 x 2.5 F 60 x 3

Screwcutting Threads To cut a 2.5 mm Lead: Formula D 30 x 2.5 F 60 x 3 2.5 25 6 60

Screwcutting Change Gears - Simple Gear Train 25 60 Change the 30z for a 25z 25 60

Screwcutting Threads