WHAT IS THREADING-FITTER ONLINE NOTE

What is screw thread?

i)- A screw thread is a rigid of uniform section formed helical on the surface of a cylindrical body.

ii)- An external screw thread is formed on the outer surface of cylindrical part. for example:- Bolts, Screws, Studs, Threaded spindles etc.

iii)- An internal screw thread is formed on the inner surface of a hollow cylindrical part. for example:- Nuts, Threaded lids etc.

iv)- External thread and internal threads are assembled together for different engineering uses.

Use of Screw Threw

Screw threw thread are used for

i)- As fasteners to hold together and dismantle components when needed.

ii)- To transmit motion on machines from one unit to another.

iii)- To make accurate measurements

iv)- To apply pressure.

v)- To make adjustment.

Parts of a Screw Thread

Crest:-

The top surface joining the two sides of a thread.

Root:-

The bottom surface joining the two sides of adjacent threads.

Flank:-

The surface joining the crest and root.

Thread Angle:-

The included angle between the flanks of adjacent threads.

Depth:-

The perpendicular distance between the roots and crest of the thread.

Major Diameter:-

The largest diameter of the thread on the Screw or Nut.

Minor Diameter:-

The Smallest diameter of an external and internal screw thread.

Number of Thread:-

The number of roots or crests per inch of the threaded length.

Pitch Diameter:- 

The diameter of the thread at which the thread thickness is equal to one half of the pitch.

Pitch:-

i)- It is distance from a point on one thread to a corresponding point on the adjacent thread measured parallel to axis.

ii)- It is expressed in mm for metric threads.

Lead:-

The distance a screw thread advances axially for one complete revolution.

Form of screw Threads

i)- Screw threads are different forms are available for meeting the various requirements.

ii)- The basic forms of screw threads are 1)- 'V' Thread, 2)- Square Thread, 

3)- Trapezoidal Thread, 4)- Knuckle Thread, 5)- Acme Thred, 6)- Buttress Thread, 

7)- saw-tooth threads.

1)- 'V' Thread:-

i)- These threads are of a 'V' shape.

ii)- 'V' threads are different types are available.

iii)- Mostly these threads are used for fastening 

iv)- 'V' thread are most commonly used from of screw thread and is used for domestic and industrial applications like Bolts,Nuts and Spindle for micrometer.

v)- These threads are made by lathe thread milling, tap and dye etc

Types of threads Thread angle :

(a) Metric Thread

(b) International Standard Thread

(c) British Standard Thread

(d) British Standard Fine Thread (B.S.F)

(e) British Standard Pipe Thread (B.S.P)

(f) Association or B.A. Thread British

(8) American National or Seller Thread.

2)- Square Thread:-

i)- The side of square thread is at right angle

ii)- Its threads depth,breadth, crest and root are all same.

iii)- These are more stronger than 'V' shape thread.

iv)- These threads are used in heavy weaght lifting machines or instruments.

v)- Its feed and power transmission like vice, spindle, hand press,screw jack etc are used.

3)- Trapezoidal Thread:-

This is also modified square thread.

ii)- Its flank angle is 30 degree.

iii)- These threads are also strong and are used for easy engagement and disengagement in transmission.

iv)- Lead screw and machine tool are made of Trapezoidal thread.

4)- Knuckle Thread:-

i)- Knuckle thread is a modified from square thread.

ii)- In this case the creast and root are rounded to from a semi-circle.

iii)-  It is used for valve spindles, railway carriage couplings, hose connections etc. 

5)- Acme Thread:-

i)- i)- Acme thread is a modified from Trapezoidal thread.

ii)- Its angle is 29 degree.

iii)- These thread are used for feed and power transmission and easy to fit split nut due to its sides are at an angle.

iv)- Acme threads are used in lathe lead screws.

6)- Buttress Thread:-

i)- In buttress thread one flank is perpendicular to the axis of the thread and the other flank is at 45°.  

ii)- These threads are used on the parts where pressure acts at one flank of the thread during transmission.  

iii)- These threads are used in power press, carpentry vices, gun breeches, ratchets etc.

7)- Saw-tooth thread:-

i)- This form of thread has an inclination of 3 degree on one side and 30 degree on the other side.

ii)- These threads are used in places where sliding forces act in one direction.

iii)- Spindle of carpenters vice and quick release presses have saw-tooth threads.

Derivation of the formula for change gears Thread cutting on lathe.

Example 1:-

The pitch of lead a screw is 6 mm and the pitch of the thread to be cut is 1.25 mm. find the change wheel in lathe thread cutting.

solution:-

We know the formula- 

Driver teeth/Driven teeth = Pitch of the work/Pitch of the lead screw.

= pitch of the work = 1.25 mm

= Pitch of lead screw = 6 mm

= Pitch of the work/Pitch of the lead screw=  1.25/6 =5/4 X 1/6

We know that the engine lathes are equipped with a set of gears ranging from 20 to 120 teeth in steps of 5 teeth(20,25,30,35,......110,115,120 and 127 available). Hence we must compound it.

= Driver gear/Driven gear= 5 x 10/4 x 10 = 50/40 gear available.

= Driver gear/Driven gear= 1 x 20/6 x 20 = 20/120 gear available.