Thread Manufacturing Processes

 What are Threads?

 Grooves of equal measurement or shapes made on a round cylindrical article or pipe are known as threads. In other words, threads of equal pitch and equal depth made on the parallel round cylindrical surface or conical round surface are called threads.

A thread is considered as a raised form known as a helix. Or A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the straight threads and later known as tapered threads.

The thread can be external on a bolt or internal inside a nut. threads are an important part of the screw as a simple machine and also as a fastener.

Types of Threads

1.     RightHand Threads

These threads are tilted towards the right-hand side. These are fastened in a clockwise movement. These are known as right-hand threads.

2.     Left-Hand Threads

These threads are tilted towards the left hand. Left-hand threads are tightened in an anticlockwise direction. For example, if a screw has left-hand threads, so that threads are to be cut on the bolt, screw, or nut which has to be turned in anticlockwise direction fasten it. Their use is comparatively less.

Some of the applications of left-handed fasteners are:

·        Automotive: Some tires’ bolts are threaded in reverse to keep torque from loosening

•         Heating and plumbing: Pipes are often fitted with both right- and left-handed threads. these are used to provide an extra layer of safety from leakage
•         Safety: left-handed threads are used in gas supply valves to separate them from the valves that control the oxygen.

3.     Taper Threads

Threads cut on around but taper surface is called taper threads. These threads are used for shafts like spindles. These threads can be right hand or left hand.

The above threads are used for the following purpose:

•          two parts of a machine-like nut-bolt and stud can be joined using taper threads.
•         Chain pulleys, jacks etc also have these threads.
•         Instruments like micrometres which are used for accuracy also have these threads.
•        In jobs of transfer the power to another. To control or reduce speed.

 

4.    “V” Shape Threads

 

This type of thread is in the shape of the English alphabet “V”. It is most widely used. These types of threads can be cut or made by tap, die, lathe machine and milling, etc. 

 

5.     British Standard Whitworth Threads

British Standard Whitworth threads are originated from great Britain. This thread is invented by sir joseph Whitworth, a citizen of Great Britain in 1841. This thread provides the facility of interchangeability. These threads are used in ordinary nut-bolts and other jobs. It haves an angle of 55°. Its crest, as well as root, are in round shape. It is known as B.S.W. Thread.

·       Thread angle = 55 °

·       Depth = Pitch X 0.6403

·       Radius = Pitch X 0.1373

·       Core diameter = D- (1.28 X Pitch)

·       (D= outer diameter of bolt)

 

6.     Seller Threads

Seller threads are named after the American national thread. Also, they have been recognized by the American standards institute. It is kind of a “V” shape thread. It has a 60° angle. Its roots and crest are flat. The American standard Institute also has invented national fine thread, which is thin.

 These threads are used in motor vehicles and airplanes etc.

Thread Angle = 60°

Depth of thread = 0.649519 X Pitch

Diameter = 0.866025 X Pitch

Crest = D/ 8

Root = 0.125 X Pitch

 

7.     Square Threads

Square threads have a very stable root. Its appearance is square. These types of threads are used for heavy loads lifting purpose, transfer of the power, to exert pressure on objects, screw jack, lead screw of different machines like press machines and vice spindles. But the specific measurement and the number per inch are not fixed in these.

·       Thread Angle = 90°

·       Depth of Thread = 0.5 X Pitch

·       Crest and Root = 0.5 X 5

 

8.     Acme Threads

Acme threads are similar to square threads but their shape is more like taper i.e., flat bottom and thin at the top. It is in comparison easier to cut it on a job than the square type threads. It is also always considered stronger. As acme threads have slight sloppy ends, it is easier to fix or unfasten the split nut on it. There is no backlash in it. These threads have an 29° angle.

·       Thread Angle = 29°

·       Depth of Thread = 0.5 X Pitch + .01″

·       Crest = 0.3707 X Pitch

·       Root = 0.33707 X Pitch – 0.0052″

 

9.     Buttress Threads

It is type of a screw thread which is triangle but which also has one side faced at right angles to the screw axis, the second face haves a slope. It has more stronger pull than square thread. This is the reason why it is used in cases where the excessive pull is needed on one side and also it must be absorbed as in a ratchet.

·       Root and crest = 0.125 X Pitch

·       Thread Angle = 45°

·       Depth = 0.75 X Pitch

 

10.  Knuckle Threads

This type of thread is also very strong. Knuckle threads are mostly used in heavy and light jobs which are tough nature. This thread appears half round both at root and the crest. It can  be made by casting in a mold.

 

 

·       Depth of Thread= 0.5 X Pitch

·       Root and crest Radius = 0.25 X Pitch

These threads are specially used for coupling of rail-coaches.

 

11.  Worm Threads

The shape of these threads is almost similar to Acme Threads but these are more deeper than Acme thread having an 29 °angle. This thread can be used in wheel of motor vehicles for the power transfer at 90° angle. As the three teeths of worm wheel fits into the shaft with worm threads, these also fit well.

·       Thread Angle = 29°

·       Depth of Thread = 0.6866 X Pitch

·       Root = 0.32 X Pitch

·       Crest = 0.335 X Pitch.

 

12.  Single and Multi-Threads

In a work piece, it is possible to have some individual and free threads running along with it. Therefore, there are single-threaded screws and multiple or multi-start threaded screws available. The independent threads are called as starts, and have an single-start, two-start, three-start, etc.

There is a movement of one thread for one complete turn round of the screw which is called a single-threaded screw. The screw is called multiple or multi-start threaded screws when there is a movement of more than one thread.

In the case of a three-start thread, for one complete turn, the thread advances three times as far if it were a single thread. Multi-start threads are used in the cases where fast motion is needed like in Fountain where they are widely used.

 

Screw Thread Terminology

Following are the terms used screw thread terminology:

The Following definitions refer to the various term used in screw threads. The different elements of a screw thread are shown in figure below. The external thread is the thread which haves a cut on the outer surface of a rod. The internal thread haves a cut on the inner surface of a hole.

1. Root

It is the bottom part of the surface of a thread, either flat or rounded which joins the sides of the adjacent threads.

2. Crest

It is nothing but the top portion of the surface of a thread, either flat or rounded which joins the sides of the same threads.

3. Flank or Side

It is the surface of a thread that joins the crest with the root and also it gives the surface contact with its counterpart .

4. The Angle of The Thread

It is the angle measured on an axial plane between the sides of the two adjacent threads.

5. The Depth of The Thread (h3)

It is the distance measured normal to the axis on an axial plane between the crest and the root of a thread.

6. Nominal Diameter

 The threads are cut on the cylindrical rod is the nominal diameter. This diameter specifically signifies the size of the screw.

7. Major Diameter (D,d)

 The diameter of an imaginary coaxial cylinder that brings the crest of an external thread or the roots of an internal thread together is the major diameter.

8. Minor, or Core, or Root Diameter

 The diameter of an imaginary coaxial cylinder which brings the crest of an external thread or the roots of an internal thread together.

·       D1 = minor diameters of the internal threads

·       d3= external threads

9. Pitch Diameter

The pitch diameter is the diameter which is measured with an imaginary coaxial cylinder that can be passed so that cutting the thread with gives width equal to the width of the groove.

·       D2= pitch diameter of internal threads

·       d2= pitch diameter of external threads

10. Pitch

Pitch is the distance from a point on a the screw thread to the similar point on the next thread, measured parallelly with the axis. It also indicates the distance from crest to crest, or from root to root.

11. Lead

Lead is the axial distance advanced by a screw for one complete turn over a threaded rod.The lead and the pitch are more similar on a single start threaded rod.

The lead is twice the pitch, and on a triple start thread, the lead is three times the pitch measured on the double started threads. Thus, the lead can also be defined as the multiplication of the pitch and number of starts.

Machining Thread Process

The following are some of the top methods for threads machining process:

Tapping: Internal threads are usually produced by thread machining process known as tapping. This process is done by using a tap which is a cylindrical shaped or conical shaped cutting tool. The tap has multiple cutting edges which are like an external thread. The internal thread is manufactured by rotating the tap while also moving it axially deeper into the bore of the metal block.

Die Threading: The process of die threading is used to produce external threads. Its method of applying force or pressure and cutting action in this process is similar to tapping process. A die is used to cut the metal block with multiple cutting points tool similar to an internal thread. Different type of die designs like solid or self-opening are available in market.

Single-point Cutting: Single-point cutting machining process is done on a lathe machine where the metal block is held and rotated continuously. The cutting tool is mounted on a carriage fed aligned to a lead screw. This process can machine both internal and external threads. Also it is slower than tapping or die-cutting. The main advantage is that only one cutting tool is required to produce different types of threads.

Chasing: The chasing process uses a tool called as thread chaser which is a several single-point cutting tools combinedly fitted together. The chaser generally is mounted on the carriage of a lathe machine which is indexed gradually to cut the thread.

 

Milling: Here single or multiple rotary cutting tools are used to thread the block. Besides rotating the cutting tool and indexing it axially as seen in tapping and die-threading processes, the cutting tool used is also revolved around the circumference of the threaded surface. Thread milling is able to machine both internal and external threads.

Grinding: Instead of cutting the block, grinding uses abrasive tools to remove metal from the piece to be machined. This is normally done in concurrence with other threading processes available. Thread grinding is used to produce precise threads and threads with a good finishing.

Rolling: As mentioned earlier, thread rolling is an external threading process that shapes the block by passing it through roller dies. The roller dies have external thread-like rollers which conjoin and deform the surface of the workpiece. Thread rolling is generally more faster than thread cutting because the shaping process only requires few rounds of workpiece passing.

Casting: Casting involves pouring or injecting the molten metal into a die or mold. The die contains the negative shape of the threaded part. This process requires secondary machining processes to produce accurate threads. although making fine threads by this process is not possible.

Three Reasons Why Manufacturers Prefer Threads from Thread Rolling Rather than Thread Cutting Processes

 

·       Superior Quality and Accuracy

The finishes of rolled surfaces are generally 32 micro-inches Ra or less. Compared to cut threads, which are barely less than 63 micro-inches Ra. A good thumb rule is that with the help of thread rolling the resulting surface finish can be twice as good as the finish we had at start on the material. For an example, if a blank haves finish of 40 micro-inches Ra then the resulting finish will be close to 20 micro-inches Ra after the thread rolling process.

·       Reducing Costs

Thread rolling stock is smaller in diameter than full size cutting block, without any of the wastage of material. This also means no scrap from regular processing. This means significantly reduced costs per thread because of higher quantity being produced per roll set.

 Thread chasing is much more efficient than thread rolling because - while producing a true thread profile. To produce high quality threads a standard thread chasing is required as compared to thread rolling. When using a thread clutch, the efficiency of thread chasing is reduced drastically and generally it is not so viable option compared to thread rolling.

This also gives advantages like reduced scrap, lower amount of labour, cheaper tools per thread produced- this is a whole formula for overall cost reduction .

·       Stronger Joints Using Rolled Threads

Cold working process can increase tensile strength of material by least 30% more than cut threads. Cold working helps increasing the strength of the thread. Additionally, threads made by rolling have improved fatigue strength by 50% to 75%. Stronger threads can be produced without the need for expensive outside heat treatment are a direct result of choosing a rolled thread over a cut thread process.

Manufacturers have many alternatives at the time of joining and assembling today’s high-performance machine parts. Rolled threads have more qualities like superior quality, accuracy, finish, improved mechanical properties, and economy of production make them an exact ideal choice when “connections matter.”

Conclusion

Producing parts with high quality requires a complete understanding of manufacturing processes. Machining thread cutting is a important aspect of manufacturing processes. Despite the process of thread machining or threading seems a little complicated, it is not advisable to totally abandon the idea.

The processes of machining threads are generally classified into three methods: subtractive, deformative, and additive. These can be varied on how the thread is shaped or formed. The main advantage of thread rolling can offer is the stronger surface of the product and dimensional accuracy. There are different types out there for thread machining that vary according to threads to be cut. 

Presented By: TY C2 Group-5

Varad Naik (40)

Amey Nandrekar (43)

Prathmesh Nikte(49)

Vaibhav Nipunage(50)

Ashwini Nirmal (51)

Guide: Prof. Ashish Mujumdar