Nobody can deny the presence of screws, nuts and bolts in machining parts. If you are a machinist you must be aware of the importance of these parts. Whenever we talk about metal work or matching the mentioning of threads is obvious. Why?

It is because threads facilitate the connection of parts with each other. While remaining cost effective they make the parts easy to use worth fasteners, connectors and fittings.

Just knowing about threads in not sufficient. You should know everything about creating the perfect threads to their types, processes and applications.

This blog is all about machining enthusiasts. It is a complex guide to thread cutting, types, applications etc. It explains all the technicalities associated with the process. It also describes the common screw machining process in detail. Continue reading to know more.

What Is Thread Machining in Part Manufacturing?

In a CNC machine center thread cutting is the most important application. But before we go into the depth of the process let’s talk about what is thread?

A thread is a continuous helical ridge that acts as a coupling agent between the motion of two parts. The thread machining process is the manufacturing of thread. This process makes two types of threads:

• External threads

• Internal threads

Both threads are manufactured in variety of sizes.

Thread machining is a subtractive method. It involves the working of a rotating tool. The rotating tool has calibrations of various depths and density. This rotating tool makes threads. The rotating tool can be in the form of a cylinder or cone.

The formation and functioning of thread can be better understood by taking the example of a water bottle cap and it’s top. The tread on the inside of the bottle cap provides seal to the bottle. It acts as a coupling agent between the circular motion of the cap and the bottle top.

Main Parameters for Thread

The main parameters of thread mean understanding the various parts of it. The various parts of thread include:

• Crest

• Root

• Helix angle

• Pitch

• Flank

Let’s get into the detail of each part of thread:

1.Crest

Crest is the top most area of the thread. It is actually the surface of the thread that combines the flanks together. Crest is the area of the thread is most distant from the cone or cylinder.

2.Helix Angle

While keeping the thread straight the angle between the thread axis and the helix is called the helix angle. For a taper thread this angle is different. In that case it is the angle which the conical spiral area of the cylinder makes with the thread.

3.Pitch

From the theoretical point of interpretation, the measurement of a pitch is equivalent to the diameter of the rotating object. The distance between the area of the two threads held parallel to each other is called a pitch.

4.Flank

The threaded part that connects roots to the crest is known as flanks. Flank is the adjacent area to the cylinder or cone. Theory describes the intersection between the axial plane and the flank as a straight line.

Types of Machining Threads

As described before thread is of two types i.e. internal and external threads. The classification of thread goes beyond that. It involves different standards as well. Spaced and machine screw threads are for fasteners. The Unified Screw Thread System use UNC (coarse) and UNF (fine) threads. This section explains the two major types of thread in detail.

Internal Threads

Internal threads are also known a female thread. Interesting isn’t it?

The machine that manufactures internal threads use a unique tool. This technique is known as single lip threading tool. The thread cutting process inside the CNC machine requires the use of another tool. That is the traditional threading cap.

The thread cutting process can only be executed on surfaces that are concave. Often machinists use hand taps or machine taps for the job of cutting the internal threads.

Internal thread cutting process involves the use of several tools and equipment such as:

• Safety goggles

• hand drill

• twist drill etc.

You need to know two things beforehand for internal thread cutting. First the diameter of the hole. Second the appropriate tap for cutting the internal thread.

External Threads

External threads are also known as screw threads. What distinguishes external threads from internal threads is that they can be cut using hands. However, you do need a round die for this purpose. Manufacturers often replace the use of internal threads with round dies.

Plug gages, screws, bolts and studs all use external threads.

The use of a lathe for the manufacturing of external threads is very effective. There are also hexagonal square dies. These types of dies find their applications in interlock systems. They make the movement of dies to inaccessible areas possible.

Apart from the round die you can use other materials for external thread cutting. These include:

• Flat tip screw driver

• Cutting spray

• Die stock etc.

How to Cut A Machining Thread?

Gaining ample knowledge of cutting threads and a machining thread is very beneficial. Thread cutting machines might not always be available. In that case you should be able to cut threads yourself. Also, you can make screws and nuts as per your choice Also you can repair them whenever you need.

Internal Thread Cutting

Cutting an internal thread requires the use of a lot of tools. These tools are necessary for your own safety. After collecting the tools, the next step is the determination of the diameter of the hole. Now choose the tap. Selection of the bottom right hand tap is crucial here. Subtraction of the thread pitch from the diameter of the hole determines the core hole diameter.

Now use a twist drill to make the hole. Use a 90-degree counter sink to make the chamfer in the core hole. Now turn the tap wrench with the left hand tap into the core hole for thread cutting.

External Thread Cutting

For external thread cutting you first need a round rod. File the edges of the rod. Chamfer it at the 45-degree angle. Make sure that the chamfer is always bigger than the depth of the thread.

Now clamp the round die. You have to put lot of pressure for optimal cutting of the external and threading dial together. Therefore, it is advisable to fix the rod affirm to avoid any unwanted motion. At the end you can use cutting sprays to maintain quality of the surface of the part.

Common Screw Thread Machining Methods in CNC Process

The manufacturing process of various internal and external threads is known as thread machining. For this purpose, thread machining tools are used. Let’s explore different methods of thread machining in CNC process:

Tapping

Thread trapping is an extensive five step process. The purpose of this process is to create threads on the internal side of a pre-drilled hole. Let’s analyze the five steps of the thread tapping process:

Following the Preliminary Steps

Of course, the first step is drilling the whole of an appropriate size in the material. The major concern here is to maintain the size of the hole in accordance with the tap. At the same time leave some room to create threads.

Doing the Alignment

This is a crucial step. Their single point should be perfect alignment between the hole and the tap. If you want to achieve straight threads than getting the perfect alignment is necessary.

Application of the Lubricant

Applying a lubricant or cutting fluid helps to reduce the friction between the tap and the hole. This step is beneficial in reducing the heat resulting from friction. It also reduces any potential damage to the threads.

Tapping

This step is the original tapping process resulting from the clockwise movement of the tap inside the hole. To prevent any clogging in the hole it is better to move the tap in reverse direction as well. Tapping results in thread cutting inside the hole.

Cleaning up

As the name suggests once the process is complete it involves cleaning up of chips or another leftover material inside the hole.

Thread tapping method has two types i.e. form cutting and cut tapping. Cut tapping involves physical tapping or cutting of the threads inside the hole. It is a traditional thread cutting method.

Form cutting is a thread cutting method that forms threads by material displacement. It does not cut thread through the material. Form cutting method is only suitable for ductile materials. Cut tapping is versatile. It is suitable for a wide variety of materials.

Thread Tapping has variety of advantages such as:

• When used with right combination of tools and techniques it produces highly precise results

• In comparison to other methods it produces stronger threads.

• In case of automation it is a very efficient process.

• It is a cost-effective process. However, you have to do a onetime big initial investment on good quality equipment.

Thread Milling

Thread milling has its name because of the use of “thread milling machine” in the process. Thread milling machines uses a milling cutter that has the shape of a disc or comb. The milling cutter has several edges.

The milling cutters of disc shapes are used for milling threads on parts such as screws and worms. The milling cutter mills the trapezoidal threads on these pieces.

Comb type milling cutters are used for milling of several external and internal threads. These include all external, internal and trapezoidal type of threads.

When the thin threaded parts are required to be produced in larger quantities we use the process of thread milling. Thread milling ensures general accuracy of the workpieces.

There are two types of milling cutters in thread milling process:

1.A Single Lip Cutter

Easy on pocket and versatile. A single cutter is compatible with wide range of pitches.

2.A Multipoint Cutter

Although expensive but it is faster and more accurate than single lip cutter. However, a multipoint cutter is exclusive for one kind of pitch. It lacks the versatility of the single lip cutter.

The thread milling process has several advantages for CNC machining such as:

• It ensures precise measurements and accuracy.

• Thread milling allows mass production of materials.

• It is efficient process.

• Thread milling cutters are versatile in nature.

CNC Threading on Lathes

CNC lathes have the ability produce high precision threads. Threading on CNC lathes produces threading cuts by using a threading toolkit. Its distinguishing feature is the production of a helical lather on the product or workpiece. It has two further classifications:

CNC thread cutting on lathes consists of two processes.

1.Straight cut cutting method

Straight cut cutting method of thread cuttings used for high precision tasks. It is most appropriate for small pitch thread cutting. It is a simultaneous process where both sides of the tool are performing. The larger cutting force causes quick wear and tear of the blades.

This method is not suitable for thread cutting with larger pitches. It is because the large cutting depth causes the blade to wear fast. This leads to an error in the diameter of the thread.

2.Oblique cut cutting method

Unlike straight cut cutting method the oblique cut method uses single side of the tool. For this reason, it does not produce precise measurements. The blade wears out easily. The thread surface of the machining is not straight. The cutting-edge angle has a poor accuracy. It assumes a typical tooth type shape.

Although it lacks precision but it is a very convenient process. It is commonly used for the removal of chips.

Thread Grinding

Thread grinding is a process suitable for the machining of hard workpieces such as lead screws, thread guages, shovel grinding hob etc. It is used in machining of precision threads on hard workpieces. Thread grinding process is ideal for the production of small batches of threaded workpieces.

The execution of thread grinding process is not very complicated though. It involves the rotation of the grinding wheel and a rotating workpiece. Between the rotating workpiece and the grinding machine the thread is ground.

In this method the grinding wheel is given the shape of the desired thread form. The pitch of the thread is formed by the relative axial motion between the workpiece and the wheel.

The distinguishing feature of the thread grinding method is the high level of accuracy.

Thread grinding is classified on the basis of the different grinding wheels. These two types are multi line grinding wheels and single line grinding wheels. The multi-line grinding wheel has further two categories. These are cut in and longitudinal grinding methods.

The width of the grinding wheel is what differentiates these two methods. When the width of the grinding wheel is less than the length of the thread the process is called longitudinal grinding method.

Also, the grinding wheel moves on the thread in a longitudinal direction for thread cutting. It can either move once or several times. This type of grinding has high productivity. However, the level of accuracy is low.

In case of single line grinding wheel the width of the grinding wheel is greater than the thread’s length. It grinds fastening threads. The chipping of large batches of taps uses single line grinding.

Application of thread

Threads have a wide range of applications. From small scale to large scale they can be used in variety of mechanical activities such as:

• They help in measurement of distance. You can use micrometer threads for this purpose.

• They can help in the adjustment of length. You can use tie rods and turn buckles for this purpose.

Threads can be used in the aircraft industry. The wing flaps on air craft and lead screws on lathes can all be actuated with threads.

You can use screws, nuts and bolts for fastening of certain objects.

You can use screw threads for multiple purposes. These include clamping between two components through bolts. You can also use them as a means of attachment between a bolt and a nut.

What is the difference between rolling and machining threads?

As a machining expert you should always know the difference between rolled threads and cut threads.

The thread rolling technique is believed to produce superior threads as compared to thread machining. It is because the thread rolling machine produces multiple threads by cold forming the steel.

The machine presses the material between two flat plates or round dies. Both plates and dies have thread pitch machines onto their faces. Therefore, the pressing produces the desired threads onto the surface of the material.

As mentioned before the threads produced by machining are made by cutting or machining away of the material. This results in the creation of thread forms. Threads produced machining threads are perceived to be of inferior quality in comparison to rolled threads.

There are many reasons behind the good quality of the rolled threads such as:

• The steel in the threads is stronger. It is because of the cold working of the thread rolling operation.

• The rolling operation causes the grains of steel to flow with the thread form

• The thread forms created have a rounded root radius.

CONCLUSION

If you want to become a manufacturer of good quality mechanical parts you should have ample knowledge about their manufacturing.

Precision and accuracy are of utmost important in manufacturing of mechanical parts such as threads. Careful manufacturing with the right tools and techniques can ensure precision and accuracy both.

Machining threads are an integral part of processes that involve connections and coupling. Only in-depth knowledge about threads and thread cutting process can lead you to informed decision making.

Although the process is complicated but not impossible to implement.