Cutting and Forming of Machining Thread

Machining thread is a fundamental metalworking process. Machining threads should not be taken for granted by any manufacturer. There is now a wide variety of connectors and fittings available in the global market. Therefore, finding the right thread for your application is becoming increasingly challenging.

Thread cutting processes also seem to have some disadvantages. However, these processes remain one of the most effective ways to produce threads. For your profit margin, it is important to explore the different thread machining technologies available. Read on to learn important information about machining threads.

1.What is machining thread in manufacturing?

A thread refers to a continuous spiral ridge. The spiral ridge has a uniform cross-section on different surfaces of a cylinder or cone. It can be formed on the nut (internal) or screw/bolt (external) surface.

The main function of a thread is to act as a coupling between two mechanisms. When you think of your water bottle cap, you have a pretty clear image of a thread. The cap and the bottle cap are coupled together to form a tight water seal.

Due to the action of the coupling, motion is transmitted. Therefore, motion helps to gain mechanical advantage.

The following terms will help you understand machining thread better:

1. Crest: This is the surface where the thread connects with the flank. The crest is the farthest area of ​​the thread protruding from the cone or cylinder. That is, it is the top of the thread.
2. Flank: The thread flank connects the crest to the root. Theoretically, the intersection of the flank surface and the axial plane is a straight line.
3. Root: The root is the thread surface that connects the flanks of adjacent thread profiles. It is the immediate area of ​​the cone or cylinder where the thread protrudes. That is, it is the bottom of the thread.
4. Pitch: This is the distance between the parallel area and the corresponding area of ​​the adjacent thread. The pitch diameter is the diameter of the theoretical cone or cylinder.
5. Helix angle: For straight threads, the helix angle is the angle formed by the helix around the thread axis. On the other hand, the helix angle of a tapered thread is the angle formed by the tapered helical area of ​​the thread.

2.Types of machining thread

There are different standards when it comes to classifying threads. On fasteners, spacer threads and machine threads are commonly used. On the other hand, UNC (coarse) and UNF (fine) threads are standard classifications of the unified thread system.

In this section, we will discuss internal and external threads.

1) Internal Threads

Internal threads (female threads) are threads on a concave surface. These types of threads are machined with single-lip threading tools. Some internal threads use traditional thread taps. A tap is a metal tool used to cut internal threads on CNC machined parts.

You need internal threads to insert the screw into the workpiece. The thread cutting process for internal threads can use either a hand tap or a machine tap.

You need the following tools to cut internal threads:

1. Twist drill
2. 90 0 countersink
3. Internal tap
4. Adjustable tap wrench (for hand taps)
5. Hand drill or box drill (for machine taps)
6. Safety goggles

First, you need to determine the diameter of the hole you need. Then, it is also important to know the correct tap for cutting. Internal machining taps are classified according to their nominal size.

You can calculate the tapping diameter using the following formula:

Core hole diameter = tap diameter – pitch.

The thread cutting process involves centering with a punch. You can then use a twist drill to drill a core hole. A 90 0 countersink helps create a chamfer in the core hole. Now you can cut the thread by placing the tap into a tap wrench and turning it into the core hole.

2) External Threads

External threads are also called screw threads. Common external threads include screws, feelers, bolts, or studs. For parts suitable for turning, external threads can extend to the length of the part.

For external threads, machining the threads on a lathe is very effective. You can also cut them manually using a round die. Round dies are usually located in a fixed die holder. Manufacturers use round dies as a counterpart to internal threads.

There are also hexagonal square dies.

These types of dies are best used for interlocking. They help fix cases and move the die to inaccessible areas.

The following tools will help you cut external threads:

1. Round dies and round bars
2. Files
3. Die stock
4. Vise (for clamping)
5. Flat head screwdriver
6. Cutting spray (to improve surface quality)

First, you need to file the edges of the round bar. Then, chamfer the round bar at 450 degrees. The chamfer should be slightly larger than the thread depth. You can then clamp the round die and hold it firmly. The round bar requires considerable pressure to cut the thread. The cutting spray helps improve the quality of the part surface while extending its service life.

3.How to cut threads

Thread cutting helps create threaded connections on parts. Connections with full screws require internal threads. Many such connections also require matching external threads as counterparts.

Knowing how to cut threads is a key work technique for single-piece manufacturing. It is also necessary for repair work. This is because machines are not always available for economic and technical reasons.

The following are the main methods of cutting threads:

1) Milling

This method involves the use of a rotating milling cutter. The shape of the milling cutter corresponds to the desired thread shape. Manufacturers use single or multiple milling cutters. With a single milling cutter, each cutting edge exists in a single plane. On the other hand, multiple milling cutters have multiple rows of annular cutting teeth.

Threads can also be cut using a hob. In this case, the teeth of the tool will be arranged along a helical line. Milling can be used to process internal or external threads. Threads cut using the milling method have high precision.

This method is best used when the thread pitch is too coarse for the die. Many machinists believe that milling is more efficient than machining threads on a lathe. Milling is the most efficient method for machining parts such as lead screws. This is because milling produces high speed and precision. In addition, milled parts can be completed in just one or two passes.

2) Machining Threads on Lathes

Another major threading process is thread cutting on a lathe. This process produces spiral ridges on the workpiece. The spiral ridges have a uniform cross-section on the part. Threading on a lathe requires continuous cutting using a threading tool kit.

Tap shanks are one of the most common methods for tapping using a lathe. The machinist clamps the workpiece in the lathe chuck. Then, smaller and larger taps are mounted using a spring loader center or dead center, respectively. Either of these two devices can be clamped on the tailstock.

After that, the tap is rotated with a handle. Another common method is to use a die handle. It is an excellent choice for external thread cutting on a lathe. Other methods on lathes include rigid tapping and single-point threading.

3) Die Cutting

Die cutting is the most common method for cutting external threads. Die cutting produces parts relatively quickly. Therefore, die cutting is an economical method for cutting threads. The accuracy and quality of this method are moderate. However, for most parts produced in large quantities, this method is acceptable.

There are generally two types of thread dies. They are:

1. Bottom or round split dies: These dies are designed to make them suitable for manual use. However, they can also be used in machines. They are round and have a screw to close their radial cutouts. This allows them to be adjusted to accommodate both loose and tight fits.
2. Adjustable dies: In this case, there are two pieces of dies mounted on a die holder. The dies can also be fixed in a chuck. The tapered arrangement also allows movement towards or away from the center. This provides different degrees of fit. Adjustable dies are ideal for manual use.

4.Design Tips for Machining Thread

Given the many benefits of thread cutting, it is not enough to just know how to cut threads. It is also crucial to understand the best ways to utilize thread cutting technology. Here, you will find some design tips for thread applications.

1. The end of the internal thread should have a countersunk hole.
2. If there is no specification, a lower height thread is better than a higher height thread.
3. It is more economical to use standard thread sizes and forms.
4. The starting end of the thread should have a flat surface, which must be consistent with the central axis of the thread.
5. Tubular components should have increased wall thickness to withstand the pressure of forming or cutting.
6. The end of the external thread of the part should include a chamfer.

5.Conclusion

The production of high-quality parts requires a comprehensive understanding of the manufacturing process. Machining thread is an important aspect of the manufacturing process. Although it seems a bit complicated, it is recommended not to give up on this idea.