A Complete Guide to CNC Lathe Threading

Almost every manufactured product contains threaded parts for joining, sealing, or moving parts. CNC lathe is used by manufacturing companies to make these threaded parts because it offers consistent accuracy and minimizes human error in manufacturing. In this guide you will learn about CNC threading tools, methods, programming and troubleshooting, so that you can make accurate threads, minimize scrap and streamline your machining processes.

What Is CNC Lathe Threading?

CNC lathe threading is a machining process used to form helical grooves on the outer or inner surface of the workpiece. The CNC programming control is used to synchronize spindle rotation and tool movement to create threads of specific pitches, profiles and tolerances.

Therefore, if you are planning to offer CNC threading services, then lathe machines offer practical and reliable solutions. They are widely used in most aerospace threading applications.

CNC Lathe Threading Tools

Whether you want to achieve oil & gas thread standards or for any other CNC lathe threading applications, it is important to choose the right tools and equipment. Among the most important tools for this process are:

• External threading inserts: External threading inserts cut threads on the exterior of components such as bolts and shafts. They assist you in creating precise and reproducible threads during manufacturing.
• Internal threading bars: Internal threading bars are used to cut threads in holes and bores. They allow you to access areas that are hard to reach but maintain the accuracy and uniformity of the thread shape.
• Full profile thread inserts: Full profile inserts cut the whole shape of the thread in one pass. They ensure you have the right size thread and achieve a smooth finish without additional adjustments.
• Partial profile thread inserts: Partial profile inserts will cut most of the shape but not the entire crest. They enable you to operate one tool with different thread sizes.
• Laydown threading inserts: Laydown inserts remain at a constant angle while cutting. They help you to handle the chips better and maintain the smooth and steady threading.
• Threading tool holders: Tool holders hold the insert in place when cutting. They minimize vibration and allow you to keep the thread aligned.
• Insert grades and coatings: Grades and coatings increase tool strength and decrease wear. They enable you to cut through harder materials and maintain the sharpness of the tool for longer.

CNC Lathe Threading Methods

CNC thread cutting requires precision. Therefore, based on the technical specifications, it is important to choose a technique that guarantees precision thread machining.

· Single Point Threading

Single point threading involves one cutting tool making a slow cut to form the thread shape in the material. A small piece of metal is removed during each pass, until the whole thread depth is achieved. Accurate and effective when using custom thread sizes.

· Multi Pass Thread Cutting

Multi-pass threading is a process that removes material in a number of small passes, rather than one deep pass. The depth of the thread increases with each pass. This decreases cutting force and minimizes the risk of tool breakage, particularly with harder materials such as steel.

· Die Threading

Die threading involves the use of a pre-shaped tool called a die to create external threads. The die is pressed and rotated onto the material to quickly form the thread. This is a simple but less flexible process than CNC-programmed threading.

· Tap Threading

Tap threading is the process of turning a tap into a hole to obtain the internal threads. The tap is used to cut the thread as it is wound in. This is a good technique to use for small holes or simple internal threading jobs.

· Thread Milling

Thread milling is performed with a rotating cutter that rotates around the material to create threads. The cutter is programmed to cut the thread into a specific form.

· Rigid Tapping

Rigid tapping is used to coordinate spindle rotation and tool feed to ensure that the tap moves smoothly into the hole. This decreases the risk of tool breakage and enhances the accuracy of the threads in mass production.

· Thread Whirling

In thread whirling, several cutting edges rotate around the workpiece to cut the threads. It is frequently applied to long and accurate threads, where surface quality and accuracy are of critical importance, like medical screws.

Step-by-Step CNC Lathe Threading Process

Custom threaded parts manufacturing involves a series of processes that must be followed in a predetermined order. This is an effective way to achieve precision thread machining.

By following these steps, you can machine different thread configurations with the tight CNC machine.

Step 1: Check the Thread Drawing and Specifications.

Threading starts with the engineering drawing as each number on the drawing influences the way your part will work and fit together. The major diameter, pitch and form of the thread (M20 × 2.5 or 1/2-13 UNC) dictate the operation of the CNC program during cutting.

If you correctly interpret these values early, then you have a lower risk of programming errors that would result in rejected parts. Using the wrong pitch value could result in the entire thread being completely unusable during assembly.

Step 2: Choose the Thread Type and Standard

Choosing the right standard for the thread type is crucial as it determines the performance of the final product during operation. Many industries use ISO metric threads like M16 × 1.5 because it allows parts to be fitted by different manufacturers.

UNC and UNF threads are used in mechanical assemblies, ensuring a secure fastening. NPT and BSPT threads are used in fluid systems to create tight seals. Using the wrong thread in the standard can cause assembly problems or leaks.

Step 3: Determine the thread dimensions

The thread dimensions will tell you how much material your CNC lathe should remove to create the desired shape. The pitch, for example, 1.5 mm or 2.0 mm, determines the travel of the tool with each spindle turn.

The final profile depends on the thread depth, the thread pitch and the minor diameter. Loose or tight threads may be created if you use the wrong values. The dimensions must be determined accurately for better machining consistency and to ensure that the finished thread fits the mating part.

Step 4: Choose the correct Threading Insert

The shape of the thread is determined by the threading insert and the accuracy and surface quality are affected by it. When a full profile is required, a full profile insert is used and is suitable for precision applications. The partial profile inserts are used to cut different pitches, while the metric threads use inserts with a 60-degree profile angle, which gives the flexibility in production.

Step 5: Set Up the CNC Lathe

A stable machine setup will produce accurate and consistent threads. The workpiece will need to be firmly mounted in a chuck or collet, otherwise the quality of the thread may be compromised. The alignment of the tool is also important because when it is misaligned it might lead to an uneven depth of thread. To achieve accuracy, it is important that spindle runout is kept to a minimum, ideally < 0.02 mm.

Step 6: Set Tool Offsets & Work Coordinates

On the CNC Machine, tool offsets are used to set up the location of the cutting tool in relation to the workpiece. Correct X-axis and Z-axis offsets are used to ensure correct thread depth and length.

Work Coordinates are used to set a reference point for the machining operation, such as G54. The machine should be calibrated to ensure accuracy of the programmed values.

Step 7: Make the Threading Program

All the movements in the threading process are executed by the CNC program. G76, G92 and G32 are the popular threading cycles that can be used to automate the threading process. Feed rate should also be proportional to the pitch, for instance, if the pitch is 2 mm, the feed is F2.0.

Bad programming can cause poor thread shapes and losses in materials. A good program ensures a correct spindle synchronization and cutting depth. This is useful for consistency in your thread during the process.

Step 8: Configure Threading Parameters

The cutting quality of the thread mainly relies on how easily the material can be cut, which is influenced by the threading parameters. Different materials have different response to cutting force, hence spindle speed, feed rate and cutting depth should be changed according to the material to be cut. For example, stainless steel can be machined at 150 to 400 RPM, as lower speeds generate less heat and slow the tool wear.

Step 9: Conduct a Simulation or Dry Run

The use of a simulation allows you to run the program before cutting and save on costly errors. The simulation shows the tool’s motion and can identify any programming errors or collisions. Dry run brings the tool into the proper position above the part without actual contact to ensure its position and clearance. At this point, problems will be easily identified, saving material and machine time.

Step 10: Make the First Threading Pass

The first threading pass checks that you have your setup, tooling and program properly set up. The tool will generally remove a small amount of material, from 0.2 mm to 0.5 mm, depending on the material being cut.

Step 11: Check Chip and Cutting Conditions

The control of chips is an important part of thread quality since long chips can damage the thread surface. Coolant also helps to break chips into smaller fragments and helps to reduce heat around the cutting edge.

Step 12: Complete Finish and Spring Passes

Finish passes are passes that remove a small amount of material, typically less than 0.1 mm, to achieve the desired accuracy of the thread. These passes are for smoothing the thread profile and surface. Spring passes repeat last tool paths without any depth change to compensate for deflection caused by the tool and machine.

Step 13: Measure the Thread Dimensions

The thread measurement is used to ensure the final product has the correct size. Different devices such as thread micrometers, pitch gauges, three wire measurement systems are adopted to verify critical dimensions. Even a slight deviation of 0.02 mm can lead to problems in assembly.

Step 14: Check Thread Fit using Gauges

Thread gauges are used to quickly check if a thread is working properly. A GO gauge must be able to pass the thread with ease, meaning that dimensions are within acceptable limits. A NO GO gauge should stop before full engagement to see that the thread is not oversized.

You can also learn: How to Cut Left Hand Threads on a CNC Lathe.

Types of Threads Used on CNC Lathes

• External Threads: Threads formed on the outside surface of a part, such as bolts and shafts, for joining parts together and securing assemblies.
• Internal Threads: These are cut inside holes that allow screws or bolts to fit securely and keep mechanical parts together when assembled.
• Straight Threads: Threads with uniform diameter over its length used in cases where sealing is not a necessity, but only simple fastening operation is required.
• Taper Threads: Tapered threads which become smaller in diameter to form a tight fit, typically found in pipes and high pressure systems.
• Right Hand Threads: Standard threads which tighten when turned to the right (clockwise) and are used in most mechanical fastening applications.
• Left Hand Threads: Tightens on anticlockwise rotation and does not loosen under reverse rotation in parts subject to reverse rotation.
• ISO Metric Threads: Standard millimeter based threads for consistent fitting in most engineering and manufacturing systems worldwide.
• UNC Threads: Coarse American threads, which are strong, easy to assemble and used in general mechanical applications.
• UNF Threads: Fine American threads that provide stronger grip and higher accuracy in precision mechanical assemblies.
• UNEF Threads: Extremely fine threads, designed for precision instruments requiring very tight and accurate fitting.
• NPT Threads: Tapered pipe threads used in fluid systems for sealing without need of any sealants.
• BSPT Threads: British tapered pipe threads are applied in plumbing systems to achieve leak-free pressure-tight connections.
• BSPP Threads: British pipe threads that use washers or seals rather than taper, to prevent leaking.
• Acme Threads: Threads for lead screws and machines where smooth movements and transfer of heavy loads are necessary.
• Trapezoidal Threads: Metric power transmission threads that convert rotary motion to linear motion efficiently in machines.
• Buttress Threads: Asymmetric threads that minimize friction and increase the capacity of the thread to carry heavy loads in one direction.

Common CNC Threading Problems and Solutions

In any custom threaded components production, even the lightest mistake can lead to many rejected parts which can be costly. Therefore, in any threaded component manufacturing, it is important to anticipate problems and design possible solutions.

Below are common challenges during thread manufacturing process and the possible solutions.

· Incorrect Thread Pitch

Incorrect thread pitch is when the feed rate is not the same as the programmed thread specification, resulting in parts that are unable to fit in assembly. You fix it by adjusting the spindle feed so that it is equal to the pitch value you set in the program.

· Drunken Threads

Drunken threads are caused by misalignment or machine backlash causing the path of the thread to wobble around the centerline, creating uneven threads. You correct this by proper alignment of the tool, and rigid workholding.

· Chatter Marks

Chatter marks are the result of vibration disturbing the stability of the cutting process and causing a rough, uneven surface on cut threads. It is corrected with a decrease of tool overhang and stabilizing the machine.

· Poor Surface Finish

Poor surface finish is caused by inappropriate cutting speed, feed rate or insert condition for the material, resulting in rough threads. It is corrected by choosing the proper cutting speed and by using a sharp insert.

· Undersized Threads

Undersized threads are caused by cutting a thread too shallow, which causes it to be too tight or difficult to assemble. To fix it, increase depth of cut and check program values.

· Tool Deflection

Tool deflection is the bending of the tool during machining, resulting in poor thread geometry. The solution is to use shorter tools and stronger tool holders.

· Insert Wear

Insert wear is due to the cutting edge becoming blunt over time, which affects the accuracy of the thread and the surface quality. The solution is to replace the inserts before they wear out too much.

· Built Up Edge Formation

Built up edges are formed when material adheres to the cutting edge and deforms the surface of the thread. The solution is to change cutting speed and employ coated inserts.

· Thread Taper Errors

Thread taper errors are caused by misalignment or set-up problems and result in a variation in thread diameter along the length of the thread. It is resolved by correcting machine alignment and positioning tools.

· Thread Runout Issues

Thread runout is caused by poor tool exit and/or incorrect programming. You correct it by making changes to the path of the program and the retraction of the tools.

Differences Between CNC Lathe Threading and Other Thread Manufacturing Methods

FAQs

What is a threading cycle in the CNC lathe programming?

A threading cycle is a programmed CNC command that is used to automate the passes for thread cutting, depth control and feed synchronization.

What is the difference between external threads and internal threads?

External threads are found on the external surface of a part while internal threads are found on the inside of a hole or bore.

What are the main types of thread cutting methods used on a CNC lathe?

The most common are the single-point threading, multi-pass threading, tap threading, die threading, milling, rigid tapping, and thread whirling processes.

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