How to Tap Internal Thread of Stainless Steel Pipes

  • 2024-05-24

Tapping internal threads on stainless steel pipe fittings is highly technical, typically done using a tap. Stainless steel is highly viscous and has poor chip-breaking properties, so it is easy to cause thread scratches on the workpiece or chip the tap.

This not only affects the processing efficiency, but also affects the thread quality. To prolong the service life of screw taps and improve the quality of threading, CTO of WayKen manufacturing company mentions the following points.

The Design and Manufacturing of Taps

The design and manufacturing of taps directly affects their service life and threading quality. Selecting the appropriate material and coating technology can significantly enhance the tap’s performance. In addition, the optimization of the rake angle and amount of eccentric relief, as well as reasonable heat treatment methods cannot be ignored.

1. Selection of Better Tap Material

Adding particular alloying elements in ordinary high-speed tool steel can greatly strengthen the wear resistance and toughness of taps. These alloying elements can enhance the hardness and impact resistance of the steel, thus extending the service life of the tap.

2. Application of Coating Technology

Titanium nitride coating on the thread surface of taps can significantly improve the wear resistance, heat resistance and lubricity. The titanium nitride coating can reduce friction, lower the cutting temperature and prevent chips from adhering to the tap. It improves the quality of thread processing.

3. Optimization of Rake Angle and Amount of Eccentric Relief

Appropriately increasing the rake angle of the tap can improve the cutting efficiency, but the rake angle is too large. It results in the tap chipping and tapping threads prismatic when backing out. Similarly, increasing the amount of tapping eccentric relief will help chip removal. But too much will cause chips to be stuffed into the back angle of the tap, which will affect the threads finish.

4. Selection of Tool Heat Treatment Method

Reasonably choose the method of tool heat treatment, to ensure the hardness and toughness of the tap. Heat treatment directly affects the tap performance. Quenching and then tempering the tap in time assures good toughness and wear resistance.

Pre-machining of Workpiece Bottom Holes

Pre-machining of workpiece bottom hole is one of the key steps in the tapping process. Correct hole size and machining method can effectively reduce the difficulty and risk of tapping, and improve the quality and accuracy of the thread.

1. The Hole Size of Pre-Machining

For accelerating the tapping, first machine the pilot hole to the specified dimensions. Especially for American standard NPT threads and British standard PT threads, the taper is 1:16. 

Most stainless steel manufacturers create straight holes as the tapping bottom hole according to the diameter of the thread’s small end, resulting in more processing in the latter part of the tapping. It leads to the increase of tapping torque, tap breakage or chipping and other undesirable consequences.

2. Optimization of Bottom Hole Size

The bottom hole size is preferably the minimum diameter of the machined thread, plus a margin of 0.1 ~ 0.2mm. This size can ensure the smooth discharge of chips during the tapping, reduce the risk of scratching the thread surface and improve the thread finish.

3. Reaming Method

It is preferred to use custom reamers for reaming before tapping. For American Standard NPT threads and British Standard PT threads, they should be processed into tapered bottom holes. 

Many companies including WayKen manufacturing company use waste taps to manually grind off the threads to make reamers, so that waste taps can be used twice. When converting reamers, grind out the clearance angle, and keep the rake angle of the tap blade straight.

The Rational Use of Taps

Reasonable use of taps is the key to ensure the quality of thread processing. When processing different types of workpieces, it is necessary to consider the direction of the chip, to ensure the concentricity of the tap and the workpiece. At the same time, pay attention to the quality of the tap regrinding.

1. Consideration of Chip Direction

When machining various types of workpieces, different chip directions should be considered. If necessary, the new tap also need regrinding. It is important to discharge the chip from the chip groove, to prevent scratching the processed thread. For example, when processing the coupling, it is best to use vertical tapping machine. And grind the rake angle on each flute of the tap, so that the chips are discharged in a trumpet shape.

2. The Solution to Non-Concentric Problem

The non-concentric of tap and the workpiece will lead to uneven force in the tapping process, resulting in premature fatigue damage or even fracture of the tap. 

The non-concentric is mainly caused by the machine spindle, tap joint, tap shank, the processed part and the fixture quality. The solution is to use qualified tap joints and fixtures. Then, carefully adjust the machine tool and move the tap joints to maintain the tap's self-centering.

3. Quality of Tap Resharpening

The resharpening quality greatly influences the tap’s service life. Resharpening taps should pay attention to maintaining the geometry of the front guide part, it is best to use the grinding machine of tap spade for grinding. 

For example, while processing British standard PT and American standard NPT thread on elbows and tees, the end faces of the two taps may touch when tapping to threads in line with the size. You need to shorten the tap face by grinding, and maintain the original geometry.

Selection of Suitable Tapping Oil

The selection of tapping oil is critical in the tapping process. Choosing the right tapping oil can greatly enhance the quality and efficiency of thread processing, while also increasing the tap's service life.

1. Functional Requirements of Tapping Oil

The tapping oil must possess excellent cooling, lubricating, and anti-corrosive properties. Inadequate lubrication can result in poor thread finish and increased resistance during tapping. Similarly, insufficient cooling can cause stainless steel chips to bond easily to the tap threads and rake face.

2. Tapping Oil Selection and Configuration

Using special tapping oil for stainless steel is more effective, can be used directly. In addition, you can also use tapping oil concentrate and oil to configure their own, but it's crucial to ensure the ratio is correct. Reasonable  choice and configuration of tapping oil can significantly improve the tapping effect, quality of threads and processing efficiency.


Thread tapping of stainless steel presents greater challenges than conventional aluminum parts due to its unique material properties. Through the above measures, the processing quality and efficiency of internal thread tapping on stainless steel pipe fittings can be effectively improved, and the service life of taps can be extended. With many years of experience in metal thread machining, WayKen manufacturing company can provide expert manufacturing advice and machining solutions.