Rigid Tapping Explained: Easy Guide

Tapping is a machining process used to produce internal threads in cylindrical workpieces.

But what exactly is rigid tapping? How does it differ from standard tapping operations? What are its process parameters? 

Rigid tapping is a machining process in which the feed rate is synchronized with the spindle rotation, in order to attain a particular thread pitch. This process eliminates the need for a floating tap holder and provides the ability to tap precise threads even with a solid tap holder.  

This article provides a comprehensive guide on rigid tapping by discussing its operation, parameters, advantages, and factors to consider when performing rigid tapping operations.

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Rigid Tapping Explained

Rigid tapping is a type of tapping operation that provides the ability to cut threads with high precision and accuracy with regard to the pitch of the threads.

Generally, rigid tapping is performed on CNC machines that provide the ability to monitor the spindle speed and regulate the feed rate accordingly to match the required tap pitch.

Manual machines do not provide the ability to constantly monitor the process and automatically regulate the parameters.

This leads to thread damage as the tap rotates at a constant speed and maintains a constant feed throughout the process, irrespective of the pitch of the required thread.

Most modern CNC machines, such as Haas CNC machines provide the ability to program the machine for rigid tapping by using G95 live tooling code.

Rigid tapping is also achievable through floating tap holders that possess springs to control the tension and compression during the tapping operation. 

Applications of Rigid Tapping

Rigid tapping is a versatile operation with applications ranging from carpentry to metalworking industries.

The furniture and cabinetry industries commonly utilize rigid tapping to machine threads onto table slabs, tables, etc., providing a fastening option for interlinked components.

For instance, rigid tapping is performed to produce accurate threads on the table slabs for fastening vertical supports via screws or bolts.

Rigid tapping is also used in the construction industry for producing specific hole patterns in walls, ceilings, or floors, which enables the fastening of lighting or other fixtures.

Furthermore, the automotive industry also uses rigid tapping to produce threaded holes in different interlinked assembly components. 

An example involves rigid tapping on the vehicle body to make threaded holes, which facilitates the mounting of mud flaps.

Machining Parameters for Rigid Tapping

There are various parameters that control the tapping operation.

Parameter	Optimal Value
Spindle Deceleration	Quick
Minimum RPM	150-250 RPM
Spindle Rotation	Proportional to Feed Rate
Coolant	Recommended to use at a moderate flow
Tap Feedrate	20-30 IPM

Spindle Deceleration/ Decelerating Time

This parameter is critical as it signifies the deceleration of the spindle once it reaches the runout of the threaded surface.

The workpiece and tap are damaged if the spindle is still rotating once the runout is reached.

This parameter is measured in seconds and is usually a small value.

A higher spindle deceleration value indicates that the spindle and tap immediately decelerate to a stop once the threads are machined.

Minimum Rigid Tapping Spindle Speed (RPM) 

A very high RPM can lead to unwanted tapping, damaging the threads, whereas a low RPM will induce vibrations and risk damaging the cutting tool or workpiece.

Therefore, it is important to optimize the spindle speed to achieve accurate threading with minimal tool damage.

Generally, the minimum RPM value for rigid tapping ranges from 150 to 250 RPM.

Spindle Rotation

The spindle rotation should be proportional to the feed rate to ensure synchrony between them.

The angle covered by the spindle while it linearly moves from one thread pitch to another should be proportional.

Using G95 live tooling code automatically adjusts the feed and speed, based on the pitch of the thread, to get the desired threads.

A rotary encoder measures and monitors this value as any fluctuation would indicate an error and inefficient tapping process.

The stepper motor and the spindle should be appropriately programmed and synchronized to ensure that the spindle RPM and the tap feed rate are synchronous.

Any slight error in programming or minute flow in the motor can result in an inefficient tapping process.

Coolant Flow

An appropriate coolant with an optimal flow rate should be chosen to eliminate frictional heat that may result in workpiece surface or tool damage. 

Ideally, the coolant flow rate is kept below 5 liters per minute, as a faster flow rate may interfere with the efficiency of the tapping operation.

Chlorinated, low-foaming synthetic fluids with low viscosity are usually employed as the coolant.v

Tap Feed Rate

The tap feed rate is defined as the product of the thread pitch and spindle RPM.

Generally, a tap feed rate of 20 to 30 inches per minute is recommended.

The feed rate should be constant during the in and out movement of the tapping tool.

For better performance, the tap feed rate is often maintained at a slightly lower value than the calculated value to allow for fluctuations in RPM.

Advantages of Rigid Tapping

Faster Operation

Rigid tapping is a quick operation, ensuring the proper formation of threads in lesser time.

This aspect renders it suitable for bulk production, e.g., bulk production of threaded nuts.

Low Cost of Tooling

The tool cost of a rigid tap is cheaper than a floating tap holder, making it more feasible for small-scale businesses and hobbyists.

Durability

Rigid tap holders are more compact and durable, especially when machining large workpieces.

They can also perform more efficiently under higher coolant flow rates and pressure, ensuring proper tapping of the hole surface.

Their longer tap length enables smoother chip removal, especially at higher spindle RPMs.

Accuracy and Depth Control

Besides having a longer tap life, rigid taps ensure a better and more accurate surface finish.

They are ideal in applications where tapping depth needs to be controlled carefully in blind holes. 

In the case of floating tap holders, or manual hand taps, a trial and error method is implemented to achieve the final depth, which may result in tap breakage or surface damage. 

Things to Consider During Rigid Tapping

Always ensure that before the tapping operation, the correct type of coolant with an optimal flow rate is chosen to prevent surface or tool damage due to frictional heat. 

The spindle torque or RPM should not fluctuate or exceed a specified upper limit, as that can lead to thread breakage or tap wear and tear.

Ensure proper inspection of the tap before executing the process because a worn-out tap can damage the workpiece and produce improper threads.

Furthermore, thread milling should be opted for as an alternative for applications that can damage the tapping tool, such as threading blind holes.

Final Thoughts

Although there are various types of threading operations, rigid tapping provides the ability to produce accurate threads with precise pitch.

As a result, the thread cuts by rigid tapping ensure tight tolerance and perfect fit, making it ideal for applications that require high precision, such as lead screw drive systems.

However, a single deviation in the machining parameters can damage the threads and ruin the workpiece.

Therefore, it is important to regulate all the machining parameters precisely during the process to get the desired threads.

Frequently Asked Questions (FAQ) 

What is a thread pitch?

The thread pitch is the linear distance between two successive crests or roots of the thread. This distance is measured parallel to the thread axis.

Can non-metals undergo tapping?

Yes, non-metals can undergo tapping. A prevalent example involves wooden workpieces undergoing tapping operations in the furniture industry.

Can tapping be performed manually?

Yes, tapping can be performed manually with a hand tap, a mechanical tool involving mechanical force to feed it into a hole and out of it for forming threads.