Posted April 14, 2021

Thread Milling vs. Tapping: The Pros and Cons

thread milling vs. tapping

Almost every industry has parts that attach to other parts with screws, bolts, and fasteners. This necessity creates an immense demand for threaded holes in CNC machined components. As CNC machining is a subtractive manufacturing method, threaded holes must be cut into the part using either thread milling or tapping. Both methods have distinct advantages and disadvantages that design engineers should understand. Knowing when to use thread milling vs. tapping will benefit the production of the ideal thread in the part you are having manufactured.

Understanding Thread Milling and Tapping

Threaded holes facilitate the attachment of other parts as long as the hole is precisely machined to match a targetted screw or bolt. One can use CNC turning to create a thread on the exterior of a part in the same way that a screw or bolt forms, but the fabrication of a threaded hole requires milling. CNC milling machines use thread mills and taps as the tools to manufacture these custom threaded holes. Since tapping is the more traditional method of creating a threaded hole, it’s a good starting point.

Using a tap to cut threads in a hole

A tap fabricating a threaded hole

Tapping

“Tapping” refers to cutting the internal threads of a drilled hole in a workpiece using a specialized tool called a tap, which rotates as a regular drill bit does. The tap’s cutting edges are its threads, and the recessed portions are known as flutes, either containing or drawing up the material the cutting edges remove. This process has been around for some time and can be done by hand or with power tools. Machinists prefer the tapping process for high-speed machining of materials such as aluminum and steel. CNC machines, including lathes with live tooling, drilling machines, and vertical mills, use taps to create threads that can reach up to 20 diameters deep. The different types of taps used with these machines include:

Straight flute taps (Hand taps)

  • Most commonly available taps with straight flutes
  • Manual option but preferred for CNC application

Spiral flute taps (Gun taps)

  • Straight flute with angular cutting edges on the end
  • Provides excellent precision

Spiral pointed taps (Bullnose taps)

  • Suitable for high-speed machining
  • Spiral and angled flute provides excellent shearing capabilities

Roll form taps (Forming taps)

  • “Chipless” taps for creating rolled threads
  • Metal is rolled and compressed to form a thread rather than removing chips 

 

A thread mill tool used for creating threaded holes

A thread mill tool used to fabricate threaded holes

Thread Milling

The process of “thread milling” cuts both internal threads in a hole or external threads around the workpiece. When used for fabricating internal threads, it differs from tapping. Instead of spinning the tool, it uses a CNC machine to rotate the tool in a helical or “corkscrew” pattern. With the computer-controlled machines’ precision, a thread mill will produce nearly any type and size of thread according to the desired specification. Machine shops use thread milling on large workpieces for materials such as titanium, stainless steel, or rigid plastics. In general, thread mills can produce threads of at least ⅛” (3.175 mm) and about three diameters deep. The precision of threads depends on the types of thread mill tools used for the process and include:

Helical flute thread mill

  • Used in high-speed applications
  • Suitable for use with thin-walled or tough materials
  • Ideal for smooth finish requirements

Straight flute thread mill

  • Appropriate for standard threading applications
  • One tooth per thread design on each face
  • Ideal to use with strong materials and fast-cutting application 

Single profile thread mill

  • Preferably used for lower speed applications
  • Provides more control in cutting and excellent finish
  • Ability to create a thread with varying pitch ranges, diameters, and lengths

 

Thread Milling vs. Tapping: Comparing Advantages and Disadvantages

Both thread milling and tapping produce threads, but differences exist between the two processes. Here is a breakdown of the advantages and disadvantages of both:

 

Tapping

Thread Milling


Speed

Tapping is a fast and efficient process. For the same size hole, tapping can complete at a much quicker rate than thread milling.

Thread milling takes a long time to machine, but the machine controller also spends significant time ensuring accuracy and precision.

Efficiency

Tapping creates threads in smaller holes, but it requires a different tap for each hole size. This complication comes with the higher cost of set-up time. Additionally, if a tap breaks inside a part while machining, its removal may damage the workpiece and introduce more time and expense.

A machine shop can use fewer tools for holes of different sizes—thread milling’s main advantage. It also creates interior and exterior threads. These abilities significantly reduce the investment in a wide variety of tools and the time and cost associated with frequent tool changes.

Thread Fit

A tap can only create the thread one designs it to produce. For every additional size and pitch of thread needed, the manufacturer requires a different tap.

One can use programming in a CNC machine to adjust the thread size and pitch. This flexibility provides the added benefit of achieving tight tolerances and a smooth finish.

Thread Quality

Tapping tools and the threads they produce are equal in size and require a lot of torque to do their work. Not only does this result in broken taps, but it can also affect the final finish and tolerance of the threads, resulting in a comparatively low-quality component.

CNC thread milling allows precise adjustments to the machining process to produce the highest accuracy and precision in the cut threads. Additionally, thread milling uses tools smaller than the final thread, allowing ample clearance for the removed materials’ safe evacuation. With controlled speed and careful operation, thread mills can produce high-quality components with an excellent finish.

Choosing a Right Thread Machining Methodology

Choosing the right tool for thread machining is essential to reduce waste and improve productivity. Many find thread milling the best option if flexibility and quality finish is the requirement of the application. For fast and efficient machining, tapping is suitable. Proprietary software such as  Plethora’s online quote system can help make an optimal choice of thread milling vs. tapping for your thread cutting application. 

Plethora is an ISO 9001-certified machine shop specializing in precision parts using advanced CNC machining tools and techniques. If you need more information to make a selection between thread milling vs. tapping, give us a call today at 415-726-2256. You can upload your design files to Quote My Part and get started on your project today.

Quote My Part

the-plethora-team

The Plethora Team

The Plethora team is your go-to CNC manufacturer for hardware done right the first time. We have the tools and experience needed to create high quality custom parts quickly and with precision, whether you need a prototype or production run.

Topics: Design, Manufacturing, finishing, CNC machining, Quality, manufacturing trends

Comments