by Sam Brown
Threaded holes are a small but essential detail in CNC part design. If they’re not specified correctly, they can lead to fit issues, tool breakage, or even part failure during assembly. This guide explores the difference between tapped and threaded holes so you can make confident design choices.
You’ll learn how each threading method works, when to use them, and how to avoid common pitfalls. We’ll also touch on materials, tooling limitations, and how to clearly communicate your thread specs. Whether you're prototyping or placing a production order, this guide helps you get threads right the first time.
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What’s the Difference Between a Tapped Hole and a Threaded Hole?
A threaded hole is any hole with internal threads that allows a screw or bolt to be inserted and secured. A tapped hole is one way of creating those threads, using a tool called a tap that cuts into the material. In simple terms, all tapped holes are threaded, but not every threaded hole is made using a tap.
Understanding this difference helps when choosing the right machining method for your part. Some holes may be better suited to thread milling or thread forming, depending on the material or the shape of the part. Being specific about the method avoids confusion and potential errors during manufacturing.
Methods for Creating Internal Threads in CNC Machining
There are a few different ways to create internal threads, and each one has its own strengths. The method you choose depends on the material you're using, the shape of the hole, and how precise the threads need to be. In CNC machining, the three most common options are tapping, thread milling, and thread forming.
Making the right choice can improve your part’s strength, save machining time, and reduce the risk of tool breakage. Some methods are better for tight tolerances, while others are more cost-effective for standard threads. Let’s take a closer look at how each one works and where it fits best in your design.
Tapping
Tapping is the most widely used method for creating internal threads. It involves using a tool called a tap, which cuts threads into a pre-drilled hole using sharp edges. This technique is quick, efficient, and a good fit for many standard threading applications.
Tapping works well in materials like aluminium, brass, and mild steel. It’s especially useful for simpler geometries and moderate production volumes. Many machine shops prefer tapping because it's a fast setup with readily available tools.
But it’s not without its downsides. Taps are more likely to break in harder or more brittle materials, which can lead to scrap parts. Once broken, a tap is very difficult to remove and may halt production entirely.
Thread Milling
Thread milling uses a rotating cutting tool that moves in a circular path to create threads. It gives you greater control over thread size, pitch, and depth, making it ideal for custom threads. This method is popular for parts that require high accuracy or non-standard thread types.
One thread mill can be used to cut a range of thread sizes, saving on tool changes. It also reduces cutting pressure, which helps prevent cracking or distortion in delicate parts. This makes it a preferred method in industries like aerospace and medical.
Thread milling does need a CNC machine capable of helical interpolation. It’s usually slower than tapping for large batch jobs, but it offers more flexibility. If precision matters more than speed, thread milling is often worth the extra time.
Thread Forming
Thread forming doesn’t cut into the material; it reshapes it by displacing the metal into the thread form. The tool pushes the material outward, creating a strong, chip-free thread. Since no chips are produced, it's ideal for parts where cleanliness or automation is important.
This method works best in soft, ductile materials like aluminium, copper, or certain plastics. It also results in stronger threads thanks to cold working, which compresses and strengthens the material. Another bonus is reduced tool wear and longer tool life.
Thread forming isn’t suitable for every situation. It struggles with hard or brittle materials and demands tighter hole tolerances for good results. If your material is compatible, it can be a clean, efficient option for long production runs.
Pros and Cons of Internal Threading Methods
Tapping
Pros:
- Fast and widely supported across machines
- Cost-effective for standard threads
- Ideal for through holes in soft metals
Cons:
- Prone to tool breakage
- Limited to fixed sizes and thread types
- Struggles with deep blind holes
Thread Milling
Pros:
- Flexible across sizes and thread forms
- Better control over thread depth and quality
- Handles hard or brittle materials well
Cons:
- Slower for bulk production
- Requires helical interpolation support
- Higher tooling cost
Thread Forming
Pros:
- Produces strong, chip-free threads
- Great for automated systems
- Long tool life in ductile materials
Cons:
- Not suitable for brittle metals
- Needs precise hole sizing
- Limited to specific materials
Understanding the strengths and limitations of each method is only part of the puzzle. To get the most from your design, your technical drawings also need to be clear and precise. For that, refer to our guide to geometrical dimensioning and tolerancing, which helps ensure every feature on your part is communicated effectively.
How to Choose the Right Threading Method for Your Part
Choosing the best threading method starts with understanding your material and how deep the threads need to go. Tapping is a solid choice for aluminium housings and standard through holes, but it can struggle in hardened or brittle metals. For blind holes or high-precision parts, thread milling often gives better control and reduces the risk of tool failure.
Say you’re working on a titanium part; tapping might lead to breakage or even scrap the whole piece. In that case, thread milling provides smoother cutting action and more accurate results. On the other hand, thread forming works well if you're using aluminium and want stronger threads without generating chips.
For more technical advice, check out our guide on how to specify thread depth and fit. It breaks down thread classes, engagement percentages, and how to clearly communicate what you need. Taking the time to specify threads correctly helps avoid costly delays or machining issues.
Let Us Help
At Penta Precision, we know that every thread detail matters, especially in high-spec industries. Our engineers can review your designs and advise on the best approach for tapping and threading based on your material and part geometry. Whether you need tapping, thread milling, or forming, we’ll guide you to the right choice.
We’re proud of our ISO 9001:2015 certification and full-service capabilities, including our CNC turning service. From machining and finishing to design advice and tolerancing, we handle it all under one roof. That means fewer suppliers to manage and more reliable results for you.
We’ve built strong, long-term partnerships with customers in the medical, aerospace, and manufacturing sectors. Whether it’s solving a design challenge or helping with a tight deadline, we’re here to support you. Let us help you get the results you need, every time.