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Threaded holes are among the most common features in CNC machined components. They allow parts to be assembled with screws and bolts without requiring additional fastening hardware such as nuts.

However, a threaded hole that looks simple on a drawing can create unexpected manufacturing or assembly problems if it is not designed properly.

Insufficient thread depth, incorrect thread specifications, limited tool access, and unsuitable hole dimensions can all affect machining quality and assembly reliability.

Whether you are designing aluminum brackets, stainless steel housings, brass fittings, or plastic components, understanding basic thread design principles can help reduce manufacturing costs and avoid unnecessary production issues.

In this guide, we explain the key considerations for designing threaded holes in CNC machined parts.

1. Choose the Correct Thread Standard

Before machining begins, the thread specification must be clearly defined.

Two common thread systems used in CNC machining are Metric threads and Unified threads.

Metric Threads (ISO Metric)

Metric threads use millimeters for their nominal diameter and pitch.

Examples include:

• M3 × 0.5
• M4 × 0.7
• M5 × 0.8
• M6 × 1.0
• M8 × 1.25

The first number indicates the nominal thread diameter, while the second indicates the thread pitch.

For example, M6 × 1.0 means a nominal diameter of 6 mm and a pitch of 1.0 mm.

Metric threads are widely used in industrial equipment, automation systems, consumer electronics, and mechanical assemblies.

Unified Threads (UNC and UNF)

Unified threads are commonly used in the United States and other markets following inch-based thread specifications.

• UNC: Unified National Coarse
• UNF: Unified National Fine

Examples include:

• #4-40 UNC
• #6-32 UNC
• 1/4-20 UNC
• 1/4-28 UNF
• 3/8-16 UNC

UNC threads generally have a coarser pitch, while UNF threads have a finer pitch.

Which thread standard should you use?

The choice usually depends on the product's target market, existing fasteners, assembly requirements, and engineering specifications.

For international projects, avoid specifying only a nominal diameter. Always include the complete thread designation and standard on the technical drawing.

2. Understand the Difference Between Through Holes and Blind Holes

Threaded holes are generally divided into two types: through holes and blind holes.

Through Threaded Holes

A through hole extends completely through the component.

Advantages include:

• Easier chip removal during machining
• More straightforward tapping operations
• Greater flexibility for screw length
• Easier inspection and cleaning

Through holes are often suitable for mounting plates, brackets, and structural components where the design allows a screw or bolt to pass through the entire part.

Blind Threaded Holes

A blind hole does not extend through the entire component. It has a defined bottom.

Blind holes are commonly used in:

• Aluminum housings
• Mechanical enclosures
• Крепежные кронштейны
• Precision fixtures
• Compact assemblies

However, blind holes require more attention during design.

The drill point creates a tapered section at the bottom of the hole, and a standard tap may not produce full threads all the way to the bottom.

Therefore, the drilled hole depth should generally be greater than the required usable thread depth.

For example, if a design requires 8 mm of usable thread engagement, the drilling depth needs to account for the tap lead and drill-point geometry.

Do not assume that the total hole depth is equal to the usable thread depth.

3. How Deep Should a Threaded Hole Be?

Thread engagement depth is an important factor in assembly strength.

If the engagement is too short, the internal threads may strip under load. If it is unnecessarily deep, machining time and cost may increase without providing meaningful benefits.

As a preliminary design guideline, the required thread engagement depends on the fastener material, internal thread material, loading conditions, and thread specification.

For many general-purpose applications, approximate starting points are:

D represents the nominal thread diameter. These are preliminary design guidelines, not universal engineering rules.

For example, an M6 thread in an aluminum component with an engagement length of approximately 9 mm represents 1.5D.

However, actual requirements should be verified based on the applied load, material strength, fastener grade, thread geometry, and safety factor.

For highly loaded assemblies, consider engineering calculations or testing rather than relying on a simple engagement ratio.

4. Don't Confuse Drill Size with Thread Size

One common drawing mistake is specifying the nominal thread diameter as the drill diameter.

For tapped holes, the pre-drilled hole must be smaller than the nominal thread diameter to leave enough material for the internal thread profile.

For example, a typical tap drill size for M6 × 1.0 is 5.0 mm.

Some common metric tap drill sizes are:

These are commonly used starting values. Actual drill selection may vary depending on the required thread percentage, material, tooling, and machining process.

If your technical drawing specifies an internal thread such as M6 × 1.0, the machining supplier can normally determine the appropriate pre-drill size.

For engineering drawings, clearly specifying the thread standard and required depth is more important than manually listing every machining step.

5. Consider Thread Quality and Material Differences

Different materials can behave differently during tapping and thread machining.

Алюминий

Aluminum alloys such as 6061 and 7075 are widely used for CNC threaded components.

They are generally suitable for tapping and thread milling. However, proper tool selection and lubrication are important to avoid built-up material on cutting edges and damaged threads.

For frequently assembled or disassembled aluminum parts, consider whether a threaded insert is appropriate.

Нержавеющая сталь

Stainless steel can be more challenging to tap because of its strength and tendency to work-harden.

Correct cutting speeds, sharp tools, suitable lubricants, and chip evacuation are important for maintaining thread quality.

Латунь

Brass is generally easy to machine and is commonly used for threaded fittings, connectors, and precision components.

Tool geometry and cutting conditions should still be selected according to the specific brass grade.

Инженерные пластики

Materials such as POM, Nylon, and PEEK can also be machined with internal threads.

However, plastic threads may be more sensitive to tightening torque, creep, temperature, and repeated assembly cycles.

For demanding applications, threaded inserts or metal fastening elements may provide a more suitable solution.

6. Tapping vs. Thread Milling: Which Machining Method Is Better?

CNC machined threads can be produced using different methods. Two common options are tapping and thread milling.

Пробивание

Tapping uses a cutting tool called a tap to form internal threads.

Преимущества:

• Fast machining for many standard thread sizes
• Suitable for high-volume production
• Widely available tooling
• Efficient for common threaded holes

Potential limitations:

• Tool breakage risk, especially in small or deep holes
• Chip evacuation can be difficult in blind holes
• A broken tap can damage an expensive component

Thread Milling

Thread milling uses a rotating milling cutter to generate the thread profile through a controlled tool path.

Преимущества:

• Flexible for different thread sizes
• Better control over thread diameter
• Useful for larger or deeper threaded holes
• Can reduce the risk associated with conventional tap breakage
• Suitable for certain difficult-to-machine materials

Potential limitations:

• Requires suitable CNC equipment and programming
• May take longer for some small standard threads
• Tool selection and machining strategy are important

The best method depends on thread size, material, hole geometry, production quantity, and required quality.

7. Common Thread Design Mistakes That Increase Manufacturing Costs

A well-designed threaded hole should be easy to machine, inspect, and assemble.

Here are several common mistakes to avoid.

1. Insufficient Wall Thickness Around the Thread

A threaded hole placed too close to an external edge may weaken the surrounding material or cause deformation.

Maintain sufficient material around the hole based on the component geometry and loading requirements.

2. Threading Too Close to the Bottom of a Blind Hole

The tap may not produce full threads at the bottom of a blind hole.

Allow additional drilling depth and account for the tap lead when specifying usable thread engagement.

3. Using Unnecessarily Tight Thread Tolerances

Standard thread tolerance classes are sufficient for many general-purpose assemblies.

Special thread tolerances should only be specified when the application requires them.

4. Forgetting Chamfers or Thread Entries

A small chamfer or countersink at the entrance of a threaded hole helps guide the fastener into the thread and reduces the chance of cross-threading.

It can also remove sharp edges left by machining.

5. Not Specifying Thread Requirements Clearly

A drawing that only shows a hole diameter without a complete thread callout can lead to misunderstandings.

Include the thread designation, tolerance class where required, and usable thread depth.

8. What Should You Include in a CNC Machining RFQ?

If your component contains multiple threaded holes, providing clear drawing information helps your manufacturing partner quote and produce the parts more accurately.

Before sending your RFQ, check that your drawings include:

• Complete thread specifications, such as M6 × 1.0 or 1/4-20 UNC
• Through-hole or blind-hole identification
• Required usable thread depth
• Thread tolerance class, if critical
• Countersink or chamfer requirements
• Material and temper
• Surface finishing requirements
• Critical dimensional tolerances

For complex assemblies, providing a 3D STEP model together with a fully dimensioned 2D drawing is especially helpful.

Conclusion

Threaded holes may be small features, but they play an important role in the performance and reliability of CNC machined components.

Choosing the correct thread standard, defining suitable engagement depth, understanding material behavior, and selecting the appropriate machining method can help prevent assembly problems and reduce manufacturing costs.

At AluCarbon Tech, we support custom CNC machining projects involving aluminum, stainless steel, brass, copper, titanium, and engineering plastics.

Whether you need precision threaded holes, custom mounting brackets, mechanical housings, or complex CNC machined components, we can review your drawings and support your project from prototyping to low- and medium-volume production.

Have a part with threaded features to manufacture?

Send us your 2D drawing or 3D model for a quotation.

Website: https://www.alucarbon-tech.com
Электронная почта: info@alucarbon-tech.com