How to Design Threads for CNC Machining: Sizes, Depths, and Practical Examples

Threads allow CNC-machined parts to connect with screws, bolts, fittings, and other components. However, a threaded hole that looks correct in CAD can still cause assembly problems if the thread size, pitch, depth, or material requirements are unclear.

Understanding how to design threads for CNC machining helps engineers create reliable connections while avoiding unnecessary machining costs and production delays.

For most custom CNC parts, the key is simple: specify the correct thread standard, provide sufficient thread engagement, and make sure the hole can be machined and inspected properly.

1. Choose the Correct Thread Type

The first step in CNC thread design is choosing a thread that matches the fastener and application.

Common options include:

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

By comparison, 1/4-20 UNC refers to a nominal diameter of 1/4 inch and 20 threads per inch.

Do not choose a thread standard based only on the approximate diameter. A metric thread and an inch-based thread may look similar but still be incompatible.

For fluid fittings, NPT and BSPP are also different systems and should not be treated as interchangeable.

2. How to Choose the Correct Tap Drill Size

Before an internal thread is cut, a smaller pilot hole must be drilled. This is commonly called the tap drill hole.

A useful preliminary formula for standard metric threads is:

Tap drill diameter ≈ Nominal thread diameter − Thread pitch

For example, an M6 × 1.0 thread has a nominal diameter of 6 mm and a pitch of 1 mm.

6 − 1 = 5.0 mm tap drill diameter

This is a standard starting value for a conventional M6 × 1.0 tapped hole.

Common Metric Tap Drill Sizes

These are common starting sizes for standard coarse metric threads, not universal values for every thread form or required thread percentage. Confirm the final drill size against the applicable thread chart and machining requirements.

A hole that is too small can increase tapping torque and tool-breakage risk. A hole that is too large can leave insufficient thread engagement.

3. How Deep Should a CNC Thread Be?

Thread depth should be determined by the connection's load, material, fastener strength, and assembly requirements.

A common preliminary guideline for many metal fastening applications is approximately 1.0–1.5 times the nominal thread diameter. For 6061 aluminum, around 1.5D is a useful starting point for many general-purpose connections, but actual requirements may be lower or higher depending on the design.

Here, D means the nominal thread diameter.

For example, an M6 screw engaging an aluminum housing with 9 mm of usable thread has a thread engagement length of 1.5D.

This is a preliminary design guideline, not a strength guarantee. The correct engagement depends on the aluminum alloy, fastener, load, and failure criteria.

Longer threads do not always make a connection proportionally stronger. Once sufficient engagement has been achieved, adding more threads may increase machining time without delivering a meaningful improvement.

Thread Depth Is Not the Same as Drill Depth

This distinction is especially important for blind tapped holes.

Thread depth: The usable length of the finished thread.

Drill depth: The total depth of the hole before or during threading operations.

A standard drill creates a conical point at the bottom of a blind hole. The tap also needs room to start and finish cutting the thread.

As a result, a hole requiring 9 mm of usable thread generally needs to be drilled deeper than 9 mm.

For an illustrative M6 × 1.0 blind hole, a design may require 9 mm of full thread engagement and roughly 14 mm of total drilled depth. The final dimensions must be confirmed against the chosen drill, tap geometry, and available clearance.

4. How to Design Blind Tapped Holes

Blind holes stop inside the workpiece instead of passing completely through it.

They are common in aluminum housings, mounting plates, brackets, and machined enclosures.

The main challenge is providing enough room for the required thread while allowing space for the drill point and tapping tool.

When designing a blind threaded hole:

1.Specify the required usable thread depth.

2.Allow additional drill depth below the usable thread.

3.Avoid forcing the tap to reach the physical bottom.

4.Consider chip evacuation and tool access.

5.Identify any critical bottom thickness or remaining material.

For example, a mounting plate that needs an M6 screw to engage 9 mm of thread should not simply specify “M6, 9 mm hole depth” without clarifying what that depth means.

A clearer drawing distinguishes the required thread depth from the total drilled depth.

5. Coarse Threads or Fine Threads?

Coarse threads are a practical default for many general CNC fastening applications. They are usually easier to start and are commonly available with standard tooling.

Fine threads may be suitable when the design requires finer adjustment, a particular engagement characteristic, or a specific established fastener standard.

However, fine threads are not automatically better at preventing loosening. If vibration is a concern, consider the entire joint design, including preload and the appropriate locking method.

For most custom aluminum brackets and housings, use a standard coarse thread unless the application requires something different.

6. Internal Threads vs. External Threads

Internal threads are cut inside a hole to accept a bolt or screw.

External threads are formed on a shaft, stud, connector, or fitting.

Both can be produced through suitable CNC tapping, thread milling, or turning operations, depending on the feature and part geometry.

When designing external threads near a shoulder, ensure the thread can be machined fully. A suitable relief feature or sufficient unthreaded clearance may be required if the mating component must seat closely against the shoulder.

Also consider an entry chamfer to help the fastener start without damaging the first thread.

7. Should Aluminum Parts Use Threaded Inserts?

Direct tapping is often suitable for aluminum components when the thread size, load, engagement, and service conditions are appropriate.

However, aluminum threads may wear or strip when fasteners are repeatedly removed and installed.

A threaded insert may be worth considering when the design requires:

1.Frequent maintenance or repeated assembly

2.Greater resistance to thread wear

3.A more durable fastening point in a lightweight housing

4.A defined fastening system that uses an insert

For example, an aluminum automation housing that is opened repeatedly during maintenance may benefit from inserts at selected fastening points.

In contrast, a bracket assembled once and rarely serviced may work perfectly well with directly tapped holes.

Choose the connection based on the actual duty cycle rather than adding inserts to every hole.

8. How Does Anodizing Affect CNC Threads?

Anodizing changes the surface of aluminum and can affect thread fit and assembly.

If a threaded hole is critical, specify whether the thread must remain uncoated, be masked during finishing, or meet a defined requirement after anodizing.

Do not assume that a thread will automatically retain its original fit after surface treatment.

This is particularly important for precision adapters, fittings, mating components, and frequently assembled parts.

Practical Example: An M6 Thread in an Aluminum Automation Bracket

Illustrative shop-floor scenario — not a published customer case.

Imagine a US equipment buyer sends a 6061 aluminum automation bracket with several blind M6 holes.

The drawing states “M6, depth 10 mm,” but does not clarify whether 10 mm means the usable thread depth or the total drilled depth.

This creates an avoidable manufacturing question.

A clearer specification could be:

1.Thread: M6 × 1.0 – 6H

2.Tap drill: 5.0 mm starting size

3.Usable thread depth: 9 mm

4.Total drilled depth: Approximately 14 mm, subject to tool and geometry review

5.Finish: Black anodizing, with critical thread requirements clearly specified

The benefit is not simply better machining. The buyer and supplier now agree on how the thread is manufactured, what the screw can engage, and what must be checked.

For a relevant product example, see our Aluminum CNC Machined Threaded Adapters and Precision Fittings. These components use internal bores, external threads, and other connection features according to customer drawings.

What Should You Include in a CNC Threading RFQ?

When submitting a threaded component for quotation, provide:

1.2D drawing and 3D CAD file

2.Thread standard and size

3.Pitch and fit class

4.Usable thread depth

5.Total drill depth for blind holes, where required

6.Material and grade

7.Quantity

8.Surface finish

9.Critical inspection requirements

Our Custom CNC Machining Services support drawing-based components requiring CNC milling, turning, drilling, and threading.

A clear thread specification helps the supplier choose suitable tooling, avoid unnecessary clarification, and manufacture the connection to the intended requirements.

Conclusion

Good CNC thread design starts with the correct thread standard, tap drill size, pitch, and usable engagement length.

For blind holes, always distinguish the required thread depth from the total drilled depth. For aluminum parts that undergo frequent assembly, also consider whether direct tapping or a threaded insert is more suitable.

The goal is not to make the deepest or tightest possible thread. It is to create a reliable connection that can be manufactured and inspected consistently.

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