CNC Threaded Holes machining Services

1. Understanding Threaded Holes in CNC Machining

A threaded hole is a hole with internal threads machined into a part, primarily used for fastening bolts or screws. In CNC machining, internal threads are typically produced using cutting taps, forming taps, or thread milling tools.

Compared with using separate nuts or other fasteners, machining threaded holes directly into a part can simplify the design, reduce the number of components, and make assembly easier. However, threaded hole design must account for more than just connection requirements. Tool selection, machining depth, hole positioning, and part geometry also affect manufacturability.

Several key parameters should be considered during the design process:

1. Thread Size:  Includes the nominal diameter and pitch. For example, M4 × 0.7 indicates a metric thread with a nominal diameter of 4 mm and a pitch of 0.7 mm.
2. Tap Drill Diameter:  The hole diameter drilled before tapping. It directly affects tapping resistance and the resulting thread profile.
3. Thread Engagement Percentage:  The ratio of the actual thread height to the theoretical thread height. It affects both thread connection performance and machining loads.
4. Thread Engagement Length:  The actual length over which the internal threads engage with the external threads of a bolt or screw. It directly affects the load-bearing capacity of the connection.

Internal threads in CNC machining are commonly produced using cutting taps, forming taps, or thread milling tools. Understanding Justway different CNC machining services  and their applicable processes can help you account for manufacturing requirements more effectively during the design stage.

(Thread Parameters)

2. Choosing the Right Thread Specifications

Thread specifications are generally determined by the nominal diameter, pitch, and thread standard. Common thread types include metric threads and imperial threads. Imperial threads are further divided into series such as UNC (Unified National Coarse) and UNF (Unified National Fine).

For example, M5 × 0.8 indicates a metric thread with a nominal diameter of 5 mm and a pitch of 0.8 mm, while 1/4"-20 UNC indicates an imperial coarse thread with a nominal diameter of 1/4 inch and 20 threads per inch.

When selecting thread sizes, consider the bolt specifications, connection strength, and available part dimensions. Larger threads generally provide greater load-bearing capacity but also require more machining space and material. For small parts with limited space, avoid selecting oversized threads unless they are necessary for the application.

The following table shows Justway's thread machining capabilities:

3. Determining the Correct Tap Drill Size

Before CNC tapping, a pilot hole must first be drilled. A tap is then used to cut the internal threads. The tap drill size affects the resulting thread height as well as the cutting load during tapping.

If the pilot hole is too small, the tap must remove more material, increasing cutting resistance and the risk of tool wear or breakage. If the hole is too large, the resulting thread height may be insufficient, reducing connection strength and increasing the risk of thread stripping.

(A smaller pilot hole leaves material for thread cutting)

The following table lists reference tap drill sizes for common metric coarse threads and UNC imperial coarse threads:

These values are reference sizes for conventional cutting taps. When using forming taps, the required tap drill diameter is different and must be determined according to the specific thread specification and tool manufacturer's recommendations.

Tap drill size is also related to the percentage of thread engagement. For ISO 60° threads, the following formula can be used to estimate the thread engagement percentage:
Thread Engagement Percentage:

E = Thread engagement percentage
D = Major diameter of the thread
d = Tap drill diameter
P = Thread pitch
Thread Engagement Percentage:

A higher thread engagement percentage generally produces a deeper thread profile but also increases tapping resistance. It is not always necessary to aim for 100% thread engagement. Instead, balance thread strength and machining reliability according to the material, applied loads, and tapping method.

(Higher thread engagement increases tapping force. 75% is commonly used)

4. Controlling Thread Depth and Designing Blind Holes

Thread depth is an important factor affecting CNC machinability. When designing threaded holes, consider not only how far the bolt needs to extend into the part but also the actual machining limits of the drill and tap.

(Cross-sectional comparison of through holes and blind holes)

Three different depth measurements should be distinguished:

1. Drilling Depth: The total depth of the drilled hole, including the conical area formed by the drill tip.
2. Tap Depth: The depth to which the tap can cut internal threads.
3. Effective Thread Depth: The length of fully formed threads that can effectively engage with a bolt or screw.

If the drilling depth is set equal to the required effective thread depth, there may not be enough clearance at the bottom of the hole. This can result in insufficient full thread length or prevent the tap from reaching the specified depth.

Chip evacuation can also become a challenge in deep blind holes. When chips cannot be removed efficiently, tool loads increase, raising the risk of tap breakage.

Therefore, through holes are generally easier to machine and are preferable when the part design does not require a closed bottom. If blind holes are necessary, provide sufficient clearance for the drill tip and tapping operation, and confirm that the selected tools and machining processes can meet the requirements.

5. Positioning Threaded Holes Properly

The positioning of threaded holes affects both the structural strength of a part and the ability of CNC tools to complete the machining process.

Maintain Sufficient Distance from Part Edges:

If a threaded hole is too close to the edge of a part, there may not be enough material surrounding the hole. This can lead to deformation, cracking, or thread failure during fastening.

The required edge distance should be determined based on the thread diameter, material, part thickness, and actual loading conditions rather than relying on a single fixed ratio.

• Maintain Adequate Spacing Between Adjacent Holes

If multiple threaded holes are positioned too close together, the remaining material between them may become too thin, compromising structural strength. Similarly, check whether threaded holes near slots or other openings could affect machining or assembly.

• Avoid Tool Interference

Threaded holes should be positioned where machining tools can easily access them. If a hole is located near tall walls, deep cavities, or narrow areas, the tap, tool holder, or even the fixture may interfere with the part.

For these features, check the tool approach direction and available operating space in advance. Adjust the hole position or part geometry if necessary.

• Add Appropriate Chamfers at Hole Entrances

A suitable chamfer is generally recommended at the entrance of a threaded hole. It helps guide bolts or screws into the hole and reduces burrs around the opening.

However, the chamfer should not be excessively large, as this may reduce the effective thread engagement length.

6. Choosing Thread Inserts Based on Application Requirements

For softer materials such as aluminum alloys, directly tapped threads may not provide sufficient strength for certain high-load applications or assemblies requiring frequent disassembly.

Repeated tightening and loosening can wear down the internal threads and may eventually cause thread stripping.

Common types of thread inserts include:

• Wire Thread Inserts

Wire thread inserts are made from helically wound metal wire and installed into pre-machined threaded holes. They provide wear-resistant internal threads in the parent material.

They are commonly used in softer materials such as aluminum alloys and can also be used to repair damaged threaded holes.

• Solid Thread Inserts

Solid thread inserts feature a rigid structure and can provide more robust threaded connections in certain applications. Their selection should take into account the parent material, applied loads, installation space, and operating environment.

It is important to note that thread inserts are not necessary for every threaded hole. They introduce additional machining and installation steps, so their use should be determined by actual connection requirements. The pilot hole and installation dimensions must follow the insert manufacturer's specifications.

Wire thread inserts are usually made of 304 or 316  stainless steel material . For aluminum parts, anodize the part before installing the insert.

7. Specifying Thread Requirements in Engineering Drawings

Even if threaded holes are already included in a 3D model, it is recommended to clearly specify the complete thread requirements in the 2D engineering drawing.

For example:

M5 × 0.8-6H, 10 mm deep.
(This indicates an M5 metric internal thread with a pitch of 0.8 mm, a 6H internal thread tolerance class, and a required effective thread depth of 10 mm.)

An imperial thread can be specified as:

1/4"-20 UNC-2B, 0.5" deep.
(This indicates a 1/4-inch nominal diameter UNC internal thread with 20 threads per inch, a 2B tolerance class, and a required depth of 0.5 inches.)

(Example of thread specifications in a 2D engineering drawing)

When submitting CAD files, it is generally unnecessary to model actual helical thread profiles in STEP files. A cylindrical hole can be used instead, while the engineering drawing specifies the thread type, size, depth, and tolerance requirements.
In addition, if the part requires surface treatments such as anodizing, electroplating, or powder coating, clearly indicate whether threaded holes need masking or protection. Surface coatings can change thread dimensions and affect subsequent assembly.

8. CNC Threaded Hole Design Checklist

Before submitting CNC machining files, review the following points:

• Are the thread specifications, pitch, and tolerance class clearly defined?
• Does the tap drill size match the thread specification and tapping method?
• Is the effective thread engagement length sufficient for the connection requirements?
• Do blind holes have enough clearance for drilling and tapping?
• Is there sufficient distance between threaded holes, adjacent holes, and part edges?
• Can the cutting tool and tool holder access the machining position without interference?
• Are chamfering and deburring required at the hole entrance?
• Are the hole positions, dimensions, and depths consistent between the 2D drawing and 3D model?
• Have the effects of surface treatments on threaded holes been clearly specified?

Proper threaded hole design improves connection reliability while reducing tool wear and machining risks in CNC manufacturing. By considering tap drill sizes, thread depth, hole positioning, and tool accessibility during the design stage, engineers can make parts easier to manufacture and reduce unnecessary design revisions and assembly issues.

If you have CNC machining requirements, submit your 3D CAD models and engineering drawings to  Justway to receive a professional DFM review and machining quote.