CNC Hole Design: Through, Blind, Tapped, and More

Holes might look like one of the simplest features in a part, but they can be tricky to machine because the cutting tool has to work inside a confined space, where access, chip removal, heat, and tool rigidity all become more difficult.

Design choices like depth, diameter, and placement can affect accuracy, machining time, and cost. Knowing the different types of CNC machining holes, and when to use each one, makes it easier to design parts that do what you need without adding unnecessary complexity. Here is an overview of the most common hole types, practical design tips, and a few common mistakes to avoid.

At Konlida, our CNC machining services handle a wide range of hole features—from simple through holes to complex threaded and reamed holes—with automated DFM feedback to help you optimise your design before machining begins.

Common Types of CNC Machining Holes

Most engineers are familiar with holes—black holes and rabbit holes included—but there are a few more practical types worth knowing if you want to avoid tumbling down the metaphorical ones.

Some holes provide clearance for fasteners, while others align mating parts with locating pins, carry fluids, or create a threaded connection. And although we often talk about “drilling” holes, that is only one way to make them. Depending on the diameter, tolerance, material, and finish you need, a CNC hole might be drilled, interpolated with an end mill, or reamed.

Here are the most common types you will run into:

Through Holes

A through hole extends completely through the part. These are usually among the simplest holes to machine because the tool can pass all the way through and chips have somewhere to go. They are commonly used for clearance holes, alignment features, fluid or air passages, and weight reduction. If your design works just as well with a through hole as a blind one, the through hole is usually the easier option.

Blind Holes

A blind hole stops before reaching the opposite side of the part. That is useful when one surface needs to stay intact or when a through hole would interfere with another feature. The trade‑off is that chips cannot simply exit through the bottom, so machining takes a little more work. If you are designing a blind threaded feature, proper depth planning is essential.

Tapped (Threaded) Holes

Tapped holes contain internal threads so a screw or bolt can fasten directly into the part. Specify only the thread depth the connection needs, since extra depth can drive up machining time without really improving holding strength. At Konlida, internal threads are cut with a thread mill rather than a traditional tap, allowing one tool to cut multiple thread sizes that share the same pitch and reducing setup time.

For detailed guidance on thread types, depths, and manufacturability, see our threading options for CNC machining .

Counterbore Holes

A counterbore adds a larger cylindrical recess above a smaller hole. This lets socket‑head cap screws and similar fasteners sit flush with or below the part surface, so the head does not stick out or interfere with other components.

Countersink Holes

A countersink uses a conical recess instead of a cylindrical one. It is designed for flat‑head screws that need to sit flush with the surrounding surface. Just make sure the angle matches the fastener you are using, since common standards include 82°, 90°, and 100°.

Reamed Holes

A reamed hole comes into play when a standard machined hole is not accurate or smooth enough. The hole is made slightly undersized first, then a reamer removes a small amount of material to bring it to its final size. Reamed holes are often used for dowel pins, precision shafts, bearings, and alignment features. Keep in mind that reaming adds another operation, so only use it when you need that level of precision.

Best Practices for Designing Holes in Machined Parts

A few practical design choices can make it a whole lot easier to machine holes while helping control cost and accuracy.

Drill Sizes

Generally, it is a good idea to choose standard hole and fastener sizes when they meet your requirements, since added complexity can bloat your budget and stretch your schedule. At Konlida, however, most milled holes are interpolated with an end mill, so engineers can specify the diameter their design needs.

Hole Diameter and Depth

Deep holes are harder to machine because chips are more difficult to clear and longer tools are more likely to flex. Keep holes only as deep as needed, and use a through hole instead of a blind hole when the design allows.

For a comprehensive overview of CNC design principles—including hole depth optimisation and feature accessibility—see our guide to optimising CNC part design .

Tool Access

Holes inside deep pockets or close to tall walls may require longer, less rigid tools, which can hurt accuracy and increase machining time. Give the tool a clear path to the feature whenever possible.

Materials

Hole machining behaves differently depending on the material. Harder metals can increase tool wear, while some softer plastics can flex or deform, making it harder to maintain tight tolerances. For guidance on how different materials affect machining, see our 1018 vs. 4140 steel guide or aluminium alloy comparison .

Tolerances

Tighter tolerances take more time to machine and inspect. Reserve them for holes where fit is critical, such as bearings, dowel pins, or precision shafts.

For an in‑depth look at tolerance selection and GD&T, see our understanding CNC machining tolerances guide.

Edge Distance

Holes placed too close to an edge can weaken the part and make machining more difficult. When checking edge distance, consider the largest diameter of the entire feature, including counterbores and countersinks.

Chamfers

Use a chamfer to help guide screws, pins, or shafts into a hole. If you only need to remove a sharp edge, call for an edge break instead of tightly controlling the chamfer dimensions.

How to Choose the Right Hole Feature

When choosing between hole features, consider the required fit, fastener type, available material thickness, tool access, and tolerance. In general, use the simplest feature that meets the functional requirements, since simpler hole designs are generally faster to machine and easier to inspect.

If you need to…	Consider using…	Why
Pass a fastener through a part	Through hole	Usually the simplest option when the hole can extend through the part
Thread directly into the part	Tapped hole	Eliminates the need for a separate nut
Recess a socket‑head fastener	Counterbore	Lets the fastener head sit flush with or below the surface
Flush‑mount a flat‑head screw	Countersink	Matches the tapered head of the fastener
Achieve precise alignment	Reamed hole	Provides tighter diameter control and a smoother finish
Keep the opposite surface intact	Blind hole	Stops the hole before it passes through the part

For more ways to improve manufacturability, see our design validation with SOLIDWORKS guide .

Common Mistakes to Avoid

The most common issues when CNC machining holes come from adding more depth, precision, or complexity than the feature actually needs, or overlooking how the tool will reach and machine the hole.

Watch out for these pitfalls:

• Specifying unnecessarily deep blind holes
• Choosing non‑standard fastener sizes without a functional reason
• Applying tight tolerances to non‑critical features
• Positioning holes too close to part edges
• Forgetting to specify countersink angles
• Calling for unnecessary thread depth
• Designing features with poor tool access

When designing parts with complex hole features that require multiple set‑ups, understanding how jigs and fixtures can improve accuracy and repeatability is essential.

The choice between 3‑axis and 5‑axis machining also affects how holes can be accessed and machined. For an overview of these options, see our 3‑axis vs. 5‑axis machining guide .

Summary

Hole Type	Best Use	Key Consideration
Through hole	Fastener clearance, fluid passages	Simplest option when possible
Blind hole	When opposite surface must remain intact	Chips cannot exit through bottom
Tapped hole	Threaded connections	Specify only necessary thread depth
Counterbore	Flush fastener heads	Allows cap screws to sit below surface
Countersink	Flat‑head screws	Match angle to fastener standard
Reamed hole	Precision alignment, dowel pins	Adds extra operation; use only when needed

Ready to Start Your CNC Machining Project?

Konlida’s automated DFM analysis reviews your uploaded CAD model and flags hole features that may be difficult to manufacture—giving you a chance to optimise the design before machining begins.

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