Designing Internal Features for CNC Machining

In the world of precision manufacturing, the external geometry of a part often receives the most attention. However, the design of internal features is equally critical for functionality, performance, and manufacturability. Mastering the design of these internal elements is key to creating efficient, reliable, and costeffective components through CNC machining.


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One of the most common internal features is the blind hole. A fundamental rule is to design holes with a standard drill bit size to avoid costly custom tooling. More importantly, the depth of a blind hole should not exceed 12 times its diameter. Deeper holes increase the risk of tool deflection, breakage, and inaccuracies. For internal threads, always specify a thread depth of no more than 3 times the hole diameter. Deeper threads offer no mechanical advantage and significantly increase tapping difficulty and cost. Furthermore, leave an unthreaded length at the bottom of a blind hole, at least equal to the pitch of the thread, to allow for chip accumulation and ensure clean, full threads.

Cavities and pockets are another essential internal feature. To ensure successful machining, it is vital to consider tool access. Avoid designing internal cavities with sharp, inaccessible corners. Instead, specify a corner radius (fillet) that is at least 1/3 the depth of the pocket. A larger radius is always preferable as it allows the use of a larger, more robust end mill, leading to faster material removal, better surface finish, and longer tool life. Designing pockets with vertical walls and flat bottoms is ideal; draft angles on walls and radii on floorwall junctions complicate machining and drive up costs.

For parts requiring internal pathways, such as fluid channels, undercuts can be a solution but require specialized tooling. If an undercut is necessary, design it with standard tool dimensions in mind and ensure there is sufficient clearance for the tool to operate. Whenever possible, redesign the part into multiple components to eliminate the need for complex internal undercuts, which often simplifies assembly and reduces machining time.

At our company, we specialize in providing expert CNC machining services for a global clientele. We understand that a welldesigned part is the foundation of a successful project. By partnering with us and applying these designformanufacturability (DFM) principles for internal features, you can significantly reduce lead times, minimize production costs, and achieve superior part quality. Let us help you optimize your designs for manufacturability and bring your most complex projects to life with precision and efficiency.