I just received this unique question from a toolmaker in China…
We’re trying to make a collapsible core with threads using a CNC lathe. The specification requires the thread start at half way around the circumference, which is impossible because the lathes have to complete a full circle before making a screw thread. Is my team wrong on this? If not what kind of machinery would we need? We thought about a screw grinder but the part is too big, overall length is 11.25inch.
Although CNC lathes are typically used to machine threads in typical machining situations… they are not commonly used for machining threads on injection molds. This is especially true for applications, such as yours, where specific thread positions are required.
The most common method of machining such threads is CNC Electric discharge machining (EDM). In such a machining method, a conductive electrode with inverted detail is manufactured using CNC machining processes. This electrode is then placed into the EDM machine and the detail is removed, or burned, from the tool steel using an electric spark between the electrode and the workpiece.
For more information into the EDM manufacturing process… please visit wikipedia:
If you have access to a 4 axis CNC EDM machine, you can create even better threads by rotating the electrode about the core as it moves up or down. This may create more accurate thread detail on the core.
For training on this topic, consider our Mold Design and Mold Making Series.
Mold makers have also used 5-axis CNC machining stations to create this type of detail… but the setup and expense of using such equipment for such a small task is generally not justified.