We’re excited to begin a new series on our blog spotlighting Design for Manufacturing (DFM) best practices. We provide DFM services as part of our holistic approach to manufacturing success so that we can help customers improve cost, performance, and/or time to market while ensuring that they are fully prepared for the next step in the manufacturing process.
Today we’ll cover best practices for thread design, but be sure to check back each month for a new tip.
4 Best Practices for Thread Design in Complex CNC Machining
Internal and external threads are most commonly produced using CNC machining as opposed to injection molding or casting, because it’s much easier to add threads to milled and turned parts.
While machining threads is a fairly straightforward process, we’ve found that there are sometimes discrepancies between the design intent of a part and the reality of producing it on the shop floor. Here are a few best practices for thread design that will streamline communication and help you get an optimal thread for your application:
Know your thread depth. Thread depth is defined as the distance from an adjacent surface to the center line of the last full thread. Contract machining shops typically treatthread depth as a minimum—unless the customer indicates that thread depth should be a maximum measurement or provides instructions not to break through. For DFM purposes, it’s generally best to specify whether your thread depth should be treated as a minimum or a maximum before sending the part out to quote. It’s also important to specify if you do not want the thread to break through.- Don’t specify drill depth. While shops generally like as much information as possible about a part, drill depth is one specification you should actually avoid. Drill depth depends on the specific tooling a shop chooses, making it difficult to determine in advance. Drill depth also needs to be deeper than thread depth, but how much deeper is, again, dependent on the tooling and the hardware being used. Since the critical dimension is the thread depth, focus on providing that information instead. Your contract machining partner will use that information to determine the best tooling for the job.
- Avoid threads that are too deep. Contrary to popular belief, designing threads that are excessively deep won’t increase the strength of your part! When features have a small diameter and a long reach, special tooling is required to machine the part. These long-necked tools cost significantly more than standard tools and are prone to breaking and causing chatter during machining. Unless deep threads are absolutely necessary for the function of your application, stick to a maximum thread engagement length of 1x thread diameter for steel and 2x thread diameter for aluminum.
- Clarify rolled vs. cut threads. Threads are either rolled or cut, and it’s important to clarify your preference, if you have one. Industries like aerospace often require the additional strength of a rolled thread, but that can vary from part to part. If you don’t specify what type of thread you’d like, you will likely end up with a thread based on your manufacturer’s choice for ease of machining. Since engineers typically have more information on hand about part functionality, they’re the best ones to determine the kind of thread needed for optimal part performance.
We hope you found this first DFM tip insightful. If you have questions about thread design, our expert engineers are happy to discuss your part with you at any point. Contact us to get the conversation started!