Stripped threads can occur in engine blocks, drone frames, and any assembled part. Stripped threads are those holes where you stripped out first bolt so it is now worthless. The solution is threaded inserts. However, the mistake newbies make is they think all threaded inserts fits all holes. Nope. Millimeters matter in the difference between a lasting repair and a failure. Why? Because different materials reacts differently to stress.
Wire thread inserts strengthens holes and look like coiled springs. They can be used on soft aluminum to return stripped holes to like new strength, and as a strong steel connection for metal fabrication. The trick with this technique is precision. There are special STI drills and taps that is larger than normal to allow for the size of coil. Too much drilling, no winding of the insert. If hole is too large, it won’t grip the threads and will pull right out under torque.
How to Fix Broken Threads in Different Materials
On steel parts, a depth of one times diameter is often enough. On softer metals such as aluminum & magnesium, it needs to engage deeper into part. Sometimes this is even 2X the diameter due to the material around the insert crushing easly. This extra depth are needed if there is vibration.
When assembling with plastics, the physics is different, thermoplastics will deform rather than break cleanly. For most plastic (such as Nylon or ABS), there is usually a choice between heat set inserts and knurled press-in inserts. With the press-in variety, they depends upon friction to hold them in position. The hole needs to be snug without being too snug so it doesn’t crack the plastic when you go to press item in place.
With the heat set style, the insert melts into surrounding plastic to form a permanent mechanical lock. The plastic material fills out the knurls on the outside of the brass insert. Each size has a specific diameter, as indicated on the chart and your hole should be just smaller than outside diameter of the insert. This creates an interference fit for holding power.
For rigid plastic such as PLA you want a snugger tolerance. With flexible plastics like soft Nylon or TPU, you want more space so that the material doesn’t split open as it expands under heat. Try to tighten the bolt and if the insert spins then you’ve got the ratio off. It’s annoying but easily avoided by controlling the temperature and using the correct drill bit.
Hole size is one thing; how you install that screw is another. When installing with a heat set insert, use your soldering iron with patience. Don’t force an insert in place till plastic melts; you’ll be fighting the threads and potentially tilting the insert. It should of been yielded, back off and wait. It bonds in the cool down. Torquing on it hot will lead to grief later.
If you’re making lots of parts, ultrasonic installation saves time over the heat set method by generating quick heat via friction. For smaller volumes or the home workshop, a temp controlled iron is still the most sensible solution.
Know your material: Choosing the proper insert depends on how it behaves. Metal require good depth so threads engage and precision cutting tools can work. Plastic demands temperature control, close tolerances and attention to make a permanent joint. The chart is meant as a handy guide but knowing why the tolerances are there is key. Respect the material, take time with the pilot hole and always check fit prior to final commitment. Your solid assembly begin well before that twist of wrench.
