We’ve discussed basic rules for thread engagement, and how that will vary depending on the material being tapped. Softer materials like aluminum will need to be tapped deeper than materials like steel in order to achieve strength enough to properly preload the bolt without stripping the threads out of the tapped material. What if we wanted to use an even softer material, like a plastic? We can introduce a third element, a tapped insert, with all the strength benefits of nut, but the mechanical application of a tapped hole.
When you fall and scrape your knee, what product do you apply to the wound? Would you say a Band-Aid? Or is it more likely that you use a generic pharmacy brand adhesive bandage? Johnson and Johnson have attained the Holy Grail of marketing and brand recognition: the proprietary eponym, when an entire class of product is referred to as a single brand name. The makers of Heli-Coil have achieved the same success.
Imagine an accurate CAD model of a bolt. Remove the head, then extrude cut a cylinder the same size as the minor diameter of the threads. All you have left is a helical coil, the threads themselves. That is essentially all a Heli-coil is, nothing but threads. This coil of threads is threaded onto the end of a special tool and then screwed into a specialty tapped hole specific to that coil insert. This action slightly tightens the coil and reduced its outer diameter, and when the tool is screwed back out, the coil opens slightly, locking in place. With a Heli-coil in place, a part made of softer materials can have tapped holes with a tensile strength and durability comparable to that of the material the coil is made out of.
There is a higher upfront cost to outfitting a manufacturing facility to efficiently use helical inserts, as every thread size and series will require both specialty taps, and well as specialty installation tools. This cost is readily absorbed, however, by the reduction in manufacturing and maintenance errors prevented by not utilizing full strength fastener in softer materials that can’t handle the fastening loads.
Heat-set inserts contain threads on their interior face, and some interference geometry on the outside diameter. They are first heated on a mandrel and then pressed into a smooth hole in the part. As the heat transfers from the insert to the part material, the part melts and the melt pool fills the voids of the interference geometry. When the melt pool cools, the part is retained and prevented from turning.
All heat set inserts are made for some kind of polymer or plastic, as and temperature which would be hot enough to melt a metal part would also cause embrittlement of the part, insert threads, or both.
Press-Fit Inserts function similarly to heat-set inserts in that the outer body of the nut has some interference geometry, which when filled with material from the base part, traps the nut and prevents rotation. Unlike heat-set inserts, the mechanical plastic deformation of the base part is due to insertion force.
These are commonly used on sheet metal parts which are too thin to be used with other types of threaded inserts. They come in more varieties for use with thru holes than other types as well, and often require an anvil to press against to cause the retention deformation.
Self tapping inserts