A thorough thread specification can avoid problems.
Figure 1. Common specification for a metal thread.
The design of a plastic thread is dictated by the earlier specifications developed for metal threads. The evolution of metal threads is discussed in Parts 1 and 2 of this series.
Over time these metal thread specifications became so comprehensive and so well recognized that they simplified the design, drawing, and specifying of a thread. For example, Figure 1 contains all of the information required to produce a metal thread. Any engineer who saw this illustration would immediately know that this was a .750-inch-long, 1?2-inch-diameter American Standard machine screw, National Course series with 13 threads per inch. The exact thread profile, tolerances, and other details necessary to produce that thread could be found in the Machinery’s Handbook.
Figure 2. A typical metal thread start and stop.
Molded plastic threads are often specified using this convenient abbreviated method. The problem with this approach is that relying on the Machinery’s Handbook will result in a thread design for metal and not plastic. This specification would result in a 60º machine-screw-thread profile with sharp corners at the roots of the thread. An Acme, or modified buttress, would be a better thread profile for molded plastic.
A metal outside thread typically extends all the way to the free end of the part. The helix angle on the thread results in a sharp edge where the thread runs off the end of the part.
Figure 3. The proper way to stop and start a molded plastic thread.
This sharp edge can cut fingers. It is also difficult to align with the mating inside thread. These problems were avoided by providing a chamfer on the end of the part, as shown in Figure 2. This same shape can be machined into an injection molding cavity, but it is difficult and therefore costly.
The tool that cuts, or forms, the metal thread shown in Figure 2 cannot cut a full-depth thread all the way up to the shoulder. This leaves a rough, unfinished appearance. With expensive products such as cameras or scientific instruments, appearance is important. In such instances the threads beside the shoulder are removed and this provides a cleaner finished appearance. The same shape can be injection molded, but removing the last few threads produces an undercut that cannot be unscrewed from the cavity. This shape can only be produced if the cavity has a parting line parallel to the length of the thread.
Figure 4. A highly stressed threaded assembly.
The thread shown in Figure 3 eliminates these problems at the start and the stop of the metal thread shown in Figure 2. This is a better design for a molded plastic thread. With this design the thread stops short of the free end of the part. The helix angle on the thread does not create a sharp edge. The blunt end at the start of the thread is strong and easy to engage with the mating inside thread. This shape is also easy to mill or grind into a cavity.
Injection molding a male thread of this type allows for a full-depth thread all the way up to the part’s shoulder.
Figure 5. A reliable method of specifying a molded plastic thread.
However, the helix angle on the thread creates a sharp corner where the thread abuts the shoulder. This also leaves a fragile, knife-thin piece of metal on the cavity in this same location.
Providing a radius at the junction of the thread and the shoulder strengthens both the molded thread and the cavity. A male thread designed in this manner can be unscrewed from the cavity without difficulty. The start and stop of inside threads on plastic parts should be designed in the same way the assembly shown in Figure 4, the start of the outside and inside threads is stopped short of the open ends. This is an ideal design. Providing a radius at the junction of an outside thread and the rest of the part is an important strength consideration.
On the other hand, the stop of the male thread in Figure 4 has not been designed for maximum strength. The helix angle on the thread has created a sharp corner at the thread stop. Tightening the nut will create a tensile load that tends to stretch the outside thread. This constant tensile load will become concentrated at the weakened location along the length of the outside thread. That weakest point will be at the sharp corner where the thread joins the rest of the part. Many threaded parts fail in this area. Providing a large radius in this location will strengthen the junction between the thread and the rest of the part.
Designing the perfect thread
Figure 1 shows a simple method of specifying a male thread. This method appeals to designers, as it simplifies the design, drawing, and specifying of a thread. This method is satisfactory for metal threads, but it is not ideal for a molded plastic thread.
This is especially true now that a lot of molds are being designed and built outside of North America. Many of these offshore moldmakers are not familiar with, or have ready access to, the Machinery’s Handbook. The most reliable method of designing and specifying a molded plastic thread is shown in Figure 5.
This method defines and tolerances the length, root, and crest diameters of the thread. The number of threads per inch, the thread profile, and a radius at the root of the thread are clearly shown.
Of equal importance, the start of the thread has been set back .030 inch from the free end of the part. A .250-inch radius has been provided to blend the root diameter of the thread into the full diameter of the part where the thread stops. Figure 5 provides a toolmaker with all of the necessary information required to machine a good-quality thread cavity. This method of drawing and specifying a thread is more work. It does, however, provide the designer the best chance of getting what is wanted (aerial), but we haven’t seen a lot of it. No one can afford to spend several years to do so. We do the best job we can to get the best possible pricing with our material suppliers.”
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