By Jaewook Lee and Byoungkon Roh
Pem-nuts are used to attach circuits to a chassis-base, but they are too expensive for regular use because of the numerous processes required to produce them using a traditional machining process. The authors used TRIZ (Theory of Inventive Problem Solving) tools and the application of the 40 inventive principles to design a mold-form to improve production and reduce the cost of pem-nuts. Figure 1 shows the circuit components used in the pem-nut production process (PDP).
| Figure 1: Circuit Components Used in PDP|
Standoff nuts are used for printed circuit boards (PCB) and other assemblies. They are manufactured in various shapes and formed in a variety of ways based on how they will be used. Most standoff nuts are built using a machining style and are called pem-nuts after the developers – the Penn Engineering and Manufacturing Company (PEM). Pem-nuts are made from free-cutting steel that improves machining ability. Automatic lathes form the pem-nuts through a machining process and the cutting tools are changed as necessary to get the proper shape.
| Figure 2: Standoff Nuts Built Using a Machining Process|
| Figure 3: Standoff Nuts – Cross Section and Actual Object|
There are several forming methods – the use of an injection mold, stamping and forging. This project's inventors did not include the injection mold method on the target techniques list, because the injections affect the electric conductivity of the components. The stamping and forging methods, however, maintain the key properties of the raw material. Stamping is the best forming method for productivity.
The goal was to reduce costs and improve the productivity of nut manufacturing by implementing a stamping method of forming the nuts instead of machining standoff nuts, so they applied TRIZ to stimulate idea generation for solutions on the best forming method for shaping parts for this project.
System Analysis and Structure
During the system analysis stage the researchers referred to the system as the "nut injection process." The nut injection process includes adding nuts to the main base chassis to fix the PDP circuit to the panel and requires the use of nuts and the chassis. The system structure is shown in Figure 4. The function of each component is as follows:
- Guide: Allows for the smooth assembly of the PCB
- Screw: Firmly holds the PCB
- Body: Determines the height and maintains the hardness of the nut
- Groove: Under-cut to strengthen the coherence between the nuts and the base chassis
- Base chassis: Keeps the PCB assembly strong
- Head serration: Prevents the nut from turning and enforces the injection strength
| Figure 4: System Structure|
Current Injection Structure
The under-cut is important functionally. The base chassis moves to this groove, and the nut and the base chassis are mechanically joined. Figure 5 represents the "before nut injection."
| Figure 5: The Before Nut Injection|
The "after nut injection" is shown in Figure 6. Because the base chassis is deformed in volume due to its plasticity, it rushes into the head of the nut at point A. The nut is fixed firmly against both the upward and downward forces to separate it from the base chassis.
| Figure 6: The After Nut Injection|
When using the current stamping method, it is impossible to form the groove. But, it is also necessary because the groove is key to the injection function. Therefore, the authors formulated a contradiction – the groove must not exist for stamping but rather for strength.