This signature enables engineers to study the precise manner in which the various parts interact during assembly. It can also be used to accurately determine whether an assembly has been correctly processed. 

For example, the signature indicates the position at which the assembly was first contacted. It also shows the amount of force applied as the piece was inserted, headed or otherwise changed; the maximum applied force; the rate of change for the force; and the final position of the ram.

By studying the force-distance curve for a particular assembly, engineers can quickly identify whether the process was correctly executed, and if not, what went wrong. If a press fit is too tight, for example, the curve will be higher than it should be. If it’s too loose, the curve will be too low. If a part is missing or a part is misaligned, this fact will also be readily evident.

Before any of that can happen, however, the characteristics of the data curve for a good assembly must be determined. This can be calculated theoretically by engineers, and adjusted as more data becomes available, or it may be determined with the use of a master, or known-good, component, which is used to generate an ideal data curve against which future components are gauged. In the absence of a master component, an ideal curve can be generated by running several acceptable components and using their data to calculate averages, as well as acceptable highs and lows.

Software is used to set up limits and evaluate all curves generated during production against the ideal. It can also be used to respond to the data it is generating and then adjust its behavior accordingly. The result is a system that can accommodate normal variation in the parts, creating assemblies that function correctly in the field even if the parts aren’t “perfect” to begin with.

Pressing to a Hard Stop vs. Pressing to a Position

In many automotive assembly applications, simply pressing to a hard stop is sufficient. These applications include press-fitting bushings and bearings, and inserting clips and other fasteners.

The graph in Figure 1 shows the key characteristics captured in the press-fit curve for such an application.

This graph shows the force-distance curve for press-to-a-hard-stop application. Source: Promess Inc.

In other applications, such as medical device assembly, the stakes can be higher. If there are added levels of regulation, or if there are requirements for verification data and traceability, simply pressing to a hard stop is not an option. In many instances, pressing to a position, whether absolute or relative, is required.

The graph in Figure 2 shows the key characteristics captured in the data curve for that type of application.

This graph shows the force-distance curve for press-to-a-position application. Source: Promess Inc.

Besides press-fit assembly, there are many applications that could benefit greatly from having the ability to generate in-process data and evaluate the curves produced. Examples include crimping, staking, riveting, forming, spring testing, bending and straightening.

Bearings Application

To get an idea of how this works, it’s useful to look at a real-world application. An automotive manufacturer needed to press a bearing into a hole machined in a part. However, the company was experiencing quality issues with the incoming parts. The holes had a lot of variation, which led to faulty assemblies. Some of the poorly assembled components were not caught before being shipped to the customer, resulting in recalls, damage to the company’s reputation, and tremendous expense.

Figure 3 illustrates the different curves displayed depending on the part.

These graphs show how force-distance curves can reveal quality problems with press-fit assemblies. Source: Promess Inc.

To solve the problem, the company upgraded its system to one capable of evaluating the press-fit curve during assembly. This solved the problem by verifying quality in-station. Suspect assemblies could be removed and fixed before they caused headaches down the line and long before they caused problems for the customer or end-user.

For more information on press-fit assembly, read these articles:
Best Practices for Press-Fit Assembly
Nondestructive Disassembly of Press Fits
Optimizing Work Cells for Press-fit Assembly