The Way It Was
For many, many years, inspection was something done after production or assembly, not during. Traditionally a sample quantity, typically 10 percent, was inspected visually by “Bob” who had the requisite experience to differentiate good products from bad by appearance alone.

If an acceptable percentage of the sample looked good to Bob, then the entire batch was shipped. If not, the whole run was re-inspected and/or scrapped.

Some argue that Bob has been checking every assembly off the line for the last 30 years and he can tell by looking at the product if it is acceptable. Not to knock Bob’s experience, but can he really detect a ten micron difference with his eyes? And, even if he can, what do you do when Bob retires?

Believe it or not, this still goes on today, and where it does, many sub-standard assemblies reach the end user. Compound this error potential with a product consisting of several sub-assemblies and the result is either a big pile of scrap that never makes it off the production floor or, worse yet, a product that reaches the consumer and fails prematurely.

The Way It Is
Consumer demands for improved product quality have led many manufacturers to adopt quality control and part traceability requirements based on 100 percent in-process assembly monitoring as the primary verification procedure. In its simplest definition, in-process assembly monitoring is the practice of using real-time sensor feedback to analyze 100 percent of the assemblies as they are put together.

Several companies now supply standard monitoring solution packages engineered for plug and play installation. For the most part these packages are easily fitted into existing equipment or designed into new applications.

This ‘off the shelf” approach to assembly monitoring has been fully developed and is currently being used by manufacturers worldwide. However, even among these companies, there are competing ideas on how to monitor the force/distance assembly process effectively.

European manufacturers most commonly use a ‘box’ strategy that allows the end-user to place a limited number of boxes around areas of the anticipated force/distance curve reported during the assembly operation. As long as the assembly falls within the prescribed areas of the boxes, the assembly passes.

Some American companies “thought outside the box” (pun intended) and came up with a different approach, typified by the 100 percent differential monitoring method developed by Promess Inc., of Brighton, Mich. This strategy utilizes a ‘teach-in’ of a known good, nominal part.

Every subsequent part that is produced is then compared over the entire cycle to the taught signature. Adjustable limits can be set to allow for minor part variation, but still offer the ability to catch suspect parts based on differences to the normal.

The Way It Will Be
Using the common operation of pressing a pin into a bore as an example, assume that the design specification calls for a minimum press force of 250 lbs. and a maximum press force of 750 lbs.

The first question to ask is “At what point during the cycle?”

In reality, the press force will climb as more of the pins surface area engages the bore until it reaches full engagement. Then it will level out slightly before spiking as the pin bottoms out.

Many people assume that checking the force at one position is sufficient and while this provides some information, it is not really adequate feedback to ensure a high quality assembly. Checking the values at several critical points is a better approach, but judging each data point collected against the same force limits will lead to false failures.

The best approach is to monitor the force versus distance signature over the entire process and extract data from critical points on the curve, in real time.

The best approach is to monitor the force versus distance signature over the entire process and extract data from critical points on the curve, in real time.

Some points that can provide relevant information:

• Where did the pin first contact (Touch Point) the bore?
• What was the final depth the pin was pressed to?
• What is the distance from Touch Point to Final Depth?
• What was the force prior to bottoming out?
• And, what was the actual bottom out force?

The unique force vs. distance signature generated for each part along with the numerical answers to these questions will tell you if the assembly is good or not. A system that provides this information as well as the ability to store and easily review it will also be very helpful in building a historical database of information for typical parts.
For more information please contact: Promess Inc. at 810-229-9334, visit www.promessinc.com or e-mail promess@promessinc.com.