TPT May 2009

I nspection, M easuring, T esting & M arking

Process variation monitoring (PVM) in mandrel bending

possible to compare the signature of each production cycle with a reference of known good parts as a way to monitor the manufacturing process. This is known as process variation monitoring, or PVM. The fundamental theory behind PVM is simple: if a process (including inputs, force, and tooling) is known to be capable of producing only good parts, and the process is consistent and repeatable, then the output of good parts should also be consistent. If any of these process variables change, such as raw material (ie thickness, hardness), lubrication, tooling condition, and even machine condition (bearings, clutches, setup, etc), the resultant signature will change. An effective PVM system detects the change and provides an alert. Process variation monitoring represents a necessary practice to detect broken mandrel links or deteriorating quality issues such as loss of lubrication, galling, tube slippage, out-of-spec material, or a change in material hardness that can lead to breaks. The simple benefits include reduced scrap and increased revenues, elimination of unnecessary downtime, and avoidance of repair. Some vehicle manufacturers have specified that an objective system be utilized to ‘see’ that the mandrel assembly remains intact, leading some manufacturers to focus attention on the mandrel with vision detection or complicated mechanical devices. Initiating an effective monitoring system requires examination of the entire process

Unseen, unknown, and unresolved process issues in mandrel bending lead to premature tool wear, unexpected equipment damage, and costly downtime. Even worse, undetected mandrel faults or poor quality parts can be produced and shipped without

any realization of a quality issue. Current QA practices may not be adequate. Detection of a broken mandrel link is critical, so manufacturers try to have some system in place to verify the presence of

 Figure 1 – Loss of lubrication

the intact mandrel. However, even when these systems are successful in detecting a broken mandrel, they do nothing to alert the operator ahead of time of a process change that could result in either a broken mandrel link, or the production of poor quality parts. A different approach to quality is to use an in-process monitor to capture the ‘signature’ of the process. It is then

 Figure 2 – Misalignment of the setup

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M ay 2009

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