Tube bending is a highly automated process to produce high volumes of complex tubes, each one formed to very precise specifications.

Unseen, unknown and unresolved process issues in mandrel bending lead to premature tool wear, product quality issues, equipment damage and costly downtime. 

Periodically, bending operations can be hit with a rash of mandrel link failures, potentially leaving a ball behind in a tube. 

Detection of a broken mandrel link is critical so manufacturers try to have some system in place to verify the presence of the intact mandrel.  However, even when these systems are successful in detecting the presence of a broken mandrel link, 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 bad quality parts.

Frequently, the broken mandrel link is the result of a quality problem, not the cause of the problem. In such cases, any number of tubes may have been produced and shipped or further processed prior to the one tube being detected as defective.

Process variation monitoring is now proving to be not only an effective method of detecting broken mandrel links due to excessive pull force; it is also an effective way of detecting the conditions that increase tension on the mandrel and could result in a broken mandrel link:
     • Loss of lubrication
     • Galling of the mandrel
     • Tubing imperfections
     • Wiper die wear
     • Mandrel misalignment
 
How It Works:
Process monitoring for mandrel bending involves sensing the pull or tension force on the mandrel rod during every bend cycle.   Pull force (or lack of) on the mandrel rod is representative of the stress on the mandrel links that retain the articulating mandrel ball assembly.  A change in this pull force or stress on the mandrel balls that exceeds the normal control limits is a direct indicator of process variance and degradation.  Some examples of process variation are loss of lubrication, galling of the mandrel balls, tube slippage or misalignment, material out of specification, and/or change in material hardness.

These are typical conditions that can occur and ultimately lead to failure of the mandrel links in a manufacturing environment.  Process monitoring will alert the operator to process changes that might otherwise go unnoticed and lead up to a mandrel failure and machine downtime.

The following chart is actual data captured from a bender that produced 6 tubes, with 5 bends each.  The first 5 tubes produced indicate relatively consistent pull force on the mandrel for each of the 5 bends.  The machine was then stopped, and the lubrication removed from the mandrel balls.  The 6th tube was produced without lubricant, resulting in a 56% increase in pull force on the mandrel links:

 

This second example shows the correlation between normal process and improper adjustment of the wiper resulting in visible wrinkling of the tube.  This resulted in an increase in pull force or stress on the mandrel links of 185%, a near doubling of normal process force:

In monitoring the mandrel pull force through every bend cycle without adding time or processing steps, the PVM2000-MB™ Mandrel Bend Monitor system provides an alert to potential issues before they become real problems.  The early detection of a degrading process reduces scrap, and saves unnecessary downtime and machine repair.