Oh The Places We Go, You Don’t Want To Know

We make hundreds of frames that fit into freight trucks – exactly. The welded assemblies, stacked end to end, can only be off by 0.250” over a 144 inch span.  For those of you who don’t know, welded assemblies like to move with the welding heat.  Sometimes the internal stresses of the metal are relieved and dimensions change.  Other times, the amount of weld wire (or amount of heat) can cause dimensional changes.  We know how to address this with fixtures and controlled welding parameters for each job.  So what happens when all these things are controlled and our frames are not meeting spec.?

This is precisely what happened to us this week. After months of successful shipments, we were faced with several rejections in a quick time period.  At first we thought this was due to the extrusions we were using.  Maybe a new lot of material was a different size or thickness?  We checked and that was not the issue.  Maybe a new welder or new weld wire was causing an issue?  That turned out not to be the case.  What the heck was the problem???

We ended up doing a full dimension check on the frames and found out that the main extrusions were twisting with the welding heat during the last welding process.  As we welded the cross supports, the welding was done only on the outside part of the extrusions, and the imbalance caused the angle extrusion to twist towards the heat.  It looked fine when we visually inspected it, and the top/bottom dimensions were fine, but in the middle, without the support, there was a slight twist.

Our first step was to implement 100% inspection because we know that residual stresses in extrusions can change depending on the mill’s run speed and cooling process. Some lots will pass no problem, but other lots (or parts of lots) will have these residual stresses that will “relieve” when heated and twist.  We were able to quickly build an inspection fixture for our shipping team to use, thereby freeing up the welders to weld.  Any frames that failed were reviewed by engineering and reworked.

Our next step was to verify that the inspection fixture not only ensured the frames met all dimensions, but also that the frames met the customer’s use. It turns out that the custom uses specially designed freight trucks to maximize their load.  Tomorrow the customer is sending a truck up and we’ll be loading the inspected frames onto the truck to make sure they fit with the tolerance the customer needs.  If there is a problem, the fixture will be modified (and the print will be modified) to match the customer need, not to match the customer spec.

Wish us luck!!

from ETM Manufacturing http://etmmfg.com/3676