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The LDH (Limiting Dome Height Test) is widely used at Ford Motor Co. stamping plants laboratories to monitor the formability of incoming sheet materials. Although the LDH test is very easy to implement and interpret, variability of the results and poor reproducibility between laboratories limit its acceptance. In this investigation, some of the causes of variability and differences between plant laboratories are discussed. Much of the experimental work was done at plant laboratories and the results are directly applicable to quality control (QC) machines. It was found that the binder force and the binder shape have a big influence on the results, and they should be carefully controlled. The binder cleaning procedure is also relevant to the test variability. Punch temperature has a much greater influence on QC machines than on research machines and a method for controlling the punch temperature in QC machines is presented.

There are quite a few studies on the influences of coatings and base materials on the results of LDH (Limiting Dome Height) test.(1) Although these influences are hard to quantify, and the interdependence of one variable with the other makes interpreting the results even more complicated(2), a basic understanding has been achieved. The influence of temperature is not clear, but there are some indications that increasing temperatures have a detrimental effect on the LDH values. A previous investigation carried out at Ford Motor Company(3) on a QC LDH machine reveals that the LDH decreases with increasing temperature of the system (basically three temperatures were monitored: punch, binder and hydraulic oil reservoir). Such a behavior can have important consequences in the industrial implementation of the LDH test: if temperature has a significant effect, then the statistical value of the results is reduced.