Crimping & Crimp Quality - Issue 4
Given that the crimp applicator is the center of every crimping process and largely determines crimp quality, one might conclude that the choice of crimping press has little bearing on the end result. However, this is incorrect. The crimping press also plays a very important roll in the crimping process.
In comparison to the crimp applicator (for which the numerous set-up possibilities lead to numerous influencing factors), a crimping machine's press force, repeatability and cycle times are important factors. Other important criteria are the seamless interaction of the press with the crimp applicator and the crimp force monitoring system.
Basic Function
The crimping press moves the crimp applicator into the lower “bottom dead center" position, deforming the crimp terminal with the conductor at the set crimp height and brings the tool back into the initial position at "top dead center."
Shut Height & Press Stroke
The basic prerequisite for being able to use a crimp applicator in a crimping press is that the shut height, press stroke and interlock system are identical for the press and the crimp applicator. The shut height is the distance between the upper and lower applicator mounting positions of the press in the closed position (= height of applicator in closed position). Every crimp applicator to be used on a given press must have an identical shut height and stroke. In order to be able to use the widest possible spectrum of crimp applicators, a press with the international standard shut height of 135.788 mm is recommended. This height has more or less become the norm in both Europe and the USA.
Example shut height
The distance between the open and closed positions of the crimping press is described as the press stroke. As the vertical movement of the applicator is carried out exclusively by the crimping press, the press strokes should concur precisely. Depending on the crimp applicator manufacturer, this varies as a rule between 39 and 42 mm, but 30 mm is also common in USA and Asia. Therefore, Schleuniger offers crimping machines with a stroke of 40 mm as well as 30mm (and adapters for different strokes). An exact stroke of the press, especially with mechanical crimp tools, is of central importance for the terminal feed, as here the press stroke is translated directly into a horizontal terminal transport movement.
Crimp Force & Operation Modes
The force of the press – usually given in Kilo-Newton (kN) or tons – determines the cross-sectional area that can be processed. In order to achieve an optimal deformation, the required minimum force varies, depending on wire cross-section, the type of wire (solid or stranded) and the terminal to be processed. The UniCrimp 200 from Schleuniger has a high press force of 33 kN so that it can be used with a wider variety of wires and terminals.
As far as crimp drives go, the eccentric crank drive predominates, with the rotational movement of the drive motor being transformed into a linear movement through a crankshaft with an eccentric crank arm. The advantage of this design is that the "bottom dead center" position of the press is precise and repeatable. The maximum force occurs a few degrees before bottom dead center and continues until dead center. The deformation of the terminal must therefore be completed upon reaching the bottom dead center position. A second maximum force, occurring shortly after bottom dead center is reached, does not contribute to deformation.
UniCrimp 200 programmable crimp machine
A combined knee-lever-crank-drive, such as with the Schleuniger UniCrimp 500, has the advantage that the closing procedure takes place very quickly, thus favoring short cycle times. The actual deformation process is carried out more slowly in order to achieve an optimal result without requiring longer cycle times. There are also crimp machines on the market with pneumatic linear or knee-lever drives.
Cycle Time
More and more, short cycle times are determining factors when purchasing a new crimping press. Especially with mechanical crimp applicators, high press speed automatically effects the feed / transport velocity of the crimp applicator and thereby influences the precision of positioning of the terminal. Depending on conductor and terminal, the material also needs a certain time during the deformation to “flow” optimally. Insufficient time can lead to reduction in quality or faulty crimps. Therefore, the optimal set-up of the press and crimp tool depends on the material (steel, brass) as well as the coating (gold, silver, pewter). In order to achieve the best possible result, individual setting of the press velocity is necessary.
Repetitive Precision
The repetitive precision of the press is an important characteristic. Otherwise, the initial fine-tuning becomes partially lost during operation. If the crimping process doesn’t take place with constant speed and force, it can lead to undesired variations.
If a crimp applicator is used in different presses, it is worthwhile to make sure that the shut height is re-set to the identical height regularly using a calibration device in order to achieve the same results independent of which press is in use. The shut height can be adjusted with most crimping presses.
Stability of bearings and precision of the guides are also important points to consider in regard to achieving long-term high-quality production. The wear and tear on the mechanical parts of old presses leads to more play and thereby to a reduction in crimp quality.
Crimp Force Monitoring
Crimp force monitoring is a measuring device used to monitor the crimp quality during the crimping process. It monitors how the crimp press, crimp applicator and terminals are "working together" during the process. During the crimping procedure, a sensor measures the press force applied and reports this to the electronic control system, which converts the recorded force into a curve over time. The aim is to find out where the curves deviate. Known good crimps are used to "teach" the system the characteristics of a good crimp and to calculate a reference force. Each and every subsequent crimp is compared with the reference crimp force. Deviations of the crimp force that lie outside of the pre-set tolerance limit are registered as “bad”.
Today, two out of three crimp presses have an integrated crimp force monitoring system. Other crimp quality characteristics include only one parameter such as crimp height, crimp width and pull-out force. On the other hand, crimp force monitoring includes forces that are influenced by the entire system. Each of these components can have an effect on crimp characteristics.
Changeover Time & Flexibility
With changeover and maintenance, it is an advantage if the press is accessible and not too small. The applicator change should take place simply, and if possible, without additional tools. If the working area is too small, the use of certain standard applicators may not be possible.
For maximum flexibility, the crimping machine, such as the UniCrimp 200 from Schleuniger, should have an adjustable press velocity, a valve for pneumatic crimp applicators and a split-cycle function for the processing of closed barrel terminals. Only with the perfect interplay of crimping press, crimp applicator and crimp force monitoring system can an optimal result be achieved. At first glance, the crimp applicator seems to be more important because of its complexity. However, upon more careful examination, the extensive influence of the crimping press on the quality of the product to be processed becomes evident.
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