Over the course of this series on upset control, we’ve discussed the repeatability of upset control and part variation in rotary friction welding. Remember, upset is the amount of shortening you get in the part as a result of friction welding. Upset is different than overall length, which is the total length of the part after welding.
If we fix the amount of energy that goes into the part during friction welding, then with perfect parts we can get very repeatable upset control. In the real world, however, we know that there may be slight variations with incoming parts that will in turn lead to variations in upset.
Instead of fixing the amount of energy that goes into the part, we could change our control technique to allow the amount of energy to vary in order to control upset. By doing so we can overcome incoming part variations, but this does not totally solve our problem because we still get some upset variations at the end of the friction welding process due to variations during the forge and braking phases. So while this technique is better than fixing the amount of energy that goes into a part, it is still not the best upset control.
Put simply, using variable energy as a control technique can also be used to control the overall length of the part. We’re allowing the amount of energy used to vary while we’re rotating, in order to overcome different incoming part lengths. This is the same idea used above for upset control, but we use a different measurement to trigger the end of the weld.
With longer parts, we want to weld for a longer period of time – thereby producing more upset – so at the end of the friction welding process the distance between the two ends of the part will be the same every time.
Conversely, with shorter parts we don’t need to rotate quite as long in order to get the same part length. This means we need less energy input, producing less upset, but we still get a very similar welded length. Instead of triggering the end of the weld on a time value or an upset value, we use a Slide Position value which represents the distance between the end of each part being welded.
The welded length is still going to vary in the forge phase because we’re changing the amount of energy from part to part, so there will be some variation there. But, we also get some variation as a result of changes in the braking phase.
Varying the amount of energy and welding to a position is the best length control technique in direct drive friction welding. While there is still some variation in the length control, this technique overcomes incoming part variations.
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