Page 17 - ITAtube Journal 1 2022

Technical Papers
easily be calculated is the amount by which the rolls spring together on the forward stroke after they have passed the ‘C’ point. On the return stroke they will spring out by a similar, but not identical amount.
Optimisation
The formular for the early development
of these profiles was developed in Europe largely by Mannesmann-Meer and use
of the formular proposed in 1969 will
still produce results. However, the use of computers – especially powerful lap tops has led to remarkable productivity gains for those prepared to push the boundaries and fully understand the process detail.
The principal working length of the die is normally split into forty segments from the start to the ‘c’ point. There are three princi- pal parameters that need to be calculated for each segment. These are the ‘Q’ factor (which is a measure of the wall strain rel- ative to the diameter strain); The percent reduction (or the total strain if you prefer): and the strain disparity (This is the differ- ence in the strain between the outside tube circumference and the inside circumfer- ence). Together these define the strain path.
Controlling these parameters in an intel- ligent way can result in much larger reductions, freedom from lap type inside diameter cold pilger defects and improved dimensional control. Metals with certain crystal atomic configurations can have key mechanical properties controlled. The larger reductions can lead to very large extension of the tube hollows which has significant productivity benefits especially where machines do not have automatic continuous loading. So, Fig 3. shows how these parame- ters might change as strain path progresses. The horizontal axis is the same as for the previous chart but stops at the ‘C’ point. Note that when the ‘Q’ factor goes below 1 and the strain disparity goes negative there is an increased risk of bore defects.
Roll force determination and side relieve implications
Readers who are following this article will be aware of the importance of side relief issues. Usually, operators use both radial
ITAtube Journal April 2022
Fig. 3 Key Strain Path Parameters
side relief and tangential side relief or sometimes parabolic side relief. Calculating the side relief needed to accommodate the forward movement of the reducing tube can be found in the literature but there is not a lot of practical information available which explains how much of the calculation to use either as part of radial side relief or tangential side relief. Sometimes it can be beneficial to change the amount of tangen- tial relative to radial side relief through the work length.
Calculating the roll force is also possible given the same base calculation as for the above charts but the formula is compli- cated. The amount of strain hardening has to integrated into these values. The force can be calculated as far as the ‘C’ point
on the forward stroke but needs load cell measurement to verify its accuracy and also dimensional measurement to verify the resulting roll separation for use in additional side relief calculation. The Roll separation occurring from the ‘C’ point and the end of the sizing section also needs to be measured.
If operators can perfect the dimensional measurement procedure, they can in theory avoid the use of load measurements but it is easy to see why it would be useful to have both. For instance, for calculating the force on the mandrel and hence how
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