PROBLEMS ASSOCIATED WITH THE PROTOTYPE RIG

As previously mentioned the rig had first to be assembled on the compression ' testing machine before each test could be carried out. After each test the rig then had to be dismantled in order that the formed component could be removed. All this involved moving heavy lumps of metal about. To make things more difficult the die blocks split laterally in relation to the tube. During the bulge forming process, as well as the tube bulging in the unrestricted area, bulging also occurred down the rest of it's length to a small extent, forcing the tube wall against the die wall. This meant that the tube was stuck firmly into the two halves of the die, making separation of the dies and removal of the component extremely difficult. In order to free the component the two die halves had to be removed as a unit from the rig and separated with the aid of drifts and a stout hammer. Because of this each test could take over half an hour to complete from set-up to extraction of the formed component.

Another problem was the method used for sealing the tube ends. This involved the use of 'O' rings located at the end of the stepped plungers as shown in Fig. 4. These worked alright initially, but because of their position the 'O' rings invariably became damaged by the ends of the copper tube and would have to be replaced after only one or two tests. During some of the tests carried out complete sealing could not be achieved. In such cases the internal pressure had to be maintained during the test by the constant use of the hydraulic hand pump. In some cases, however, the rig had to be dismantled and the 'O' rings replaced before the test could be carried out.

Leakage was not so much of a problem, though, when large compressive axial forces were obtained during the process. These occurred when forming tee and cross pieces especially when using the thickest walled tube. The high pressure acting between the step of the plunger and the end of the tube ensured a good seal. This, however, sometimes led to another problem. The large compressive axial force being used caused deformation of the ends of the tube. Consequently the tube would form into the recess at the end of the plungers containing the 'O' rings. This would prevent the plungers from being withdrawn from the tube at the end of the test. The removal of the plungers was again obtained with the use of a hammer, but with caution this time since the springs between the die block and the guide plates were held in a compressed state.

These heavy duty springs located between the die block and the guide plates were used in order to keep the two halves of the die block together during forming. A secondary use was to keep the die block central between the two guide plates so that the bulge would form centrally

on the tube. Unfortunately this did not always work and the resulting bulge would be formed off central, due to one plunger meeting a greater resistance than the other during forming. The bulge thus formed would also be mishaped as well as not central due to more forming occurring at one side than the other.

As well as these problems in assembling and dismantling the rig there was also a lack of control during the actual bulging process as well as a lack of instrumentation. This caused a large variation in the components formed and made accurate recording of the process difficult. The strain rate for the axial compression could be set/altered on the compression testing machine. However there was little control over the

internal pressure during the process apart from setting its maximum value with the pressure relief valve and its initial value. With the variations in the quality of the tube sealing for each test, it was extremely difficult to carry out a series of tests with similar internal pressure conditions'.

The only values recorded during the test, apart from the dimensions of the copper tube, were the :

i. initial and final internal pressure, ii. initial and final axial force.

The internal pressure was taken from a pressure gauge mounted on the hand pump and was not very precise. If leakage was occurring the pressure would be changing all the time, making recording the final value very difficult if the maximum pressure set by the relief valve was not achieved.

In order to overcome these problems the following points were needed to improve the prototype bulge forming rig :

1. improved machine design in order to make tests easier and quicker to perform,

2. better tube sealing arrangement# 3. greater machine/process control, 4. better instrumentation.

At this stage it was decided to design a new bulge forming rig taking into consideration the problems encountered with the prototype rig.