Tube in tube

To make a tube feed through in a bigger size tube, you start with preparing the larger tube by blowing a hole the exact size of the tube you want to fit in there. Then, you insert the smaller tube in there and melt it to the wall on the one side. After this the other side needs to be airtight connected to the tube and this is done by heating up the entire (small) tube and blow it up until the two parts are connected and air tight. This is all temporary measures as the proper weld will be made after. The other side now can be blown open and the connection can be nicely made. The other attachment can now be taken off (we don't need to blow anymore) and cleaned up nicely. The trick is to be fast and keep also the glass hot enough to prevent cracks..