When beginning to do tube settings, the first thing anyone should do is buy a “Setters Tube Holder” which appears in many tool supply catalogues. I have two of them for easy access.
Expose only the very top of the cullet in the tube holder. It will grip securely and will prevent the sides from collapsing while preparing the item for setting. The clamp holder has seven sizes of chucks so that you can pick the chuck that will hold the cullet securely as you worked on it. Insert your jewelry carefully and tighten very securely! Then place the diamond on top and check if there is enough metal protruding around the diamond to securely set it. If not, spread the top open with an awl.
Next start the cutting for the pavilion seat using a #156c undercutting bun that is about 112 to 314 the size of the stone size. Make a bearing low enough so that the table of the stone will be flush with the top of the bezel. Cut the bearing with slight increments of cutting so that it takes about 4-5 series of cuts to make the seat. Once this is accomplished, try to place the diamond into the hole. If it does not fit, ream out the top part of the bezel. Cut only the area from above the seat to the top of the cullet. The diamond should just fall into place inside of the hole and stay in the newly made bearing. If it still doesn’t, ream it open a tad more, and use a copper stone pusher to press the stone into the seat if needed.
I usually file around the top so that I won’t have so much metal to push over. I usually push the metal over the stone with a thin steel pusher that is serrated on the front end so the pusher won’t slide off. Once this is started, check for any bezel spaces on the crown facet. I want all the metal over and absolutely touching the stone.
The next step is to shape the top and sides evenly. I will use a Pumice wheel, of 180 grit and I also use a triangle file #4 cut for this procedure. Then I use a right-sided onglette graver or a flat #39 and cut a bevel on the inside of the metal that is over the stone. This gives it a professional look, and it has a clean finished effect.