I’m curious, if you have a preform, what method/tools do you personally use to center the dop? I’m open to hearing new tricks or things I had not thought of.
I first mark the mounting centre then place the pavilion of the stone in a large cone or flat dop and secure with a non adhesive fixing, something like Bluetac or plastecine that allows me to move the stone.
Then I place in a transfer jig and starting with a very fine dop on the other side of the jig, align the centre. Then switch the fine dop for a large flat and press this against the stone to flatten the table. It may take a few goes to get the positioning right, but once positioned with both the flat and fine dop, I then mount using a 5 minute epoxy to grab the stone, then depending on the size of the rough I might add more epoxy for a solid fixing.
This technique may take some practice initially but once mastered you may find yourself using it regularly to minimise wastage on high value stones.
Once you have a pretty good by eye preform and have cut a temporary table, for rounds, ovals, trilliants, squares, emeralds and so on, use a cone dop to push the stone onto the flat dop and that will usually get you pretty well centered. For pears, some hearts and various other stones it’s a bit more complicated. Lightning Laps sells a Magdop which helps center, using a magnetic dop and some silicone putty, for about $30. UltraTec has a GUIU dop that centers by means of four set screws, but it is expensive, about $250+. There are some transfer fixtures which have a pointer at right angles to your dops which can help you center if you put the stone up to the pointer and rotate it…but you will need some silicone putty or similar there, too. I have an old one of these, but wouldn’t know where to get one now…a machinist could add such to some transfer fixtures easily. That’s all I know, one or more of these should help. In some situations, you might have to make a few test cuts and re-dop. However, you can get too precise in many situation, so unless the stone is very expensive rough, as my mentor, the late Will Smith used to say, “just cut the damn stone!” You lose a little weight (maybe!) but you make it up in time saved cutting the next one…