i made a few mandrel holding devices out of wood and long bolts and butterfly nuts, that i can clamp onto my bench…
i got two pieces of wood…top piece shorter than bottom piece…which create horizontal vise jaw.
then i notched them in the middle to accept mandrels,
then drilled holes down on each side, countersunk the bottom of the holes, pushed long 5/8” bolts up, and screwed on butterfly nuts to tighten the vise jaws…
i knotched one with shallow V’s to accept rounded mandrels, one with a square notch for my sinusoidal stake, and one with a large rect notch for my oval bracelet mandrel.
(i actually made the square and rect by gluing on additional wood to create the space for the mandrels…wood slightly shorter than the mandrel so that i could tighten vise jaws down without contacting the additionsl wood.
they feel very stable.
i clamp to the bench with C-clamps.