I built a spider for my Atlas Clausing 12X36 to chamber rifle barrels for various rebarreling projects. The spider is a light press fit on the end of the spindle with two rows of 4 socket head cap screws evenly spaced.
if I¡¯m working on a barrel that is prefinished, I just use coke can aluminum under the screws, build a concentric barrel plug for each end (a snug fit between the lands of the bore) and dial in both ends with a DTI via a 4 jaw and the spider.
Since my spindle diameter isn¡¯t large enough to pass a full barrel through, I also employ a steady rest on the breech end, and thread, face the breech, set the shoulder, and headspace before taking anything down. ?Concentricity is the name of the game, and I always attempt to shoot for (pun definitely intended) 1/2 a thou in remaining concentric on all linear and perpendicular mating surfaces.
John W.