What's the Best Way to Hold Round Bar Stock and Square Stock on the Same 5-Axis Trunnion Setup?

Use a modular fixture plate on the trunnion table with interchangeable jaw sets: a self-centering vise jaw for the round stock, standard parallel jaws for the square. Don't go looking for one workholding device that grips both shapes equally well. The trunnion itself doesn't care what shape you're holding. The jaws do all the work, and swapping jaws is faster and more repeatable than compromising on a universal fixture.

Why one fixture rarely does both jobs well

A vise built to grip square stock clamps on two flat faces and relies on those faces staying parallel to get consistent clamping pressure. Round stock against the same jaws only contacts along a line, so clamping force concentrates instead of distributing, and the part can rotate slightly under cutting load even when it looks tight.

Going the other way, a chuck or self-centering vise built for round stock will grip square stock off-center or on two corners instead of two faces. That throws off your zero and can mar the part. Trying to make one setup serve both shapes usually means neither one is held as well as it could be.

What a subplate system buys you

A subplate mounts once to the trunnion and stays there, with quick-change jaws or modules bolting to it. It separates the "which shape am I holding" problem from the "is my zero repeatable" problem. You dial in the subplate's position on the trunnion once, and every jaw set that bolts to it inherits that same known location. Swap from square jaws to a self-centering round-stock jaw and you're not re-touching off the whole coordinate system, just verifying the new jaw set landed where it's supposed to.

This matters more on a trunnion than a 3-axis table. A trunnion's rotational axes make a workholding error compound as the table tilts. A part that's slightly off center in a chuck reads fine in a straight-on view and swings visibly out of position the moment the trunnion rotates 45 degrees.

Get the fixture's zero locked down once, on a subplate that doesn't move between jobs, and that source of error goes away.

Picking the actual jaw hardware

For round bar under a few inches in diameter, a self-centering vise or a modular V-block pair gets the part on axis without a full chuck. Larger round stock, or anything that needs to index rotationally, is better served by an actual chuck mounted to the subplate. Square and rectangular stock stays with standard parallel jaws, soft jaws machined to the part's profile for a repeat job, or a fixture plate with dowel-located stops if the run is long enough to justify a dedicated jig.

The setup that works in production: one subplate that lives on the trunnion, a library of jaw sets bolting to known locations on it, and a five minute changeover between round and square jobs. No full re-fixture and re-zero every time the stock shape changes.