Figured out my fourth axis setup for turned titanium parts - sharing what worked

I went through almost the same head-scratching a couple years back with a batch of aluminum spacers that needed slots indexed around the outside. What saved me was cutting the soft jaws in the rotary itself, but I bore a shallow pocket so the part sat at a known depth instead of flat against the jaw face. Every blank dropped into the same spot and I never had to re-touch off between parts, just once at the start of the run.

One thing I learned the hard way: torque the jaws the same way every time. I got lazy once and snugged them down differently than when I machined them, and the part sat a couple thou off center. You could see it in the finish on the slots. Now I tighten them the same order and the same feel every setup, and it's been repeatable since.

I also played around with letting the rotary spin through features versus re-clamping, and the spun version wins easily. Less chances to introduce error, and my tolerances between features stayed consistent across the whole batch. I was on an older Sherline rotary on a router table at the time, so if it worked with that gear it'll work with most rigs.

Another small habit that helps me is cutting a witness mark on the jaw and the blank so if anything shifts I can spot it fast before wasting material.

I recently ran into a similar setup with a high-tolerance titanium valve body that needed a central bore and multiple complex radial features cut. My issue wasn't indexing, but holding the part perfectly square while the boring operation was running. The standard rotation chucks felt a little loose under the pressure of the heavy milling cutter on the outside of the material.

Instead of just relying on the soft jaws for friction, I ended up building a quick fixture using two hardened steel clamps mounted around the base of the part. These clamps didn't touch the cutting face but held the main body of the titanium firmly in place against the vise jaws. It essentially locked down the whole assembly.

The benefit was keeping the micrometer readings rock steady during the initial offset setup. I know the other reply mentioned making a shallow pocket in the jaw, which is smart for consistent Z height, but for my specific job, adding that extra mechanical restraint seemed neccessary to eliminate any slight runout when I was boring deep into the material.

When running those heavy cuts in titanium, the issue often shifts from pure indexing accuracy to managing thermal expansion and material rigidity during the machining pass itself. I found that if the setup piece is machined out of a dense steel or even carbide, rather than aluminum, it acts as a much more stable thermal mass, reducing the likelihood of micro-shifts that ruin the tolerances you are trying to acheive. Furthermore, simply gripping the flange often doesn't provide sufficient lateral restraint; ensuring the clamping force acts against the axis of rotation, not just tangentially, prevents the part from slightly walking or vibrating during heavy radial boring operations.

I've been struggling with how the setup holds up when I cut the titanium. It seems like the whole thing shakes or jumps a little bit when the cutter gets going, and that makes the lines uneven. I thought maybe I should make the piece that holds the part much heavier to stop that wobble. Like, instead of a simple steel fixture, use something super solid? For someone who has done this before, what material do you use for the base setup to keep it totally steady while it's turning?

Quick update - I tried making the fixture heavier like one person suggested. It really cut down on the wobble when the tool engages. But now I'm worried about the surface finish being too rough in the valleys, even with the new weight. Does anyone have tips for optimizing feed rate on titanium while maintaining that stability?