I’ve been bouncing between 3D printed fixtures and machined aluminum fixtures for first-article parts. Last night at 8:30 in the garage, I sketched a soft-jaw in Onshape in 18 minutes and spun a PETG jaw on the Prusa MK3S — 0.2 mm layers, done in 1 hour 40 minutes. Compare that to cutting a 6061 block on the Tormach 770M: 2–3 hours including setup and probing, plus $28 of stock. Printed won on speed and design feedback.
I use printed jaws to prove geometry the same night — CF‑nylon, 6 perimeters, about 60% infill, and pressed 3 mm dowel pins hold about ±0.1 mm and tolerate a little coolant — but PETG creeps if you leave it clamped, so I cut 6061 for real torque or tight repeats the next day. Have you tried CF‑nylon or adding pins for locates?
I print PETG jaws 0.5 mm oversize and kiss the clamping faces in the mill by about 0.2 mm so they’re dead true to the spindle. Two 3 mm dowel pins into the vise keep them from creeping — plenty good for first-articles before I commit to aluminum.