I'm machining some small (1" x 1.5") aluminum brackets and want these to be cosmetically nice. IOW, I'd like the machined surface to be unmarked by clamp or vise marks. How can clamp these things in a vise so that they won't get marked by the jaws and stay put while I'm cutting them? Soft (aluminum, brass,) jaws or spacers of some sort?
The marks left aren't hugely obvious but I'd like to get as close to perfection as I - and my limited skills - can.
The usual soft metals or firm plastics in thin strips. Aluminum from old instrument cases, the kind that has pebble-grained vinyl bonded to the panels. Automotive wet/dry sand paper, with the plain paper side against the workpiece, and then cleaning the abrasive particles off the vise when done. Fibreglas tape or circuit board stock, foil tape, heavy paper stock such as file folder paper.
I'm wondering what might be the problem with the hardened, steel jaws in your machine vise. Assuming that is what your have, that is. Are the parts an odd shape or very delicate? I often use alum soft jaws cut to the desired shape to hold the workpiece. The main thing in not marking up your parts is to keep everything CLEAN. That doesn't mean just blowing air, though that is often all that is needed. You must have the part and the jaws absolutely clean, no residue. Also, a vise can generate a lot of clamping force, don't be ham-handed. There is a time and place for leaning on the handle, and for not leaning on it. If your jaws have irregularities, that may mean there are depressed and protruding deformities. Use an Arkansas stone to rub out high spots. If you don't have one, or one in good shape, use a tool bit (lathe) and wrap some fine emory paper around it.
Good luck with the cosmetics, a little care and you should get nice results.
In bench vises, I've used leather backed by plywood, sections of small extruded aluminum angle, chunks of rubber from heavy truck mud flaps and some magnet-backed nylon jaw faces made for the task. In a machine vise, I haven't used anything. I suppose some copper sheet shims, if clean on both sides, would work. Any of the things I've used in a bench vise might let your piece wiggle around too much at the wrong time in a mill. If your machine vise jaws are clean and smooth, there shouldn't be much for marks left on the workpiece.