I have a piece of rough round aluminum stock that I want to insert into the 3 jaw chuck. The stock is kind of rough, do I insert it all the way into the chuck until it firmly touches the end? It spins kind of wobbly. What can I do to correct it?
Shim it, indicated it, or center drill the end and support it with the tailstock center. What is the diameter and length. If it is not to long I would probably indicate it true by tightening and tapping a little at a time, then lightly face the tail stock end, skim cut the OD for at least the jaw depth, then reverse it and grab on the skim cut diameter with the cut face bottoming on the chuck end. This will give you a much tighter chucking.
You don't mention the ratio of diameter to length. If the length is at least as much as the diameter (assuming a diameter large enough to require you to reverse the chuck jaws to grip it), here is what I would do. (And what I *have* done, with a 6" diameter by 6" long workpiece on a 12" swing lathe.
What *I* would do is to insert it most of the way in, snug it up lightly, and check runout at the free end. Tap it a bit as it slowly rotates, until it is pretty close to true. then center-drill the end, and put in a live center to support it.
Once that is ready, slack the chuck a little, use the tailstock live center to push it deeper into the chuck, and re-snug it firmly.
Once you are there, up the speed a bit, and face the end, from the outside at leat as far in as the jaw steps would contact. I would probably go as close to center as I could manage with the live center in place.
Also -- Turn the OD down enough to make it smooth all the way around, for at least enough length so the chuck jaw steps will grip on a nice cylindrical and centered surface.
Then, once it is faced cleanly, reverse the workpiece, pushing the faced against the chuck jaws, and tighten. Check runout again at slow speed, and adjust that if necessary until true. (If you turned the OD as well, you will probably be pretty close without tweaking.
Center drill that end, and bring the live center in again to support the free end.
Face that end as close to center as you can get, then back out the live center and finish to full center. Redrill the center hole if you've taken too much off the length. Replace the live center, and use it to support while you turn rest of the diameter clean.
If you need the full diameter to be a single clean turning, what I would do to accomplish this is to drill a hole into one end off-center far enough to line up with a faceplate slot, and thread a stud into the workpiece to drive it by the faceplate while holding it between centers. That way, you could finish the OD full length in a single pass. (Assuming that it is small enough in diameter to pass over the carriage on a Sherline.) Of course, this is a bit of a problem if you can't allow a hole in one end for the stud.