Aluminum tends to string like that with light cuts and slow feeds. This is due to the high ductility of the material; it bends but doesn't break unless encouraged to do so by either a chip breaker or taking a cut big enough to produce a really thick chip. In the latter case, a heavy cut and a faster feed rate produces chips instead of strings. I am not sure your lathe, with that tool, is capable of such a cut.
Cutting fluid really helps with chip evacuation, cutting temps and finishes. You still string but it won't pile up as badly on you. WD-40 works well for aluminum; just brush it on. Cutting fluid also reduces a Built Up Edge (BUE), which is that deposit you're finding on your tool. The mechanism behind how this occurs is somewhat controversial but you can look at it as a welding of the aluminum onto the tool. Accordingly, it can be really hard to get it off unless you grind it off. Trying to pick it off usually results in a chipped tool. Best to take a deeper cut, use some WD-40 and pick up your feed rate.
The reason that second tool seems to work better has to do with the lead angle of your tool. See the side of the tip where the cutting is taking place? That edge is called the side cutting edge - pretty imaginative name, eh? Okay, engineers are not known for originality.
Anyway, that side cutting edge is at an angle, called the side cutting edge angle. The angle of your tool's edge is canted back a bit more than the typical brazed tool and is actually angled similar to what a good HSS side cutting edge angle is. At this angle, the edge tends to cut or shear a chip rather than plow (like the typical brazed tool does) when you take a light cut. What also happens is that
the chip follows a path that is perpendicular to the side cutting edge of the tool so chip evacuation is a bit better. That is why the chips don't seem to pile up - the cutting forces run perpendicular to the side cutting edge and the chips will flow along that path, perpendicular to the angle of your side cutting edge. Make sense?
Don't get too hung up on this stuff for now. What I wanted you to know is that that side edge angle on the tool is NOT how a brazed tool is usually used but it will work better for you on a light lathe taking light cuts so use it.
The other thing to note is that the tool has a very sharp corner at the nose. That is, it has no nose radius and this is typical of brazed tools. It will work better and definitely finish better if you stone a tiny radius at the very tip. Just use a diamond stone and round the nose just a little. I think you will be surprised at how much better the tool both cuts and finishes.
So, my suggestions to you are:
- Get your tool on center height - get it precisely there, not close. I know other guys will tell you to go higher or lower or whatever but trust me, center height on a small lathe is where you should be.
- Sharpen the tool edges with a diamond stone. Follow the angle under the cutting edges (called the relief angles) and hone them evenly. Then flatten the top of the carbide insert, then put a tiny radius at the tip of the tool.
- Angle your tool (the one in the picture) slightly toward the chuck to rough and slightly toward the tailstock to finish. See what happens to your ability to take heavy and light cuts when you do this, and notice what happens to the finish.
- Use WD-40 for all cuts. It helps.
- Take bigger/deeper cuts to rough and lighter cuts to finish.
- Play, have fun but be observant. See what happens when you vary depth of cut, speed and feed and see if you can fine-tune things to get what you want.
- Learn to grind HSS tools. For your lathe, this will easily outperform those carbide tools.
I've been doing this for what seems like a very long time but I still enjoy watching a chip peel off exactly the way I want it to, and I still take pleasure seeing a fine finish produced on a piece that comes in on size. I no longer think much about what I'm doing to get these results but I can tell you that it is exactly what you're doing now - paying attention to tool angles, sharp edges, cutting fluids, speeds/feeds/depth of cut and so on. Basics, Sir, basics.