I'm not sure if you're asking for help/suggestions, but if you are, here are some ideas.
Make your setup as rigid as possible. That would mean taking as much slop out of the cross slide and compound ways as possible. If you haven't already, I would recommend removing the gibs on both control surfaces, hone them on a stone to remove any burrs, and check them for flatness. Assuming they are flat and free of burrs, I would then fit them and adjust them using a dial indicator to remove as much sponginess as possible and still have the surfaces free moving. Here is a link to another thread where one technique is documented to check the rigidity of the compound after adjusting the gibs.
Go here.
Looking at some of your surface finishes, I suspect you have some rigidity issues, which is why I mention the first point about the gibs. But you can improve rigidity by pulling in the compound so that the QCTP is sitting directly over the swivel-mount for the compound, and 90-degrees to the spindle axis.
Further along in that same thread, the problem he was having was lack of rigidity because the compound was not sitting flat on the cross slide. You can
see that here. Rigidity is king, so check out your setup with some dial indicators - who knows, you may have the same warped compound base that TX Cowdoc did.
I am not a fan of the type triangular insert you are using. It has a relatively weak cutting point. I would favor an 80* rhombus style insert and tool holder - CPMT style. But, if you want to use that triangle-shaped insert, I would pick one graded for use with tougher materials. Assuming the I.C. of your insert is 3/8", something
like this would be a better choice. I would also tweak the lead angle of the tool by rotating the QCTP 10° or so CCW from where you show it in your videos, so that the radiused tip is doing most of the work and the cutting forces are toward the chuck rather than across the spindle axis. With decent rigidity, shifting the lead angle to balance the tool forces toward the chuck, and a proper grade insert you should be able to peel of 0.100" per pass no problem and get close to mirror finish.
ddickey suggested a good post by Ray C on how to hit target diameters without trying to creep up on final size with tiny skim cuts. Creeping up is definitely asking to overshoot your target OD and leave questionable surface finish. I use a balanced cut method, somewhat similar to what Ray C is trying to convey but without all the math. It's fully described in much simpler terms
in this video, and I use the DRO instead of the dials. Using this technique on my PM1340, I can routinely hit target dimensions to within 2/10ths, and there is no way this is possible creeping up on the final OD with shallow finishing passes. Granted, I have a solid tool post which helps, but even with the compound, this method works. Watch Stefan's video to the end, and realize that the technique is even more accurate using a DRO. He is also demonstrating the technique on 4140 with a similar sized lathe, using the style of insert (CPMT) that I would pick for your material and machining condition.
Hope this helps.
