The only other thing that comes to mind is "slip stick". Richard King talks about this in his classes a little bit. This happens when the sliding surfaces are pretty much wrung together. And when they move, they jump a little in small "jumps". Usually it stops once the sliding member starts moving, but not always. I see that in the picture of the saddle vee and flat. The surfaces are flat smooth. This has to be broken down by scraping. And that vee is a bear to scrape on the Sheldon. I just did mine on the 15" I reconditioned recently. See if you can get some of the experts over on PM to tune in and see what they say.
Man. I need you to work on mine. The tool is high positive carbide insert thats designed to cut aluminum and brass. I'm also taking very light cuts and the cutting action is smooth as silk. I don't think i'm getting slip stick. I use top of the line way oil and the carriage slides like butter when the lead screw is out. no friction at all. I't also sits solid no noticeable rocking or movement. I also lubed the ways manually several times with more than enough way lub, and obviously after cleaning the blue off they are clean as can be so I don't think I am sticking. The way lube is Mobile Vacuoline 1409 which is formulated like the old stuff. This stuff is makes any surface its on almost frictionless. The tailstock glides with near zero resistance when unlocked. I would really like to have the surfaces scraped but I don't have the setup or knowledge yet to do the work. I also think it's to uniform for stick slip. Our big summit lathe suffered from this a years ago mostly due to the operator thinking it was full of way oil, but really the glass was just stained and it was bone dry. It did something similar but it was not uniform in any direction and in reverse (outward cut) it would dig in then smooth out.
Are you using a insert like a CCMT or a "C" style? If so, the shavings could be rubbing causing that pattern. Usually when that happens with that style insert, its much worse! Try a different turning tool and see if it goes away. And I mean a totally different style insert like a triangle or diamond. Better yet, a old piece if Rex 95 HSS.
I'm going to move your thread over to the "Questions and Answers" section for a while and see if we can get some more participation. And maybe gets some more "heads" involved and maybe someone else out there has experienced this and can tune in and offer suggestions.
It's a DPGT and it does it in titanium with CNMG and Brass with CNMG and Aluminum with CNMG, DPGT, HSS. The insert I'm using now roles the chip off without any problems. And the pattern is the same in all materials, even stainless. When I run stainless I use a very high dollar insert that cuts wonderful on the other machine, cuts fine on this one just leaves ridges.
I also have a set of Class 3 bearings on the way, so one way or another were going to find out eventually if thats it. I plan on exhausting everything before that, I would rather not loose the ones that are in there but. I am also looking into replacing the other bearing in the headstock the non-spindle bearings. Any chance they were class 3 or above. The book calls for some to be Timken and some to be New Departure, but no numbers. But I can see those without tearing down the headstock.
The "official" terms are coefficient of static friction and coefficient of dynamic friction. For most material pairs when dry, the former is noticeably larger than the latter. Lubricants both reduce them and bring the two closer together.
I had a similar vibration problem with a Nardini lathe. Found the motor sheave was loose and had damaged the motor shaft.
The loose sheave couldn't be felt until the belt tension was released.
The company didn't want to send the motor out to have the shaft repaired. I had to file it round, about 15 mill undersized, and bore a new sheave to fit.
It's surprising how close one can true a shaft by file. Had less than 1/2 thou. TIR, I think it was 1/4 thou TIR, when done.