What Would Make This Pattern On The Lathe?

Paul likely has the root cause identified. If you were watching in slow motion and under magnification, you would probably see chips "welding" and building up on the tip. When the cutting stresses are enough, that little blob of metal gets knocked off and any pushoff you were seeing is instantly removed as the tool digs in for a revolution or two. Then the process repeats itself, at a sort of random interval. It's part of dealing with that particular material. You could use some of the suggestions here to limit this effect, but unless you have enough machine (and trust) to make a pretty hefty cut at high speed/feed with coolant and carbide, you will find it difficult to impossible to machine a nice finish.

btw, are you sure about the material? From here it doesn't look like hot rolled material. And I rarely see people running HRS in a collet. Generally it's not round enough to hold properly. Looks more like CRS to me, but that could be my eyesight.
 
Dan_S said:
I'd agree with Gadget, if you are going to use hss/cobalt you will most likely get a better finish with a vertical shear tool.

If you use carbide and run close to the maximum sfpm, with feed & doc both over 0.010" you can get an almost polished finish, but the chips will becoming off blue/purple and the carriage will be moving fast enough to make you uncomfortable.
Click to expand...

If you are moving fast and the chips are blue, that can be a burn hazard. I try to keep the part well covered with coolant or cutting oil. NOTHING like trying to watch your cut and have a hot chip land on the back of your hand.
 
Your pic looks pretty normal for 1018crs or hot rolled steel without coolant or oil.These steels are notorius for being gummy and sticking to the tooltip thus cutting a groove,,then when the build up gets long and hot from friction as it is not cutting but wearing,,it breaks off. etc., etc., down entire lenght of your part.I machined many years before carbide inserts were in wide use. For these steels we used hss toolbits hand ground with proper clearance angles . A small radius stoned on helps hss bits last longer between sharpenings. With a good cutting oil or coolant and proper speeds and feeds smooth finishes were obtained. This method of lathe work was actually CUTTING the metal. I am now 71. My lathe a 15 x 54 LeBlond. My tool holders are drop-on quick change. I use some carbide and a lot of hss. Depends what I am doing. Stihl brand chainsaw bar oil is produced to cling to hot, fast moving steel. Best all round cutting oil in my shop. Among other jobs I worked for Leblond Lathe as a field service technician fo 10yrs. Good luck to you
 
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