What Would Make This Pattern On The Lathe?

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Lynrob

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Today I was turning some 3/4" 1018 HSR using a tangential tool holder and a freshly ground "crobalt" tool. The finish was pretty good for HSR but when I looked at it closely I found randomly spaced rings several thousands deep. The finish actually looks pretty good until I try to sand it and then the rings are revealed. Any ideas of what would cause these rings? I thought maybe something was loose so I locked my compound but that didn't seem to help and the cross slide and tool holder both seem tight. I am pretty new at this and I'm not sure if this is normal or if something is wrong.

Sorry for the poor quality of the photo but it should give you an idea of the rings I'm talking about. I got these rings to show up better for the photo by sanding slightly. The rings are deep enough that I can't easily sand them out. I had to file them out before sanding and polishing. Thanks for any help you might be able to offer. If it matters, this is on a PM1236 lathe. The cut was done at about 700 RPM with a feed rate of 0.004"/rev and a 0.020" DOC (0.040" off the diameter). The cut seemed to go well with no straining or complaining.

ba7137fa579e562d400d8055403313d4.jpg


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It looks like you might have a problem with tool rigidity. Loose gibs on the cross feed or compound? Play in the crossfeed?
 
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Just had this happen to me today also. Found I was running the machine too slow. Once I shifted to a faster speed, the finish improved immensely.
 
You are probably not going to believe this as it flies in the face of everything you have been taught, but try setting the tool below center, a lot, maybe 0.030 or so, maybe even lower. Try different heights. Also sometimes this can be corrected by increasing the feed, 1018 HRS is a real bear to get a good finish on anyway.
 
It looks like the kind of results I normally get. I would try increasing the feed speed and maybe use a more rounded cutting bit.

Tim
 
The live center may be causing it, try with just a dead center or no center. . How does it look with a finish cut < .005"
 
That material is not likely produce a nice surface finish ever, it tears leaving the artifacts that you are seeing. Either use another material which will be more expensive and less easily purchased or live with the finish and polish it with abrasives afterwards.
Using flood coolant and turning up the speed and feeds will also help.
 
more feed, maybe more speed and larger nose radius would be my suggestion. Conversely you could slow the speed down, go with finest feed you have and use vertical shear tool for finishing taking cuts of .001" at a time. Nice article on the tool in Machinist Workshop 2 years ago.

michael
 
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