Trouble getting good finish when turning

gearhead

gearhead

Active User
Registered
Joined
Jan 5, 2012
Messages
62
The short of it is that I'm brand new to machining, just starting on my "new" Atlas 3996 12"x36" that I got a few months ago.

Problem: In turning, I consistently am getting a less than desirable finish on the part. You can feel the ridges in it. Sometimes it looks smeared, and sometimes it just looks like a million tiny threads rather than a nice, smooth finish that I'd like on a final pass. Faster speeds with light cuts provide better results, the sort of "fine thread" look, but still not up to par, and that's with the lathe at it's quickest speed taking a very slight cut and feeding very, very slowly by hand. Slower spindle speeds give the "melted" or "smeared" look to the cut, faster spindle speeds give the fine thread look.

I am using HSS bits that I'm practicing sharpening. There may well be something to that in the issue. I have ground a small radius on the tip, and that seemed to help a little, but still issues.

The tool is set so that it is just slightly below the center line of the work piece. There is no chatter or noise from the bit, and the chip color is good, no bluing or anything like that.

I hope I've given enough information, though I know there are many variables.

By the way, I give not thanks to the goofball who first put the machine together (under drive bench) back in the 70s...because he installed all the pulleys backwards. Yes, figuring out why the spindle speed never seemed remotely correct was tough!
 
Feed rate too high? Reduce feed rate, increase speed. What metal are you turning?

Sent from my Nexus 10 using Tapatalk
 
Like Silverbullet said, you want a radius on the tip. More radius, mo better :tranquility:
Actually for finishing you want a nice radius to blend the steps in.you have to realize you are moving over x thousands per rev. So a sharp point will cut at that point. And it will look like a minor diameter of a thread. Smooth the radius out and it starts to take away that V and now you have smooth jazz ...
 
gearhead said:
The tool is set so that it is just slightly below the center line of the work piece.
Click to expand...

I suggest you put the tip of the tool dead on center height. A nose radius of 1/64 - 1/32" is enough to finish well. Angle the tip of the tool more toward the tailstock (larger lead angle) and it will finish better.

It would help to see a pic of your tool and your set up.
 
Thanks for all the suggestions! I am feeding it as slow as it can possibly go (the auto feed is too fast, so I'm feeding it by hand, and I do get better results doing so) and using cutting fluid. The tool is maybe a bit below center. I have an old turret tool post, and I've reached my limit with shims for bringing it higher. Not sure what I can do about that. I'll see what I can grind out for a radius on the tool. I wondered if that was having an effect. I'm turning into a shoulder, so I can't use a very large radius.
 
It depends on 4 things.
  • Diameter of work piece
  • Sharp tool bit
  • Feed-rate (IPM)
  • RPM
I can accurately cut to a diameter, but getting a quality finish each time is not my strong suit. Like most machining, many factors come into play. In my experience, operating at a little above the calculated RPM and slowing down the feed rate has helped me with finish quality. For example, if you've calculated the RPM at 436, run it at like 525 and try a .004 feed rate.
 
If power feed too much change gearing to improve it.

Best finish is with smaller feed.

Just remember the "cut" looks exactly like the tip of your cutter.

And say the tip of your cutter is 1/32 inch radius and you are feeding greater than that rate then you will be cutting a groove instead of a flat.

Depending on many other variables the tool could cut at higher rate of feed and still come out well but the lathe in use may not be suitable to pull that off so the cutter gets pushed away from the work and leaves some behind.

More practice on shaping your bit may improve the results but hand feeding even on our L&S 16 X 54 does not work as well as power feed unless we are very carefull.

Sent from my SAMSUNG-SGH-I337Z using Tapatalk
 
gearhead, You've already been given some very good answers, but at this point they are all just guesses. You can't expect to get any real help without giving us some more info. Make and size of your lathe, what is the slowest feed rate, please explain why you can't raise your tool on center, include a pic of your tool, and also of the tool mounted in the tool post, what material are you turning (some metals don't finish very well). Believe me, No one on this forum is going to criticize you, but you ARE on the right forum to get all your newbe questions answered. Good luck, and happy machining, JR49
 
gearhead said:
Thanks for all the suggestions! I am feeding it as slow as it can possibly go (the auto feed is too fast, so I'm feeding it by hand, and I do get better results doing so) and using cutting fluid. The tool is maybe a bit below center. I have an old turret tool post, and I've reached my limit with shims for bringing it higher. Not sure what I can do about that. I'll see what I can grind out for a radius on the tool. I wondered if that was having an effect. I'm turning into a shoulder, so I can't use a very large radius.
Click to expand...
Auto feed too fast??? Are you using the power feed or the half nuts?
 
You must log in or register to reply here.
Back
Top