Bad Lathe Finish

[Derek13]

Often times i will experience a poor lathe finish when turning 01 steel... usually when turning smaller diameters. At my work we really only do lathe work for roughing out pins which are then ground, so really the finish/appearance/accuracy of the lateh work means nothing so long as you can finish grind it.

But i want to know how to do better on the lathe as i hate the finish i often get. As the pins are only usually 1.5 - 2.5 inches long, a tailstock isn't used.

The finish i often recieve looks like it has like a bunch of shiny spots on it and dull spots also... this is using carbide insert tooling turning at 1000rpm... it cant be a dull tool, since even when i change to a new insert i still often get a poor finish.

If any of you could tell me things that cause this and solutions i would be much obliged ... I will post more information if needed.

Thank you!

[Ian B]

Derek,

Could it be a centre height issue? If the tool's too high, its front face may be rubbing against the work. This would be particularly noticeable on small diameters.

Just a thought,

Ian

[tattoomike68]

((shiny spots on it and dull spots also))

sounds like slow feed rate , it cuts and it rubs then cuts some more.

I do most turning at .006" - .012" feed rate.

If you are cutting dry try a few drops of cutting oil.

[Daminer]

Stickout is "1.5 - 2.5" W/O a tailstock; What's the "small" diameter of the pin.....

More than a few diameters of stickout can bring work deflection, vibration and harmonics into play.....

Try a tailstock for support and rigidity, might make a difference.....

Good luck, Jim

[Davis In SC]

I have fought the same problem, on 0-1, S-7, L-6, etc... & this is on a like new Hardinge... Center height is very important... I like to grind a tool with lots of clearance, & run it just a tiny amount above center. This prevents thin work from trying to climb up on the tool. I have also found that a high-speed steel tool often cuts better than carbide does.

[Tinkerer]

"The finish i often recieve looks like it has like a bunch of shiny spots on it and dull spots"

When I run into this finish... it's usually caused by slight slippage of the work in to chuck. Try wiping the jaws with a rag before installing work. To see if this is your cause place a mark on the bar and line it up with the edge of the #1 jaw. Make some cuts if this finish starts to show stop the lathe and check the mark to see if it has moved. I have a habit of marking the work and aligning with the scribe line on my #1 jaw. That way if I have to remove the work I can more easily get it back into the chuck at the same position.

As the others have pointed out... could be height of tool and relief.

[japcas]

Derek, could you tell us what the o.d. of the pins you are cutting are and also what is the depth of cut that you are taking? Also what is the nose radius on the carbide insert you are using? You should be taking a depth of cut per side that is bigger than the nose radius of the tool to help prevent deflection. This puts the cutting pressure toward the chuck instead of away from the cutting tool.

[Derek13]

The OD of the pins are anywhere from 1/4" to 1" typically... most frequently landing in the 1/2" to 3/8" range.

Not sure on the nose radius... but we have two different types of inserts/holders... one a triangle insert and one a diamond.

I find if i take say a .020" cut, it will give a better finish... but it springs and deflects and i have to take a clean up pass at the same depth and this is usually (but not always) when the poor finish occurs...

Thank you all so far for your information! I hadn't thought of putting the tool slightly above center... i figured it was typical to run it slightly below if anything, but i will give that a try for sure.