turning unwanted tapers

[big job]

And then some smaller machines like my 9A SB just dont like carbide, it wants to act
more like a parting tool. I know this sounds way out there, but, if you have a quick
change tool post/holder, stick a HSS twist drill in it. Its really nothing new, for years
twist drills were used with shapers. Use the fattest that will go in there keep it pretty
well choked up, and a small flat stock on the top to tighten it. Then you (turn) its
cutting edge to "dial it in". Then try it with a piece of 'junk' you will get the idea to
dial it in.

[uncle pete]

One more thing to try? Raise the feed per rpm. Yeah I know it sounds counter intuitive for a finish cut, but sometimes upping the feed per rev gives you a better surface finish. Your carbide type, tip radius, depth of cut, etc. could or may be right on the verge of starting to chatter. At a micro level it may be already doing so. Increasing the feed loads the cutting tool better on the lighter depths of cut. It's worth a try anyway.

Pete

[loose nut]

Use a tool-post grinder with an aluminium oxide wheel - but see that the wheel doesn't "clog up" and keep it well "dressed".

Good idea, if only I had one.

[vpt]

Personally I use an assortment of files to finish off projects all the time. With some parts I will turn down to within .001-4 of the final finish and then file in the rest. One stroke of my bigger file will take SS down about .0005, the smaller files obviously less. A nice clean file can leave a near mirror finish if done right.

[Mcgyver]

If you are using the tailstock and tool pressure is the culprit then the taper should be "hour glass" shaped. .

hmmmm, maybe. as you go on to note "or its moving". I think that is more the case. Even on a half decent lathe, set a tenths indicator up and lean on the barrel....he only has to be out .0003" to get that taper.

Are you using a rotating centre; newish Rohm, beater offshore? The centre through its bearings can also allow some movement.
Ultimate accuracy is best with a dead centre.

I to would go with very sharp hss, oil and slow speed as a dodge. I suppose the full professional answer would be 1) make the set up as rigid as possible, 2) pick your final desired DOC, 3) through some trial cuts adjust the tailstock such that there is no taper at that DOC.

It should be better than you are getting, but keep in mind eliminating taper to a small number tenths over a long length is both difficult and not often required.

[vpt]

Maybe use a follow rest?

[loose nut]

Maybe I'm expecting to much??

[darryl]

Not expecting too much- you're blessed with a challenge, that of fine-tuning your expertise in machining

[rohart]

I disagree with lakeside saying that if the taper is caused by tool pressure you'll get an hourglass shape. If the work bends away from the tool midway, you'll get a bobbin shape, wider in the middle, because the work bends away from the tool resuling in less removal, and a thicker final product.

Otherwise, I agree with all the above - HSS for finishing, or Jaako's suggestion of sharpening carbide with diamond.

But the idea of finding exactly the right DOC with HSS to remove the work-hardened layer but still get a finish sounds like the kind of work you'd if you were in a production environment. It doesn't seem to fit the home shop.

If you have a carbide you can sharpen to a larger radius, you might be able to give it enough to do with a higher feed but lower DOC, but that's a technique that I haven't tried yet.

I have a carbide insert that's very picky - lower the feed a little and the finish is terrible. Up the feed and it's all shiny again. But often the shiny is an illusion, and it needs HSS or paper to get a polished finish.

Of course, grinding is the answer, SS or not.

[lakeside53]

lol.. you are right! ok, inverse hour-glass! Sorry, right idea but I wasn't thinking of the right word. Wonder if I got it right when I said my wife has an hour glass body?