Lathe cutting aggressive taper

[Lee KennedyParticipant @leekennedy29914]

Evening guys,

 

any advice on this?

my lathe is cutting a taper of 0.25mm over a 25mm distance.
approx half way along the cut there is a significant change in the finish and this is where the cut gets heavier. The finish changes from good to a horrible tearing. It also damages the cutting tool.

 

i didn’t even attempt to start looking at bed alignment etc. Retreated in defeat for the evening. Seems strange that there’s a definite point where the finish changes and the cut gets heavier.
I can’t detect any slop or excessive play anywhere. My thoughts are this is more a mechanical issue than an alignment issue.

 

thanks

Lee

[alecsParticipant @alecs]

Pictures of your set up and of the job and its surface finish are really needed to make any meaningful comments.

But it’s unlikely to be bed alignment for that severe a taper. Possibly material with a hard spot? Try a test with a different piece of material. Or it could be faulty tool bit. Try changing that too. Or it could be toolpost loose and moving. Or it could be a couple of dozen other things.

 

[Lee Kennedy]

On 13 June 2026 at 00:09 Lee Kennedy Said:

this is where the cut gets heavier. The finish changes from good to a horrible tearing. It also damages the cutting tool.

 

 

As Alec says, we really need more info. We don’t know what you’re trying to cut with what tool nor how you’re trying to cut it, except with a lathe, which isn’t much to go on.

[JasonBModerator @jasonb]

What diameter are you turning?

Could well be a tool height problem with work flexing away from the tool and cutting at the unsupported end, this would explain it being a larger diameter at the start of the cut

But as you get closer to the chuck the work can’t move so you are over/under ctr height and that will give a poor finish and damage the tool.

[HuubParticipant @huub]

I assume you cut using the top slide and not using the power feed.

If you cut using the top slide, than the top slide is not aligned properly, set at an angle or the gibs are not tightened evenly along the whole length of the gib. Try cutting using the power feed and check if it is cutting better.

If you cut using the power feed than check the gibs. I suspect the gibs are not tightened right (must be lose at 1 side). You must tighten the gib screws (top slide & cross slide) until you feel a very little drag. Than check if it still turns a taper without locking the gib screws.

Reduce the depth of cut by 50% and check if it gets better.

 

[Pete RimmerParticipant @peterimmer30576]

The first thing I would do is change material. Sudden changes in finish suggest that what you’re turning has a definite change in it’s condition, either by constitution or temper.

Actually, first of all move the part further into the chuck and go again. if the change of finish moves with the part, then it’s the material. If not, it’s the machine.

[alecsParticipant @alecs]

And if it’s not the material, a bellmouthed old chuck could be to blame. What type of lathe have you, and how old is it?

[Dave HalfordParticipant @davehalford22513]

Lee,

Unfortunately we don’t know,

Direction of cut.

stick out, tail stock support or not.

material, some is dire and won’t turn.

tool type.

 

If this is still your GH600 at least it won’t be wear or a bad chuck just yet.

[Author]

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