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.

[Howard LewisParticipant @howardlewis46836]

As Dave Halford says, we need more information.

What machine are you using?

We assume that the workpiece is being held securely, and unable to shift.

Both work and cutting tool must be held securely

What is the material? HSS will have a job cutting hardened steel. You need carbide for that.

How does the lathe behave with different material?  It could be irregularities in the material.

Is the tool sharp?

What sort of tool is is?  A form tool, having a greater cutting edge might be more liable to chatter that a single point knife tool.

Is it at centre height?

Is it set to present just the cutting point to the work, rather than a long edge?

Are the gibs tight enough to prevent movement?

Whar depth of cut are you using?

What feed rate are you using?  If either of these is excessive for the tool/material combination, you may well have problems.

What cutting speed are you using? Too high a peripheral speed will shorten tool life, and cause problems.

Finding the answers to some of these questions may help you to find the solution, rather Forum members without the detailed knowledge that only you can have, being on site.

Howard

[Neil WyattModerator @neilwyatt]

If it’s ‘where the cut gets heavier’ this suggests you are trying to cut the taper all in one go or with a heavy depth of cut.

Try keeping the DOC quite modest and cutting the taper in several passes.

While the single pass is attractive if you need several matching tapers, you are better off zeroing the cross-slide dial at the ‘finished’ setting and using the cross-slide to apply in-feed. A;ways finish with a second pass to take out any spring in the system.

Neil

[John HinkleyParticipant @johnhinkley26699]

Neil,

One of us has misunderstood the OP’s post. I read it as the lathe cutting an unwanted taper whereas your reply assumes that he’s trying to cut a taper. I have just reread the original post and I am still confused, though now tending towards the intended taper interpretation . As so many have said, more information is required before any meaningful suggestions can be made.  If I were to guess, I’d say low tool height and/or a long unsupported workpiece.

John

 

[alecsParticipant @alecs]

The AWOL OP mentioned bed alignment, leading me to assume his taper was non intentional.

 

[Nicholas FarrParticipant @nicholasfarr14254]

Hi Lee, definitely need more info, like diameter and length of material, and if you are using a centre or a steady to support what you are cutting.

Regards Nick.

[Martin ConnellyParticipant @martinconnelly55370]

Sloppy spindle bearings letting the workpiece climb over the tool at some point as the forces change? Loose top slide being pushed down as the forces change? So many possibilities when everything needs to be well adjusted on a lathe.

Martin C

[BazParticipant @baz89810]

About time we heard from the OP, his lathe could be anything from a Cowells to a ten foot between centres Dean Smith and Grace, if he cannot be bothered to provide a bit more detail as requested why are you all wasting time speculating on what iffs and maybes.

[alecsParticipant @alecs]

Just another drive by.

[old martParticipant @oldmart]

I would cut tapers starting at the small end with relatively small depths of cut in several passes until the tool reaches the large end of the taper. That way the depth of cut stays about the same rather than increasing. Supporting the work to minimise any deflection helps. With tapers, it is important to set the tool height exactly on the centreline.

[speelwerkParticipant @speelwerk]

It seems the taper was a bit to aggressive since the OP is silent.

[alecsParticipant @alecs]

Crickets.

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