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Lathe Cutting a Taper When Using Carbide Tooling

P

pdu

Plastic
Joined
May 18, 2020
Hello Everyone,

I have a new 1440 lathe, which cuts a taper when using carbide tooling. I am using TNMG carbide inserts with a 0.8mm tip radius. I am locking the cross slide and compound slides when making the cut using power feed. I checked with a fresh insert and took light cuts of about 0.1mm and it is still doing it. The taper is 0.02mm over 70mm. If I use a nice sharp HSS tool I get virtually no taper. I thought that it may be the spindle bearings but I get a nice shiny finish and when I push and pull on the bar the chuck only deflects about 0.005mm. I am using a 30mm bar for the test.

My question is this normal for carbide tools to push a 30mm bar that much that I get a taper or do I have a problem somewhere?
 
Last edited:
You answered your own question - sharp HSS, no taper, carbide gives you a taper.

"Sharp" is the critical issue, if you look at most carbide inserts, all but those meant for plastics and aluminum or other soft non-ferrous materials, you'll see that they're all purposely "dulled" by some amount, with a corner break or radius on the cutting edge.

If you don't have a very rigid lathe, or are working with small diameter stock, this dulling causes a pushing-away between edge and part, leading to tapers, bad finishes, chatter, etc.

You need to learn to find the right insert for the job, or change to a sharp insert before a finish pass.
 
Milland is correct. Carbide tooling generates much more tool pressure than HSS. Carbide also likes more speed than HSS. Carbide tooling is more expensive to use than HSS tooling. Carbide tooling requires more horsepower and machine stiffness than HSS tooling. Carbide tooling is great for production work. You have a manual machine so you are probably not doing production work. So why are you using carbide? Today's HSS tool alloys are much better than what was used years ago. Modern cobalt HSS is very tough and stays sharp for a very long time.

My best guess is you don't know how to grind HSS tools or you don't have the right equipment to grind HSS. If you must use Carbide, you need to find more stiffness in your setup. If your new machine is a Chinese special, that stiffness may be elusive.
 
pdu said:
Hello Everyone,

I have a new 1440 lathe, which cuts a taper when using carbide tooling. I am using TNMG carbide inserts with a 0.8mm tip radius. I am locking the cross slide and compound slides when making the cut using power feed. I checked with a fresh insert and took light cuts of about 0.1mm and it is still doing it. The taper is 0.02mm over 70mm. If I use a nice sharp HSS tool I get virtually no taper. I thought that it may be the spindle bearings but I get a nice shiny finish and when I push and pull on the bar the chuck only deflects about 0.005mm. I am using a 30mm bar for the test.

My question is this normal for carbide tools to push a 30mm bar that much that I get a taper or do I have a problem somewhere?
Click to expand...


IME it can be, what type of chipbreaker & edge prep is on your TNMG?

IMO an Asian type 1440 lathe can still be a bit light for some of the negative rake inserts.
 
Another example of why I seldom bother to look at a youtube link.
This one, [HOW TO MAKE A TREPANNING TOOL. - YouTube]. I click the link and for the first several minutes I see a boring op then a simple drilling op, So I quit. NOT one mention or look at a Trepanning operation. BAH
...lewie...
 
My advice is increase the rpm and reduce the depth of cut. A stiffer machine can take deeper cuts. Maybe the last few passes need to be light spring passes.
Bill D.
 
Lewie said:
Another example of why I seldom bother to look at a youtube link.
This one, [HOW TO MAKE A TREPANNING TOOL. - YouTube]. I click the link and for the first several minutes I see a boring op then a simple drilling op, So I quit. NOT one mention or look at a Trepanning operation. BAH
...lewie...
Click to expand...

Your loss. He's not a entertainer, he's a great machinist being kind enough to document a series of tricks and methods for doing a rather arcane aspect of our field. It's stuff you could try to figure out on your own for years and never get, and he shows you in one video.
 
Lewie said:
Another example of why I seldom bother to look at a youtube link.
This one, [HOW TO MAKE A TREPANNING TOOL. - YouTube]. I click the link and for the first several minutes I see a boring op then a simple drilling op, So I quit. NOT one mention or look at a Trepanning operation. BAH
...lewie...
Click to expand...

Try looking at this one Nimonic Alloy. what's the cost 🤔 to help a friend 🍺 - YouTube it's trepanning from the off.
 
pdu said:
Hello Everyone,

I have a new 1440 lathe, which cuts a taper when using carbide tooling. I am using TNMG carbide inserts with a 0.8mm tip radius. I am locking the cross slide and compound slides when making the cut using power feed. I checked with a fresh insert and took light cuts of about 0.1mm and it is still doing it. The taper is 0.02mm over 70mm. If I use a nice sharp HSS tool I get virtually no taper. I thought that it may be the spindle bearings but I get a nice shiny finish and when I push and pull on the bar the chuck only deflects about 0.005mm. I am using a 30mm bar for the test.

My question is this normal for carbide tools to push a 30mm bar that much that I get a taper or do I have a problem somewhere?
Click to expand...

I suppose a fella could do lots worse than to mount up an indicator so it runs against the ways as the cut progresses. Perhaps a second one that runs along the work.

It'd give you a solid visual indication of what is moving, anyways.

I really like the xxGT inserts for light duty lathes. Stupid sharp edge, like, slash your hand open and swear a lot, sharp.
Intended for aluminum and plastics, they work well enough on steel, unless you are in the habit of bumping the edge against the work to set zero. It takes very little of that to chip off the edge!
 
Negative rake insert and you are advising him to take lighter passes? Maybe if it rubs on the end of the insert, it will burnish the part.

As stated by others, negative rake and light lathes do not get along. And I'm guessing a shaft unsupported at the tailstock end, probably sticking out of the chuck a ways.
Of course it is cutting a taper.
OP, you need to learn to grind HSS, if for nothing else, the education you will get on what works, what doesn't, and why.
 
OP What diameter is the material? Less than 20MM try using a center in the tail stock. Good rule to go by ,any more than 4 times diameter use a center. Does your insert have a chip breaker? If not try a insert with one. I assume your taper is smaller closer to the chuck. If so that means cutting pressure is causing deflection at the un-supported end. I recently bought this set and they work well on mild steel,cheap too.
4pcs 12mm Lathe Turning Tool Holder Boring Bar + 10 DCMT070204 Carbide Inserts | eBay
I would think you could find the same in AU.
 
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