Trying to turn stainless steel taper.

[tcarrington]

FWIW, I see a relatively high length to diameter ratio on the shaft your are turning. That will increase the tendency to chatter. It will also limit the depth of cut you can achieve. The real trick here is getting the RPM up to near "entitlement" for stainless. From there your feed and depth of cut will probably favor a higher feed and small depth of cut. I would favor a carbide solution to allow a higher RPM. I will caution that it is entirely possible on a workpiece that looks as long and thin as yours to pull it up and over the tool.
But, be brave and send it. it is very easy to not remove enough material at a time and work harden it to near impossible state to machine.

 

[macardoso]

I didn't see the difference between CCMT and CCGT explained. The M stands for molded insert, the G stands for ground. The ground insert will have a sharper cutting edge than the molded one. This has nothing to do with the suitability for machining a particular material, that is determined by the grade of carbide.

While you are technically correct, I’d like to explain my reasoning.

The carbide grade refers to the particular mix of different carbides and binders and determines the hardness vs. toughness as well as temperature resistance. Grades are designed from the manufacturer for particular materials depending on their machining requirements (high heat, abrasiveness, hardness, etc.). I believe that coatings also get mixed into the grade for the same reasons as above.

But the grade is only part of the story. The edge prep, chipbreaker, insert geometry, and insert clamping all play a big role in cutting performance. Manufacturers typically pair up grades, edge preps, and chipbreakers for specific materials or even specific machining operations of one material. Unfortunately these are always done for large production CNC equipment, or are offered as generic geometries.

Here is my opinion based on my experience. The inserts made for specific materials seem to often cut best with more rigidity, surface footage, and horsepower than my lathe (12x36, 1.5HP, 1000 lb) can provide. Positive rake medium inserts barely break a chip correctly in many cases before my lathe chatters, and most inserts create massive cutting forces that make hitting tolerances difficult. I think this is why many people turn away from inserts and go to HSS.

Inserts are great in production CNC machines as there is a ton of rigidity to back them up, but on our smaller hobby machines the bluntness of the edge (great for tool life) limits the effectiveness of the insert tool.

This is where the xxGT or xxGX inserts come in. The ground and polished edge is super sharp and cuts with extremely low horsepower and cutting forces. They are comparable to sharp HSS tooling. Their tool life in nonferrous is excellent, but I’ve also found good use for them in medium alloy steel, stainless, and even cast iron. The inserts are extremely free cutting and produce a good surface finish even in tricky materials like 1018. They also allow a very fine minimum depth of cut (only a few tenths) without rubbing. This allows you to turn precision tolerances on parts.

I’ll admit the tool life in ferrous materials is not great, but at $0.50 per edge, they are very affordable and I reach for them every time I have a critical dimension or need a good finish. When roughing is required, I always choose a tougher insert, ideally one designed for my material if I can.

 

[FTlatheworks]

While you are technically correct, I’d like to explain my reasoning.

The carbide grade refers to the particular mix of different carbides and binders and determines the hardness vs. toughness as well as temperature resistance. Grades are designed from the manufacturer for particular materials depending on their machining requirements (high heat, abrasiveness, hardness, etc.). I believe that coatings also get mixed into the grade for the same reasons as above.

But the grade is only part of the story. The edge prep, chipbreaker, insert geometry, and insert clamping all play a big role in cutting performance. Manufacturers typically pair up grades, edge preps, and chipbreakers for specific materials or even specific machining operations of one material. Unfortunately these are always done for large production CNC equipment, or are offered as generic geometries.

Here is my opinion based on my experience. The inserts made for specific materials seem to often cut best with more rigidity, surface footage, and horsepower than my lathe (12x36, 1.5HP, 1000 lb) can provide. Positive rake medium inserts barely break a chip correctly in many cases before my lathe chatters, and most inserts create massive cutting forces that make hitting tolerances difficult. I think this is why many people turn away from inserts and go to HSS.

Inserts are great in production CNC machines as there is a ton of rigidity to back them up, but on our smaller hobby machines the bluntness of the edge (great for tool life) limits the effectiveness of the insert tool.

This is where the xxGT or xxGX inserts come in. The ground and polished edge is super sharp and cuts with extremely low horsepower and cutting forces. They are comparable to sharp HSS tooling. Their tool life in nonferrous is excellent, but I’ve also found good use for them in medium alloy steel, stainless, and even cast iron. The inserts are extremely free cutting and produce a good surface finish even in tricky materials like 1018. They also allow a very fine minimum depth of cut (only a few tenths) without rubbing. This allows you to turn precision tolerances on parts.

I’ll admit the tool life in ferrous materials is not great, but at $0.50 per edge, they are very affordable and I reach for them every time I have a critical dimension or need a good finish. When roughing is required, I always choose a tougher insert, ideally one designed for my material if I can.

I use them for thou doc for finishing, but it still leaves lines, no matter what shank I use. How do you achieve excellent finishes with them? What shank?

I’m afraid with my lack of experience I don’t know what can be achieved with tooling on the lathe, or sandpaper and file.

 

[mikey]

I just wanted to chime in to clarify something. The aforementioned CCMT and CCGT inserts are both available as molded inserts. What seems to be confusing folks is that CCGT inserts also come with an AK chipbreaker; inserts with an AK chipbreaker are typically ground with sharp edges, are uncoated, have a very positive rake and are best suited for aluminum and non-ferrous materials. The molded CCGT inserts do work well with stainless but these inserts are typically available with a different chipbreaker morphology. So, when you see someone referring to "ground sharp edges with high positive rake" on a CCGT insert they are referring to an insert with an AK chipbreaker. The grade of the insert is something else entirely.

 

[macardoso]

I just wanted to chime in to clarify something. The aforementioned CCMT and CCGT inserts are both available as molded inserts. What seems to be confusing folks is that CCGT inserts also come with an AK chipbreaker; inserts with an AK chipbreaker are typically ground with sharp edges, are uncoated, have a very positive rake and are best suited for aluminum and non-ferrous materials. The molded CCGT inserts do work well with stainless but these inserts are typically available with a different chipbreaker morphology. So, when you see someone referring to "ground sharp edges with high positive rake" on a CCGT insert they are referring to an insert with an AK chipbreaker. The grade of the insert is something else entirely.

Thanks Mikey - you are correct.

 

[macardoso]

I use them for thou doc for finishing, but it still leaves lines, no matter what shank I use. How do you achieve excellent finishes with them? What shank?

I’m afraid with my lack of experience I don’t know what can be achieved with tooling on the lathe, or sandpaper and file.

I use them with a SCLCR 1/2" shank. It should not matter much. Any insert will leave lines in tough to finish materials. Higher speed and a very steady feed rate (power feed) seem to do the most. If the feed is unsteady, the insert will cut deeper when the tool is feeding slowly, and deflect more when feeding quickly.

 

[FTlatheworks]

I use them with a SCLCR 1/2" shank. It should not matter much. Any insert will leave lines in tough to finish materials. Higher speed and a very steady feed rate (power feed) seem to do the most. If the feed is unsteady, the insert will cut deeper when the tool is feeding slowly, and deflect more when feeding quickly.

I’m still learning, so I apologize, but I mean angle provided by the type of shank. Does this make sense? Feel free to tell me what’s wrong about my terminology. It’s either run my lathe, work, and school, or read about lathes, work, and school.

 

[macardoso]

I’m still learning, so I apologize, but I mean angle provided by the type of shank. Does this make sense? Feel free to tell me what’s wrong about my terminology. It’s either run my lathe, work, and school, or read about lathes, work, and school.

Well the inclination of the insert (Rake) into the cut is defined by the insert rake and holder. The CCMT/CCGT inserts are positive rake so the top of the insert is flat (parallel) to the floor and the cutting edge is provided by the rake ground into the insert. Negative rake inserts require holders that tip the insert into the work since the insert has no rake built into it.

The second angle is the angle of the holder relative to the work. With insert tooling, I personally keep the toolpost square to the work (and thus the tool holder square to the work) as all the clearances are built into the holder geometry in this configuration. Special cases may force you to rotate the tool post.

Finally the last angle is the included angle of the insert. CCMT/CCGX is an 80 degree diamond. VCMT/VCGT is a 35 degree diamond (different holder). The smaller the included angle, the lower the cutting forces, the lower the likelihood of chatter, and the weaker the insert.

Others have other opinions about all this, but this is what works for me!

 

[FTlatheworks]

Hopefully this post hasn’t been dead too long. Can anyone help out with the turning long tapers. I keep getting horrible chatter and finishes. I also keep bending the part trying to part it off. It is a 1.5 degree taper 5-7 inches long.

 

[FTlatheworks]

FWIW, I see a relatively high length to diameter ratio on the shaft your are turning. That will increase the tendency to chatter. It will also limit the depth of cut you can achieve. The real trick here is getting the RPM up to near "entitlement" for stainless. From there your feed and depth of cut will probably favor a higher feed and small depth of cut. I would favor a carbide solution to allow a higher RPM. I will caution that it is entirely possible on a workpiece that looks as long and thin as yours to pull it up and over the tool.
But, be brave and send it. it is very easy to not remove enough material at a time and work harden it to near impossible state to machine.

Is there a better way to turn tapers than how I am doing it? I feel like my stock being on two ball bearings is the problem.