Lathe Insert IC Relationship to HP/DOC

Had a question for the indexable experts here.

Does solely the size (I am talk IC size) of an insert have an inherent bearing on the cutting requirements of the insert?

For instance, two inserts:

CCMT431
CCMT321

(the difference in thickness is only because I am not aware of any CCMT331 inserts)

If price was equal, and the DOC did not exceed 3/16", would there be any reason to use the 3/8" IC insert over the 1/2"? Does chipbreaker/feedrate/DOC optimal geometry change with IC size? Perhaps a more simple way of stating: If a low HP lathe works well using a CCMT321, a .050" cut, and a .010" feed, would you expect the same performance from a comparable CCMT431 on that same lathe? Or, would the CCMT431, even with the same nose radius, generally have a top geometry that would require a deeper cut, or a faster feedrate to get the same surface finish and chip generation quality? Further yet, perhaps this "matched" performance is possible, but do manufacturers simply assume that a 1/2" bit will be used in heavier operations, and therefore make the cutting surfaces accordingly?

I have a lower HP machine that would likely work well with some of the smaller positive geometry tooling. However, I have a reason to stick with (physically) larger tooling that I currently use with a larger higher HP machine. What I don't want to do, is end up with inserts that are too big (IC size) to make some of the more modest cuts I plan to make on the this machine. If I need to go to a 3/8" or even 1/4" IC insert, the deal is done. If I can stay with 1/2" IC and still get the smaller cuts to work equally well, I have reason to do so.

Thanks for the info.

Not really. If going from larger IC to smaller, as long as the insert is strong enough so that it doesn't crack/fracture from the force of the cut the smaller insert will work fine. Note in your example the larger IC insert is also thicker, which adds strength.

Going the other direction, as long as the nose radius is equal and the geometry of the top of the insert is the same there should be no difference in performance. The larger IC insert should perform just the same as the smaller one.

eKretz said:

Not really. If going from larger IC to smaller, as long as the insert is strong enough so that it doesn't crack/fracture from the force of the cut the smaller insert will work fine. Note in your example the larger IC insert is also thicker, which adds strength.

Going the other direction, as long as the nose radius is equal and the geometry of the top of the insert is the same there should be no difference in performance. The larger IC insert should perform just the same as the smaller one.

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I was thinking along the same lines. Thermal conductivity and mass might have some small effect if you push things enough far.

Finishing inserts are harder to find and generally more expensive in large/huge sizes but CCMT4 size should have still okay offerings.
Less finishers available in CNMG64 size..

eKretz said:

Going the other direction, as long as the nose radius is equal and the geometry of the top of the insert is the same there should be no difference in performance. The larger IC insert should perform just the same as the smaller one.

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I see where you are going, but that is exactly my question: Are the nose radius and top geometries (readily?) available in CCMT4xx the same ad those available for CCMT3xx, or even CCMT2xx? I am looking for inserts more agreeable to shallow finishing/low HP cuts.

MattiJ said:

Finishing inserts are harder to find and generally more expensive in large/huge sizes but CCMT4 size should have still okay offerings.
Less finishers available in CNMG64 size..

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This is what I was looking for! If finding these inserts is going to be difficult in a 1/2", would it be best to stay in a smaller size (3/8" or even 1/4") to have more available inserts for these light shallower cuts I know this smaller lathe will be making?

Let me validate my questions with a situation: I recently had a job turning 12L14 and boring 1045 to H6/h6 fit. I roughed the material with my WNMG432, but when it came to getting some of the last thousandths, few of my inserts worked well (as expected). The CCMT (3/8") in my boring bar fared the best and I hand fitted the remaining tenths. All the OD turning I finished with HSS to get a decent surface finish with such small cuts.

I would like to be able to take deep cuts with my existing larger holders, then do these .050" and less (down to "dust passes")cuts with an insert, particularly on harder materials. If I can do that with a CCMT 43x insert, great. If a CCMT 21.5x insert would simply fit the bill better and have more readily available insert geometries for this kind of operation, then I will accomodate.

For high finish and light cuts - look at Cermet as a opposed to carbide inserts,

Alternatively you can get HSS inserts Standard and Specialty Inserts

Gundraw said:

I see where you are going, but that is exactly my question: Are the nose radius and top geometries (readily?) available in CCMT4xx the same ad those available for CCMT3xx, or even CCMT2xx? I am looking for inserts more agreeable to shallow finishing/low HP cuts.

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In my experience, most are. When you get above 4xx in IC is when things start getting to be lower in options.

Gundraw said:

If price was equal ....

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Ah, but it is not equal, so you have this exactly backwards. You want to use the smallest insert you can, because it is much cheaper. When depth of cut gets to be too much for the insert to handle, then you go to the next bigger size.

I guess I am a little confused about what is actually being asked. First I thought it was whether the larger inserts could be used and work as well as smaller inserts because the OP already had a surplus of larger inserts and didn't want to buy smaller ones.

Then OP mentions the desire to use smaller inserts for lighter finishing cuts...

Yes there are different insert geometries available for roughing and finishing in 2xx 3xx and 4xx. Most are available in all three of those sizes readily.

If you have the choice, definitely go for smaller inserts for finishing operations, they are cheaper.

I keep thinking there was a ROT about max. depth of cut not to exceed 1/3 IC.

There is, but at most finishing depths of cut you could use a little more than a sliver and be good most of the time.

digger doug said:

I keep thinking there was a ROT about max. depth of cut not to exceed 1/3 IC.

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I don't know why that would be. I routinely ran .250-.300 DOC with 1/2 IC inserts.

Indexable drills use the entire side of the insert.

The smaller IC holders usually come with smaller shanks too. If the lathe is made for 1-1/4" shank tools, you probably won't find a 3/8" IC holder that doesn't have to be shimmed...

It's because of the inserts with through holes. For solid inserts I don't think it matters. If you're babying feedrate probably doesn't matter either. Same goes for smooth vs. interrupted cut etc.

guest said:

I don't know why that would be. I routinely ran .250-.300 DOC with 1/2 IC inserts.

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Sounds like you run like me .... I don't mind a feed of thirty or forty and plenty of speed but I've seen parts where the feed must be .125"/rev That'd probaly be hard on the smaller inserts ... I'd always heard half the ic as well.

eKretz said:

It's because of the inserts with through holes. For solid inserts I don't think it matters. If you're babying feedrate probably doesn't matter either. Same goes for smooth vs. interrupted cut etc.

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I was running TNMG432's, they all had a hole. I normally roughed at .012-.014 IPR, that's where I ran out of horsepower. If I dropped my DOC down to .200, I could bump up to .016/rev.

4340/EN30B

My Komet drills used a 1/2 IC trigon insert, they had a hole too- and used the entire side of the insert.

EmanuelGoldstein said:

Ah, but it is not equal, so you have this exactly backwards. You want to use the smallest insert you can, because it is much cheaper. When depth of cut gets to be too much for the insert to handle, then you go to the next bigger size.

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Dually noted. Just realize that sharing a consistent holder and size may be beneficial to me in other ways other than just the price of the inserts, especially considering the frequency of use for these finishing inserts. What I am trying to avoid is investing in shallow cutting 1/2" inserts that could be used in my existing holders, only to find they don't get down to the small takeoff amounts I want, then having to invest in a new holder and inserts.

eKretz said:

I guess I am a little confused about what is actually being asked. First I thought it was whether the larger inserts could be used and work as well as smaller inserts because the OP already had a surplus of larger inserts and didn't want to buy smaller ones.

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I appreciate all your feedback, let me clarify: I already have 1/2" IC holders and roughing/medium inserts that perform terribly with finishing cuts. I would like to setup another insert (and holder if necessary) style for shallow finishing passes: 1/2" would be nice as I could use the holders I already have (or another identical one), so I can share between machines and insert styles. However, if I will find the 1/2" IC will limit the selection/availability I have for these shallow finish passes, I need to move down to another size. Your comment above makes it sound like I can readily find 1/2" IC inserts for just as shallow of a cut as I could with 3/8" or 1/4" IC insert.

digger doug said:

I keep thinking there was a ROT about max. depth of cut not to exceed 1/3 IC.

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I have always heard 1/2 of IC and have never tried to exceed that. I have also never heard credible sources confirm this.


My intention for these new inserts would be a the smallest DOC possible when needed, often on one-off parts. Any hints on a insert style/chipbreaker design to achieve this, and a minimum DOC in which they can be run?

Gundraw said:

What I am trying to avoid is investing in shallow cutting 1/2" inserts that could be used in my existing holders, only to find they don't get down to the small takeoff amounts I want, then having to invest in a new holder and inserts.

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That doesn't have much (if anything ?) to do with the size of the insert. It has to do with the material and the geometry of the insert. For instance, you're going to play holy hell getting a shiny finish on 8620 with any light cut unless you kick the speed way up. You've got to treat the material differently and/or use different geometry if you want to take light finish cuts. Positive / negative, chipbreaker geometry, grade, tool nose radius, feed, speed, coating, that stuff will make a difference. But insert size doesn't matter.

Now watch Carbide Bob come along and tell me I'm an idiot

Gundraw said:

My intention for these new inserts would be a the smallest DOC possible when needed, often on one-off parts. Any hints on a insert style/chipbreaker design to achieve this, and a minimum DOC in which they can be run?

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Slow speed, small DOC, gummy material inserts have been covered in dozens of topics previously but here is some starting points:

-Any old fart here is going to recommend HSS
-Look for CCGT inserts. Usually sharper than CCMT even if its strictly speaking a tolerance designation. Typically also ground vs. molded(sintered).

Sumitomo T1500Z cermet grade would be one starter for light cuts but it's available only in smaller sizes
https://www.sumitomotool.com/fileadmin/user_upload/Blaetterkataloge/catalogs/E-125/pdf/complete.pdf
CCGT 09T301 RFX T1500Z should be razor-sharp cermet insert with higly positive rake angle. Dunno if anyone stocks them or if you need to order from Japan.

For my hobby shopping sharp aluminim specific inserts also work remarkably well. IE Korloy H01 grade with AK chip breaker.
AFAIK it's straight tungsten carbide grade and wears really fast on steel with normal carbide speeds. But works well when you can't archieve normal carbide insert finishing speeds.

EmanuelGoldstein said:

Now watch Carbide Bob come along and tell me I'm an idiot

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Hell....I can doo that ....

But, I understand, it works both ways.

Kennametal makes a good geometry for fine finish cuts, I think they call it "FF" geometry IIRC.