# Triangle vs Diamond inserts



## nnam (Jan 28, 2020)

With triangle shape inserts, we can use 3 corners.  With diamond, we can only use 2, although it has 4 corners.
Those are obvious, but what make you choose which kind of inserts to use?  I recently bought a set of 4 insert holders using diamond shape.  It's kind of nice that they all use the same inserts, so I don't have to buy different kinds.  However, some set uses a mix of shapes.

Some kind souls care to enlighten me on this?

Example of 3 different types:  https://www.amazon.com/Excircle-Indexable-MTJNR1616H16-MWLNR1616H08-MCLNR1616H12/dp/B076WTPBVK

Same type of inserts, the one I had: https://www.amazon.com/gp/product/B07M6WNLGY  (this one shipped me a wrong size by the way)


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## Mitch Alsup (Jan 28, 2020)

There are tool holders that allow you to use the "other" two corners.

The std tools are labeled with 80 (degrees) the other tools are labeled 100 (degrees).

I use diamond in 80 and 100 tool holders to get 8 corners out of an insert. (CNMG)


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## llamatrails (Jan 28, 2020)

Different degree insert corners will get into different degree angled shoulders when cutting. 
Larger degree insert cutters can take heavier cutting forces for deeper cuts or harder materials.


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## darkzero (Jan 28, 2020)

Generally triangle shaped inserts require less HP. Diamond requires slightly more HP but is a stronger insert. Then there's positive vs neutral negative rake. Positive rake will perform better on smaller machines. Neutral negative rake require more HP & machine rigidity.

A common mistake IMO is people choose indexable tools based on how many cutting edges you get per insert. Those starting out will look at neutral negative inserts cause they can be flipped giving you twice the amount on cutting edges than a positive insert. But if they have a small lathe they'll quickly find out that the neutral inserts won't perform well. I too have made that mistake before.

Another common mistake is people will buy a tool cause it's cheap. Many times they don't research the inserts first & may find out the inserts are really expensive or obselete.

I choose insert shape & geometry base on performance, application & availability/cost, not based on how many cutting edges per insert. I'm no expert though, just a hobbyist, & I get by ok. I use diamond the most now, mostly positive but sometimes neutral negative. When I had a mini lathe I used triangle more but still used diamond as well.


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## benmychree (Jan 28, 2020)

darkzero said:


> Generally triangle shaped inserts require less HP. Diamond requires slightly more HP but is a stronger insert. Then there's positive vs neutral rake. Positive rake will perform better on smaller machines. Neutral rake require more HP & machine rigidity.
> 
> A common mistake IMO is people choose indexable tools based on how many cutting edges you get per insert. Those starting out will look at neutral inserts cause they can be flipped giving you twice the amount on cutting edges than a positive insert. But if they have a small lathe they'll quickly find out that the neutral inserts won't perform well.
> 
> ...


I quite agree with all that you say here, and I have voiced the same opinions previously, only thing, it is not NEUTRAL  rake, it is NEGATIVE rake one leans back, the other leans forward to the cut; NEGATIVE rake tools require much more horsepower and ridgidity than POSITIVE rake tools.


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## darkzero (Jan 28, 2020)

benmychree said:


> I quite agree with all that you say here, and I have voiced the same opinions previously, only thing, it is not NEUTRAL  rake, it is NEGATIVE rake one leans back, the other leans forward to the cut; NEGATIVE rake tools require much more horsepower and ridgidity than POSITIVE rake tools.



Yep, you are right. They are technically negative tools. Don't know why I said neutral.... I was just looking at another thread & they were talking about neutral inserts vs positive. Thanks for the correction.


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## rwm (Jan 28, 2020)

I want someone to make a 10 sided insert with negative clearance angle on both surfaces!!! It would look like a torx driver with a skinny middle on cross section...
R


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## bakrch (Jan 28, 2020)

darkzero said:


> Yep, you are right. They are technically negative tools. Don't know why I said neutral.... I was just looking at another thread & they were talking about neutral inserts vs positive. Thanks for the correction.


It's easy to conflate this.  Negative rake allows the insert itself to have neutral edge or no clearance.

Funny thing.  I worked on lathes for 23 years, but when I tooled up my PM1236 I went with WNMG321 as the workhorse. It did not perform as I had hoped, obviously.

Thought I was being clever because it's 6 edges.

Mind you,  15HP was the average HP lathe that I worked with.  Sure,  I stepped up to weaker lathes, but the tooling was already established for it.  You guessed it, positive geometry for roughing on those. 

I did read it a million times in kennametal catalogs, about the HP requirements for negative rake tooling... but I still had to learn this for myself coming from a professional arena into the hobbyist.


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## nnam (Jan 28, 2020)

Is it true that the diamond shape has better rigidity on the holder than the triangle, or it's more like: if it's not rigid, then something is wrong with the setup (mismatch insert/holder, etc).


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## MontanaLon (Jan 28, 2020)

rwm said:


> I want someone to make a 10 sided insert with negative clearance angle on both surfaces!!! It would look like a torx driver with a skinny middle on cross section...
> R


While it may be possible to form an insert with that geometry, it likely wouldn't be strong enough to last very long. Carbide, even in the tougher grades is still brittle  Test it for yourself, take a triangle insert and you can cut steel with it, now take it and drop it from a foot high onto that same steel and it will crack. 


nnam said:


> Is it true that the diamond shape has better rigidity on the holder than the triangle, or it's more like: if it's not rigid, then something is wrong with the setup (mismatch insert/holder, etc).


I think that comes down to surface area under the portion of the insert that is supported by the tool holder.

Many of the tool holders for triangle inserts leave the point of the triangle doing the cutting dangling off the edge with little support. The diamond shapes don't seem to do that in my experience. FWIW, I find it easier to break triangle inserts by pushing them hard. But they are pretty sweet for giving a good finish all else being equal.


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## Cadillac (Jan 28, 2020)

rwm said:


> I want someone to make a 10 sided insert with negative clearance angle on both surfaces!!! It would look like a torx driver with a skinny middle on cross section...
> R



They do sell round ones which you just rotate/spin to a new cutting surface


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## darkzero (Jan 28, 2020)

Cadillac said:


> They do sell round ones which you just rotate/spin to a new cutting surface



Actually just used mine a few hrs ago.


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## mikey (Jan 28, 2020)

nnam, the tool holders you linked to in your first hyperlink are negative rake tool holders. Therefore, the inserts have ZERO relief; all the relief is provided by the bedding angle of the tool holder. Note that there is an anvil, an extra cabide piece, under the actual insert. This is commonly found on negative rake tooling. The number of points is immaterial; all of them are negative rake inserts because the tool holder is.

As noted above, negative rake tooling requires a lot of rigidity and power to use. If you have it, they cut and finish well but typically do not allow for micro-cuts so they take more skill to be accurate with. These tools are not appropriate for smaller lathes, smaller being under 5HP. 

Please listen to the guys. You are better off with positive rake tooling.


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## nnam (Jan 29, 2020)

This is mostly for learning and information purposes.  I bought the 2nd set, and they sent me a wrong set.  So I sent back and bought a same set that were sent correctly from a different seller.
I didn't know the 1st set has 0 relief.  The question is mostly about the shape of the inserts between diamond and triangle.


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## macardoso (Jan 29, 2020)

I'm going to throw my personal experience into the mix here. Lets assume you're going to stick with positive rake single sided inserts (CCMT for example). The next question is what shape. I think there are two properties that come with the shape and are on opposite ends of the spectrum. These are *Strength *and *Applied Cutting Force*.




An insert with a large included angle (S, C, W, etc) will be a stronger insert preferred for general purpose roughing and turning. The tool will perform better in materials that have a tough scale (cast iron) or in interrupted cuts. These are my first choices. The downside is that they *all else held equal* will create larger cutting forces (directed into the workpiece) than an insert with a smaller included angle. 

For fine finishing, turning accurate diameters, long and slender parts, thin walled parts, and low rigidity setups (boring), I will opt for a narrower insert (D or V) to take my finishing passes. These tools will cut smoother and be far less prone to chatter. The narrow tip to the insert also allows machining of undercuts or grooves where a wider tool wouldn't fit. They are great for work near the tailstock. These inserts will not handle roughing well and the tip is prone to chipping in interrupted cuts. In general the smaller the included angle, the smaller the cutting forces you are working with.

Another crucial factor is the insert nose radius. A large nose radius will be stronger and provide a better finish at a given feedrate compared to a smaller nose radius - this is good for productivity. A large nose radius will produce much higher cutting forces and be more prone to chatter than a smaller one. 

I will select large corner radii for heavy cutting and use very sharp cornered insert for most other cases.

Mike


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## rwm (Jan 29, 2020)

I have a round insert tool but I get tremendous chatter with it due to the large cutting surface. My SB 10K is not rigid enough to use it effectively.
Robert


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## macardoso (Jan 29, 2020)

Another thing to consider is that you can have a negative rake tool using neutral rake inserts with a an aggressive chip breaker. This allows the *effective rake* of the tool to be positive while using a negative rake tool. Many finishing inserts and inserts for non ferrous metals are made this way.  These inserts will still require a lot of horsepower and rigidity than positive rake inserts, but are not nearly as demanding as neutral rake inserts with a roughing chip breaker.


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## nnam (Jan 29, 2020)

There are several references to "rake" above.  I think they meant to say "relief".  Is the "rake" is for breaking the chips (on top-horizontal angle)?
Most of these inserts has a tiny bump around the edge to give them positive rake.   However, I don't know how sharp that bump, so it would be considered neutral rake and probably requires more force and horse power.

On the 1st link, the second letter is "n", which means 0 relief/clearance if mounted horizontally, and can be flipped over to use both sides.  The 2nd link, the second letter is "c", which means 7 degree relief.


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## mksj (Jan 29, 2020)

Rake with regard to insert pocket is the angle of the insert pocket which is either negative, neutral or positive. The insert pocket dictates the type of insert that can be used, they are not interchangeable. In addition there is the insert lip/cutting edge characteristics, so those that are for lighter cutting/finishing tend to have a more positive (upward) rake. So the cutting edge angle is a function of both the inset pocket angle and the insert edge angle.

This chart from Carbide Depot gives some overall properties of the different insert shapes and general properties. There are numerous other factors that effect the application and cutting properties of the inserts. Typically diamond shapes are more fragile so used more for profiling and cutting where other inserts will not fit. As a day to day insert you are better off with a CCMT or TCMT (i.e. neutral rake holder). As far as cutting edges, it is a moot point in the realm of the hobbyist because you are not doing production levels where you will wear out the insert quickly. I can get several months of work out of an insert and typically buy them for $3-4 a piece for Iscar, Kennametal, etc. It is much more important as to getting an insert that works in your machine as far as insert characteristics and coatings for the type of turning you are doing. 

Many of the insert styles like the square I use primarily for edge chamfer, others for grooving, etc. I primarily use CCMT for steel, and since I have a heavier lathe WNMG for aluminum. I use sharper positive rake inserts for boring. The DCMT is primarily for profiling and edging, but does chatter with deeper cuts.

Inserts for face mills are a totally different category so some of my inserts are octagons and have 8 cutting edges, so good for shallow cuts but not for removing a lot of material.


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## mikey (Jan 29, 2020)

mksj said:


> Rake with regard to insert pocket is the angle of the insert pocket which is either negative, neutral or positive. The insert pocket dictates the type of insert that can be used, they are not interchangeable. In addition there is the insert lip/cutting edge characteristics, so those that are for lighter cutting/finishing tend to have a more positive (upward) rake. So the cutting edge angle is a function of both the inset pocket angle and the insert edge angle.



nnam, the quote above should clarify the rake issue. In order to really help you, perhaps it might help to more clearly state what you're after. Perhaps you can answer the following so we can answer you more clearly:

Are you trying to sort out which tool holder and insert combination would be most suitable for your lathe?
Which lathe do you have? How much HP?
What materials do you work with most of the time?
You may require multiple tool holders and inserts, depending on your needs for access to the part. Is there a particular need you have in mind?
Is your QCTP fixed or can you move it to suit the tool holder/insert shape?
Just in general, you choose the insert shape to allow you to access the features on a part that you need to cut. If you have a hobby class lathe and do mostly general turning and facing then a 80 degree diamond works well. The popular SCLCR tools are a good example. The older triangle shaped tools can do similar work but require more tool holders to do the same work as an SCLCR can; hence, they tend to come in sets. Essentially, the shape of the insert and the cutting edge you use (side or end edge) depends on the features of your part or work piece. In most cases, this is most useful for a CNC lathe so unless that is what you have, it is likely that a general purpose diamond insert will work for most things.

There are tons of edge geometries available but rather than try to go through a slew of them, I suggest you home in on one or two cutters and focus on those. Pay attention to the nose radius and what the manufacturer recommends for roughing, medium cuts and finishing because THAT is what determines the edge geometry of your insert. The anticipated depth of cut is determined by the inscribed circle of the insert; the deeper you need to go, the bigger the IC and the more rigid and powerful the lathe must be.

Finally, the nose radius is what determines the accuracy of your cuts because it, along with your depths of cut, greatly impact on cutting forces. You kind of need to understand how that relationship works but for now, try to ensure the inserts you choose allow for a wide range of nose radii, especially towards the small end.

At the end of the day, how many points an insert has has very little to do with the value of the tool. In a hobby shop, an insert can last for months and months. I suggest you focus on the other factors that will allow you to choose the right tool for you.


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## wachuko (Jan 29, 2020)

I am learning soooo much.


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## mikey (Jan 29, 2020)

@nnam, my last post was off the top of my head. I just re-read it and it is all true but the wording might be "above your head" if you don't yet understand inserted cutting tools. I apologize; I didn't intend that. Let me try to be a bit more useful, and in doing so I am going to assume you are not running a CNC lathe.

Negative rake tooling requires a big, powerful, rigid machine. These tools do not require speed as much as they require rigidity and power. A negative rake tool like a Trigon negative rake tool used in a big lathe, say 20", will put a beautiful finish on a part while also taking what would be a monster cut on a hobby lathe. Put that same tool in a 10-12" lathe and the tool will not cut nearly as well. It will cut but your depths of cut will be limited and your finish will suffer some. Not only that, your accuracy will suffer if you cannot take a big enough cut to bury the nose radius. This is why most of us are trying to steer you away from negative rake tooling. I don't know what kind of lathe you have so take this with a grain of salt, okay?

The vast majority of hobby guys will be better served with positive rake tools. The term "positive rake" can be confusing because the insert holder or the insert can have positive rake. For the tool holder, a positive rake means the insert is angled up a bit. For an insert, the cutting edge and the slope of the top of the insert behind the edge can slope up, meaning it has positive rake. All of these inserts will have a relief angle ground under the cutting edge; this is characteristic of positive rake tools. Note that you can have a neutral rake tool/insert holder and use positive rake inserts in it; this will give you a net positive rake cutting tool.

Positive rake tools cut with lower cutting forces but rake is only a small part of the cutting forces the tool generates. Your cutting conditions (depth of cut, speed and feed) also impact on the forces the tool generates, as does the nose radius. The nose radius is perhaps the most important factor to consider when choosing inserts and the tools that will hold those inserts. If you are using a light lathe to take small cuts on common (not hard) materials then you want a positive rake tool and insert with a small nose radius, get it?

Note that the class of insert you use in a given tool holder, like a CCMT insert for an SCLCR tool holder, will have different edge geometries depending on which operation you are doing. Roughing inserts will usually have a wider land between the cutting edge  and the raked part of the insert; medium cutting inserts will have a slightly narrower one, while finishing inserts usually have almost no land behind the cutting edge; the raked part is right behind the cutting edge. Of the three types of CCMT inserts, the finishing insert will produce the lowest cutting forces. If you also use the smallest nose radius you can find in a finishing insert then you will be able to take smaller cuts on a smaller, less rigid and less powerful lathe.

I hope this sort of helps to visualize what we've all been saying here. If I had to recommend a single tool holder for general use on a hobby class lathe, it would be an SCLCR tool holder that uses CCMT and CCGT inserts. Both of these inserts are positive rake insert. I would focus on finishing inserts that can rough, take medium cuts and finish well for you. I would also choose the smallest nose radius you can find for that tool. Note also that bigger tools are not always better because small nose radii are not found on bigger inserts. If accuracy is important then pay close attention to the size of the nose radius; the bigger it is, the more it will deflect. The most accurate cuts will use a depth of cut at least equal to the nose radius so sometimes smaller is better, especially on a small lathe.


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## nnam (Jan 30, 2020)

Thank you all for taking time explaining the in and out of inserts.  It turns out that the shape does not matter as much as the others.

I was lucky not fully aware of this  but bought the recommended CCMT inserts and holders.

They have a 0.4mm nose radius.  I just ordered a set of 10 inserts  CCGT with 0.2 mm nose radius for finishing cut and aluminum cuts.

My lathe is 15x30 Leblond regal.  It has 3hp I think, but some are equipped with 5hp.


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## macardoso (Jan 30, 2020)

Awesome! Nice lathe. That would be at the bottom end of being able to actually use negative rake tools. Your lathe is bigger than most on this site.

My lathe is a 1000lb 12x36 with 1.5 HP. Just a bit too small for a CNMG432


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## bakrch (Jan 30, 2020)

nnam said:


> Thank you all for taking time explaining the in and out of inserts.  It turns out that the shape does not matter as much as the others.
> 
> I was lucky not fully aware of this  but bought the recommended CCMT inserts and holders.
> 
> ...



Jealous.

I get to play with one of these at work, great lathe.


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## rsicard (Jan 30, 2020)

mksj said:


> Rake with regard to insert pocket is the angle of the insert pocket which is either negative, neutral or positive. The insert pocket dictates the type of insert that can be used, they are not interchangeable. In addition there is the insert lip/cutting edge characteristics, so those that are for lighter cutting/finishing tend to have a more positive (upward) rake. So the cutting edge angle is a function of both the inset pocket angle and the insert edge angle.
> 
> This chart from Carbide Depot gives some overall properties of the different insert shapes and general properties. There are numerous other factors that effect the application and cutting properties of the inserts. Typically diamond shapes are more fragile so used more for profiling and cutting where other inserts will not fit. As a day to day insert you are better off with a CCMT or TCMT (i.e. neutral rake holder). As far as cutting edges, it is a moot point in the realm of the hobbyist because you are not doing production levels where you will wear out the insert quickly. I can get several months of work out of an insert and typically buy them for $3-4 a piece for Iscar, Kennametal, etc. It is much more important as to getting an insert that works in your machine as far as insert characteristics and coatings for the type of turning you are doing.
> 
> ...


Would like to meet up with you.  Also live in Tucson.  In the process of renovating a Bridgeport Milling Machine.  Also have a 13-40 WWII Southbend lathe.  Hope to hear from you soon.  Thanks. Rolly


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## mksj (Jan 30, 2020)

Hi Rolly,
Moved from Tucson to Fallbrook CA last year, after 11 years in the heat we decided it wasn't for us. Just getting everything setup at our new house.
Mark


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