Inserts and their holders are matched sets.
The letters call out a specific shape to the insert. This insert needs to be used in a holder designed for this shape. You would not use a TNMG in insert in a milling cutter made for a TPG, it just won't work.
Cutting geometries are divided into 2 main classes. Positive and Negative.
In a positive geometry the cutting tool drags across the surface. In a positive geometry it slices into the material. Negative inserts are stronger and have twice as many cutting edges (the top and bottom surfaces are the same size). Positive inserts are weaker but use less cutting pressure.
Aluminum cutting inserts are high positive rake (20 degrees) and are set at extreme angle in their holder.
Imagine a cold chunk of butter and you have a butter knife.
Hold the knife so that the top twisted towards you and you drag it across the surface. This is negative cutting.
...... /
....../ -->>> negative cutting
...../---------
__/
Now twist the knife so the bottom edge is towards you and pull it towards you. This is positive cutting.
. .\
. ...\ -->>> positive cutting
. .....\-----------
_____\
The top surface of the insert is positioned in a pos or neg direction by how the pocket is oriented (twisted) in the holder. Flat holders are called neutral but from a design standpoint this is negative cutting (hold your butter knife straight up and down).
Soft materials like aluminum do not like negative cutting actions.
On top of this carbide comes in different flavors. This is done by changing the recipe and adding things like titanium carbide to the mix which make a harder carbide for finishing and tantalum carbide which increase heat resistance. On top of this you place a variety of coatings, each of which has a specific use. This is the wide variety of grades you see in the marketplace. I make over a hundred different grades of carbide inserts.
Tooling designed for cast iron cutting does not work well on aluminum.
A tool designed for aluminum will chip very quickly when cutting steel.
A TPGB will be normally be set up for steel finishing. It will work on aluminum but not as well as a TEC tool (which would take a different holder)
Yes it's all very confusing at first. All the manufactures have info on the correct use of their tools and recommendations for which grades work best on different materials.
General grade usage: C-2 honed for cast iron, C-5 and C-6 for steels, C-3 grades with a up-sharp edge and polished top for aluminum.
All around do everything inserts: C-2 uncoated up-sharp inserts.
Note that this is a drastic generalization of the subject and you can get away with all sorts of non-optimal cutting conditions if you take it easy on the tool. However some combinations just don't work. For example you would not want to try cutting aluminum with a heavy honed and coated steel roughing insert.
Bob
You can always spot the pioneers -- They're the ones with the arrows in their backs.