There are many misconceptions about carbide tooling as well as arguments about positive rake vs. negative rake inserts. Lets compare two inserts: (Neutral Rake) CNMG432 and (11 degree Positive Rake) TPG322. The insert in the picture to the far left is an 80 deg. diamond CNMG432. "C" indicates an 80 deg. Diamond in the ANSI and the ISO nomenclature system. "T" in the second insert indicates a Triangular insert. So, the first letter in all major manufacturer's inserts is the shape of the insert. The second letter indicates the relief angle on the Flank of the insert. N=Neutral, so in order to make a neutral rake insert progress in the direction of feed, generally, it is necessary to tip the insert to 5 degrees axially and 5 degrees radially. A neutral/negative rake insert will typically be double sided and provides greater economy by delivering additional cutting edges on the opposite side of the insert. There are exceptions however. The most common exception is an insert with two M's, such as CNMM432, which is a heavy duty insert with topography on the top, but, flat on the bottom for full tangential load support all the way to the outer edge.
The TPG insert is single sided and because there is a G in the third position, the insert has been Ground, usually top, bottom and periphery. This is typically more costly to purchase and will have only three cutting edges vs. four on the CNMG insert. "G" tolerance inserts generally will have a very sharp edge, but that will only be the case on uncoated and PVD coated inserts. If an insert is CVD coated there is little chance that the edge will be dead sharp. CVD coatings REQUIRE a honed edge in order to limit the build up of coating at the intersection of the flank face and the rake face. Milling inserts are also often honed, even with PVD coatings, in order to improve resistance to chipping.
Topography:
Fortunately, the work piece cannot distinguish between a negative rake insert and a positive rake insert. There are many who will disagree with this statement and they would be correct IF the inserts have no topography molded in to the top of the insert. A common positive topography negative insert is a CNGP style. This insert is peripherally ground with a 10 degree positive slope at the cutting edge. If the TPG insert is tipped at 5 deg. positive in the tool holder and the edge uses the other 6 degrees for clearance we are almost exactly at the same slope with the CNGP tipped at 5 degrees for clearance and a resulting 5 degrees of positive rake on the top surface. A load meter on a CNC machine will not indicate a measurable difference between the two. If we were to compare a ground flat top negative with a ground flat top positive insert, YES, there would be a difference. I would prefer high positive topography in all boring. Negative inserts are tipped up to 13 degrees in small hole boring to achieve radial relief causing greater deflection, and yes, more tool pressure than their positive counterparts.
My first choice of inserts for manual machines would be either "C" clearance or "P" clearance triangles. TCGT321, TPGT321 screw down inserts. Just Be sure it is *uncoated or *PVD coated. (TiCN, TiN, TiAlN, AlTiN, etc.) Make sure it has a "G" in the third position which indicates that the insert has been ground and offers a *sharp edge.
I would like to discuss grades of carbide with everyone, but that would keep me up all night. Go for a tough grade if possible vs. a hard grade.
Best Regards, Gary
