# 16x40 Lathe Indexable Tool Holders...positive Or Negative Rake?



## coolidge (Mar 15, 2015)

So I did some research into the 3/4" - 1" tool holders for a 16x40 lathe I'm considering, Grizzly G0670 made in Taiwan. There are just about as many negative rake tool holders as there are positive rake. Watching some video's of larger manual lathes I see people using negative rake indexable tool holders and they seem to work fine, but the lathes were a bit larger than this 16x40. My 12x36 doesn't seem to be rigid enough for negative rake, the positive and neutral work better. Anyone have any thoughts on which way to go? This is a CXA tool post by the way.


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## darkzero (Mar 15, 2015)

It really depends on the materials you work with. You mainly see negatives used on larger lathes cause the larger lathes are used for larger work, heavier DOCs. I use negatives on my 12x36 just fine. I only use negatives for larger diameters for ferrous metals, stainless, & for heavier material removal. Negatives generally don't like small DOCs. They do leave an awesome finish for alloy steels if you take deep enough cuts. I mainly use positives as I work with smaller stuff most of the time, 1"-2" diameters. I work with titanium, stainless, & non ferrous the most. Positives aren't as strong so they generally don't last long for heavy machining.


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## G MORSCH (Mar 20, 2015)

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


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## Tony Wells (Mar 20, 2015)

Good info, Gary. Thanks for posting it. The CNMG432 is my go-to insert on about 95% of my work, unless I need a lower angle than 80°. Then I switch over to a D profile. I do also like the fact that the 100° corner of the C profile can be used in some roughing applications as well as in mill work.


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## Cactus Farmer (Mar 20, 2015)

Years ago I bought used Kennametal Face mill that used CNM? inserts. One of the dog bone screw heads was stripped and I called the dealer, the nice lady sent me a dozen and 4 shims for nada. What customer service! I sometime later found a flea market dealer that had 10-12 full boxes of CNMP inserts. TiN coated too!. Not remembering the size of the ones I needed I bought a box and rushed home. It was a fit and I returned to buy all the rest. Someone had bought one  but I got all that was left. Double sided and positive rake even in a negative rake tool holder. I had never heard of such a thing before, I am now educated on this wonderful system. Low force cutting even on smaller machines and an awesome finish on my parts. I now have several 43? size lathe tools for these inserts plus the face mill. Now I'm looking for an inexpensive face mill that uses the CNM? inserts turned 90 degrees to take advantage of  the other 4 cutting edges. Negative rake inserts with the positive rake cutting edge are not very common in the used markets around here but I find a few from time to time. Our local machine shops are geared to oilfield work, BIG and strong machines turning big heavy iron. My little machines are like toys to these guys, some of their chips will hurt you with their weight ,plus they are almost always blue........600 degrees cooks live meat! 
There are square and triangular inserts with this geometry so I am on the lookout for tooling that fits my holders and the inserts to fit them. I love treasure hunts!
And now with my new/old 13X40 gearhead lathe I'm stepping up to a CXA main block and holders. Larger tools are in my future......life is good.;>)


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## coolidge (Mar 20, 2015)

I'm going to have to find the time to research this subject further. Right now I don't know enough to ask an intelligent question and that makes me uncomfortable.


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