Are CNMG inserts actually negative rake?

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Lucas E

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Just ordered a new multifix tool post and am looking at getting some indexible tooling for my South Bend Heavy 10 lathe. I think I'm leaning towards CCMT or CNMG so I can use one insert for external and boring operations. I know CCMT is often recommended for smaller lathes due to it's positive rake, however the SCLC holders you use for them are actually flat with no positive rake at all. It seems all of the positive rake is built into the insert it self. On a CNMG the tool holder is tilted down for negative rake so you can run a zero clearance insert, but the inserts seem to have a similar positive rake as the ccmt inserts. Does the tilting down of the CNMG holder have that much of an effect on horsepower and rigidity needed?

I'd like to use the CNMG as you can flip the insert and get double the amount of cutting edges, but I'm looking for an insert that will give me the best surface finish on my 1 HP southbend 10". I'm just confused as ccmt is always referred to as positive rake when it really seems like it's neutral rake, and CNMG is referred to as negative rake when it uses the same style positive rake insert that makes a CCMT be considered "positive rake".

Does anyone have any experience with these they can provide? If a CCMT will indeed provide a better finish then I'll go that route, but I really only want to go with 1 style of insert.
 
rake is the angle of the cutting edge presented to the work, so it's a combination of both the angle at which the insert is held AND the shape of the chip breaker molded/ ground into the insert. So CCMT inserts in a zero degree insert pocket are positive rake due to the chip breaker. CNMG can be negative, neutral or positive depending on the insert. The negative degree insert pocket is purely there to provide clearance for the zero degree clearance (double sided) inserts.
 
I went with CCMT because the positive rake will give less tool pressure, not a better finish. Tool pressure is the key. You can work with different feeds and speeds to get a nicer finish.
 
Batmanacw said:
I went with CCMT because the positive rake will give less tool pressure, not a better finish. Tool pressure is the key. You can work with different feeds and speeds to get a nicer finish.
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What lathe are you running? Are you happy with the CCMT?

Also, what brand tool holders are you using? I'm shopping around trying to see if any of the imports are worth it, or if I need to spend $65+ for each holder from one of the major brands like Mitsubishi, Kennametal, iscar, etc.
 
Lucas E said:
What lathe are you running? Are you happy with the CCMT?

Also, what brand tool holders are you using? I'm shopping around trying to see if any of the imports are worth it, or if I need to spend $65+ for each holder from one of the major brands like Mitsubishi, Kennametal, iscar, etc.
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I absolutely love the ccmt insert design. I use Mesa tool holders for regular turning. They can be hard to Google so look for Mesa tools boring bars and they seem to come up easier. I've got two double end tools with right/left or right/chamfer.

I don't have many import holders but I think they work fine. I've got lots of Mitsubishi Chinese inserts that work very well and I've got better quality as well. It is often hard to tell the difference in use. I use both the 21.51 and 32.51 with some 21.52 and 32.52 on hand. I really only use the smaller radius.
 
I have both CNMG and CCGT inserts for the same tool holders.
CNMG for the harder metals, CCGT for the softer ones.
 
Mitch Alsup said:
I have both CNMG and CCGT inserts for the same tool holders.
CNMG for the harder metals, CCGT for the softer ones.
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Do you mean CCMT and CCGT? I also use some CCGT inserts in my CCMT holders. My CCMT holders don't have the clearance angles to use CNMG.
 
There are several types of chipbreaker on **MG inserts. Shars catalog gives a good description of the different chipbreakers. Select a chipbreaker for finishing. The inserts for roughing have too big a negative hone on the cutting edge for use on light lathes.

A better choice would be **MP inserts with a positive rake molded in. Kennametal makes them. Carbide Depot sells them.
 
Batmanacw said:
Do you mean CCMT and CCGT? I also use some CCGT inserts in my CCMT holders. My CCMT holders don't have the clearance angles to use CNMG.
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No, I use the turned up edge of CCGT in a CNMG tool holder to give the cutting edge a positive rake.

Note: CCGT is a CCMT that has polished edges.
 
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