What's the difference between MGMN and MGGN parting/grooving inserts?

MGMN and MGGN are both insert designations from South Korean carbide manufacturer Korloy, however this designation is also used in the wider industry for generic inserts modelled on the same design. The letters each mean:

• MG: Multi-grooving square
• M/G: Tolerance class
• N: Neutral handed (not left or right handed)

So the key difference is in the third letter:

• M = M class tolerance
• G = G class tolerance

However, when we compare MGMN and MGGN inserts in practical use, there are clearly many differences not summarized by a simple difference in the ISO designation. This is similar to our article on the difference between CCMT and CCGT inserts.

Here are the common difference between MGMN and MGGN inserts that aren't indicated in the simple 4 letter designation:

1. Back rake angle

• MGGN: Often has a sharply positive back rake angle. This reduces cutting forces generated at the cutting tip, at the expense of providing less support for the cutting edge. This is advantageous on smaller lathes that can struggle with the high cutting forces generated when parting/grooving. Note the back rake terminates in a curve upwards that helps facilitate chip removal.
• MGMN: Tend to have a flat back rake angle, or very minimally inclined 5 degree angle. This prioritizes the level of support of the cutting edge, at expense of increasing the level of friction and so cutting force generated.

2. Sharpness

• MGGN: MGGN inserts have a ground sharp edge. This dovetails with the commonly found positive back rake angle to minimize cutting forces
• MGMN: Most often a honed or chamfered edge, designed for durability and resistance to chipping or breaking, rather than to minimize cutting forces. This would be especially desirable when doing interrupted parting or grooving.

3. Coating and surface finish

For the same reasons a positive back rake is common on MGGN inserts, MGGN inserts are often uncoated and polished. This is to provide as smooth a cutting surface as possible which in turn produces as smooth a finish as possible, especially in cutting soft materials like aluminium.

However, this is not always the case. In the above photo we see a coated MGGN insert next to a coated MGMN insert. This would be most commonly used on small lathes for parting/grooving on harder materials like steel, where the reduced cutting forces are still desirable (due to small lathes difficulty in handling the immense cutting forces generated when parting) but superior finish is sacrificed for superior durability on harder materials that tend to cause more wear than softer materials.

MGMN by comparison are almost never uncoated, since the same reasons you would want a flat rake and lower tolerance insert are the same reasons you wouldn't care about finish - i.e. they're used on rougher cutting in harder materials where strength and durability of the cutting edge take precedent of ease of machining and quality of finish.

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