The Ultimate Guide to Choosing Carbide Insert

Last but not least is the whole topic of insert nomenclature. It's a deep subject, one filled with exceptions and cutting tool-specific rules. Regardless, most manufacturers follow the ANSI or ISO tool identification system (and sometimes both). We won't get into the details here except to say that it uses an alphanumeric code to specifies an insert's shape (round, square, triangular, etc.), clearance angle (neutral to positive), tolerance (some inserts are pressed to size, while others are ground), the size of the locating hole (if any) and clamping method, its size and thickness, corner radii, and various other defining features (see the chart above for an example).

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Complex naming systems aside, however, choosing the right insert for your machining application isn't as difficult as it might appear. That's because cutting tool manufacturers have developed online tool advisors that walk machinists and programmers through the tool selection process. For example, Kennametal's NOVO prompts users to answer questions about the metal removal process (milling, turning, or holemaking), the machine tool that will be used, workpiece material and removal amount, and expected depths of cut. It then generates a machining strategy along with insert and toolholder suggestions, ordering information, product availability, feed and speed recommendations, and more.

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Long story short, carbide insert selection is much easier than it once was, even though the number of cutting tool options has grown exponentially since the days of brazed carbide and HSS tool bits. Download a catalog, log in to NOVO, or give your local cutting tool representative a call. You'll be making chips in no time.

Without being at your lathe and observing, it's difficult to give you a precise answer, but based on what you have already said I can make a few reasonable comments/suggestions. You didn't say what your depth of cut is, nor the manufacturer of the insert, so I'll speculate a bit. A CNMG 432 GM 324 insert would be mounted in the tool holder at something like a 5 or 6 degree negative rake. The "GM" indicates the insert is intended for general machining (that's somewhere between fine finishing and roughing), and since it has an "M" tolerance, I can guess without seeing it that it's molded with a blunt cutting edge. The 324 is the manufacturers grade, so without knowing who made the insert, it's difficult to know what that grading designation means. But with those aspects alone, that insert is best suited for use on steel and cast iron. It has a 1/32" nose radius, which suggests that the ideal depth of cut would be >0.015" at a feed rate of <0.015 when used on steel or cast iron.

When that type of insert is used on aluminum, given the negative rake and blunt cutting edge, relatively high tool pressures are being generated since the insert is not free cutting. When facing, the tool pressures are axial - toward the chuck/spindle with considerable rigidity. When turning, the tool pressures are radial - across the axis of rotation and against material being cut which has less rigidity pushing against the chuck/spindle. So when turning, the material is likely to start flexing during the cut, and this can set up a vacillation that shows up in the surface finish as waviness or chatter. In a turning operation, you can alter the direction of the tool pressures some by increasing the feed rate. Increasing the feed rate will shift the tool pressure more towards the chuck rather than mostly against the material being turned. This might help smooth out the surface finish, although with that insert, the aluminum is being torn, not cut. Increasing the depth of cut will also produce more consistent "cutting" action when turning with this specific type of insert on aluminum and could improve surface finish, but it will also increase the radial tool pressure suggesting a higher feed rate as well. The best thing you could do is switch to a neutral or positive toolholder with an insert for aluminum. Hope this helps.