Keeping an assortment of brazed carbide tool bits and solid carbide rounds in the tool crib is probably a good idea. You never know when you may need to hand-grind a special tool for an oddly shaped part or hard-to-reach area.
For most turning operations, though, indexable carbide tools make machine setups both faster and easier while significantly reducing the downtime associated with tool changes. Which still leaves one problem: deciding which insert is right for the job.
Peruse a supplier’s tooling catalog or website and you’ll quickly find an overwhelming variety of indexable tooling. Profiling tools, groovers and threaders, boring inserts, indexable and modular drills, cutoff tools—there’s no shortage of high-quality turning tools out there, all designed to make lathe turning operations more profitable and part quality more consistent.
But here again, how do you know which one (or ones) to choose?
Reading the ISO Code for Inserts
A good place to start is by learning the ISO 1832 standard, which, according to the International Organization for Standardization, “establishes a code for the designation of the usual types of indexable inserts for cutting tools in hard cutting materials or any other cutting materials, in order to simplify orders and specifications for such inserts.”
Ready for a quick ISO introduction? Pick up a “typical” box of turning inserts—some CNMG432 roughers, for example, or VPGR331 profiling tools—and follow along:
- Shape: The “C” in CNMG denotes the insert shape, which in this case is an 80-degree diamond (or rhombic), just as the “V” in VPGR331 signifies a 35-degree diamond. The former is a go-to geometry for heavy facing and turning operations (as is the trigon, or W shape), while the latter is limited to light finishing cuts in either direction. Other popular shapes include the D (55-degree diamond) and T (for triangle), although depending on the manufacturer, up to a dozen additional letters are in use.
- Relief (or clearance) angle: In our CNMG example, the N means the insert has a 0-degree relief angle, making it a double-sided insert. But because all cutting tools need some small amount of clearance to function, the toolholder must therefore tip the insert down slightly in the pocket to avoid rubbing. The P in VPGR, on the other hand, has an 11-degree clearance angle and sits perfectly horizontally in the toolholder. Generally speaking, the greater the angle (G is the highest at 30 degrees), the weaker the cutting edge.
- Tolerance class: It’s logical to think that the M in CNMG identifies it as a molded (or pressed) insert and the G in VPGR as one that’s been ground to size. And while this might be true in many situations, the M actually signifies the tolerance range—looser, in this case, compared with the more stringent G—used in the insert manufacturing process.
- Insert type: This fourth placeholder covers a lot of territory. An A here indicates that the insert has a hole in the center, B means there’s a hole and one countersink (allowing the mounting screw to sit flush with the top surface), G signifies a hole and chip grooves on the top and bottom surfaces (as in our double-sided CNMG), and R is a clamp-style insert with no hole and a chip groove on the top surface (like most 35-degree diamonds). Check the standard’s chart. There are plenty more.
- Size: Every turning and boring insert has an inscribed circle (IC) value, signifying its size in 1/8-inch increments as viewed from the top. For our -432 insert, the circle measures ½ inch (4 x 1/8 inch), just as a -331 is 3/8 inch (3 x 1/8 inch) across. Also, it’s important to note that inserts with an IC of less than ¼ inch use 1/32-inch increments and rectangular inserts require two digits—one for length and one for width.
- Thickness: Similarly, the thickness of inserts with an IC of ¼ inch or less is measured in 1/32-inch increments, while anything larger uses multiples of 1/16 inch—for instance, a CNMG432 and VPGR331 are both 3/16-inch thick, whereas a TCMT221 boring insert measures 1/8-inch thick.
- Corner rounding: It’s tempting to think of this seventh digit as the corner radius, and most of the time you would be right. That’s because a CNMG432 has a 1/32-inch radius (2 x 1/64 inch), a VPGR331 has a 1/64-inch radius, and any insert with a 0 in this placeholder is considered sharp (0.005-inch radius or less). That said, some inserts have chamfers rather than radii (A, D, E, etc.), and others are flattened or truncated (N and P).
That was a very abbreviated tour through the ISO 1832 standard, now on its sixth edition and spanning 24 pages. You’ll find it a helpful starting point on your road to insert selection.
Beyond the ISO Code: Proprietary Cutting Tool Inserts
Unfortunately, you’ll also find it chock-full of exceptions. In their ongoing effort to address the machining world’s ever-evolving landscape, cutting tool manufacturers continue to develop novel, often proprietary solutions that don’t fit neatly into any standardized set of guidelines.
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