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Buying Guide: Four Things to Know About Milling Cutter Selection

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Given the huge variety of solid and indexable milling cutters available online today, it can be challenging to choose the right one for a machining application. Here’s a quick primer to get you started.

One of the first lessons in many machine shop programs is learning how to set up a fly mill or fly cutter. Used primarily on manual knee mills, this most basic of cutting tools is equipped with a hand-sharpened bit that's secured to the tool body with a series of set-screws. As the tool rotates, the machine operator engages a mechanism that moves the workpiece beneath it, removing a small amount of material from its top surface with each pass. 

What is a Milling Cutter?

The fly mill is just one example of a milling cutter. As with drill bits, these are rotary tools used on CNC machining centers, live-tool lathes, and yes, manual milling machines. But where a drill bit's cutting edges are positioned at the very end of the tool, limiting it to axial material removal (i.e., holemaking), a milling tool also boasts cutting edges along its periphery, allowing it to remove material radially, from side to side. A huge variety of such tools exists:

  • The fly mill just discussed is a type of face mill, used to produce flat surfaces on metal and plastic workpieces. They are rarely found on CNC machining centers.
  • Slotting mills and slitting cutters are just as their name describes, able to cut slots and narrow slits respectively in the tops and sides of parts.
  • Dovetail tools are a special type of milling cutter that machines the angled grooves needed to join many machinery components.
  • Similarly, gear cutters (also known as hobs) are used to cut gear forms, splines, and sprockets. For the most part, they require dedicated machinery.

Each of these is an application-specific cutter, limited to relatively small amounts of material removal and the creation of distinct part features. In a moment, we'll talk about the more general-purpose tools—the end mills, shell mills, and other workhorses of the machining industry. For now, though, it’s important to recognize that all milling tools (with one or two exceptions) are just as described previously—multi-toothed rotary cutters that utilize a shearing action to remove material along the periphery of the tool and, in some instances, can act like a drill as well.

Slotting mills such as Kennametal KNS® Slotting Mill are  able to cut slots and narrow slits respectively in the tops and sides of parts.

Classification of Milling Cutters

In the world of industrial milling, two broad classes of cutting tools exist. These are solid milling cutters and indexable milling cutters. As the name implies, the first of these is made from a solid piece of cutting tool material—usually tungsten carbide, although high-speed steel (HSS) and cobalt milling cutters are also available for hobbyists and low-volume or light-duty machining operations.

Due to its material cost, solid carbide tools are generally smaller than an inch or so in diameter. Above this size, almost all milling cutters use indexable carbide inserts attached to a steel cutter body via small screws or clamps. They provide an effective yet affordable solution for the vast majority of all machining operations, although solid carbide tools are still used for small part features and for finishing work.

Straddling the fence between these two are so-called modular end mills, which employ a solid carbide head that threads onto a steel body. As with modular carbide drills, these cutting tools provide the performance and accuracy of solid carbide but at a far lower cost. And because a worn head can be swapped out in a few seconds, modular tooling also increases productivity.

Modular end mills such as Kennametal DUO-LOCK employ  a solid carbide head that threads onto a steel body.

Types of Milling Cutter

One of the most commonly used of all milling cutters is the end mill. Solid carbide, indexable, and modular versions are available, as are ones with square, ball, and radiused cutting ends. They are used to machine slots and shoulders, mill pockets, and cut the inner and outer perimeters of various part features. As suggested earlier, some end mills have center-cutting capabilities, allowing them to plunge like a drill or ramp into a workpiece surface.

Shell mills or shoulder mills can be thought of as an end mill's larger, heftier cousin. They cannot plunge or ramp, but they can remove large amounts of material quickly. Due to their large size, most are indexable, often filled with dozens of carbide inserts. So are face mills, modern, multi-fluted versions of the venerable fly mill that opened this article, used to true up horizontal surfaces and make them both smooth and flat. And copy mills are similar in shape and function to ball-nosed end mills. These are a favorite of moldmaking shops, which use them to rough out large cavities before finishing with a solid carbide or modular end mill.

There's some overlap between all of these. For example, it's no problem using an end mill to machine the top surface of a small metal workpiece, but the wider and longer that surface becomes, the more reason there is to use a face mill. And some face mills can be used to cut square shoulders like a shoulder mill would, provided that shoulder is fairly shallow in depth. As stated previously, there’s a huge variety of milling cutters out there. The trick is knowing which one to use and how to apply it.

 

Shell mills such as Kennametal Mill 4-15™  cannot plunge or ramp but can remove large amounts of material quickly.

Bestselling Milling Cutters

That's where Kennametal comes in. Granted, they're not the only cutting tool supplier out there, but they are filled with experienced applications and support people. The tooling manufacturer also carries an extensive lineup of most every cutting tool mentioned here.

Its Duo-Lock series of modular end mills, for example, provides a best of both worlds solution to anyone needing solid carbide performance at an indexable price range. There are also high-feed “Beyond” style milling tools, the Mill 4 and KSSM series of shoulder mills, Fix-Perfect face mills, and much more. Most are available in material-specific or general-purpose carbide grades, and all are available either online or in-person through an authorized distributor. Long story short, Kennametal has milling cutters covered. 

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