Pros and cons of indexable tools

Author Christopher Tate
October 04, 2016 - 10:45am

Cemented-carbide cutting tools have been in use since the late 1920s and are now ubiquitous in the metalcutting world. Around 1950, tools that accept cemented-carbide inserts became common in metalworking and are still considered a basic necessity.

Indexable cutters are typically constructed of a steel body that has machined features, allowing for the accurate positioning of inserts. Steel tool bodies also provide a method for clamping inserts along with a method for attaching the assembled body and inserts into the machine tool. 

Benefits Over Solid Tools

Before the proliferation of carbide inserts, it was always necessary to remove solid cutting tools from machines to sharpen them. Tool sharpening was such a significant amount of work that large manufacturers often had grinding departments dedicated to tool sharpening. Therefore, the single most significant benefit offered by indexable tools is the ability to renew the cutting edge without having to remove the cutting tool from production. Renewal of the cutting edge is typically done by unclamping the insert and turning or flipping it (indexing) to a fresh cutting edge or replacing a completely worn insert with a new one.

This facemill accepts inserts with 16 cutting edges, making the inserts cost-effective. Image courtesy C. Tate.

Solid drills, taps and other types of cutting tools are still used, depending on the application, but the indexable turning tool is the predominant tool of modern turning operations. Indexable tools have effectively displaced HSS, brazed-carbide and custom form tools, which were common prior to the proliferation of CNC machines. An indexable turning tool paired with the multiple-axis motion of a CNC machine tool simplifies the creation of complex shapes, virtually eliminating the need for dedicated turning tools. 

In addition, applying indexable drills on a lathe provides some distinct advantages over traditional solid HSS or carbide twist drills. It is common for a programmer to make a hole with an indexable drill and then step off center and cut the hole to a larger diameter, which can eliminate the need for a boring bar. CNC lathe turrets frequently suffer from misalignment because of wear and tear, and misalignment of the machine can damage standard drills. On the other hand, indexable drills are more forgiving than standard drills when used on machines with questionable alignment. 

Groove-turn inserts, which have made grooving tools multifunctional, like indexable drills, present a significant development in turning tools. Old grooving technology allowed cutting in only one direction; groove-turn inserts allow the user to make radial as well as axial cuts. 

The Common Touch

There is a tremendous amount of commonality among indexable turning tools. Turning inserts are manufactured in common shapes, such as diamond, square and round. The shapes and sizes of these inserts are made to standards governed by bodies such as ANSI and ISO. Their commonality gives the programmer and machinist an almost unlimited selection of insert grades and cutting-edge geometries. All of the major cutting tool manufacturers produce turning inserts to these standards, so finding an insert that gives the best performance is relatively easy.

Unlike turning tools, indexable milling tool bodies tend to require insert shapes and geometries that are not common among manufacturers, forcing users to purchase inserts made specifically for the brand of mill they use. However, toolmakers have been developing and offering families of milling tools that utilize their proprietary shapes across a range of cutter bodies. Having a family of cutter bodies that accept the same insert reduces tool inventory while providing some flexibility to programmers and machinists. 

Most of the major toolmakers manufacture custom indexable form tools. Unlike form tools for turning, form tools for milling are still a viable way to complete a complex geometry, especially when a shop is trying to reduce cycle time and cutting tool inventory. Parts with features such as multiple steps, radii and chamfers require the use of multiple tools to create them. The advantages of custom, combination indexable form tools are reduced cycle times, reduced tool inventory and improved part quality.

This boring bar for a lathe allows different cutting heads to be bolted on, so multiple styles of inserts can be applied. Image courtesy C. Tate.

Unfortunately, custom indexables are expensive and often require modified inserts, making the inserts expensive as well. For these reasons, custom indexable tools are usually reserved for high-production environments or for manufacturing very expensive parts. 

A significant indexable milling innovation was the development of plunge milling for roughing large cavities like the ones machined in molds. Tools used to rough deep pockets or shoulders can be quite long. In traditional roughing operations, radial loading of long cutting tools from side-to-side cutting motion induces chatter. Plunge-roughing is done by feeding the tool axially, which directs cutting forces into the spindle taper, where the machine is most rigid. Plunge-roughing results in much higher metal-removal rates when long tools are needed. 

On the Other Hand …

Indexable tools do have some drawbacks. Inserts are usually made by pressing carbide powder and binding materials into a die under high pressure. After forming, the inserts are heated to high temperatures and sintered, binding the powder and other materials together and giving the insert its strength. This process results in a cutting edge that is stronger than the edge of a ground cutting tool. Although stronger, the formed cutting edge is also less keen, or sharp, which can limit the ability of the tool to effectively take a shallow DOC, which can make finishing difficult. 

Variation in insert pockets and insert size can cause the cutting edges of multiple-insert milling tools to lie in slightly different planes. The result is often less-than-desirable surface finishes or noticeable steps in shoulders. In these instances, the user may be forced to use another type of tool for finishing. 

As industry continues to demand modularity, lower inventory levels, higher productivity, increased tool life and universal tooling solutions, indexable cutting tool technology will continue to make advances and replace obsolete cutting tools.

Out with the Cam-out Effect

When tightening or loosening screws with conventional conical profiles, such as the cross-recessed Phillips head, the cam-out effect is a common problem. In accordance with the triangle of forces, some of the energy cams out of the screw, causing the screwdriver or bit to slip out of the screw head. The user has to counteract cam-out by applying additional force.

Profiles with straight sidewalls, like hex, Torx and Torx Plus drive systems, eliminate the cam-out effect. The basic difference between the Torx and the Torx Plus designs is that the former incorporates a 15° drive angle and the latter has an almost 0° angle, which ensures optimal torque load is transmitted to the screw. Although the straight sidewalls of the Torx drive prevent cam-out, its 15° drive angle prevents full engagement of the drive bit and fastener and still permits a small amount of radial stress, which can reduce bit life.

Sloky USA offers preset torque T-handles and screwdrivers for Torx and Torx Plus screws to fasten carbide inserts to tool bodies. Image courtesy ACSC International Inc./Sloky USA.

With the hex design, the points of contact cause stress risers to develop. In addition, damage to the screw head is almost inevitable, and the transferable torque is relatively low in comparison with the level of force that has to be applied.

The danger when tightening and loosening screws is the high stress concentration that occurs when turning the fastener. High stress occurs mainly in hex screw heads, which can cause stress in the tool and screw. The elliptically based geometry of the Torx profile and the associated lack of angular edges minimize stress concentration. This, in turn, protects and extends the life of the screw and tool. The elliptically based geometry also provides a small tolerance range between the bit and screw, improving force closure.

The Torx Plus profile further improves the force closure via its working angle of almost 0° and its larger core diameter. This allows a higher torque transfer and prevents the screw and tool from suffering damage.

Producers of carbide inserts use the Torx or Torx Plus system to fasten inserts to tool bodies, but the trend is toward Torx Plus because it more efficiently transmits the torque generated by the driver to the screw. With the Torx Plus drive, little or no end load is required to keep the driver engaged in the recess, reducing operator fatigue and muscular stress during manual assembly. As a rule, carbide insert manufacturers prefer hand tightening over power-driven systems. 

—Alan Richter, based on information provided by torque-screwdriver supplier Sloky USA, El Monte, Calif.

The Torx Plus profile (right) transmits higher torque to the screws than Torx (left) or hex profiles. Image courtesy ACSC International Inc./Sloky USA.

Related Glossary Terms

  • boring


    Enlarging a hole that already has been drilled or cored. Generally, it is an operation of truing the previously drilled hole with a single-point, lathe-type tool. Boring is essentially internal turning, in that usually a single-point cutting tool forms the internal shape. Some tools are available with two cutting edges to balance cutting forces.

  • boring bar

    boring bar

    Essentially a cantilever beam that holds one or more cutting tools in position during a boring operation. Can be held stationary and moved axially while the workpiece revolves around it, or revolved and moved axially while the workpiece is held stationary, or a combination of these actions. Installed on milling, drilling and boring machines, as well as lathes and machining centers.

  • chatter


    Condition of vibration involving the machine, workpiece and cutting tool. Once this condition arises, it is often self-sustaining until the problem is corrected. Chatter can be identified when lines or grooves appear at regular intervals in the workpiece. These lines or grooves are caused by the teeth of the cutter as they vibrate in and out of the workpiece and their spacing depends on the frequency of vibration.

  • computer numerical control ( CNC)

    computer numerical control ( CNC)

    Microprocessor-based controller dedicated to a machine tool that permits the creation or modification of parts. Programmed numerical control activates the machine’s servos and spindle drives and controls the various machining operations. See DNC, direct numerical control; NC, numerical control.

  • fatigue


    Phenomenon leading to fracture under repeated or fluctuating stresses having a maximum value less than the tensile strength of the material. Fatigue fractures are progressive, beginning as minute cracks that grow under the action of the fluctuating stress.

  • gang cutting ( milling)

    gang cutting ( milling)

    Machining with several cutters mounted on a single arbor, generally for simultaneous cutting.

  • grinding


    Machining operation in which material is removed from the workpiece by a powered abrasive wheel, stone, belt, paste, sheet, compound, slurry, etc. Takes various forms: surface grinding (creates flat and/or squared surfaces); cylindrical grinding (for external cylindrical and tapered shapes, fillets, undercuts, etc.); centerless grinding; chamfering; thread and form grinding; tool and cutter grinding; offhand grinding; lapping and polishing (grinding with extremely fine grits to create ultrasmooth surfaces); honing; and disc grinding.

  • grooving


    Machining grooves and shallow channels. Example: grooving ball-bearing raceways. Typically performed by tools that are capable of light cuts at high feed rates. Imparts high-quality finish.

  • high-speed steels ( HSS)

    high-speed steels ( HSS)

    Available in two major types: tungsten high-speed steels (designated by letter T having tungsten as the principal alloying element) and molybdenum high-speed steels (designated by letter M having molybdenum as the principal alloying element). The type T high-speed steels containing cobalt have higher wear resistance and greater red (hot) hardness, withstanding cutting temperature up to 1,100º F (590º C). The type T steels are used to fabricate metalcutting tools (milling cutters, drills, reamers and taps), woodworking tools, various types of punches and dies, ball and roller bearings. The type M steels are used for cutting tools and various types of dies.

  • lapping compound( powder)

    lapping compound( powder)

    Light, abrasive material used for finishing a surface.

  • lathe


    Turning machine capable of sawing, milling, grinding, gear-cutting, drilling, reaming, boring, threading, facing, chamfering, grooving, knurling, spinning, parting, necking, taper-cutting, and cam- and eccentric-cutting, as well as step- and straight-turning. Comes in a variety of forms, ranging from manual to semiautomatic to fully automatic, with major types being engine lathes, turning and contouring lathes, turret lathes and numerical-control lathes. The engine lathe consists of a headstock and spindle, tailstock, bed, carriage (complete with apron) and cross slides. Features include gear- (speed) and feed-selector levers, toolpost, compound rest, lead screw and reversing lead screw, threading dial and rapid-traverse lever. Special lathe types include through-the-spindle, camshaft and crankshaft, brake drum and rotor, spinning and gun-barrel machines. Toolroom and bench lathes are used for precision work; the former for tool-and-die work and similar tasks, the latter for small workpieces (instruments, watches), normally without a power feed. Models are typically designated according to their “swing,” or the largest-diameter workpiece that can be rotated; bed length, or the distance between centers; and horsepower generated. See turning machine.

  • metalcutting ( material cutting)

    metalcutting ( material cutting)

    Any machining process used to part metal or other material or give a workpiece a new configuration. Conventionally applies to machining operations in which a cutting tool mechanically removes material in the form of chips; applies to any process in which metal or material is removed to create new shapes. See metalforming.

  • metalworking


    Any manufacturing process in which metal is processed or machined such that the workpiece is given a new shape. Broadly defined, the term includes processes such as design and layout, heat-treating, material handling and inspection.

  • milling


    Machining operation in which metal or other material is removed by applying power to a rotating cutter. In vertical milling, the cutting tool is mounted vertically on the spindle. In horizontal milling, the cutting tool is mounted horizontally, either directly on the spindle or on an arbor. Horizontal milling is further broken down into conventional milling, where the cutter rotates opposite the direction of feed, or “up” into the workpiece; and climb milling, where the cutter rotates in the direction of feed, or “down” into the workpiece. Milling operations include plane or surface milling, endmilling, facemilling, angle milling, form milling and profiling.

  • milling machine ( mill)

    milling machine ( mill)

    Runs endmills and arbor-mounted milling cutters. Features include a head with a spindle that drives the cutters; a column, knee and table that provide motion in the three Cartesian axes; and a base that supports the components and houses the cutting-fluid pump and reservoir. The work is mounted on the table and fed into the rotating cutter or endmill to accomplish the milling steps; vertical milling machines also feed endmills into the work by means of a spindle-mounted quill. Models range from small manual machines to big bed-type and duplex mills. All take one of three basic forms: vertical, horizontal or convertible horizontal/vertical. Vertical machines may be knee-type (the table is mounted on a knee that can be elevated) or bed-type (the table is securely supported and only moves horizontally). In general, horizontal machines are bigger and more powerful, while vertical machines are lighter but more versatile and easier to set up and operate.

  • plunge milling

    plunge milling

    Highly productive method of metal removal in which an axial machining operation is performed in a single tool sequence. The tool makes a series of overlapping, drill-like plunges to remove part of a cylindrical plug of material one after another. Because of the increased rigidity of a Z-axis move, the tool can cover a large cross-section of material.

  • tolerance


    Minimum and maximum amount a workpiece dimension is allowed to vary from a set standard and still be acceptable.

  • turning


    Workpiece is held in a chuck, mounted on a face plate or secured between centers and rotated while a cutting tool, normally a single-point tool, is fed into it along its periphery or across its end or face. Takes the form of straight turning (cutting along the periphery of the workpiece); taper turning (creating a taper); step turning (turning different-size diameters on the same work); chamfering (beveling an edge or shoulder); facing (cutting on an end); turning threads (usually external but can be internal); roughing (high-volume metal removal); and finishing (final light cuts). Performed on lathes, turning centers, chucking machines, automatic screw machines and similar machines.