Turn with the Right Tool

The latest cutoff and plunge-and-turn tools on the market allow machinists to do multiple lathe operations without sacrificing tool performance or making frequent tool changes.

When faced with a cutoff or grooving and turning operation, a machinist typically doesn’t use a tool designed specifically for the job. To separate a slug, blank, or other workpiece from its original stock, a machinist probably will choose a brazed tool or indexable insert not designed for cutoff. When grooving and turning are combined, the machinist might try to do both with an indexable insert designed just for turning or just for grooving.

Using a cutting tool to perform a task other than that for which it is designed may be a common practice to avoid tool changes between lathe operations, but it compromises the performance of the tool. Toolmakers have developed indexable inserts specifically designed for cutoff and plunge-and-turn inserts capable of both grooving and turning. When these tools are used appropriately, the results exceed anything possible with improvised tools.

Cutoff Tools

Lathe cutoff operations are not as simple as they may seem. They require highly specialized, carefully engineered tools. Indexable cutoff tools have been designed to efficiently and accurately separate parts from larger workpieces or to separate identical parts from each other. These tools may be used in place of a separate sawing operation or in applications involving barfeeders.

Choosing a tool of the correct width is the key to a successful cutoff operation. To offer enough stability, most cutoff inserts are at least 0.062" wide. A narrower insert wastes less material, and may even allow for the production of a few more parts per bar. However, if the width of the insert is reduced, the width of its support blade must also be reduced, because the blade must always be narrower than the insert for clearance. Unfortunately, as the blade gets narrower, it becomes less rigid. Therefore, it is important to choose an insert and holder wide enough to offer sufficient rigidity, not necessarily the narrowesttool available.

Although holder designs for cutoff inserts vary from manufacturer to manufacturer, the two main styles are integral-shank and blade (Figure 1). The integral-shank style is a slight variation of a standard holder. It has a standard shank with an insert pocket on one end to hold the insert. The holder may have a screw-down top clamp or a spring-force clamping device. The integral-shank style is more rigid than the blade style. However, the diameter of the part that can be cut off using the integral-shank style is limited by how much of its head has been relieved for clearance. The blade style is a less expensive, more versatile holder. It typically uses spring-force clamping and fits into a tool block that mounts to the machine. Blade-style holders can be designed as double-end tools to provide twice the useful blade life.

The basic shape of a cutoff insert doesn’t change much from manufacturer to manufacturer, but insert designs vary depending on the clamping style of the holder. Unlike most turning inserts, which use screw-down top clamps, cutoff inserts use spring-force clamping. The most common style is the self-locking single-end insert, but variations such as double-end inserts and through-coolant inserts are presently on the market.

Insert manufacturers have developed a wide range of grades for almost all materials in cutoff applications. While coated and uncoated carbide inserts are the most popular grades, advanced technology has allowed cermets to be used in an increasing number of cutoff applications. Multilayer-coated carbide inserts and high-nitrogen-content cermet inserts have been developed specifically for cutoff and grooving.

Figure 1: Blade-style and integral-shank-style insert holders for cutoff applications.

Plunge-and-Turn Tools

The introduction of plunge-and-turn tools has made it possible for users to perform multiple tasks without having to change tools between operations or try to make do with a tool designed for only one of the tasks. In addition to grooving and turning, plunge-and-turn tools are designed for back turning, facing, and face grooving.

When a plunge-and-turn tool is used in an application that previously required two or three conventional tools, the tool paths, depths of cut (DOC), and feed rates have to be changed. Because one tool can perform multiple operations, however, these tools offer many advantages over conventional tools. Tool space on the turret is made available for dedicated tools. Inventory in the toolcrib is reduced, and setup time is saved. Better surface finishes are achieved when turning with these tools, because the insert functions as a wiper flat.

Holder designs for plunge-and-turn inserts vary from manufacturer to manufacturer, but most are integral-shank types with some sort of screw-down top clamp. One design incorporates a “V” clamping surface on the top and bottom to center the insert and provide more rigid clamping. This enables the tool to machine in three directions. When turning with plunge-and-turn tools, the holders are designed to deflect a small amount, allowing clearance that reduces cutting forces.

The insert geometries vary more than the holder designs. Single-end, double-end, full-nose-radius, front-relief-angle, and face-grooving styles are available. While each geometry is intended for a specific operation, such as producing rounded grooves or deep grooves, they all have the ability to plunge and turn. Therefore, fewer tools are needed compared to conventional methods.

Insert grades for grooving and turning are similar to those used for cutoff applications. Coated carbide inserts are the most popular, and multilayer-coated carbide grades are available. Cermet grades are suitable for plunge-and-turn applications as well.

Machining Requirements

The condition of the lathe has a direct impact on the performance of cutoff and plunge-and-turn tools. The machine should have sufficient rigidity and horsepower to prevent vibration and chatter. CNC machines are preferred for their precise movements. Fixturing should be as rigid as possible. The barfeeder must be in good condition, because bar slap is detrimental to tool life. The machine should be able to supply a generous, uninterrupted flow of coolant to the cutting edge of the insert. This is particularly important for cutoff operations, which generate a lot of heat.

Proper selection of cutoff and plunge-and-turn tools is also critical. First, select the appropriate style of insert holder based on machine and workpiece requirements. For cutoff, a general rule of thumb is to use a tool that has a cutoff capability (OD max.) just larger than the diameter to be cut off; in other words, a tool with the smallest possible overhang. This overhang is more adjustable for a blade-type holder than an integral-shank type. For grooving and turning, choose the holder with the highest rigidity and the largest shank.

The next step is to select the insert width and hand. For cutoff, a narrower insert wastes less material, but it must be sufficiently rigid. For grooving and then turning with the same tool, select the largest allowable insert width. The hand of a cutoff insert affects tool life and part quality. In most cutoff applications, a neutral insert offers the best tool life. However, a handed insert will leave a smaller burr or nub and may eliminate secondary operations. Then, determine the appropriate insert grade for the application.

When installing the tool, ensure that the holder or blade is perpendicular to the centerline of the workpiece. For cutoff operations, check the cutting height of the insert; it should be 0.006" to 0.008" above center. If possible, mount the holder upside down to aid in chip removal.

To use cutoff and plunge-and-turn tools properly and most efficiently, it is essential to determine the correct cutting speeds and feed rates. Table 1 shows the recommended speeds and feeds for various cutoff applications. When in doubt, use 70% to 80% of the speed recommended for turning and a feed of 0.004 ipr. Once the tool has cut to the point where the part diameter is equal to the insert width, reduce the feed rate to 0.002 ipr or less.

Table 1: Machine recommendations for cutoff with indexable cutoff tools.

Table 2 and Table 3 recommend speeds and feed rates for grooving and turning with plunge-and-turn tools. For these operations, it is important to determine the correct feed rate and DOC for the insert and toolholder being used. Do not exceed the manufacturer’s recommendation for the maximum load for the insert.

Before performing the actual cutoff or plunge-and-turn operation, review the tool paths and programming. For cutoff operations, cut as close to the chuck as possible, and never cut away from center. Make sure that the coolant stream is aimed directly at the cutting edge and the flow is sufficient. Coolant applied to both the top and bottom of a cutoff insert will extend tool life.

Table 2: Machine recommendations for grooving with plunge-and-turn tools.

Table 3: Machine recommendations for turning with plunge-and-turn tools.

For cutoff or grooving and turning, be careful not to push the insert to failure. Due to the holder design, insert failure will cause damage to the holder. Obviously, it is more cost effective to replace the insert than both the insert and holder. When indexing the insert, check the insert pocket for burrs or debris. If the insert pocket is damaged, the holder must be indexed or replaced.

With all the recent developments in indexable inserts, machinists may get confused about which tool to use. Technological advances in insert grades, especially in cermets, have made indexable inserts more applicable to cutoff and plunge-and-turn operations. Insert designs allow cutoff tools to take advantage of spring-force clamping and plunge-and-turn tools to perform a variety of operations. These advanced grades and designs of indexable inserts shouldn’t confuse machinists but make their job easier and less time-consuming.

About the Author

Brent Lindsey is an applications engineer in the Ceratip Technical Center at Kyocera Industrial Ceramics Corp., Mountain Home, NC.