Holemaking Using High-feed Milling

Dry cutting with an air blast through the spindle allows chips to be easily evacuated when using a high-feed cutter, such as the Tungaloy TXP (shown here)

Many shops today have discovered the benefits of high-feed milling, which uses extreme chip thinning to allow for feed rates of 0.080 in. per tooth and higher.

Also, since the cutting forces created by high-feed milling are directed up into the spindle rather than perpendicular to the spindle, the tool can be used for long-reach applications.

When high-feed milling was first developed, mold shops began using it to rapidly remove large amounts of material for mold cavities. This was particularly helpful given the need for long-reach tools in many of these cavities. The technology then spread to general machining, a scenario in which high-feed milling is often used for face milling and pocket work.

One often-overlooked operation that high-feed milling can be successful in, however, is holemaking.

Since high-feed milling cutters are capable of ramping, they have become an option for quickly producing holes.

Many of today’s CNC machines can perform helical interpolation, an operation that moves the tool in a circular motion in the X and Y axes while simultaneously moving downward in the Z axis. When programming for helical interpolation, care should be taken not to exceed the maximum depth of cut restriction or the maximum ramp angle of the cutter.

Costs Involved

Prior to the adoption of high-feed milling, the popular way to produce large-diameter, long holes required a family of tools. These tools consisted of a basic head, inner cartridge, outer cartridge, pilot drills, shanks, extensions, and inserts, all of which required a significant outlay of capital. The high-feed cutter, on the other hand, requires only the cutter body, an adapter, and some inserts.

Speed

The conventional method of drilling a hole requires movement in the Z axis only, and at a relatively modest feed rate. The high-feed cutter requires helical movement but at faster feed rates. Therefore, the distance a high-feed cutter travels is longer than the travel required in conventional drilling, but is done much faster.

The feed rate of a high-feed cutter is measured in hundreds of inches per minute, but it is taking small laps of only 0.040-in. to 0.060-in. depth of cut per lap. When this is translated into the time required to drill a hole, the operator can expect high-feed milling to be two to three times faster than a conventional drill.

Flexibility

The conventional method of drilling holes also relies on the drill body to produce the hole diameter. In high-feed milling of holes, the CNC controls the diameter that the cutter will produce.

To drill a 3-in.-diameter hole, followed by a 3.5-in.-diameter hole, and a 4-in.-diameter hole used to require three expensive, conventional drilling tools and a lot of downtime for changing tools. High-feed milling of several holes with different diameters is done by the CNC, so no downtime or extra tools are needed.

In many cases, part prints will call for through-holes or blind holes with counterbores or countersinks. These counterbores or countersinks can be produced using the same high-feed tool at the same time and in the same cycle as the drilling operation, reducing cycle time, the need for additional tools, at no extra costs for tooling purchases.

Even tapered holes can be produced with high-feed milling. The accuracy of the hole is controlled by the CNC, not by the tool itself, increasing the accuracy of the operation.

Torque

In drilling operations all of the inserts are engaged and fully working throughout the duration of the cycle. High-feed milling of a hole, however, is a milling operation, and only some of the inserts are engaged at any one time, thereby drawing less torque.

Chip Removal

Since the high-feed cutter is a smaller diameter than the hole being produced, there is room for the chip. The cutter is never lifted off the material being cut, which also makes it difficult to recut a chip. This being said, it is still important to try to evacuate the chip from the hole. The best way to do this is with an air blast through the spindle.

An external air blast may also help evacuate the chip, as can high-pressure coolant through the spindle. Chip evacuation is made easier if the holes are produced on a horizontal machine rather than a vertical machine.

Surface Finish

One drawback to high-feed milling to create holes is the surface finish produced: It is considered to be rough given that the tool is traveling at hundreds of inches per minute.

For more information, visit www.tungaloyamerica.com.