A Rough Guide to Better Milling

July 5, 2006
Simple Techniques Put Chips on the Floor and Parts Out the Door.

Growing in popularity, high-feed milling is adopted easily into existing machine programs.

High-feed milling multiplies feedrates as much as four times.

Tangential milling may double or triple insert edge life while allowing for higher material-removal rates.

TWO SHOPS HAVE FOUND THAT THEY can improve their rough-machining operations by adopting relatively simple milling techniques. Tri-Core Mold & Die Co. now rough cuts parts 33 percent faster, and Harrison Steel Castings Co. has eliminated recurring catastrophic cutter wrecks in roughmilling operations by implementing these new techniques.

Tri-Core (www.tricoremold.com) in Machesney Park, Ill., uses the cutting strategy known as high-feed milling, while Harrison (www.hscast.com), of Attica, Indiana, capitalizes on tangential milling. High-feed milling involves taking shallow depths-of-cut at above-average feedrates — 400 ipm and faster. The main benefit of this technique is that it lets shops rough mill down closer to net part shapes, and do it as quickly as possible.

Tri-Core builds plastic injection molds and machines 98 percent of its own mold bases. When roughing out large core and cavity blocks using 2-in.-diameter button-type cutters, the shop could not exceed feedrates of 80-ipm on depths-of-cut of 0.030-in. on 40-taper machining centers. When fully engaged, the company's cutters generated a lot of chatter. On top of this, cutter inserts only lasted about one-and-a-quarter hours before needing to be indexed.

After initiating high-feed milling, and moving to tooling from Ingersoll Cutting Tools (www.ingersoll-imc.com), Tri-Core ramped its feedrates up to 100 ipm and increased its depths-of-cut to 0.050 in. Charles Lunsford, lead programmer at Tri-Core, said the biggest benefit is that the new high-feed milling inserts don't require indexing as often as the shop's standard button-type inserts.

"We were stopping machines every hour to index inserts. Now, the machines run up to 4 hours without indexing inserts, so we get a lot more unattended machining time," Lunsford says.

Most cutting tool manufacturers agree that high-feed milling works most effectively with cutters and inserts designed specifically for the technique. Inserts typically feature large sweeping radii, shallow backdrafts and positive rakes. These geometries make for an extremely strong insert that reduces cutting forces and directs them inward and up machine spindles, which minimizes lateral cutting forces and vibration.

A high-feed milling insert's shape is the key to its success. When the Ingersoll cutter moves into a corner, there is no spike in the machine's spindle load, Lunsford says. The shop previously machined corners using big enough radii to accommodate button-type cutters, it now does not have to do that.

"When machining molds, for instance, shops spend a large amount of time picking material out of corners and detailed areas, and any tool or milling technique that allows them to achieve net shape faster, and in the roughing process, is beneficial. Since high-feed milling cutters generate less cutting forces, shops can run longer cutters that hang out further from machine spindles to reach part corners and details that may be too deep for conventional roughing cutters," says William Fiorenza, die and mold product manager at Ingersoll.

In addition to cutters and inserts designed for high-feed milling, using the right machine tool plays a key role in getting the most benefits out this rough-milling technique. For instance, Ingersoll estimates that Tri-Core easily could feed its 2-in-diameter high-feed milling cutters at 300 ipm for core and cavity roughing if the shop used its Makino S 33 machining center instead of its 40-taper machines. This is because machines used for high-feed milling need to have such state-of-the-art features as adaptive controls with ample high-speed machine look-ahead features, feedrate capabilities of 400 ipm and rigid machine platforms that are able to handle sharp and abrupt changes in multidirectional forces.

"Older machines that can't feed over 150 ipm may not provide the full benefits of high-feed milling as newer machines can," says James Minock, milling product manager at Seco-Carboloy (www.carboloy.com). He also says that length of cut is another machine consideration for high-feed milling.

If a shop is roughing a short pocket or face, the machine tool must accelerate and sustain velocities of feedingat 300 ipm and faster in relatively short time periods. "It boils down to the number of teeth/inserts a cutter has. The fewer and smaller the inserts, the lighter the achievable depths-of-cut. Running smaller cutter sizes, a lot of machines could handle high-feed milling, but most shops want to use large cutters with more inserts," says Minock.

In test cuts, Seco-Carboloy's 6-in.-diameter high-feed milling cutter roughed out, from solid 4340 steel, a 12-in.-diameter, 14-in.-deep hole in less than 20 minutes. However, such a cut required a large machine with ample horsepower and good chip evacuation. The key to high-feed milling is feeding cutters fast enough to keep them ahead of average chip thicknesses, Minock says.

In addition to the availability of cutters and machines that can handle it, high-feed milling also is growing in popularity because it is easy to implement in existing machine programs. A lot of shops have dedicated CNC programs, and adjusting them for a toolpath change would take a long time. But with high-feed milling, only feedrates change, not toolpaths. "Other techniques, such as plunge milling, for instance, require program changes to allow for Z-axis movement, and many shops have a tough time adapting their CNC programs to such a milling technique. For highfeed milling , machine operators can make necessary adjustments on-the-fly at the machine tool," says Minock.

PRIOR TO INCORPORATING TANGENTIAL MILLING, HARrison Steel Castings used conventionally positioned round inserts to rough mill 0.625 in. of material over a 500-square-in. area on cast steel pads for bulldozers. Round inserts are renowned for having the strongest cross section of any insert shape.

However, Harrison ran two roughing and two semifinishing passes and had to change inserts after each pass. Edge failures overloaded the cutter, forcing the shop to repair blown pockets or replace an entire cutter about once a week.

"An insert would fail in a flash, and before anyone could react, the inserts would be plowing the metal rather than cutting it. All told, the four roughing passes took an hour of chipmaking time, and stoppages for edge changes simply stretched out the operation," says J.D. Gray, process control coordinator at Harrison.

After switching to tangentially orientated, double-sided square inserts from Ingersoll, the shop found that its inserts lasted through all four passes on each part, and cutter wrecks no longer happened.

Tangential-milling cutters orientate inserts so that they lie flat in the cutter rather than standing up, as in a conventional configuration. This aligns the insert's strongest cross section with the main cutting force vector. The change in configuration can double or triple edge life and improve process security in rough-milling applications while enabling higher material-removal rates, according to Ingersoll.

The technique requires special cutters and inserts that cost about the same (size for size) as conventional roughing tools. Most shops often use this technique for slab milling and face milling, taking heavy depths of cut where radial tool pressures are relatively excessive. But it should be noted: This technique is more suitable for milling flat plains, not contouring as in captive pocket milling.

Shops can do tangential milling on many different types of machines, but vertical machining centers are the machines most often used for this work. Because it is a roughing technique, it can be demanding on a machine's spindle, and it requires that machine size, cutter diameter and horsepower be considered and matched.

According to Ingersoll's Fiorenza, the depths-of-cut a shop can take using tangential milling are proportional to machine horsepower. Even shops with low-horsepower machines of 25-hp can benefit from tangential milling. Lower horsepower would limit such shops to taking cuts of 0.100-in. deep with a cutter that could handle 0.300 in., but they would still benefit from the technique's relatively higher material-removal rates.

"Roughing with conventional inserts, shops can experience breakage due to high cutting forces," says Fiorenza. "With tangential cutters, these same shops can feed at high rates, and the inserts will survive."