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The power of positive inserts.

Insert geometries not possible just a few years ago are now offering users multiple benefits for milling operations.

Chuck Petrosky makes chips--about 40 tons of them last year alone. Chips, in fact, are his only product.

As manager of Kennametal's customer support machining laboratory in Raleigh, NC, and grade development lab in Latrobe, PA, Mr Petrosky has had a hand in evaluating the performance of literally thousands of Kennametal and competitor's insert designs for both turning and milling. It's his job--and the job of the other workers at the two laboratories--to test candidate insert materials and geometries, and to help customers sort out sticky machining problems.

As you might imagine, that experience has left Mr Petrosky with some pretty definite opinions about what works and what doesn't. And when it comes to milling, it pays to think positive, he says.

He's not alone in that assessment. Advances in carbide insert materials and manufacturing technology are making possible insert/cutter body combinations with large amounts of positive axial and radial rake--60 to 65 degrees is not uncommon.

What's good about positive geometries? Wolfried Mielert, president, Krupp Widia Div, Dapra Corp, Bloomfield, CT, explains it this way: "The positive inclination of the cutting edge--axial rake--lifts the chip and curls it away from the workpiece. Positive radial rake cuts softer and with less pressure, using less horsepower, creating less heat, and resulting in less strain on machine bearings, ways, and spindle." He compares positive-geometry milling to a planer on wood, where the work material is lifted up and peeled off.

Mike Gadzinski, milling product manager, Iscar, is a little more specific. "A general rule of thumb is that for every degree more positive the tool becomes, you have about a 2% decrease in horsepower consumption," he says. So if you're going, for example, from a tool that's five degrees negative to one that's five degrees positive, you can expect approximately a 20% decrease in horsepower consumption."

Design drivers

The advantages of positive geometry in milling operations have been known for a long time, but insert manufacturers have only recently begun to develop products that more fully realize the many benefits of thinking positive.

What's driving the current work, says Mr Gadzinski, are fundamental changes in workpiece characteristics and in the way they're being machined. "The near-net shape processes we have today mean there's less and less metal being removed in a lot of applications," he says. "We rarely see workpieces that start with a solid billet and are roughed down to a finished product; instead, we're dealing with things that are preformed almost to a finished part. So you have less and less machining being done.

"Another big consideration is the machine tools themselves," he continues. "The majority of machines being sold today are in the 15- to 20-hp range, and are much lighter and smaller than older equipment. They're designed for high-speed machining, for finishing parts with much less metal removal."

Gary Coslett, milling product manager, Sandvik Coromant, Fair Lawn, NJ, agrees that responding to evolving machines and machining tasks is important. And he can tick off several other factors that are driving the design of milling inserts.

"Versatility is important," he says. "Customers want tools to do as much work as possible while they're in the spindle. They want machines to run as unattended as possible, so cutting tools have to be predictable. Even ergonomics is a factor. We're being asked to produce quieter cutters. And they have to be gentler and easier on the machine tool, but perform better than they did in the past. All those things have driven us toward positive geometries because it's the cutting edge itself that creates noise, breaks, and dictates cutting forces."

Kennametal's Mr Petrosky hears about customers' environmental and surface finish concerns. "Coolant is a big issue. Everyone's asking for dry milling," he says.

Any product involving that many variables and trade-offs is bound to give design engineers headaches. Insert manufacturers, however, have responded to customer demands with an array of sophisticated products that can improve the productivity of face, slot, pocket, and peripheral milling of nearly any workpiece material.

Positive thinking

The main developments making positive-geometry milling possible are insert materials and manufacturing techniques that simply didn't exist just a few years ago.

"Carbide today is a much better and more sophisticated material than it was a few years ago, and coatings are improved," says Mr Coslett. "Not only are current grades tougher, but they're more wear-resistant at the same time. That combination enables us to do things with the top rake of an insert that we weren't able to do years ago."

The toughness comes mainly from higher cobalt contents in insert substrate materials--10% to 12% Co, as opposed to 7% to 7.5% a few years ago. But wear resistance is a function of the sophisticated, multi-layer coatings now being used on carbide milling grades. Besides TiN, common coating materials now include TiCN, TiA1N, and aluminum oxide.

The big change in insert manufacture is the ability to press positive geometry directly into the insert itself. "The ability to mold inserts into complex shapes without having to do a lot of grinding allows you to do some rather unique things," says Mr Gadzinski. Iscar, for example, builds positive axial rake into its milling inserts by making them thicker at the bottom. This lets the insert stay more perpendicular in the cutter body, resulting in a stronger cutter. Positive radial rake can be pressed-in in the same way, he adds.

Let the chips fall?

All the insert manufacturers agree in the merits of positive-geometry milling. But a bit of controversy surrounds the desirability of bumps, pockets, and other chipforming features on the insert rake face.

Sandvik comes down squarely in the chipforming camp. "The design of the rake face redirects cutting forces away from the edge line and into the cutter body," says Mr Coslett. "That reduces power consumption and enhances edge strength, but it also forms and redirects the chip. That usually comes back as a benefit in terms of chip removal and maintenance."

Dapra's Mr Mielert isn't so sure. "Bumps, waves and other features generally are not as important in milling as positive axial and radial rake," he says, although "it can be helpful, when you're using large-diameter cutters and heavy depths of cut, to have some sort of chipforming features on the rake face."

Bumps and shapes on the insert rake face are very effective for chipbreaking in turning, says Jim Crockett, Kennametal's global manager for milling grades and inserts. "But the interrupted cutting inherent in milling operations already breaks chips, so chipforming is the benefit of bumpy insert topographies, which can sometimes enhance 'zone' temperatures away from the cutting edge on light cuts or shorten chips in large-diameter face milling of 'gummy' workpiece materials like low-carbon steels," he says.

"Kennametal's internal competitive metalcutting tests indicate that positive radial and axial rakes are the key to quieter milling at lower horsepower, not features such as 'chipsplitters', which actually increase the surface area of the insert/material interface. However, there is widespread user perception that bumpy geometries are helpful, so you will see them in most milling product offerings," he says.

On the edge

So now you're probably ready to go off in search of the sharpest cutting edge you can find, right? Well, as you might suspect, choosing the edge preparation that's right for you depends mainly on what you're cutting.

"How sharp an edge you want depends on the material you're cutting and on the condition of the workpiece material," says Mr Mielert.

"Most inserts have a standard hone (radiused cutting edge); very seldom will you get an insert that's absolutely sharp unless you're machining something like aluminum," says Iscar's Mr Gadzinski. "The size of the hone varies with insert size. The larger the insert, the more hone you have on it. It can vary anywhere from 0.001" or 0.002" to 0.005 or 0.006"."

Edge preparation also is a consideration in choosing feedrates. The feedrate must result in a minimum chipload greater than the insert edge preparation, says Mr Mielert. "If you've got a hone of 0.002", you don't want to try to cut with 0.001" chipload. You always keep the minimum chipload higher than the edge preparation; otherwise, you in effect have a negative cutting edge."

Using too low a feedrate actually is a fairly common problem, says Mr Gadzinski. "If the machine's vibrating and tools are wearing out, the natural human tendency is to slow it down. In a lot of cases, that's the worst thing you can do," he says. "Maintaining a proper chipload is especially important in peripheral milling. If you're using a 4" diameter cutter but you're milling 1" width of cut on the outside of a part, actual chip thickness becomes much less than the feedrate. You have to increase feedrate in some cases rather significantly to maintain cutting action."
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Copyright 1993 Gale, Cengage Learning. All rights reserved.

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Title Annotation:insert materials and manufacturing technology
Author:Destefani, James D.
Publication:Tooling & Production
Date:Nov 1, 1993
Previous Article:Mill/turn center cranks out hips.
Next Article:CNC blends surfaces better than CAD.

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