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Guide to maximum tool performance.

Guide to maximum tool performance

Whether carbide, cobalt steel, ceramic, cermet, or superabrasive, many of today's tools are built to cut specific materials or handle special workpiece challenges such as tight corners or deep grooves. Even standard tools have been highly designed to offer increased life and speed. But with higher performance comes more responsibility. Engineers must give more thought to operating parameters. New tools can be more sensitive to feeds, speeds, coolant choice, and spindle rigidity than you might have imagined.

Sounds pretty basic, but have you really applied state-of-the-art knowledge? Or did you figure it wasn't all that important? Well, it really is. For example, have you considered that coolant can wreck a tool? Some tools or tool-and-workpiece combinations just don't call for it. Others do. Coolant often is not recommended for interrupted cuts, because it adds to the thermal shock of getting into and out of the cut on each revolution of the blade or workpiece.

Check with lubricant suppliers and toolmakers before running a new setup. Do you want to use mineral oil or synthetic lubricant? Will carbide or coated HSS inserts work best? Coated inserts normally require coolant for machining exotics, but work perfectly well dry on most steel.

Maximum tool life may not provide maximum productivity. With FMS cells, for example, consistent tool life is more important than long life. Choosing the right coolant is vital here. And, adaptability may be more important than maximum life or heavy-cutting capability. Besides repeatability, FMS needs toolholding security. Choose insert systems that employ simple hardware, fewer parts. Avoid locator plates or wedges whenever possible.

Will coating always grant longer life? Indeed, hard coatings, especially glass or ceramic, may flake off. If they do, they can become abrasive particles that wear tools more quickly than if they hadn't had the coating in the first place.

Modern times

Yes, familiar rules are changing, and those that don't are becoming more strict. Fortunately, there is some flexibility. Although most carbide inserts work efficiently only at high speed, you can find a few that can work slowly if that's all the machine or setup can muster.

A few ceramic inserts have a broad range of applications, working at speeds from 80 to 3000 sfm in irons, steels, and high alloys. These can simplify insert inventory as well as machine-setup and programming details.

Teledyne Firth Sterling, Lavergne, TX 37086, offers a new chipbreaker insert with a wide range of applications for light cutting. The firm says its 2B chipgroove solves finishing problems where stringers are the culprits. A combination of double-side insert with high-positive-rake chipgroove suits it for economical positive-rake cutting of aluminum, free-cutting steel, carbon steel, alloy steel, high-temperature alloys, and stainless steel.

The 2B chipgroove controls chips at light depths of cut and light-to-moderate feed rates. The corner is configured to direct chips away from the workpiece, thus preserving surface finish. The high-positive-rake groove reduces build up edge, minimizes cutting-tool pressure, and lowers horsepower consumption. The graph shows the chipbreaking range for turning 4340 steel at 500 sfm with an SD3 CNMG-432A-2B insert.

Because of the light forces generated, tool life is increased up to three times. For example, in test turning workpieces of 8620 material at 780 sfm, 0.015 ipr feed, and 0.030 depth of cut (DOC), a new TFS Grade MP51 tool cut 1166 pieces per corner before failure. A competitive standard tool cut only 750 pieces. In turning titanium, a Grade H36 insert cut 50 parts compared with 30 for a standard tool. In facing 4119, performance jumped from 37 pieces to 167 pieces.

In boring AMS-5645 material with Grade SD5 inserts at 500 sfm, 0.006 ipr feed, and 0.030" DOC, performance increased from two to five parts per corner.

Good advice

The publication, Tooling Progress, published by GTE Valenite Corp, PO Box 3950, Troy, MI 48007, offers advice to insert users. Here are some guidelines for cutting ferrous metals:

"You can achieve maximum productivity and cost efficiency by running setups at the largest depth of cut and highest feed rate the machine tool and workpiece will allow. Neither depth of cut nor feed rate have a significant effect on tool life. And, with faster metal-removal, you can reduce costs through shorter machining cycles and higher production rates.

"In many things, less is more. But in metal removal, larger usually is better than smaller when it comes to three tooling dimensions: First, a larger cutting-edge entry angle helps produce thinner chips, dissipate heat, protect the nose radius, and reduce insert notching. Second, a large toolholder shank will help minimize deflection and reduce overhang ratio. Third, an insert with a larger nose radius will help produce thinner chips, dissipate heat, improve finish, and provide greater tool strength.

"On the other hand, to cut costs, you should select the smallest insert size that conditions allow, because the price of inserts goes up 25 to 40 percent for each increase in inscribed-circle (IC) size. And, to get more parts and reduce costs per insert edge, select the strongest shape the workpiece will allow. In order of increasing strength, available shapes typically include 35-, 55-, 80-, and 100-degree diamond; then triangular, square, and round.

"To get double the number of cutting edges, use negative-rake geometry whenever the workpiece and machine tool will allow. Negative-rake inserts also will provide greater edge strength and help dissipate heat."

Treatment or coating?

Not every drill and end mill on the market is top grade. Those sold with unrelieved stress caused by poor machining or heat-treating processes can benefit from special stress-relieving techniques. For example, some engineers find that cooling tools to -100 F before using them results in longer cutting life. Others say you have to go down to -300 F for maximum benefit.

If you don't like cold processes, Innovex offers FluxaTron magnetic-pulse treatment to do the same thing as deep cold - relieve stress. Once you've got the base metal straight, you might want to add surface treatments such as nitriding (as opposed to hard coating).

Under the right conditions, these extra processes do indeed extend cutting-tool life and improve workpiece quality, sometimes even with good tools. For regrinds, they can boost performance of tools that have been abused in service.

Magnetic treatment can be invaluable with special tools. For example, Innovex loaned a FluxaTron magnetic-pulse unit to a potential customer to try it on a drill. The one trial paid for the unit. It boosted drill performance from between 6 and 15 holes per tool to 100 holes per tool. It was a metric HSS drill, altered by the user, machining 0.03" -dia holes in M-4 tool steel.

Or, is coating the answer? Engineers at Weldon Tool Co, Cleveland, OH, know that hard coatings have their limitations. They have tested them all and found that TiN (gold-color titanium nitride) is certainly a good material, but not always worth the extra cost. You need to check it out for each application. Of course, Weldon will supply tools with almost any coating the user wants. They also have an optional peening treatment to improve the surface of high-performance tools such as Crest-Kut end mills.

When they do order hard coatings for M-7 and M-42 steel tools, Weldon engineers specify the PVD (physical vapor deposition) process. That's because CVD (chemical vapor deposition) is too hot - even the so-called low-temp CVD. Temperatures exceed safe levels for highspeed steel, softening the base metal and unfortunately negating any coating advantage.

Coatings for carbide inserts, however, are applied by the CVD process, because there is a larger throughput, and the higher temperatures don't soften carbide.

Weldon engineers point out that temperatures and adhesion coincide somewhat. That is, they would rather run a PVD process at its highest end of function without destroying the tool - and a CVD at its lowest end of function to avoid destroying the bond. CVD is faster, and it provides a coating with more density or coating load.

Hot news on cool coatings

Krupp-Widia Corp, 19868 Haggerty Rd, Livonia, MI 48152, hopes to change tradition with its new plasma-assisted CVD coating denoted PCVD. It's said to combine the benefits of both the CVD and PVD processes. The patented process uses low-pressure discharge of a reaction gas to allow chemical activity at lower temperatures. It causes chemical reactions that normally require 1000 C in conventional CVD processes to take place at only 500 C in the PCVD chambers.

Krupp-Widia says that the low coating temperatures stop most diffusion, phase transformation, and exchange reactions between substrates and coating materials. Thus substrates retain toughness, even after coating. The process also provides more-uniform surface coating.

Measurements show the PCVD coating displays the same or higher bond strength than CVD coating, solving problems of spalling. The bottom line is longer tool life or increased cutting speeds with less thermal load. For milling and turning with interrupted cuts, expect less thermal cracking and chipping.

End-mill operating hints

Don't even think of using small carbide end mills unless you have new machine tools with well-maintained spindles and toolholders. Spindles must be rigid. If end mills (or drills) are coated with ultrahard materials suac as amorphous glass, the coatings will likely flake off when the tools bend. We've already noted how this material then becomes a an excellent grinding compound - at at the wrong time and place!

Along similar lines, don't machine titanium twice. It gets hard! Be sure chips get out before the cutting tool hits them a second time. Use coolant-hole drills, for example.

Make sure chucks hold end mills securely. High-helix tools such as Crest-Kuts tend to act like screws, (and out of toolholders). That's why you can't use collects to hold them, and you must use rigid fixtures to hold the work. Don't use older machines with quills that "hop up and down" under the influence of high-helix tools.

Even the markings on end mills can be a problem. Weldon uses a laser to inscribe identification on tool shanks without leaving raised metal (not even a micron) on surfaces. This method is essential to ensure that shanks fit tightly in holders. The laser replaces stamping and electrochemical etching, eliminating the need to neutralize marked surfaces.

New wave

Roughing end mills with sinusoidal tooth form have cut heavy metal successfully for many years. These knuckle-form hogging mills are efficient, but often produce a rough surface that must be removed by a finishing pass with a conventional end mill.

The ideal tool, then, is one that can remove material at roughing feeds and speeds, but also produce a finish comparable to a conventional end mill. Combination roughing and finishing end mills with flat or truncated crests are available, but they have never achieved the popularity of sinusoidal tools, according to Brubaker Tool Corp, Millersburgh, PA 17061.

Brubaker engineers have created the Starchip family of end mills for roughing and finishing in one operation. The basic geometry consists of a raised tooth having a flat radial relief and radial clearance angle. This means it can be resharpened like a conventional end mill. However, it also allows the cutting geometry to be precisely adjusted.

Chipbreakers are placed in a helically overlapping pattern whose hand of helix is opposite that of the flute helix. This design balances forces. For example, when an end mill has a right-hand helix and right-hand hand cut, the resulting cutting forces try to pull the tool out of its holder. However, the cutting forces resulting from the chipbreakers, which are placed in a left-hand helix, want to push the tool into the holder.

The balance of forces produces a free-cutting action that reduces horsepower requirements. Also, the opposed-helix design strengthens the form, reducing the tendency of the cutting edge to chip.

Finishing touches

How about exotic tool materials? Don't get too excited. We've heard a lot about Chinese tool steel, for instance. Weldon Tool Co studied the material and found aluminum content. The firm asked its own steel suppliers to provide it for those applications where it seems to work well. In other words, you can get it from local specialty steel produces. And, it may not be that exotic after all.

On the other hand, don't limit yourself to just the tried-and-true methods. There are few easy solutions to difficult problems, but it's worthwhile to explore new tools and rules. One insert maker says, "We like to see large manufacturers contract to smaller shops more willing to try new tools and machining methods!" What should be constant is management and engineering willingness to explore technology.

PHOTO : A single 2B chipgroove insert from Teledyne Firth Sterling (TFS) can handle a wide range of feeds and speeds for light cutting.

PHOTO : Inserts from Kennametal come in many styles and materials.

PHOTO : Brubaker Starchip end mills rough cut and finish in one operation. They do the job best if they work in a spindle with low runout and deflection. Runout may produce a series of parallel lines on workpiece surface.

PHOTO : Careful check of land wear determines tool life in tests at Innovex.

PHOTO : Quantum [TM] 10 whisker-reinforced-ceramic insert performs a deep roughing cut on an Inconel 718 roll. Round insert is strongest possible shape.
COPYRIGHT 1990 Nelson Publishing
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1990 Gale, Cengage Learning. All rights reserved.

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Title Annotation:cutting tools; includes related articles
Author:Miller, Paul C.
Publication:Tooling & Production
Date:Jan 1, 1990
Words:2198
Previous Article:Systems integration is controversial.
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