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High-speed milling puts more profit in you pocket.

Although it represents only one element of the total cost and productivity picture in metalworking, high-speed machining (HSM), when properly applied, can produce significant reductions in manufacturing costs. HSM focuses on reducing machining time, improving surface finish, reducing side forces on workpieces, and reducing unit horsepower required to machine a particular work material.

HSM need not mean machining at 20,000 rpm or more. On the contrary, as will be illustrated for aluminum, a 10,000-rpm spindle of sufficient horsepower can dramatically increase productivity, thus dispelling the myth that HSM requires ultrahigh spindle speeds.

Extensive end milling tests with various size spindles on 7075-T6 aluminum test pieces, Figure 1, produced the following conclusions about HSM.

* unit horsepower decreases with an increase in cutting speed, but levels off at 5000 fpm.

* High cutting speed with lower chip load improves surface finish when machining thin ribs.

Talking with manufacturing engineers in industries that machine a lot of aluminum gave us additional insight. For example, most plants underutilize their machine tools when cutting aluminum, i.e., a horsepower meter reading of about 10 percent is common. Moreover, some reasons cited for using conservative speeds and feeds included fear of excessive noise, problems in cleaning away chips, anticipated maintenance problems, and apprehension associated with machining faster. In the aerospace indistry, where workpieces are relatively expensive even before machining, fear of scrap is a real deterrent to high metalremoval rates, at least among shop-floor personnel.

Such fears, together with basic misunderstandings that HSM means machining at exotic speeds such as 100,000 rpm, have greatly limited production applications of the technology. In an effort to help metalworkers realize the benefits of HSM for cutting aluminum, we recently developed a series of new guidelines for end milling applications incorporating 2-flute end mills with diameters ranging from 1-2' to 2'.

An example graph for a 2'-dia, 2-flute end mills is shown in Figure 2. The following constraints were used: end-mill fatigue life; chip load for economical tool life; surface roughness; side force on the workpiece; spindle power limit of 100 hp; and spindle speed limit of 60,000 rpm. here's how to use the graph.

1) For a particular aluminum alloy select the largest diameter and shortest length end mill that can be used for rough and finish machining.

2) By referring to the particular HSM graph for the selected end mill size (in our example, a 2'-dia tool), choose appropriate spindle speed, horsepower, and metal-removal rate for the various types of cuts.

3) The spindle speed, horsepower, and metal-removal rate should approach the maximum available capacity of the machine tool.

Consider the following example. For a particular part, a 2'-dia, 2-flute HSS end mill is used for roughing and finishing. For roughing, 1200 rpm is used with 7.5 hp at the point of cut. For finishing, 2000 rpm is used with 0.5 hp at the cut. The machine tool is capable of running from 150 rpm to 4000 rpm with 30 hp. How can the HSM perspective improve productivity?

The machine tool's capabilities are represented by the Figure 2 area enclosed by drawing a line upward from 4000 rpm and across from 30 hp. Notice that within this envelope a 30-hp roughing cut can be made easily at 3000 rpm, whereas a 2-hp finishing cut can be made at 4000 rpm. By selecting these spindle speeds and horsepower, you can reduce actual metalcutting time by 75 percent. Proper feed rates will be 52 ipm for roughing and 40 ipm for finishing.

This example shows how the HSM viewpoint can improve productivity on an existing machine tool. Further gains, of course, can be obtained by purchasing a new machine tool capable of delivering higher horsepower and spindle speeds.

Machine-tool showdown

Another use of the HSM graphs is determining total time required to machine a specific part, or family of parts, using various commercially available machine tools. For example, consider evaluating the machining time for end milling pockets in three different 7075-T6 aluminum parts, Figure 3. Number of pockets, pocket sizes, and tool sizes for this example are described in the table next to the bar graph. Equipment limiting factors, such as horsepower or spindle speed, were determined for each machine tool and are noted for each combination of part and machine at the base of the graph.

This example demonstrates that for large pockets such as in Part A, a machine tool equipped with high horsepower and moderate spindle speed can reduce total machining time by as much as 50 percent. This is an important insight for large aircraft parts that generally are machined on big, expensive gantry-type profilers for periods exceeding 35 hr/part.

You also can conclude from this example that, for end milling small and medium-size pockets (Parts B and C), a machining center equipped with high spindle speed and moderate horsepower can reduce total machining time by as much as 55 percent. Remember, however, the total floor-to-floor time to machine small- and medium-size workpieces may range from 5 to 10 min for small parts to 10 to 30 min for medium-size parts. To reap effective overall benefit from HSM of small- to medium-size workpieces, emphasis must also be placed on reducing total material-handling time, and on minimizing tool-change time.

For more information on high-speed machining, circle E50.
COPYRIGHT 1985 Nelson Publishing
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Copyright 1985 Gale, Cengage Learning. All rights reserved.

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Author:Aggarwal, T.R.
Publication:Tooling & Production
Date:Apr 1, 1985
Previous Article:Industry report - steel.
Next Article:Thread-milling tool tip; technique eliminates imperfect threads at end of cut.

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