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High-speed machining enters the Iron Age.

High speed machining is rapidly evolving from a high-tech specialty, largely limited to the aerospace and electronics industries, into a mainstream manufacturing technology. It offers major advantages for staying competitive in the global market--improved productivity, reduced costs, and enhanced cutting accuracy. Cycle-time reductions of 20% to 80% on aluminum parts and corresponding productivity increases of 25% to 400% are common. Now high-speed machining stands on the threshold of bringing those same breakthrough to cast-iron parts manufacturing.

Over te last five years, high-speed machining has achieved wide acceptance with producers of aluminum parts. Aerospace contractors are machining components two to five times faster than with conventional equipment. Applied to electronics parts and chassis, high-speed machining reduces cycle times and unit cost while allowing thinner walls, finer detail, and superior finish. Automotive manufacturers find the technology can provide the capacity for high-volume operations with high flexibility.

The most difficult aluminum applications have been addressed by developments in spindle design and tooling, paving the way for high-speed machining to take on tougherr metals, starting with gray iron. This discussion examines recent, as well as likely, developments. Shown here, for example, is face miling of an ABS brake manifold, showing how high-speed machining is practical for small to medium-sized cast iron parts.

Many of the major advances in high-speed machining of aluminum over the past two years have resulted from improvements in tooling. Development of new inserted cutters deserves special mention. First, the inserted tools make deep pocket machining practical and affordable, where large, long tools of solid carbide would be highly cost prohibitive. Second, insert technologies provide the key to extending high-speed machining capabilities to tougher metals.

Inserted cutters are not as free-cutting as solid carbide tools with high helix and high shear angles. This factor, combined with the long length and moment of deep pocket tools, means higherr cutting forces are exerted on the spindle bearings when inserted cutters are used. To deal with the greater cutting forces, spindles have been developed with greater power, larger bearings, higher preloads, and more rolling elements.

At the present level of tooling technology, carbides are good to 1000 surface feet in aluminum alloys, and coated carbides to 1500 surface feet. Cermet is achieving 2000 surface feet, while polycystalline diamond (PCD) edge tools are approaching 10,000 surface feet. These are general figures. Actual experience varies--both higher and lower--depending upon silicon content in the aluminum alloy and acceptable rate of tool wear.

Continued tool development should bring greate abrasion resistance for carbides and coated carbides and greater toughness for cerment. One of the most promising advances is occurring in thin-film, monocrystalline, diamond-coated tools. This holds the promise--as demand grows and cost comes down--of even higher metal removal rates than possible with PCD, as well as application for the first time to tools of complex geometry, such as form tools and taps. Diamond-coated inserts offer multiple edges and indexing capabilities for productivity advantages over single-edge inserts.

Having established its productivity advantages with aluminum, high-speed machining stands ready to bring the same breakthroughs to cast-iron parts manufacturing. Aggressive development has overcome the major technological hurdles to faster metal removal with cast iron with new tooling technology and powerful, high-speed, rigid spindles, complemented by very stable, yet responsive machine architecture.

Extensive testing and feasibility studies on high-speed machining of cast iron have produced these results, which are representative of the potential that high-speed machining offers for cast-iron parts: * Drilling-1" drill run at 15,000 rpm and 100" per minute to 0.005" true position tolerance and excellent surface finish of 63 rms. * Milling--8" dia face mill run at 2500 rpm in finished cut at 0.10" depth, feeding 300 ipm. Roughing cut with same tools made at 1000 rpm at 160 ipm at 0.125" depth of cut. * Boring--3" dia twin insert boring head, run at 3000 rpm and 0.060" radial depth of cut at 60 ipm for roughing; finish cut was made at 5000 rpm and 0.005" depth of cut at 100 ipm.

Commercially viable

High-speed machining of cast iron can today be commercially viable for many manufacturers and part producers. In support of that belief, LeBlond Makino demonstrated the high-speed machining of two cast iron automotive components at IMTS '92: * Large diesel engine block. A horizontal machining center with 40-hp, CAT 50 taperr, 15,000-rpm spindle, machined a 900 cubic inch, V-8 diesel engine block. This part was intentionally selected to prove the capabilities of high-speed machining at the largest and most demanding assignments. Feasibility studies with the engine maker have shown that one machining center with two fixtures can consolidate 26 separate transfer line operations in machining of the engine block. A cellular arrangement of multiple machining centes would allow high-volume production at capital costs very competitive with transfer lines and flexibility. * ABS brake manifold. In the other demonstration, an ABS brake manifold was machined on a 30-hp, CAT 40 taper, 20,000-rpm HMC.

Metal removal--Al vs Fe

High speed is relative and constantly evolving term. In machining of aluminum, high speed today probably signifies more than 10,000 rpm and/or 4000 surface feet. For cast iron, anything over 6000 rpm and 1000 surface feet could be considered high-speed machining. Feed forces are higher and surface footage less because cast iron is much tougher than aluminum. As a rule of thumb, the metal removal capability with aluminum by high-speed machining is 4 cubic inches per free spin horsepower. For cast iron, metal removal twin typically run 1.5 to 2 cubic inches per horsepower, though we have approached 3 cubic inches in some experiments.

In fact, we were surprised in our first tests at high-speed machining of cast iroon to find we were pulling more cubic inches than theoritically seemed possible. We attribute this to synergy of high horsepower, spindle speed, and feedrate. By sinking so much horsepower into such a small area and feeding into the material so quickly, there is not engough time for the heat to soak into the part. So much heat is concentrated in the tiny zone ahead of the cutting tool that the yield strength of the cast iron is greatly reduced. This phenomenon is what makes high-speed machining of cast iron practical.

High metal removal rates

The greatest attraction to high-speed machining in today's competitive global marketplace is its ability to improve both productivity and quality. These double-barreled benefits result from reductions in tangential load that occur with cutting at high-surface footages. Tangential load goes down as surface footage goes up, which can be clearly seen as a function of this formula:

Tangential load (lb) = 126,000 x hp/diameter x rpm (surface footage)

The higher feed forces with cast iron are best handled by reducing the feed per revolution and increasing the rpm to achieve a desired metal removal rate. In this way, the target surface footage can be attained, while reducing cutting forces and pressure on the tool. For example, having the feedrate and doubling the rpm produces the same surface footage. Further increasing the rpm or decreasing feed will increase metal removal. The following chart graphs this relationship between tangential load, surface footage, and rpm.

This ability to increase metal removal while reducing cutting forces holds a host of benefits and options for manufacturers looking to improve output and competitiveness: * Reduced heat effects--High feedrates allow material to be removes ad quickly as it is being heated. Most of the heat goes out with the ejected chips, minimizing heat absorption by the part. This avoids workpiece stress and thermal growth to allow finer details and tolerances; * Greater machining consistency and accuracy by reducing cutting forces hence sources of error from deflection of workpiece, fixture, or machine tool; * Improve surface finish for better part performance, quality appearance, and/or reductions in finishing costs and operations; * Reduced fixturing and setup requirements for direct cost, labor, and time savings. Low cutting forces can allow low-profile fixtures that greatly improve machine tool access. This permits more part faces to be machined in single setup and single operation.

Tooling requirements

Tooling presents the major difference between high-speed cutting of aluminum and cast iron. Because of the greater abrasiveness of cast iron, the solid-carbide and coated-carbide tools that are the workhorses in high-speed machining of aluminum are not effective with cast iron. High-speed machining of cast iron demands either cubic boron nitride (CBN) or ceramic-inserted tooling.

Where ceramics can be used, they are the insert of choice because they cost far less. However, their range of applications is much more limited because they are not as tough. Ceramics are prone to catastrophic failure if applied in marginal circumstances.

CBN, while harder and tougher than ceramics, is not as chemically inert. It can have an affinity for many ferrous alloys and some ductile and malleable irons, leading to accelerated wear. Ceramic inserts can perform CBNs in those niche applications. New superbrasive hybrids may prove an even better alternative. Part ceramic and part CBN, they offer most of the advantages of CBNs with most of the chemical inertness of ceramics. We have had good success with GE's new BNZ 8000 superabrasive. With the possible exception of cost, CBN is probably the ideal material for machining gray iron; chemical reaction is not a problem with gray iron.

CBNs and ceramics provide value-added ways to improve the economics. In many cases, it is possible to use cutter bodies (approved for high rpm) but with fewer inserts for high-speed machining. For example, a face mill can be used with two inserts instead of eight, then run at four times the rpm. This gives the same metal removal rate, but one-fourth the cutting foces. While each individual insert may cost more, only a quarter as many are being used. If the two cutters are ceramic, this may provide a true savings over eight conventional cutters. At present costs, CBNs could still be more costly.

However, other kinds of savings can help offset the cost difference. For example, two inserts can greatly simplify the setup of face mills. Normally, with a large face mill and a large number of inserts, adjusting all of them to exactly the same height for optimum tool life and surface finish can require a great deal of time. In many automotive shops it is common to have two or three face mills being set up while cutting with another. Using two inserts can save on labor and cutter bodies. In some cases, it is feasible for the operator to set up the tool on the machine.

"Full top" CBN--where the layer of CBN is sintered directly to the carbide substrate--is preferable for high-speed machining. We have encountered a number of applications with brazed CBNs where the CBN material could handle the cut, but the braze joint would melt and throw the tool out of the substrate. There were also problems with chipping of the CBN against the carbide substrate.

Full top inserts are more costly than conventional single-edge brazed CBN inserts because they contain more CBN material. However, full top inserts allow indexing due to multiple edges per insert, providing significantly greater life per insert. This advantage has been enjoyed for several years in lathe applications where the the tool does not rotate. Machining center applications have been handicapped by lack of a means to secure the insert to the rotating tool at high rpm. The EDM group at LeBlond Makino solved the problem by developing a process to pierce through both the CBN and carbide substrate, allowing inserts to be mechanically lock-screwed to the cutter body. For safety reasons, we recommend that every insert be screwed to the cutter body in all high rpm applications.

Based on out test experiences, here are some specific recommendations relating to high-speed machining of cast iron: * Large diameter bores and face mills--Ceramics can be highly effective and cost competitive, with the exception of parts with severe interruptions. Then CBNs will outperform ceramics due to their toughness. For high-speed boring, we recommend using a twin insert boring bar for its inherently betterr dynamic balance and balanced cutting forces compared to conventional single insert tools. The two inserts also allow greater metal removal at a given feed per revolution. * Small diameter boring--The negative rake that is required by the lack of strength for most ceramics creates problems with chip formation and chip disposal. CBNs permit more positive rake geometries, so tend to work much better. * End milling--Here, too, ceramics can force the rake to be excessively negative, so that poor cutting conditions result. * Drilling--We have not found any ceramics that are suitable for drilling of cast iron. We have obtained exteremely successful results drilling with CBN inserts in the outer pocket of a Komet inserted drill and a conventional coated-carbide drill in the inner pocket where surface footage is lower and thrus higher. * Tool Balancing--We have never had an improperly balanced tool result in spindle failure. However, tools are sensitive and improper balance can result in unacceptable tool life or failure, poor surface finish, or even unsafe condition (see Safety Recommendations for high-speed machining).

Spindle requirements

Spindle design is critical to successful high-speed machining of cast iron. Compared to aluminum, cast iron demands more powerful and more rigid spindles to deal with the greater load forces presented by the tougher metal.

Achieving high metal removal rates with cast iron--both at high-speed finishing operations and in lower rpm, greater depth-of-cut roughing operations--demands substantial spindle horsepower. As can be seen from this formula, it takes more horsepower to achieve high torque and cutting force at lower spindle speeds:

Work per unit time (power) = a constant x torque x rpm

There is a direct coupling of horsepower, torque, and spindle rpm. There is no escaping this law of physics in high-speed machining.

Power requirements for cast iron are compounded by the necessity of insert tools of negative or neutral rake, which are not as free cutting as the more positive carbide tools usable on aluminum. In general, high-speed machining of cast iron requires about double the free spin hp to achieve the same metal removal capability as with aluminum.

Machine advances

A key advance in expanding applications for high-speed machining has been the introduction of larger, high-speed machining centers capable of taking on larger jobs and higherr machining loads. More attention is being given in machine design to reducing cycle times and optimizing machine cutting time.

High machine productivity--minimizing non-cut time--requires reliability, fast spindle acceleration/deceleration, high-speed traverse and cutting feed, fast tool and pallet changes, CNC technology, and, in many cases, the capability for automated, largely untended operation.

Major advances have also been made in high-speed controls. These are particularly effective at the heavy data crunching required for highly accurate cutting at high speeds on corners, changing geometries, and circular interpolations.

Spindle acceleration, in particular, can be more central to machining productivity than maximum rpms. In high-volume, high-speed machining, how fast the spindle gets to speed can be the most critical determinant to reducing cycle times. In general, the more powerful the spindle, the greater will be its acceleration and productive capacity at high-speed machining. This factor, we find, is not well understood, and thus does not receive the weighting it should in machine evaluations.

The spindles developed by LeBlond Makino have the drive motor integral with the spindle. This eliminates gears, belts, and transmission losses to deliver full power, seamless gear change, and immediate spindle response. In the CAT 40 taper design, the spindle housing is the drive motor stators; the spindle body, the drive motor rotor. The CAT 50 taper spindle couples the motor to the spindle body. This integration of motor and spindle provides the additional advantage, from its rotating mass, of the "flywheel" effect. This puts more actual power into the cut than is produced by the spindle motor, again allowing results that can exceed the theoretical.

For more information from LeBlond Makino, Mason, OH, circle 343.

Safety recommendations for high-speed machining

* Operators must receive special training to assure that they appreciate the potential energy generated in high-speed machining and observe proper safety practices. * All tools placed in a toolchanger must be suitable for operation at the maximum spindle speed available. This guards against unsafe conditions resulting from human error in selecting speed. * All tools should be balanced to recommendations of the manufacturer. Not all tools are designed for high rpm operation. Balancing alone will not necessarily make them safe. Suitability must be confirmed with the tool manufacturer. Preferably, the entire tool assembly should be balanced to avoid possible "stacking" of tolerances. * For high-speed machining with inserted tools, every insert must be screwed to the cutter body. Clamps are not sufficient to hol inserts in place at high rotational forces. Inserts must be fastened to the manufacturer's recommended torque using a torque wrench and the threads coated with the recommended anaerobic sealing compound to prevent them from vibrating loose. Screws must be replaced routinely to avoid fatigue from extended use. * Before using brazed tools, check with the manufacturer to assure that the tool can take the temperatures of high-speed machining. Brazed joints can fail in high-speed operations. * Nothing in this discussion should be misunderstood as LeBlond Makino's recommendation for operation of machines or tooling at speeds higher than those recommended by their manufacturers. The cutting speeds described have been accomplished with elements designed to withstand the forces encountered.
COPYRIGHT 1992 Nelson Publishing
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Copyright 1992 Gale, Cengage Learning. All rights reserved.

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Title Annotation:cast-iron parts manufacturing
Author:Hyatt, Greg
Publication:Tooling & Production
Date:Sep 1, 1992
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