SECTION VI.  MACHINABILITY

Introduction
Machinability
Effect of Microsturcture
Comparative Machinability
Hard Spots
Surface Finish
Coining
Manufacturability Considerations
Machining Recommendations
References

Introduction

To succeed in the fiercely competitive international markets for all finished products, from motorcycles to machine tools, manufacturers must offer the end user the best value - the highest ratio of quality to cost. In order to maximize quality while minimizing cost, designers have added manufacturability to the list of criteria that must be met by a successful design. This trend toward increasing importance of manufacturability has been confirmed by the results of a survey of 2500 design engineers conducted by the Ductile Iron Group. When asked to rank 19 materials selection criteria in order of importance, respondents placed both ease of machining and cost of manufacture in the top six. In modem manufacturing terminology, manufacturability is an attribute which indicates how economically a component can be produced to meet customer specifications. This concept embraces the traditional indicators of machinability - tool life, power requirements, and surface finish and accuracy - and adds other manufacturability criteria such as cycle times, yield, scrap, consistency, inventory requirements, compatibility with automated NC machining, and overall manufacturing cost.

The production of most finished metal products involves machining operations to produce the desired final shape. Castings offer the designer the lowest cost route for the production of complex shapes because they can be cast to near final shape, reducing both machining and materials costs. Near net shape casting technology and improved dimensional consistency offered by competent, modern foundries yield additional savings in manufacturing costs. Ductile Iron, with its excellent castability, offers the designer all the manufacturing advantages of castings plus the added benefits of a machinability: strength ratio that is superior to other cast irons and cast steels.

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Machinability
Machinability is not an intrinsic property of a material, but rather the result of complex interactions between the workpiece and various cutting devices operated at different rates under different lubricating conditions. As a result, machinability is measured empirically, with results applicable only under similar conditions. Traditionally, machinability has been measured by determining the relationship between cutting speed and tool life because these factors directly influence machine tool productivity and machining costs. The increased use of disposable inserts has reduced tool life costs and this factor, along with a greater emphasis on quality, has increased the importance of surface finish and dimensional accuracy and consistency.

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Effect of Microstructure
Machinability is determined by microstructure and hardness. The graphite particles in Gray, Malleable and Ductile Irons are responsible for the free-machining characteristics of these materials and their superior machinability when compared to steels. Within the cast irons, graphite morphology plays an important role in machinability, with the graphite flakes found in Gray Iron providing superior machining characteristics. While the graphite particles influence cutting force and surface finish, the matrix is the primary determinant of tool life.

Hardness is often used as an indicator of machinability because of the close relationship between hardness and microstructure. However, hardness gives an accurate representation of machinability only for similar microstructures. For example, a tempered martensite matrix will exhibit superior machinability to a pearlitic matrix of similar hardness. Figure 6.1 describes the relative machinability of ferrite and pearlite. The properties and machinabilities of these and other matrix components follow.

Ferrite is the softest matrix constituent in Ductile Iron and as a result exhibits the best machinability. While not as soft as the ferrite in steel, the ferrite in Ductile Iron gives superior machinability due to the effect of silicon, which decreases ferrite toughness, and the lubricating and chip-breaking effects of the graphite spheroids. Machinability increases with silicon content up to about 3% but decreases significantly with increasing silicon content above this level.

Pearlite, which consists of an intimate mixture of soft ferrite and hard lamellar iron carbide, is a common matrix component in all intermediate strength grades of Ductile Iron. The volume fraction of pearlite and the fineness of the lamellae determine the hardness and the machinability of Ductile Iron. Although machinability decreases with increasing pearlite content, pearlitic irons are considered to have the best combination of machinability and wear resistance. Figure 6.1 shows that pearlite fineness affects machinability and that the effect of hardness decreases as pearlite fineness increases.

Carbides are the hardest constituents in Ductile Iron and have the poorest machinability. When present as thin lamellae in pearlite they are easily sheared and are in their most machinable form. When present as massive or "free" carbide, both iron and alloy carbides cause a dramatic reduction in machinability (Figure 6.2).

Martensite is an extremely hard matrix phase produced by quenching Ductile Iron. It is too hard and brittle to be machined as quenched, but after tempering martensite is more machinable than pearlite of similar hardness.

Other structures such as acicular bainites and ferrite are produced by interrupted cooling in Ductile Irons with sufficient hardenability to suppress the formation of ferrite and pearlite. Acicular microstructures have a similar machinability to martensite tempered to the same hardness.

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Comparative Machinability
Improved machinability is often one of the benefits gained when a steel component is replaced by a Ductile Iron casting. Machining Handbooks do not present an unambiguous indication of the improved machinability of Ductile Iron, and it is instructive to use practical examples whenever possible. Experience gained by General Motors during the machining of ferritized Ductile Iron blanks for the production of ADI hypoid pinion-and-ring gears revealed improvements in tool life ranging from 20% to over 900%, compared to the annealed, forged steel blanks (Table 6.1). In addition to improved tool life and reduced tool costs, the improved machinability led to significant increases in productivity. Both laboratory and shop trials at Fiat (Table 6.2) on the machining of differential bevel gears revealed that, compared to a forged 18CrMo4 steel, ferritic Ductile Iron could be machined faster with less tool wear, resulting in increased productivity and reduced costs.

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Table 6.1 Tool life improvement resulting from the replacement of forged steel gear blanks by ferritic Ductile Iron.

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Table 6.2 Comparison of the machinability of a ferritic Ductile Iron and a forged 18CrMo4 steel.

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Hard Spots
Isolated "hard spots" in castings can seriously degrade machining performance. These areas of significantly increased hardness usually consist of carbides caused by localized rapid cooling and excess levels of carbide forming elements. Undissolved inoculant, oxides (slag), refractories, dross and burned-on moulding sand can also produce hard spots that are detrimental to machinability. Most hard spots can be eliminated by the use of good foundry practice: minimum levels of carbide forming elements (including magnesium and cerium), good inoculation, minimum holding times, correct pouring temperatures, good pouring practices, hard, expansion-resistant molds and good gating practices, including the use of gating system filters. Unavoidable hard areas in complex castings caused by rapid, localized cooling can be eliminated by annealing or normalizing heat treatments.

Surface Finish
Ductile Iron can be machined to produce a very fine surface finish, with the degree of finish depending on the fineness of the grain structure and the finishing method. With grinding and honing, a surface finish of four microinches or less is possible. Table 6.3 summarizes the surface finishes that can be obtained with various machining operations and different grades of Ductile Iron.

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Table 6.3 Surface finish in machined Ductile Irons.

*Reference

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Coining
Coining is a specialized operation that can be used to both deform a Ductile Iron casting to produce its final shape, and shear off ingates, feeder necks and parting line "flash". Due to the strength of Ductile Iron, the size of casting that can be coined and the degree of deformation produced are limited. However, for small, high production castings that have been cast to near final shape and require limited further dimensional control and ingate and feeder neck removal, coining is a highly cost-effective operation that can eliminate certain machining operations.

Manufacturability Considerations
The machinability of a casting is an important component in its overall manufacturability, but there are other important considerations. Machining allowance affects productivity, yield and machining costs. Compared to steel castings, Ductile Iron requires reduced machining allowance for similar section sizes. Increased consistency of casting dimensions resulting from high density molding, and reduced surface defects can permit further decreases in machining allowance. Consistency of casting dimension is critical to obtaining the performance offered by modem automated machining centers. Consultation between the designer and foundry, the incorporation of manufacturability criteria in the purchase specifications, and the selection of a competent Ductile Iron foundry as a source of consistent castings can significantly improve the manufacturability of the component and increase the value offered to the end user.

Machining Recommendations
Starting recommendations for the machining of Ductile Iron are summarized in Tables 6.4 - 6.8, obtained with permission from the Machining Data Handbook. For more complete data the reader should consult the first three references. Additional information on the machinability of Austempered Ductile Iron and Austenitic Ductile Iron can be found in Sections IV and Section V respectively.

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Table 6.4 Starting recommendations for drilling Ductile Iron.

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Table 6.5 Starting recommendations for turning Ductile Iron with single point and box tools.

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Table 6.6 Starting recommendations for turning Ductile Iron with ceramic tools.

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Table 6.7 Starting recommendations for face milling Ductile Iron.

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Table 6.8 Starting recommendations for slab milling Ductile Iron.