Casting a part for better machining: metalcasters can reduce their machining costs through improved production methods and proper part and tooling design.
Machinability generally is based on up to five main factors.
"For some, it is the ease of machining (meaning lower horsepower to get the job done), for others it is having excellent tool life and low tool failure rates (from breaking)," said Mark Fields, manager of metallurgy and melting at Cast Fab Technologies, Cincinnati. "For others, it is the achievement of an excellent finish, and for still others it can be the overall amount of metal removed. And for some, it may have to do with the cleanliness of the removed material and it's shape. For most, it is a combination of all of the above."
Depending what machining factor your metalcasting facility wants to improve, you can take steps to clean up your melt, tighten your molding processes or adjust part design.
Machinable By Design
Metalcasters can work with their customers to improve a part's machinability through design, which often means finding ways to eliminate the need for machining at all. According to Edward Vinarcik, product engineer for a company that purchases castings, machining processes by their nature create waste. He said that after a design is completed, each machined feature should be evaluated by answering a series of questions, starting with three basic queries (Fig. 1):
[FIGURE 1 OMITTED]
* Is the feature required?
* Can the feature be cast net-shape?
* Can the feature be positioned to consolidate tools and fixturing?
Todd Farrar, vice president of iron metalcaster Farrar Corporation, Manhattan, Kan., said his company, which also performs machining in-house, makes an effort to discuss with its customers up front the different production or design options that may affect machining and, ultimately, part cost. A cast-in hole or cored cavity, for instance, can reduce machining time, but if the end-user requires tight tolerances, it can be best to machine it.
"If we can hold tolerance consistently across multiple runs, generally speaking, we prefer to do it in the casting," Farrar said. "We tell our customer up front there are different ways to get from A to Z in their part."
Some features may be added to a casting to facilitate machining when it cannot be avoided. For instance, patternmakers can design-in pads, which are projections of excess metal on the casting to facilitate machining or other secondary operations, or bosses, which are short protrusions on the surface of a casting intended for drilling or tapping. The location and proportion of pads and bosses are critical in order to avoid hot spots that may lead to shrinkage cracks. According to AFS Aluminum Casting Technology, pads and bosses located on walls not perpendicular to the parting line should be tailed to eliminate the need for sand cores, loose pieces, split metal cores and multiple mold partings (Fig. 2). The height of bosses and pads should be less than the thickness of the casting section but high enough to permit machining without touching the surface of the section (Fig. 3). When several bosses are on one surface, joining them will facilitate machining (Fig. 4). Designing sloping bosses for a straight parting of the mold or die will save money in tooling construction (Fig. 5). In permanent molds, ribs on the mold parting line can serve as feeders.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
A casting's parting line should be located away from where it will be clamped into a machine tool.
"End-users of castings are looking for tight dimensions in the areas they are calling for machining," Farrar said. "It can be tough to clamp onto a parting line because it is not always as consistent [as the rest of the casting surface]."
Cut Machining Time
Since machining cost is closely related to machining time, modifications to the part to increase machining efficiency is a bonus. Casting a few locators (flat pads on which the machining equipment can clamp) onto the part can cut down the time needed to machine. The pads then can be quickly ground off after machining, if needed.
Finding the right balance of machine stock can aid in reducing machine time, as well. Machine stock is the extra amount of material added to areas of the casting that are going to be machined. Machining operations hold tighter tolerances and dimensions than a raw cast surface, so enough machine stock should be in place to give a consistent amount of material to remove, Farrar said. If enough machine stock is not present, the machine tool may end up leaving some of the raw casting surface open.
However, a heavy hand on the machine stock results in longer machining times, more metal needed in the raw part, and more material left on the machine room floor.
"It's important to make sure that wherever there is machining need, there is enough machine stock," Farrar said. "But that will be determined by whoever is doing the machining. Machine shops have different requirements based on their capabilities and equipment."
Which machining equipment a part fits in also factors in to machining time. Smaller machines generally work faster than bigger machines. Discuss the work envelopes of your machine shop's equipment. By maneuvering the part design to fit into a smaller envelope, you could significantly cut machining cost and time.
Leave Part Features to the Machine Shop
In some instances, a metalcasting facility may simplify its casting design and leave the features requiring tight tolerances up to the machine shop. One permanent mold aluminum caster has found that adding in more machining to a part can make it less expensive.
Table 1. Effect of Iron Matrix Microstructure on Tool Life Material Matrix Type of Hardness Cutting speed * microstructure graphite m/min sfm Gray iron 100% ferrite Flake 100 270 880 Coarse Flake 195 110 360 pearlite Fine pearlite Flake 225 105 340 Acicular Flake 263 60 200 Ductile 100% ferrite Spheroidal 170 250 810 iron 97% ferrite, 3 Spheroidal 183 175 570 % pearlite 60% ferrite, Spheroidal 207 130 430 40% pearlite 60% ferrite, Spheroidal 215 110 360 40% pearlite 20% ferrite, Spheroidal 265 75 240 80% pearlite Ferritic 100% ferrite Temper 109 290 950 malleable carbon ASTM 32510 Pearlitic Spheroidite Temper 179 140 450 malleable carbon ASTM 48004 ASTM 60003 Spheroidite Temper 230 85 280 carbon ASTM 80002 Spheroidite Temper 250 80 260 carbon Material Ultimate tensile strength (ksi) Gray iron 15.7 35 45 59 Ductile 70 iron 77 84.7 93 97.25 Ferritic 50 malleable ASTM 32510 Pearlitic 70 malleable ASTM 48004 ASTM 60003 80 ASTM 80002 100 * Cutting speed for 30 minutes tool life. Source: ASM Metals Handbook, Volume 16 Machining
Aluminum castings can be machined on similar equipment to gray iron but at higher speeds and feeds. L A Aluminum, Hayden Lake, Idaho, machines 80-85% of its parts in-house and in recent years has invested in dual spindle lathes, vertical and horizontal machining capabilities, and 4- and 5-axis CNC machines. According to Michelle Richter, sales and marketing manager for the company, the emphasis on machining has made it an efficient part of the overall production process, taking out cost rather than adding it.
"We can machine accurately every time," Richter said. "Whereas castings vary over the life of the tool/mold."
With each new part, L A Aluminum aims to build a simple tool and machine-in the features. According to Richter, a complex casting will result in higher scrap rates. A feature like cast-in holes requires the use of pins in permanent mold casting. Pins often wear more quickly than the tool and eventually lead to castings that are out of tolerance due to mold wear and movement. Richter said the machine shop will always machine-in an accurate hole.
The metalcaster has found that it gains more from improving the efficiency of its machining department than making adjustments to its metalcasting process.
"Machining is where we make up for smaller profit margins," Richter said. "It is so much easier to improve the machining process with new equipment. A lot of my customers that used to do cast-in features brought machining in-house [for similar reasons]."
Clean Up Inclusions
Beyond part design, metalcasters' best method for improving casting machining is with process control. For instance, with steel castings, metalcasters must take into account the 6% volumetric shrinkage that occurs as the material solidifies. Shrinkage often is repaired by welding up the material. This makes the area of the casting hard and can break the inserts on a machine tool.
"You want to be able, as a metalcasting facility, to make sure there is not any shrinkage where there is going to be a hole drilled out or cut," said Malcom Blair, vice president of technology at the Steel Founders Society of America.
Fortunately, shrinkage in steel castings can be predicted using solidification software, and a metalcaster can adjust the part design to avoid shrinkage in areas that must be machined. However, the presence and location of inclusions, which can result in premature tool wear or breakage, are harder to predict.
Table 2. Machinability Ratings for Aluminum Permanent Mold Castings Alloy Temper Rating * 222.0 T551, T61, T65 Excellent 242.0 T571, T61 Very Good 296.0 T4, T6, T7 Good 308.0 F Good 319.0 F Good 332.0 T5 Fair 333.0 F, T5, T6, T7 Good 336.0 T551, R65 Poor 355.0 T51, T6, T72, T71 Good C355.0 T61 Good 356.0 T6, T7 Good A356.0 T61 Good 357.0 T6, T61 Good 443.0 F Fair 512.0 F Excellent 513.0 F Excellent 535.0 F Excellent 711.0 F Excellent 850.0 T5 Excellent 851.0 T5 Excellent 852.0 T5 Excellent * Machinability rating is based on a range of cutting operations Source: AFS Aluminum Casting Technology
"What makes predicting inclusions difficult is when you do the model, you have to calculate the amount of inclusions that will be produced and the rate they will grow or join up with each other," Blair said. "When that happens, you have to predict whether they will stick to the wall of the mold, slide or not stick at all. One of the biggest challenges we don't have our heads around is predicting how an inclusion reacts to a mold surface."
According to Blair, the best way to avoid inclusions is to maintain clean metal practices.
"Many years ago, we did some work in pouring with a shroud on a bottom pour ladle," Blair said. "Using the shroud, the metalcaster reduced the number of inclusions by an order of magnitude, meaning they were 10% of what they were before. One thing we did not anticipate was the machine shop found the insert life doubled or even tripled once the inclusions were reduced."
For iron castings, one of the largest factors affecting machining is the presence of hard carbides in the metal.
"Carbides are just tiny hard particles distributed in what would normally be a machinable matrix," said Charles Bates, an industry veteran who was written numerous research papers on machining castings. "They are about as hard as the tools used to cut them."
Table 3. Machinability Ratings for Aluminum Sand Castings Alloy Temper Rating * 220.0 T Excellent 242.0 T2, T571, T77 Very Good 295.0 T4, T6, T62 Very Good 308.0 F Fair 319.0 F, T5, T6 Good 355.0 T51, T6, T7, T71 Good C355.0 T61 Good 356.0 T51, T6, T7, T71 Good A356.0 T61 Good 357.0 T6 Good 358.0 F Good 443.0 F Poor 511.0 F Excellent 512.0 F Very Good 514.0 F Excellent 520.0 T4 Excellent 535.0 F Excellent 710.0 F Excellent 713.0 F Excellent * Machinability rating is based on a range of cutting operations Source: AFS Aluminum Casting Technology
Carbides most often form when trace elements are picked up in the melting scrap--often from high strength, high alloy steel. The trace elements, which also can be picked up when using the same furnace to melt batches of steel and iron, combine with the carbon to form carbides, nitrides and carbo-nitrides, Bates said, which kill the machine tool quickly.
Bates recommends keeping a close eye on the charging scrap and inoculating castings to minimize carbides in thin sections and corners.
Good molding and gating practices can improve the machinability of castings from any type of casting facility, according to Bates. Clean molds free of sand and dust will mean a casting surface free of inclusions. Good dimensional control is important in making castings that will fit properly in the machine tool for ease of machining.
"The orientation of the casting in the mold exerts a strong influence," Fields said. "Particularly for dross-prone alloys where oxides can be trapped in the cope surfaces or under 'shelves' in the mold formed by cores."
Table 4. Machinability Ratings of Copper Casting Alloys UNS Number Common Name Machinability * Group 1-Free-cutting Leaded red brass 90 alloys C83800 Leaded red brass 90 C84400 Leaded semi-red brass 90 C84800 Leaded semi-red brass 90 C94300 High-leaded tin bronze 90 C85200 Leaded yellow brass 80 C85400 Leaded yellow brass 80 C93700 High-leaded tin bronze 80 C93800 High-leaded tin bronze 80 C93200 High-leaded tin bronze 70 C93500 High-leaded tin bronze 70 C97300 Leaded nickel brass 70 Group 2-Moderately Leaded high-strength 60 machinable manganese bronze C92200 Leaded tin bronze 60 C92300 Leaded tin bronze 60 C90300 Tin bronze 50 C90500 Tin bronze 50 C95600 Silicon-aluminum bronze 50 C95300 Aluminum bronze 35 C86500 High-strength manganese 30 bronze Group 3-Hard-to-machine High-strength manganese 20 alloys bronze C95300 9% aluminum bronze 20 C95400 11% aluminum bronze 20 C95500 Nickel-aluminum bronze 20 * Ratings are based on free-cutting brass (61.5% copper, 35.5% zinc and 3%. lead) earning a 100 machinability rating. Source: AFS Casting Copper-Base Alloys
For More Information
"Machine Shop Survival," S. Wetzel, MODERN CASTING, September 2006, p. 39.
RELATED ARTICLE: It's a Material World
Alarge portion of a part's machinability stems from its material. Plain carbon steels typically machine better than low alloy steel. Aluminum alloys containing copper, zinc and magnesium machine easier than other aluminum alloys. Copper alloys with lead machine better than other copper alloys. Annealed irons can be cut at high speeds. Along the same lines, alloys with a higher hardness typically are more difficult to machine.
"You shouldn't make castings stronger than necessary to meet the customer's specifications," said Charles Bates, a metalcasting industry veteran who has written a number of research articles on machining. "Some think stronger is better, but it hurts machinability."
For some applications, metalcasters can suggest a more machinable alloy for the application. However, machinability takes a lower priority than a part's required strength.
Heat treatment also may be used to improve a part's machinability. The machining characteristics of carbon and low alloys steels can be improved through heat treatment by as much as 100-200%, according to the SFSA Steel Castings Handbook. Soft aluminum alloys, such as grade F aluminum, may produce a built-up edge on the tool face, causing machined surfaces to be rough, according to Fields. T5 or T6 heat treatment can harden the alloy enough to avoid the built-up edge.
However, heat treatment will change the microstructure and thus the properties of the material, which should be considered in determining the final production process for a part.
Shannon Wetzel, Senior Editor
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|Date:||Sep 1, 2010|
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