can be quickly ground off after machin- ing, 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 ma-
chine 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 equip- ment. 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
Table 1. Effect of Iron Matrix Microstructure on Tool Life Material
Matrix microstructure Gray iron
100% ferrite Coarse pearlite Fine pearlite Acicular
Ductile iron 100% ferrite
Flake Flake Flake Flake
Type of Hardness graphite
Cutting m/min sfm
100 270 880 15.7 195 110 360 225 105 340 263 60 200
Spheroidal 170 250 810 97% ferrite, 3 % pearlite Spheroidal 183 175 570
35 45 59 70 77
60% ferrite, 40% pearlite Spheroidal 207 130 430 84.7 60% ferrite, 40% pearlite Spheroidal 215 110 360
93 20% ferrite, 80% pearlite Spheroidal 265 75 240 97.25
Ferritic malleable ASTM 32510
Pearlitic malleable ASTM 48004
ASTM 60003 ASTM 80002
100% ferrite
Spheroidite Spheroidite Spheroidite
Temper carbon 109 290 950
Temper carbon 179 140 450 Temper carbon 230 85 280 Temper carbon 250 80 260
*Cutting speed for 30 minutes tool life. Source: ASM Metals Handbook, Volume 16 Machining
found that adding in more machining to a part can make it less expensive. 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 Alumi-
num 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 ac- curate 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 ma- chining process with new equipment. A lot of my customers that used to do cast-in features brought machining in- house [for similar reasons].”
50
70 80
100
Ultimate tensile speed* strength (ksi)
Fig. 1. This flow chart can be used to optimize a design for machinability by evaluating a machined feature. MODERN CASTING / September 2010 27
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