Fig. 2. In aluminum, free-standing internal and external bosses require the use of sand cores, loose pieces or collapsible metal cores, which will increase production costs. The added expense may be held down by tailing the bosses.
Fig. 3. It is usually best to keep the height of the bosses and pads less than the thickness of the aluminum casting section but high enough to permit machining without touching the surfaces of the section.
Fig. 4. When several lugs or bosses are on one surface, joining them will facilitate machining in aluminum castings.
Clean Up Inclusions Beyond part design, metalcasters’
best method for improving casting machining is with process control. For instance, with steel castings, met- alcasters 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.
Fig. 5. Designing sloping bosses for straight parting of the mold or die in aluminum casting will save money in tooling construction.
“You want to be able, as a metalcast-
ing 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 cast-
ings 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,
It’s a Material World A
large portion of a part’s machinability stems from its material. Plain carbon steels typically machine better than low alloy steel. Aluminum alloys contain- ing 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 writ- ten 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 machin-
ing 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.
MC 28
which can result in premature tool wear or breakage, are harder to predict. “What makes predicting inclusions dif-
ficult 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
Table 2. Machinability Ratings for Aluminum Permanent Mold Castings
Alloy Temper Rating*
222.0 T551, T61, T65 Excellent 242.0 T571, T61 296.0 T4, T6, T7 308.0 319.0 332.0
F F
T5
333.0 F, T5, T6, T7 336.0 T551, R65
T61
A356.0 357.0 443.0 512.0 513.0 535.0 711.0 850.0 851.0 852.0
T6, T7 T61
Good Poor
355.0 T51, T6, T72, T71 Good C355.0 356.0
T6, T61 F F F F F
T5 T5 T5
Good Good Good Good Fair
Excellent Excellent Excellent Excellent Excellent Excellent Excellent
*Machinability rating is based on a range of cutting operations
Source: AFS Aluminum Casting Technology MODERN CASTING / September 2010
Very Good Good Good Good Fair
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