HIGH-PRODUCTION ALUMINUM Metal / Melting Costs
Metal (based on casting weight) Melting (based on pour weight)
Molding / Pouring / Cleaning Costs Automatic molding Pouring
Cleaning (shotblasting)
Coremaking, Finishing, Dipping, Assembly Costs Coremaking
Casting Finishing Costs Trim pressing Belt grinding
100% inspection/gaging Painting
Miscellaneous Operation Costs Heat treating (based on casting weight)
Container (based on pieces/container and casting weight) Shipping
Set-up charge Other miscellaneous costs SG&A TOTAL UNIT COST
Core A B C
Cost
25.52% 3.40%
0.72% 1.36% 0.09%
24.92% 19.64% 7.67%
1.14% 2.41% 0.00% 2.06%
0.00% 0.24% 0.05% 1.26%
6.02% 96.50%
3.50% 100.00%
HIGH-PRODUCTION DUCTILE IRON Metal / Melting Costs
Metal (based on casting weight) Melting (based on pour weight)
Molding / Pouring / Cleaning Costs Automatic molding Pouring
Cleaning (shotblasting)
Coremaking, Finishing, Dipping, Assembly Costs Core Making
Casting Finishing Costs Snag grind Bench grind
100% inspect/gage Paint
Miscellaneous Operation Costs Heat treating (based on casting weight)
Container (based on pieces/container and casting weight) Shipping
Set-up charge Other miscellaneous costs SG&A TOTAL UNIT COST
Core A B C
Cost 9.33% 16.66%
4.06% 6.66% 0.23%
4.52% 3.56% 1.39%
7.95%
36.36% 0.00% 5.03%
0.00% 0.61% 0.14% 0.00%
0.00% 96.50%
3.50% 100.00% Fig. 1. According to these sample costing models from an aluminum and ductile iron facility, finishing is among the most costly casting operations.
Following is a look at five strate-
gies you can use to limit grinding and finish your finishing faster.
1. Redesign the Casting If you have the opportunity to work
with your customer early in the casting design process, redesigning features can be the best opportunity to save in the finishing room. Features that otherwise would be produced in post- processing, such as holes and passage- ways, can be cast-in, and parting lines can be moved. “Determine if there are features
you can design that will save money in the cleaning room,” Marlatt said. “You have to understand your customer’s cleanliness criteria. If a parting line of ±0.79 in. (20.7 mm) is acceptable and the process is capable of holding that, grinding will not be necessary.” According to Jitendra Shah,
president of Product Development & Analysis, Naperville, Ill., the design of a casting can have a significant effect on the repeatability of your castings, as well as the creation of metallic projec- tion defects that must be ground away in the finishing room. “Te things you can do are design
better draft and radii so you don’t have a lot of sand burn-in and burn-on and mold damage,” he said. “[And avoid] heavy sections surrounding pockets,
which can cause metal penetration and burn-on. You can save 10-15% [of the total cleaning] cost through design.” Nick Fox, manager of Galesburg Castings Inc., Galesburg, Ill., agreed that designing pockets with inad- equate radii can create a problem because operators will often tear away portions of the mold inadvertently. “Te more interaction the foundry has at the customer level in terms of designing the casting, the parting line and the shape of the casting, the less finishing will be required,” Fox said. “Te more intricate we get…we have to make sure we have plenty of radius.” Fox said design changes also can
push processing costs further down the line from the finishing room. “It might be cheaper to machine
a feature than core it,” he said. “If the core price is $3 a core, and I know it is going to cost $0.80 to grind, and you can machine it for $3…the machine shop can save us time and money.”
2. Redesign the Rigging Because so many parts can’t be
redesigned, tooling design often is the simplest place to find cleaning room savings, according to Shah. Studies have shown that finishing times are highly correlated with the size and number of gates and risers used in a mold, so they must be placed carefully.
“Te most important [thing] is the
placement of the ingate, because you spend a lot of time and money grind- ing it,” Shah said. “Ten, [you must design] the overall rigging, the gating and risering system, so that you have the least amount of turbulence and produce no defects.” According to American Foundry
Society technical director Tom Pru- cha, a cost is incurred every time an employee picks up a part. While work- ing for an Indiana-based ferrous met- alcasting facility, he and his colleagues examined strategies to make gate grinding unnecessary or change the parting line so it could be easily ground without tricky part manipulation. “We had a concept called no-
grind,” said Don Roberts, a retired for- mer coworker of Prucha’s. “We…engi- neered the finishing or grinding out of the process in two ways. One, we put the ingate in an area that would need to be machined. Two, we designed the gate so it would break flush.” According to Roberts, the customer
must be consulted if you elect to use a procedure like this, as it can add varia- tion to the casting. “Our customer worked with us so
we could gate the casting in an area they were going to machine,” he said. “With their help, we were able to reduce the number of castings that
January 2012 MODERN CASTING | 39
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