9
Weigh Cost Tradeoffs
Weigh the cost benefits of post-casting machining vs. casting to specifications with die core slides.
One of diecasting’s major strengths is its use of core slides to produce parts to final dimensional specifications as-cast. In many cases, the added investment in such tooling is more than offset by the unit cost savings in eliminating all, or virtually all, high-cost post-casting machining. Such tooling decisions, however, should always be made with a careful cost-benefit analysis. For example, depending on the final design of a feature, side-action core slides may double the number of parting lines. Tis could result in stepped parting lines with increased part trimming costs. CNC machining after casting may be the better bottom- line option in these cases. For smaller parts, the unit die tooling alternative (as discussed in No. 8), combined with post-casting machining as required, should be considered.
Superfluous print dimensions can lead to confusion, errors and higher processing costs.
10 Prints destined for toolmaking must contain all relevant part specifications for
diecasting production, post-casting machining and finishing, plus references to mating parts and their specific requirements. But irrelevant dimensions will slow the approval process and may lead to production errors. Prints should be stripped of such irrelevant dimensions and notations prior to submission for diecasting quotations and die design. Appropriate, clear print dimensioning will help ensure production repeatability and
reduce inspection time. For improved dimensional accuracy and repeatability in die cast parts, datum points should always be kept on the same side of the diecasting die.
Avoid the Superfluous
Minimize post-casting machining costs by proper hole dimensioning and placement on prints.
11 Engineers less familiar with the
requirements of the diecasting process often size and place part features in ways that entail significant additional post-casting machining costs, as well as longer casting cycles, if not modified. Hole sizes should be appropriately
dimensioned for the alloy selected, part size and length-to-diameter ratio of the hole. Preplanning hole spacing for efficient secondary machining can cut secondary costs. For example, speci- fication for 0.625-in. (15.875-mm) spacing between drilled and tapped holes can allow single set-up conven- tional machining with multiple spin- dles for lowest cost operation. Where the design will allow, more robust part features always yield superior results in post-casting operations.
12
Let Hard Facts Cut Costs
Refined casting analyses may assure meeting part specs at significantly reduced costs.
When analytical data demon-
strates stringent specifications can be met with alternative production solutions, saving production costs, such results should be evaluated by OEM engineering. For example, in the case of electronic
enclosures designed for assured elec- tromagnetic interference (EMI)/radio frequency interference (RFI) shielding, radiographic analysis of die cast housings, backed by EMI/RFI leakage testing, can establish threshold parameters for minute discontinuities in a die casting in relation to shielding effectiveness. Tese discontinuities in a casting’s micro-struc- ture can result from metal flow caused by unusual design features. Analysis of the parts has determined that when data for the castings fall above established threshold parameters, shielding integrity can be assured at specified levels. If parts can be produced at higher production rates, substantial unit cost savings can result. ■
32 | METAL CASTING DESIGN & PURCHASING | Sept/Oct 2011
Employ Holes Properly
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