Patternmaking planning and layout.

Patternmaking Planning and Layout

In the June issue of modern casting we discussed the importance of patternmaking to the foundry industry. We recognized the patternmaker's valuable skills that facilitate the most economical method of efficiently producing castings. They play an essential role in the production of patterns for the thousands of applications that rely on the strength, utility and economy of metal castings. The second part of this four part series follows a casting from conception to reality, from a patternmaker's viewpoint.

Planning the Pattern

Patternmakers are the first to evaluate the variables that must be resolved before a casting proceeds to production. There are ideas, plans, measurements, blueprints and deadlines involved, and usually little precious time available. The successful patternmaker will consolidate the facts, cross reference them with his experience, and propose a strategy to meet the casting challenges. Some key items to be included in the patternmaker's proposal might contain: * the pattern's end use (prototype, short or long run job) * designated areas to be machined * parting line location * draft allowances * tolerances * coring required * pattern type and construction * flash style and size * molding equipment/method * filling and feeding problems

Patterns can be ranked according to their complexity. The relatively simple jobs involve adding a pattern to an existing family of patterns, or replacing lost or worn-out patterns. The more complex projects, such as casting drawings, utilizing extensive coring are resolved by the skillful patternmaker possibly through the use of a different molding techniques or using a follow-board with a standing green sand core. Other patterns may require imagination and experience to build cost effectively, such as oversize and more complex castings.

Certain complicated castings may require a patternmaker to build a casting model to better understand the complexities of the tooling needed to produce the part.

Pattern Layout

One of the patternmaker's chief obligations is to make certain that the casting buyer and foundry are in total communication and they both agree that the foundry meets the buyer's expectations. In many ways, he mediates the buyer/seller relationship by making certain that each party understands what is expected to produce an acceptable casting. Interpretations of customer drawings often can be misunderstood or misleading, and it is to their mutual advantage that the patternmaker performs casting layout. This is a fairly inexpensive, accurate means of translating an engineering drawing into three dimensions. If changes are necessary they can be economically performed at this stage.

This visualization also verifies the necessary steps involved in building a production pattern, particularly for complex parts. The patternmaker's layout serves all concerned with a model casting. It is generally constructed from pine board into which he skives the exact outlines of the part. Mylar film or drafting vellum can be used to draw orthographic projections of the casting to better visualize all elevations.

The layout or drawing is always drawn to full size, though generally not dimensioned, but having great accuracy and incorporating the shrink rule. Machining allowances, coreprints and specifications that define the casting should also be included. The principal view of the part should most clearly orient the casting to the production process. Normally drawn in the ascast position with the parting line always clarified, the principal view may be augmented by as many additional views as necessary to clarify the part, adding fillets, machining allowances and draft.

The corebox is used to shape the final casting by establishing the size and shape of the core. Cores are those adjuncts to the pattern that create cavities in the casting and set limits for protrusions, machined surfaces, eliminate draft and reduce hot spots. Both the pattern and corebox are permanent, and can be reused until damaged or worn. Patterns are built to be perpendicular to the parting line therefore the corebox allows versatility for the casting's final shape. The patternmaker tries to minimize the number of cores to keep costs down and yield a precision casting.

Layout is similar to editing, in that it strives for clarification, proving concepts before approving production pattern construction. This segment of the production process mutually binds the buyer and seller to a common understanding of the job at hand.

PHOTO : A patternmaker will often construct a full scale casting layout when designing pattern equipment.