Machinery's Handbook, 31st Edition
638 DESIGN PARAMETERS Checking Casting Designs: In checking castings, the form of the pattern is studied, the methods of molding, the method of supporting and venting the cores, and the effect of draft and rough molding on clearances. Undue metal thickness should be avoided, as well as extreme differences between thick and thin sections in the same casting. All metal thicknesses should be specified, so that appropriate chaplets can be selected for supporting cores. Ample fillets and rounds should be provided and properly dimensioned directly or by note. Cores should be designed so that they can be secured in the mold with out crushing or causing interference. Allowances should be made for swelling, shrinkage, or misalignment of cores to ensure adequate machineability. Material should be added to surfaces destined to be finished. On large castings, sufficient extra material should be provided for finishing to permit “clean up” to net size in case of warpage. Checking Drawings.— The following are some rules-of-thumb and general guidelines to keep in mind when creating and reviewing engineering drawings and data models. Checking the Technique Used in Making the Drawing: Inspect the drawing to see that all regular, auxiliary and section views are made in such a way as to illustrate the most descriptive views of the form of the piece and its relationship to other parts. Selection of type and quantity of views on a drawing serve as both a carrier of the dimensional values and to represent accurately the visual nature of the part. Ultimately, drawings must communicate design intent, both numerically and visually. Checking Dimensions in General: In general, all dimensions should be checked for cor rectness. If manually drawn, dimensions should also be scaled to determine if the drawing is to scale. Where any dimension is “not-to-scale,” it is indicated by underlining the offending dimension. This is the standard technique for indicating not-to-scale. While most CAD systems automatically indicate not-to-scale dimensions, some do not. Not-to- scale dimensions are problematic on CAD drawings and models as they are indicative of dimension values not matching the geometry. In the case of a CAD drawing dimension being not-to-scale, it too should be underlined. However, this may be a serious concern, and may demand immediate correction. More and more, CAD geometry drives down stream applications such as numerical control or tolerance analysis. Hence, errors in CAD geometry cannot be tolerated if accurate downstream results are expected. ASME and ISO Drawing Checklists.— The following checklists represent questions based on industry practice and standards for engineering drawings and, unless noted, apply to both inch and metric drawings and models and both ASME Y14.5 and ISO methodology. Model Checklist: Are features built within dimensional limits and to appropriate mate rial conditions (MMC, LMC, nominal)? Are features built to sufficient numerical accuracy? Are fillets and rounds added to all non-finished corners of castings and forgings? On as-cast models, is machining allowance added to surfaces destined to be finished? Has allowance for wrench clearance been built into locations of bolts and other fasteners requiring tools? Drawing Checklist: Are all dimensions indicated only once to avoid potentially misin terpreted redundant dimensioning? Are all auxiliary and section views labeled? Are general notes added to address default conditions such as fillets and rounds, numerical significance, default tolerances? Are all finish marks indicated where appropriate? Are clearance hole diameters clearly specified to avoid potential look-up errors? Are there sufficient views so that no assumptions need be made about any feature or dimension?
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online