Chapter 1-PART DRAWINGS; A COMMUNICATION TOOL

How your prints influence the quality and cost of your sheet metal parts and stampings.

The ease of interpretation of the designer's drawing sets the tone of manufacturing success for the project. From the estimator interpreting your drawing into a cost picture, to the final inspector assuring that the programmers and craftsmen have met the specs, your drawing is the only link to your thought processes which created the product. The importance of the drawing as a communication tool cannot be over emphasized, because it is a real tool, used by many people in the complicated processes of manufacturing.

Some of the most important thought should be applied BEFORE the drawing is begun. The position in which the part is portrayed will often determine the ease of interpretation. International Standard Organization (ISO) drafting standards, for instance, demand the part to be shown the same way as it would be held in the machine during fabrication. This is not always possible, but lathe parts, for example, are always shown as they would be clamped in the chuck or collet. The operator therefore does not have to reverse the image in his mind, one less chance for error.

The following are intended to improve communication excellence. First and foremost, it is imperative to make the part features most prominent. The part must "jump out at you" from the drawing. To achieve this, use the heaviest lines for the outline and all visible lines. These should be "heavier" by a factor of three, compared to dimensional lines. Invisible edges should be shown at half the full line strength and then only, if they clarify the picture.

Cross-cut sections are one of the most informative views you can give to the interpreter of your drawing. Don't be handicapped by the "normal" projection of a cut view. If showing the view in the opposite direction from "normal" would make interpretation easier, then do so with directional arrows and an identifying letter. Cut-view lines and arrows should be slightly heavier than the outline for proper direction of the view.

Figure 1. Illustration of good drafting practice and dimensional call-outs.

Avoid "boobytrapping" your drawing. A typical example are tightly spaced dimensional lines going to different features. To eliminate this problem, offset one line to space them apart, show one dimension on a different view or add an exploded view. Centerlines which are almost in line with each other should be terminated with a short cross-line behind the last feature to which they belong. This eliminates a very common cause for error. See Figure 1.

One more ISO drafting standard which would be prudent to adopt, is to show the overall dimension for each view as the farthest dimension from the outline. If the total length, width or depth is given elsewhere in conjunction with other dimensions, list it as a reference dimension.

Don't hesitate, when developing a design, to use plain English explanatory notes to aid interpretation, make a point or further develop a detail. Avoid the use of unusual language conventions which can he misunderstood or misinterpreted.

For critical features in your design, use functional dimensions and tolerances which are directly interlinked with the related feature. For instance, if a bracket is to be used to mount a part and spacing is critical to the front and top surface, dimension the bracket directly from the front and top of the part, not from some other feature.

Always attempt to dimension features and flanges from co-planar interior datums to avoid tolerance accumulation from successive bends. When you cannot avoid establishing a flanged edge of a part as a datum (the surface is critical to the function of the part), indicate the critical dimensions through notes or tolerance additions and indicate the noncritical dimensions in the same manner.

Use drawing block tolerances where possible to indicate non-critical dimensions. Full-millimeter metric or single-digit decimal inch dimensions should be used with appropriate tolerances to locate operator-placed features such as spot welds, tack welds and self-piercing rivets.

Computer Aided Design (CAD) creates a whole new set of challenges. See CAD/CAM Chapter. Until the programs are perfected, they leave a lot to be desired. Very often the image can be read as an inside or outside view, depending on the viewing angle, because of visible lines being present in the wrong places. Such CAD creations should be cleaned up to proper standards, otherwise the time saved in drafting will be spent several times over in interpretation.

The craftspeople working on your project have spent years to hone their print reading skills. They have to rely on standards to be consistently correct in their interpretation. Changing these standards is guaranteed to cause problems--something you, the designer will want to avoid.

Technical drawings are a communication tool, no more, no less. Some people like to call it documentation. It is that only, when the drawing lies filed away, not serving it's communication purpose. Since a picture is supposed to be worth a thousand words, it will be easy to get your thoughts across with drawings, when done properly.

Making your design easy to quote and manufacture requires good communication between the designer and supplier and will speed up the quotation process. Drawings must be current and easily readable, produced to ANSI Y 14.5 specifications, with clear details.

Even the most clearly detailed prints too often fall victim to the reduction and faxing process. Convenient and expedient as this method is, details can get skewed in the process. Numerals, especially, get distorted, as is evident when an eight becomes a three and the fives turn into sixes, etc.

Binding documentation, for this reason, should never be faxed at all, unless it is immediately followed up with originals sent by mail. The exception may be an original "A" size (81/2 X 11 in.) print which should come through the faxing process without distortion if it has crisp details. When the fax process is used, as it is often done for initial quoting purposes, an engineering contact person with intimate project knowledge should be identified to field questions, which are almost certain to arise. Binding drawings for actual production must be submitted in their original size.

Table I is a guideline and explanation for the quantity of drawing sets required depending on the number of processes involved.

The lack of binding documentation for each user on each project has resulted in countless errors, delays and expenses in the past. Always supply sufficient original document sets.

An available sample part, or even a card board mock-up, is of tremendous help in the quoting process and should be supplied whenever possible. Even the best print is not as easily interpreted as a sample part, especially a complicated one.

Giving options on design features which may be fabricated in various ways will let the metalforming supplier use the best and quickest processes for economical production. Table II is a partial listing of interchangeable processes which could be given as options.

As part of a complete drawing, an itemized list of all components is a must. Components just identified at their locations lead to frustrating searches and double-checks, with a good chance of missed items.

The designer and/or buyer should also check the availability and lead times of specified components, as they are beyond the influence of your metalforming supplier. It is not uncommon to encounter lead times of up to 12 weeks for relatively minor items without which, the job cannot be completed. Finally, if a drawing has undergone revisions, an Engineering Change Order (ECO) listing these changes is of great help to the estimator when requoting a project or to the supplier engineer when changing internal documentation to create the new part.

View the Geometric Dimensioning & Tolerance Summary Fact Data Sheet...

Excerpt taken from Design Guidelines for Metal Stampings and Fabrications -- 2nd Edition copyright © 1995 Precision Metalforming Association

Purchase the new Third Edition of Design Guidelines for Metal Stampings and Fabrications copyright © 2004 Precision Metalforming Association at Marketplace today!

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