Common Details Missed on the Drawing
Not only should a design engineer consider the model, but many things often get missed on the drawing as well. Its not very often you come across a design engineer that likes making drawings, but drawings are just as important as the model because it carries tolerance, datums and inspection specifications.
Here are some common misconceptions and missing information on a drawing.
The drawing tells them how to machine it.
Not true! The machinists tell you how to machine it. Although it can inform the machinist and provide insight, the drawing provides the specifications the finished part shall meet. It is up to the machinist to figure out the best way to make the part.
The drawing tells them how to inspect it.
Also not true! Again, the drawing only provides the end-result that is required. The inspection of the part is up to the machinist to guarantee conformity.
The title block general tolerances will cover me
In most cases this may be fine, but the designer should consider the areas on the part that deviate from the general tolerances.
Tip 1: Find all the dimensions that can be larger tolerances than the general tolerances and give the machinist as much room as possible
Tip 2: Make the tighter tolerances the as clear as possible to the machinist. Sometimes a dedicated detail view of tighter tolerances features helps the machinist understand intent.
Datum scheme is not identified
You do not have to be an expert in Geometric Dimensioning and Tolerancing to specify datums. Even if the primary datum A is not called-out, a designer should make it clear what features are “base” or “origin” features where the other features are defined by.
Centerlines, extension lines and multiple same-size features
This is a grouping of missing information that is related. Often, center lines for the main body of the part, its holes and hole patterns are missing. A good machinist can infer and understand, but for complex parts, it can create confusion and drive up cost and rework. Extension lines that connect features to each other such as multiple holes that are in-line, are very important to make clear as the machinist must understand how many times a dimension applies. Multiple features of the same size or same XY locations are often defined by a single dimension. It is important to count how many times the dimensions applies and put a 2X, 3X, 4X, …nX etc. destination prefix.
Duplicate dimensions
The more information the better, right? Not in this case. Over-dimensioning the drawing is like pointing in the opposite directions at the same time when given directions. 👈 ❓ 👉 Which dimension should the machinist used to set up and inspect to? 🤷♂️
Tip: Choose which dimensions can be a reference (Reference is always in parentheses)
Circular Surfaces and Tangent Lines
This is the most undetectable error on drawings. That is because looking at the side of a circular surfaces is like looking into horizon - It’s a fading ambiguous edge. 🌅 Theoretically, there is an edge there, but it depends on the point of reference. The issue arises when the designer creates the dimension to a tangent edge but doesn’t realize it snapped to a different edge. In more complex designs, hidden edges that are close to circular surfaces can create ambiguity when the dimension placed.
Another common issue is untrue projections. This is when a complex section is not projected in a way that depicts the features as it would actually be produce. Visually, it may be correct, but the dimensions defining the features need to be presented so that the features can actually be machined. There are many examples were this can happen, but in general its circular and complex curvatures that produce tangent edge conditions prone to this issue.
Tip 1: Always show the model dimensions when possible. This method ensures the dimensions that define the model are not re-created on the drawing.
Tip 2: The designer should know their design from a mathematical model perspective. It is helpful to calculate the dimensions in question by hand to make sure it agrees with the model.
Tip 3: Do not be afraid to add an extra page or extra detail view showing the complex areas and their dimensions
Decimal places are not fully understood
In an effort to “just-get-it-done”, the designer can often overlook what the decimal values mean to the machinist. If cost is the primary consideration (and it always is), the number of decimals tells the machinists how arcuately the feature must be measured to and therefore informs the feeds and speeds of the machine.
Tip: Consider each dimension’s value individually and seek to give the machinist the most room possible. Critical dimensions should be considered carefully and questioned if multiple decimal places are actually necessary.
Bonus! Here is a good video for creating drawings for machined parts.
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