Last week for Standards Tuesday I introduced you to the application of dimensions per Section 1.7 of ASME Y14.5M-1994. If you missed it I highly recommend you go back and read it, Standards Tuesday – Application of Dimensions, since it covers such important topics such as Dimension Lines, Extension lines and spacing of dimensions. Today we will finish up Section 1.7 before we move on to Dimensioning Features next week. For the sake of full disclosure, I must tell you that all my post in regards to Standards Tuesday are merely my interpretation of the standards. I take special care in ensuring that I understand each section described in the standards before making my write up. To make it easier on those who are reading their own copy of the standard while reading my post, I try to maintain the same section headings with my own description of each. With that said, let us move on to the exciting parts.

Chain Lines

As mentioned in Rule 11 of the Fundamental Rules of Dimensioning:

“Unless you specify otherwise on the dimension, it should apply to full depth, length or width of the feature.”

When it is not appropriate to have the dimension apply to the full depth, length or width of the feature; chain lines allow you to indicate a limited length or area of a surface. You can use chain lines to apply treatment to only a section of a feature. A good example of this is when you want to add knurling to only a section of a shaft.

Applying Chain Lines

In the view that best describes the location and extent of the surface to be prepared draw a chain line parallel to the object line offset a small distance from the line(s). Add dimensions to show the length and location of the chain line if it is not clearly indicated by the mere location of the line. If you are adding a chain line to a revolved part, you can show the line on only one side of the feature. If you are apply chain lines to a view that is normal to the reader then you create a fully dimensioned boundary using chain lines. Below illustrates applying a chain line to a shaft:

Leaders

Leaders are used for local notes, symbols and some types of dimension such as diameters and radii. When pointing to an edge or point of an object, the leader should terminate with an arrow head. When pointing to a surface the leader should terminate with a dot, which indicates that the entire surface within the object line boundary is being indicated. Leaders can be drawn using straight lines at any angle but to maintain readability of the drawing, adjacent leaders should be drawn parallel to each other. The other end of the leader, the short horizontal line, should point to the middle of the first or last character of the note or dimension.

Leader Directed Dimensions

Leader Directed Dimensions such as radius callouts should be drawn individually. Although SolidWorks may give the ability to have multiple leaders when make dimension callouts it should be avoided since it adds an unnecessary degree of complication to the dimension. Also, if your drawing view contains a lot of radial dimension that may make it difficult to read, you should use flagnotes or letters to identify the dimensions elsewhere on the drawing.

Circle and Arc

When dimensioning circles and arcs, the leader must be drawn radially. In SolidWorks, this is not an issues since it automatically creates all radius or diameter dimensions radially. Making a leader radial means that if you were to theoretically extend the leader line it would intersect with the center mark of the arc or circle.

Reading Direction

The reading direction of all notes and dimensions must be consistent throughout the drawing. After all we are not doing architectural drawings here. All notes and dimensions must  be able to be read without having to turn the drawing around other then the intended direction based on the format. The one exception to this is when you are using ordinate dimensions, which are meant to be inline with the extension line. Ordinate dimensions will either be read from the bottom or the right based on whether they are vertical or horizontal. Ordinate dimensions will be discussed in detail in a future Standards Tuesday post.

Reference Dimensions

Judging by the drawings I see at my day job, everybody knows how to specify a reference dimension. Even though most don’t know how to use them properly :0). Anyway, specify a reference dimension by enclosing it within parenthesis. As I mentioned in an earlier an earlier post, reference dimensions should be used sparinly.

Overall Dimensions

An overall dimension is used to show the entire length, width or height of a part including all of the intermediate dimensions. If you do use an overall dimension then one of the intermediate dimensions must be left out of the view or shown as reference. This is because if you left all the intermediate dimensions along with the overall dimension the part would be over constrained. If it is more important to show the intermediate dimensions then the overall dimensions must be as a reference.

Dimensioning Within the Outline of a View

If at all possible, always make every attempt to place dimensions outside of the visible object lines of the drawing view. Of course there are times when this is near impossible, either because of the feature being dimensioned or the length of the extension lines. When this is the case, dimensions can be placed within the visible object lines.

Dimensions Not to Scale

Even though the writers of ASME Y14.6M-1994 make every attempt to be diplomatic, I will not. This is a huge thorn in my side when I deal with drawings from other users. With out sugar-coating it, there is no real reason to have a drawing not be done to scale when using modern CAD packages. The only excuse would be….laziness. I have a feeling I will receive a bunch of emails about this.

Well, that concludes Section 1.7 Application of Dimensions as described in ASME Y14.5M-1994. Like I said, next week we will begin coverage of the methods of dimension features. Until then, feel free to contact me with any questions, comments, or complaints. If you enjoy reading the SolidWorks Geek and you don’t want to miss anything please subscribe to my RSS feed. Its free, easy and it just might earn you a prize.

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