Dimensioning
The purpose of dimensioning is to provide a clear and complete description of an
object. A complete set of dimensions will permit only one interpretation needed to
construct the part. Dimensioning should follow these guidelines.
- Accuracy: correct values must be given.
- Clearness: dimensions must be placed in appropriate positions.
- Completeness: nothing must be left out, and nothing duplicated.
- Readability: the appropriate line quality must be used for legibility.
The Basics: Definitions and Dimensions
The
dimension line is a thin line, broken in the middle to allow the placement
of the dimension value, with arrowheads at each end (figure 23).
Figure 23 - Dimensioned Drawing |
|
An
arrowhead is approximately 3 mm long and 1 mm wide. That is, the length is
roughly three times the width. An
extension line extends a line on the object to
the dimension line. The first dimension line should be approximately 12 mm (0.6 in) from
the object. Extension lines begin 1.5 mm from the object and extend 3 mm from the last
dimension line.
A leader is a thin line used to connect a dimension with a particular area (figure 24).
Figure 24 - Example drawing with a leader |
|
A leader may also be used to indicate a note or comment about a specific area. When
there is limited space, a heavy black dot may be substituted for the arrows, as in figure
23. Also in this drawing, two holes are identical, allowing the "2x" notation to
be used and the dimension to point to only one of the circles.
Where To Put Dimensions
The dimensions should be placed on the face that describes the feature most clearly.
Examples of appropriate and inappropriate placing of dimensions are shown in figure 25.
Figure 25 - Example of appropriate and inappropriate dimensioning |
|
In order to get the feel of what dimensioning is all about, we can start with a simple
rectangular block. With this simple object, only three dimensions are needed to describe
it completely (figure 26). There is little choice on where to put its dimensions.
Figure 26 - Simple Object |
|
We have to make some choices when we dimension a block with a notch or cutout (figure
27). It is usually best to dimension from a common line or surface. This can be called the
datum line of surface. This eliminates the addition of measurement or machining
inaccuracies that would come from "chain" or "series" dimensioning.
Notice how the dimensions originate on the datum surfaces. We chose one datum surface in
figure 27, and another in figure 28. As long as we are consistent, it makes no difference.
(We are just showing the top view).
Figure 27 - Surface datum example |
|
Figure 28 - Surface datum example |
|
In figure 29 we have shown a hole that we have chosen to dimension on the left side of
the object. The Ø stands for "diameter".
Figure 29 - Exampled of a dimensioned hole |
|
When the left side of the block is "radiuses" as in figure 30, we break our
rule that we should not duplicate dimensions. The total length is known because the radius
of the curve on the left side is given. Then, for clarity, we add the overall length of 60
and we note that it is a reference (REF) dimension. This means that it is not really
required.
Figure 30 - Example of a directly dimensioned hole |
|
Somewhere on the paper, usually the bottom, there should be placed information on what
measuring system is being used (e.g. inches and millimeters) and also the scale of the
drawing.
Figure 31 - Example of a directly dimensioned hole |
|
This drawing is symmetric about the horizontal centerline. Centerlines (chain-dotted)
are used for symmetric objects, and also for the center of circles and holes. We can
dimension directly to the centerline, as in figure 31. In some cases this method can be
clearer than just dimensioning between surfaces.
ليست هناك تعليقات:
إرسال تعليق