Dimensioning

For video demonstrations of this "lecture" follow this link Videos to accompany lecture


In this session we will learn how to dimension an orthographic projection using proper dimensioning techniques.  We will also learn how to properly PLACE dimensions on a drawing.

During our discussion of "how to dimension" and how to "properly place" dimensions I will highlight certain key concepts which indicates that you will probably SEE them again.  For example when you see the following:  Dimensions affect how a part is manufactured. The highlight indicates a potential test question so study this material with this in mind.

An orthographic projection is a shape description of an object while a detailed drawing is an orthographic projection COMPLETE WITH ALL THE DIMENSIONS and specifications needed to MANUFACTURE the object.

Should we dimension an object using the dimensions that we used to draw the part?  The answer is NO, because drawing dimensions are not necessarily the same dimensions that are required to manufacture the part.  (ASME Y14.5 - 1994).

The goal of dimensioning is to include everything needed to manufacture and inspect the part exactly as intended by the designer.

Dimensions use special lines, arrows, symbols and text.
        There are three lines 1) Dimension lines, 2) Extension lines, 3) Leader lines.
              Dimensioning requires these three lines and are drawn THIN so that they will not be
              confused with visible lines (object lines)

Figure A

The dimension line indicates the extent of "value" of the measurement.  Under ideal circumstances the dimension line is broken by the "feature size" as shown.  Arrowheads are placed at both ends of the dimension line which terminates on each end at an "extension line".  You should note that the "extension lines" are ALWAYS in pairs with a "visible gap" between the extension line and the "object line" of the part being measured.  Long extension lines should be avoided.

Leader lines are a straight inclined thin solid line leading to a note.  In figure A above there are five leader lines, each of which "lead" directly to the following notes respectively, arrowheads, feature size, dimension lines, extension lines and visible gap.



Notice in figure B that the leader lines shown are terminated three different ways.  With an arrow if it ends on the outline of an object or with a dot or nothing if it ends "within" the outline of an object. 

Avoid!!! 1) Crossing leader lines, 2) Long leader lines, 3) Leaders that are parallel to adjacent dimension, extension or section lines. 4) Small angles between the leader and the terminating surface.  5) Leader lines should be straight NOT CURVED and they should point to the CENTER of an arc or circle at an angle between 30 and 60 degrees.

Figure B

 
Arrowheads are drawn BETWEEN extension lines IF POSSIBLE.  If space is limited, they MAY be drawn on the outside.

Figure C (arrowhead placement)

        There are three TYPES of dimensions 1) linear, 2) angle, 3) notes


Linear distances:  These are usually arranged either horizontally or vertically, but they may also be aligned with a particular feature of the part.  The main thing to remember here is that they are simply "a distance" between two distinct features of a part.

Angles:  Used to give the angle between two surfaces of features of a part.

Notes:  These are used to dimension diameters, radii, chamfers, threads, and other features that cannot be dimensioned by the other two methods.

        There are three things that affect dimension appearance/effectiveness, 1) Symbols,  2) spacing
           and readability, 3) placement.

With the use of CAD the legibility of the letters are not really an issue, however their uniformity and proper placement throughout the drawing are determined (not by the software) but by you.   This is also true regarding the height of the dimensions and/or other text.  The software will have options to change the height of letters comprising various notes and feature descriptions; these options should be set as discussed in this section.

Dimension lettering should be 1) legible, 2) easy to read, 3) uniform throughout the drawing.

Dimensions should be easy to read, and minimize the possibility for CONFLICTING interpretations.  Your goal as a drafter should be to place the dimensions in such a way that there is NO NEED for any interpretation.  Dimensions should NEVER be duplicated or the same information given in two different ways.


For example 1) the spacing between dimension lines should be uniform throughout the drawing.  Dimensions should NOT BE PLACED inside an object line.  2) The dimension line should not touch the object UNLESS clearness is gained.  3) Dimension text should ALWAYS be horizontal with means that it is read from the bottom of the drawing.  4)  Dimension text should not cross dimension, extension or visible lines.  5) Dimension lines should NOT CROSS extension lines or other dimension lines.  6) Extension lines and center-lines SHOULD NOT CONNECT between views.
      EXTENSION LINES CAN CROSS OTHER EXTENSION LINES OR VISIBLE LINES.

Upper case letters should be used FOR ALL LETTERING unless otherwise required.  The minimum lettering height is 3 mm (.12in).
For video and interactive examples of the above principles follow the link provided.

Dimensioning symbols:  These symbols are used to REPLACE text.  The goal of using dimensioning symbols is to eliminate the need for language translation.  The importance of symbols can be realized when we consider the number of products that are designed in the United States and the number of products that are manufactured or assembled in the United States.

Example problems:

Figure D (dimension placement mistakes)

Figure E (placement errors explained)

You will notice that even though all of the dimensions given are numerically correct, the error is in their placement.  Remember that the goal of dimensioning a drawing is "READABILITY" and doing away with the need for INTERPRETATION.  If you are careful to follow all of the rules (standards) then you will minimize the chances of creating confusion on the drawing.

Compare figure D with Figure F and notice the improvement and the lack of any need to interpret what each dimension means.

Figure F (placement errors CORRECTED)

Another example of dimensioning mistakes

Figure G

At first glance there does not seem to be very much wrong with the dimension placement in figure G, but upon closer inspection remember that readability and proper spacing are important features to ensure clarity.

Placement errors identified

The leader line is supposed to "point" to the center of a circle with a radius but to the edge as shown when identifying a diameter.  The error here with the leader line is the angle at which it points to the object line, it should be between 30 and 60 degrees.  The center line is not supposed to extend from one view into the next.  The text "feature size" should BREAK the dimension line.  Dimension lines and extension lines SHOULD NOT CROSS.

Figure H (placement errors corrected)

Some other things to consider:

     Dimensions should be grouped whenever possible
     Dimensions should be placed BETWEEN views, unless clearness is promoted by placing them
     outside.
     Dimensions should be attached to the view where the shape is shown best.
     DO NOT dimension hidden lines.

In the next example you will be exposed to other common types of dimension placement errors.

Once again, all of the correct information is present, but it is not presented correctly.  Think of a sentence that contains all of the proper words, but if the words are out of order, the meaning of the sentence can change dramatically.  For example: compare Fred eats chicken, with Chicken eats Fred.  The words are exactly the same, but the meanings are totally opposite.

 1) The dimensions should be IN BETWEEN the front and top views.  2) The leader lines should always point UP not down or in a horizontal direction.  3) NEVER dimension hidden lines.  4) This is a poor choice of location because it is difficult to see the feature that is being dimensioned.  5) Group dimensions of "related" features together.  6) The extension lines are too long, this is solved by "moving" the dimension as shown.

Dimensioning and locating features:  In this section we will learn how to properly locate arc's and circles.  We do this with the use of a note and/or a symbol and a leader line.

Figure I

Notice how the diameter is dimensioned with a leader line and a symbol.  Do you recognize an error in this figure?  Should the leader line should be pointing to the CENTER of the circle defined by the diameter or does this rule only apply to locating a RADIUS?

Dimensioning HOLES and CYLINDERS:

Figure J

 In figure J - Can you tell which is the circular hole and which is the circular beam or cylinder?  The answer is no.  The standard we will adapt for this situation is that the diameter of the hole is given in the circular view while the diameter of the cylinder is given in what we will call the rectangular view (see figure K).

Holes are dimensioned by giving their diameter and location in the circular view of the drawing.

Figure K

Just looking at figure J it is not possible to tell the difference between the hole and the cylinder so what we do to clarify this is to dimension the hole in the circular view and the cylinder in the rectangular view.

A cylinder is dimensioned by giving its diameter and length in the rectangular view BUT it is LOCATED in the circular view of the drawing.


Dimensioning other features.

The depth of a BLIND HOLE may be specified in a note and is the depth of the full diameter from the surface of the object.  Notice how the leader line slants UP.  Notice how the leader line points to the edge of the hole (because it specifies diameter).  If the dimension specified a radius it would point the the center of the hole.

If the hole goes all the way through the part then the abbreviation "THRU" follows the dimension value instead of the symbol indicating a depth of 15.

Figure L

Where the center location of the RADIUS is important the drawing must clearly show its location as shown in figure M.  However where the center location of the RADIUS is unimportant the drawing must clearly show that the ARC location is controlled by other dimensioned features such as tangent surfaces. (see figure N)

Figure M

Figure N

Do not confuse the difference between figure M and figure N.  The location of the center of the radius is important in figure M however figure N shows how an edge is rounded as opposed to the location of a specialized feature.

The SPHERE:

Their are two types of spheres - 1) COMPLETE sphere and 2) INCOMPLETE sphere.  In the figure shown (figure O) the complete sphere is a flat topped cylinder while the incomplete sphere is a cylinder with a rounded top.

Figure O

REPETITIVE feature:

Repetitive features or dimensions may be specified by using the symbol "X" along with the number of times the feature is repeated.
       There is NO SPACE between the number of times the feature is repeated and the "X" symbol,
        however there is a space between the symbol "X" and the dimension.

Figure P

Do you recognize the repetitive features?  There are four holes.  Each hole does not need to be dimensioned separately.  We will use the "X" symbol explained above.

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