1. Dimensioning Lecture 16 Instructor:
Up to this point, the focus has been on shape description. Dimensioning deals with size description.
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2. Dimensioning
Goals
Understand the principles of good dimensioning practice
Apply the standard dimensioning practices for engineering drawings
Reference
BTG Chapter 7 – Dimensions and Tolerances
Instructor:
Dimensioning is critical to go from idea to production. There are ANSI standards for dimensioning and companies may have their own standards that build on the ANSI standards.
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3. Geometrics
The science of specifying and tolerancing shapes and locations of features of on objects
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4. Geometrics
It is important that all persons reading a drawing interpret it exactly the same way.
Parts are dimensioned based on two criteria:
Basic size and locations of the features
Details of construction for manufacturing
Standards from ANSI (American National Standards Institute)
Instructor:
It is not only critical that everybody using a drawing interpret it the same way, it is important to do the dimensioning in the simplest way. There are lots of ways to dimension an object following the standards and yet have a drawing that is difficult to read. In Engineering, the ANSI standards are available for lots of different areas and they are not just about engineering graphics.
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5. Scaling vs. Dimensioning
Drawings can be a different scales, but dimensions are ALWAYS at full scale.
Instructor:
Here is an important concept. Scale deals with how you draw something – the dimensioning is always done at full scale. Remind your students that while the computer is comfortable with any scale, you want the final drawing printed at some recognizable scale. These recognizable scales would be the ones that were discussed earlier in the term. For example, if you are printing a set of house plans you probably want them in ¼” = 1 foot or 1/8” = 1 foot.
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6. Units of Measure Angle Dimensions Length
English - Inches, unless otherwise stated
Up to 72 inches – feet and inches over
SI – millimeter, mm
Angle
degrees, minutes, seconds
Instructor:
While most construction of buildings and roads is done in feet and tenths of a foot, architects are working in feet and inches. A mechanical or industrial engineer may be building products that have both inches and mm.
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7. Dimensioning Terminology
Instructor:
On this drawing, go through the labeling on each dimensioning component. You want to make sure that you and the students are using the same terminology.
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8. Dimensioning Standards
Instructor:
The computer will allow dimensioning to be done with arrows in or out or allow a dimension to be crowded into a small space. Make sure that you emphasize that clarity is important. Dimensions that are crowded are hard to read. The guidelines for spacing of dimensions from objects are very important and must be followed.
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9. Dimensioning Basic Shapes -Assumptions
Perpendicularity
Assume lines that appear perpendicular to be 90° unless otherwise noted
Symmetry
If a part appears symmetrical – it is (unless it is dimensioned otherwise)
Holes in the center of a cylindrical object are automatically located
Instructor:
There are some features of dimensioning that are assumed. Right angle that appear as right angles are 90 degrees apart. The same thing is true for symmetry in an object.
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10. Dimensioning Basic Shapes
Rectangular Prism
Instructor:
In this figure the dimensions for height, width and depth are shown. When possible all dimensions should be placed between views as long as they are not crowded. All three dimensions are needed for complete description of a 3D object.
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11. Dimensioning Basic Shapes
Cylinders
Positive
Negative
Instructor:
Cylinders are dimensioned where they are viewed as rectangles. Holes are dimensioned where you see the circle representing the hole. Note that the symbol, phi, is used for diameters and precedes the number. Note also that the phi is used whether you are looking at the circle representing the hole or whether you are dimensioning the rectangular view of the cylinder.
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12. Dimensioning Basic Shapes
Cone Frustum
Instructor:
Cones can be dimensioned in a variety of ways. Any of these are OK. Note that the diameter of the
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13. Dimensioning Basic Shapes
Circle Pattern Center Lines
Instructor:
When you have a series of holes on a circular centerline, the centerline is referred to as the bolt circle. You may want to ask the class what other dimensions are needed to completely dimension this object. Answer: the diameter and number of holes and the overall diameter of the object.
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14. Dimensioning Shows: Size Location & Orientation Instructor:
The figures on the left only show size. This figure is incomplete without adding the location and orientation dimensions.
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15. Example Problem What dimensions (size & location) are needed?
Where should they be placed?
Instructor:
A quick active learning exercise. Six dimensions are needed. H, W, D of the block and the diameter of the hole and its location dimensions from the top or bottom and from the right hand edge in the front view.
Think Individually, then share with the person next to you.
- Compare with the other pair at your table
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16. Grouping Dimensions
Dimensions should always be placed outside the part
Yes No
Instructor:
The figure on the left is dimensioned correctly but the horizontal dimensions on the steps could have been done with any two of the three steps. Here the left and center steps were dimensioned. Another important point to make here is the spacing between sets of dimensions. You do not want the dimensions to be crowded.
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17. Staggering Dimensions
Put the lesser dimensions closer to the part.
Try to reference dimensions from one surface
This will depend on the part and how the tolerances are based.
Instructor:
Note that the diameter dimensions on the left side of the figure have the numbers staggered so that it takes up less room on the page.
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18. Extension Line Practices
Instructor:
There are situations where features are so close together that arrowheads cannot be shown without have the arrow on the extension line. In cases like these it makes sense to leave a gap in the extension line.
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19. Use the Symbol ‘x’ to Dimension Repetitive Features
Instructor:
We looked at the location of holes on a circle in a prior slide. Here we use the number of holes X size of the hole to complete the dimensioning.
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20. Symbols for Drilling Operations
Instructor:
In the past, the symbols shown here were labeled by the operation that produced the feature. For example, the counterbore and countersink were operations. Now the ANSI standard say to show the shape by a graphic symbol and then it is up to the manufacturing staff to determine how that shape will be made. Make sure that the students memorize the symbols and how they are used correctly. This will be on the quiz and on the mid-term. These symbols are also part of the library of symbols in CAD packages.
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21. Example Problem What dimensions (size & location) are needed?
Where should they be placed?
Instructor:
Here is an inclass problem for your students. In this case the hole need to be dimensioned and located. Because there is a ‘counterbore’ there has to be a dimension for the size and depth of the counterbore that is co-located with the hole dimension. In addition, the H, W, and D are required.
Think Individually, then share with the person next to you.
- Compare with the other pair at your table
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22. Assignments
Dwg 31 – DIM-1
Fully dimension the objects shown using correct dimensioning practices. (part is fully dimensioned if it is possible to machine it from the dimensions provided)
Use Inch Dimensions to the second place (x.xx)
Dwg 32 – DIM-2
Fully dimension the objects shown using correct dimensioning practices.
Use one-place metric dimensions (x.x)
Bring your AutoCAD 2D Book Tomorrow
Instructor:
The first two bullets say how the exercises are to be done. The students need to bring the AutoCAD tutorial book for the next class. They will be learning how to dimension using AutoCAD or their CAD package.
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