School of Technology

TECH 13580 - Engineering Graphics I

 

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DIMENSIONING

PURPOSE

Views only explain an object's shape, but that isn't enough information to build the object. Views which have been dimensioned provide the necessary information. Dimensions are of two types; size ( how big ) and those of location ( where at ). The purpose of dimensions is to build the object. Therefore your goal is to give dimensions on your drawing necessary to build the object, not necessarily dimensions to make the drawing, which are not always the same. A reader of a drawing should not have to "scale the drawing" ( place a scale on the drawing to determine missing dimensions).

BASIC COMPONENTS

Dimension Line - thin & black line used with arrowheads on each end, 3/8" from object and 1/4" apart from any other dimension lines, if nearby and parallel.

Arrowheads - to clarify where a dimension starts and stops, 1/8" long with width 1/3 of its length, gracefully curved to meet at the exact end of the dimension, Avoid the "Flintstone" variety 

Extension Line - thin & black, perpendicular to the dimension line to establish the bounds of the dimension for answering the question, "What is it coming off of?" 1/16" gap from the object's feature being dimensioned, 1/8" overhang past the arrowhead.

Leader - a diagonal line with a short horizontal continuation, arrowhead (one only) on the end of the diagonal line used to touch the feature being "called out" in the local / shop note, diagonal can be any angle but 45 degrees most popular.

Finish Mark - a symbol used to identify which surfaces to be "finished" (machined smoothly), old method used italicized letter F while modern method uses 60 degree V with tail found on Ames Lettering Guide, located on edge lines to be machined, OK to repeat in other views, OK to be drawn on hidden lines, code numbers can be used to indicate how smooth via micro inch RMS (root mean square), located on edge view, Use FAO if all surfaces are to be finished rather than draw finish marks all over the drawing.

SYSTEMS OF DIMENSIONING

General Practice Structural Practice

Aligned versus Unidirectional - Aligned system has each dimension placed perpendicular to the dimension line with all vertical dimensions reading toward the right edge of the drawing. The newer Unidirectional system of dimensioning has all dimensions read downward toward the bottom of the drawing. Aligned Unidirectional

Cumulative versus Coordinate - The Cumulative system is satisfactory for general work when accuracy is not so critical. Start from an edge and give the dimension to the first feature such as a hole's center, then how much further to the next feature, etc. The dimensions, thus "accumulate". On the other hand, the Coordinate system gives each dimension all the way back to the starting edge, the datum, resulting in less opportunity for error build-up for greater accuracy. In either case, omit the last dimension in a string of dimensions to clarify the location of the datum. 

Cumulative Coordinate

Fractional versus Decimal versus Metric - Fractions (never smaller than 1/64") are used for materials commonly measured in fractions, such as wood. Decimals are used for all those materials, such as metals, commonly measured in decimals (either in inches or metric) and are thereby more popular than fractional dimensions. Metric dimensions continue to grow in popularity with manufacturing within a world economy. Most metric dimensions are in millimeters (mm). There are 25.4 mm in an inch.

ANSI (American National Standards Institute) versus SI (from International Standards Organization) versus MS (Military Standards) - The rules established by the SI system are replacing the other systems as a global economy impacts manufacturing.

RULES OF DIMENSIONING

1. By using extension lines, keep dimensions off the views, if possible.

2. Place dimensions between views to be visually shared. Try to avoid dimension placement below the front view, above the top view, & to the right of the r. side view.

3. Place the largest dimension the farthest from the view to avoid dimension lines being crossed by extension lines. 

4. Avoid dimensioning to hidden features. Dimension the feature in the view its visible. 

5. Holes are dimensioned by their diameter while arcs are dimensioned by their radii either by leaders or extension lines. Slots are an exception.

6. Usually best to give the over-all dimension to aid in stock procurement rather than having to add all the dimensions to determine sizes to order or design for.

7. Do not repeat dimensions in various views. Give them only once to avoid congestion.

8. Do not try to give every conceivable dimension. Also recall that your objective is to give dimension necessary to manufacture the part, not necessarily those dimensions you needed to make the drawing. Manufacturing experience helps!

9. Centerlines can serve as extension lines, never as dimension lines.

10. Avoid inch marks (") unless you are using the structural system of dimensioning.

11. Give full size dimensions regardless whether the drawing is half size, etc.

12. Dimension height usually never smaller than 1/8".

13. Follow modern SI rules - handout

14. Study the Do's and Don'ts of dimensioning - 57 rules 

SELECTED TOPICS OF DIMENSIONING

1. Angles - Dimension line drawn with a compass or CAD, not freehand

2. Arcs - by radii 

3. TYP (typical) - when all arcs are the same size, saves dimensional time and space

4. False Center Arc - when the true (actual) center is likely off the drawing 

5. General Notes versus Local Notes - General notes are notes regarding the entire part.

examples of general notes:

Local notes refer to a specific feature of the part and thus require a leader.

6. Datum - a machined surface from which dimensions are located, the starting point, common with mating surfaces,  Also datum centers possible.

7. Contour Dimensioning - placing dimensions in the view where the part's contour are most easily visualized 

8. Extension Lines Crossing - OK for extension lines to cross other extension lines, center lines, object lines (no gaps) to get to feature being dimensioned, Not OK to cross dimension lines 

9. Dimensioning Section View - no crosshatching through dimension 

10. Staggered Dimensions - to enhance legibility 

11. Finish Marks - to identify surface to be machined 

12. Dimensioning Curves - via reference points 

13. Superfluous Dimensions - avoiding unnecessary dimensions 

14. Tabular Dimensions - assigning letters rather than numerals for same shape parts 

15. Pitch / Rise & Run - for house roof, stairs, & civil engineering 

16. Geometric Breakdown - analyze part in terms of its component's geometric shapes 

17. Bolt Center ( B.C. ) - a circular center line upon which holes are located 

18. Chamfers - a beveled edge on a cylinder to aid assembly 

19. C-C ( Center to Center ) - dimension given to locate spacing of holes sharing a center line

20. NTS ( Not To Scale ) - a note to acknowledge that the drawing is inaccurate, but the dimension is correct, indicated by a bold line under the dimension or NTS for the entire drawing

21. Threads - later in the course

22. Detail versus Assembly Drawings - Detail drawings are fully dimensioned. "Stock" items are not detailed. Assembly drawings usually have over-all dimensions, but do include a BILL OF MATERIAL and leaders with balloons. 

Note: More to dimensioning than these notes (tolerancing & geometrical tolerancing).

More later…