SCALING

In ancient times, simple objects and structures were built without detailed architectural plans, andeven without established dimensions. The outline of the structures and the position of each room could bedetermined experimentally by *pacing off* approximate distances. The builder could then erect thestructures, using existing materials, by adjusting sizes and dimensions as necessary during the buildingprocess.

Today, design requirements are so demanding, and materials are so diverse, that a completedimensioned set of drawings is absolutely necessary to insure proper execution of the design as conceivedby the designer. In the preparation of these drawings, the modern designer must use reduced-size scales.The abilityto use the metric scale accurately is required not only in preparing drawings but in checkingexisting plans and details (Hepler, D.E. and Wallach, P.I.,1982).

REDUCED SCALE

The architect*s scale is used to reduce the size of a structure or object so that, it can be drawnsmaller than actual size on paper. The metric scale is also used to enlarge a detail or small object forclarityor to dimensionit accurately. .

SCALESELECTION

The selection of the proper scale is sometimes difficult. If the structure to be drawn is extremelylarge, a small scale must be used. Small structures can be drawn to a larger scale, since they willnot takeup mudi:space in the drawing sheet.

USE OF THE SCALE

Scale technique is governed largely by the requirements of accuracy and speed. Before a line canD@ drawn, its relativepositionmust be found by scaling,and the speed with whichscale measurementcanbe made willgreatly affect the total drawing time.

The scale is orilyas accurate as its user. In using the scale, do not accumulate distances. That is,always layout overall dimensions first. The width and length willbe correct and their position willnotchange if you are slightly off in measuring any subdivisions that make-up the overall dimension.

To make a measurement, place the scale on the drawing where the distance is to be laidoff, alignthe scale-in the direction of the measurement, and make a light short dash with a sharp pencil at theproper graduation mark. In layout work where extreme accuracy is required, a *pricker*, or needle pointset in a',wood handle, may be substituted for the pencil, and a small hole pricked into the pap~r in place ofthe pencil mark. It is best to start with the *zero* of.the scale when setting off lengths or whenmeasuring distances (French, T. E. and Vierck,C. J., 1953).

To avoid cumulative errors, successi\Je measurements on the same line should, if possible, be made.Without,shiftingthe scale. In representingobjectsthat are largerthan can be drawn to their naturalor fullsize, it is necessary to reduce the size of the drawing in some regular proportion (French, T. E.'and Vierck,C.J., 1953). .

The important thing in drawing to scale is to think and speak of each dimension in its full size andnot in the reduced (or enlarged) size it happens to be on paper. This practice prevents confusion betweenactual and re.presented size (French, T. E. and Vierck,C. J.,1953).

DIMENSIONING

PURPOSE OF DIMENSIONS

Dimensions are used on a drawing to supply detailed manufacturing, fabrication, orconstruction information concerning the size and location of the components of an object. After adrawing shows necessary completeness of shape, the size and relative location of its details areindicated by means of dimensions and specifications (Rising, J. S. and Almfeldt, M. V., 1959).

An important consideration in dimensioning is that the values used should be those neededfor the production of a desired object. These dimensions sometimes differ from those used by thedraftsman. For example, the draftsman uses a radius to draw a circle which describes a drilledhole. The machinist, however, must know the diameter of this same hole before he can select theproper drill (Rising, J. S. and Almfeldt, M. V., 1959).

When applying dimensions to a drawing, the student should remember the followingimportant criteria:

1. ACCURACY - that dimension values are correct.2. CLEARNESS - that each dimension is placed in its most appropriate position.3. COMPLETENESS - that there are no omissions of specifications.4. READABILITY - that lettering, numerals, and dimension lines are neat, uniform in size, and

very distinct.

SUMMARY OF GOOD DIMENSIONING PRACTICES

1. Dimensions should be placed outside of the views. Clearness, ease of reading, and shorterextension lines sometimes make it practical to place some dimensions within the outline.

2. Dimensions should generally be placed between views rather than on the outside of relatedviews. However, due to the position of the contour, shape or to other reasons, this standard maybe violated for clearness.

3. A particular dimension should apply to one view only. Do not extend lines between relatedviews.

4. Sufficientdimensionsshould be given so that the piece can be madewithout any calculationssuchas additionor subtraction.

5. Dimensions should not be duplicated.

6. Dimensions should be specified only on that view which shows the true length of thedimension.

7. Dimensions should be given from "finished" or mating surfaces whenever possible.

8. Dimensions should never be crowded. A minimum space of 3/8 inch should be left between anoutline and a paralfel dimension and a 1/4 inch space between dimension lines.

9. Numerals are inserted about. midway between the arrowheads except they may need to bestaggered on adjacent dimension lines.

10. When a series of dimensions relate to one view, the smaller dimensions should be nearer tothe view, longer ones further away. Overall dimensions should be outside of all others.

11. Center lines are extended as witness lines, but never used as dimension lines.

12. If it is possible, extension or dimension lines should not cross each other.

13. Dimensions should not be referred to invisible lines if the specification can be givenotherwise.

14. All center lines should be located by coordinate dimensions. Do not assume that the centerline is in the middle of a piece, locate it by dimension.

15. Chain dimensioning is poor practice except in special cases. If necessary to use chaindimensions, leave one "link" out of the chain and include an overall dimension. If no dimension isomitted, from the chain,.the overall should be marked REF.

16. The point of all arrowheads should touch the line to which the dimension refers. Leadersused for circle diameters should proceed radially from the circumference and terminate in ahorizontal bar at the mid-height of the note. The arrow end of the leader touches thecircumference, not the center of the circle.

17. Avoidthe useof "spokedimensions"for circlesexceptfor large diameters. It is betterto usealeader from the circular view or a dimension related to the rectangular view of the cylindricalshape.

18. Dimension a circle by giving the diameter ; arcs by giving the radius. When not obvious, theletter D, or Dia., follows a diameter dimension; the letter R follows a radius.

19. Blank out any section lines for the dimension line and numerals if a dimension must be placedwithin a sectioned area.

20. All numerals should be of uniform height, in most cases 1/8 inch. Fractions are 1/4 inch highwith the bar in line with the dimension line, never at an angle.

21. Numerals for angles in degrees are placed to read from the bottom of the sheet..

22. Finish marks should show on all edge views of machined or "smoothed surfaces".

23. If a dimension given on a drawing is not to the scale of the drawing because of a "change" oran error in the layout, rather than remaking the drawing a wavy line can be placed under thedimension thus: 4.75.

24. The four essentials of dimensioning are ACCURACY, CLEARNESS, COMPLETENESS, ANDREADABILITY (Rising, J. S. and Alrnfeldt, M. W., 1959).

2

OBJECT LINE

Define shape.Outline and deIai objects:

. HIDDEN LINE

Show hidden 1eaIures.

CENTER LINE

locate centerpoints ofarcs and cirdes. .

DIMENSION LINE

Show size or location.

EXTFNSION LINE

DefIl1Elsize or location.

LEADERCall out specificfeatures.

CUTTING PLANE

Show internal features.

SECTION LINE

Identify internal features.

BREAK LINE

Show long breaks.

BREAK LINE

Show short breaks.

O'tJECT

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HIDDEN LINE

CENTER lINE

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Define shape.Outline and detail objects:

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Show I*iden Ieatures.

CENTER LINE

Locate oenterpoints ofarcs and circles, .

DIMENSION LINE

Show size or location.

EXTFNSION LINE

DefIl1Elsize or location.

LEADER

Call out specificfeatures.

CUTTING PLANE

Show internal features.

SECTION LINE

Identify internal features.

BREAK LINE

Show long breaks.

BREAK LINE

Show short breaks.

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CENTER lINE

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THE ALPHABET OF LlNE$

Visibleline THIC,K

Hidden lineMEDIUM2----------------

Section line 3

Center line 4

THIN

Extension line EXTE'NSIOJ'~ LINE

LEADER

Dimension line 6

Dimension line!" 7

CUHing-plane lineor viewing-planeline

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CuHing-plane lineor viewing-planeline

Breakline 10 -Breakline II

Phantom line 12

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