45.132XFigure 2-22.-Proportional dividers.bar called a beam. The slide-mounted attachmentscan be locked in any desired position on the beam.Thus, a beam compass can be used to draw circlesof any radius up to the length of the beam. Withone or more beam extensions, the length of theradius of a beam compass ranges from about 18in. to 70 in.PROPORTIONAL DIVIDERSPROPORTIONAL DIVIDERS (fig. 2-22) areused for transferring measurements from onescale to another. This capability is necessary whendrawings are to be made to a larger or smallerscale. They can also be used to divide lines orcircles into equal parts.Proportional dividers consist of two legs ofequal length, pointed at each end, and heldtogether by a movable pivot. By varying theposition of the pivot, you can adjust the lengthsof the legs on opposite sides of the pivot so thatthe ratio between them is equal to the ratiobetween two scales. Therefore, a distance spannedby the points of one set of legs has the samerelation to the distance spanned by the points ofthe other set as one scale has to the other.On the proportional dividers, a thumb nutmoves the pivot in a rack-and-gear arrangement.When the desired setting is reached, a thumb-nutclamp on the opposite side of the instrument locksthe pivot in place. A scale and vernier areprovided on one leg to facilitate accurate setting.On less expensive models, the movable pivot isnot on a rack and gear, and there is no vernier.The dividers may be set by reference to the tableof settings that is furnished with each pair; theywill accommodate varying ranges of scales from1:1 to 1:10. However, it is better not to dependentirely on the table of settings. You can checkthe adjustment by drawing lines representing thedesired proportionate lengths, and then applyingthe points of the instrument to them in turnuntil, by trial and error, the correct adjustmentis reached.To divide a line into equal parts, set the dividerto a ratio of 1 to the number of parts desired onthe scale marked Lines. For instance, to dividea line into three parts, set the scale at 3. Measureoff the length with points of the longer end. Thespan of the points at the opposite ends will beequal to one-third the measured length. To useproportional dividers to transfer measurementsfrom feet to meters, draw a line 1 unit long andanother line 3.28 units long and set the dividersby trial and error accordingly.Some proportional dividers have an extra scalefor use in getting circular proportions. The scalemarked Circle indicates the setting for dividingthe circumference into equal parts.The points of the dividers are of hardenedsteel, and if they are handled carefully, thesepoints will retain their sharpness during longuse. If they are damaged, they may be sharpenedand the table of settings will still be usable, butthe scale on the instrument will no longer beaccurate.SCALESIn one sense, the term scale means thesuccession of graduations on any graduatedstandard of linear measurement, such as thegraduations on a steel tape or a thermometer. Inanother sense, when we refer to the “scale of adrawing,” the term means the ratio between thedimensions of the graphic representation of anobject and the corresponding dimensions of theobject itself.Suppose, for example, that the top of arectangular box measures 6 in. by 12 in. If youdraw a 6-in. by 12-in. rectangle on the paper, thedimensions of the drawing would be the same asthose of the object. The drawing would, therefore,be a full-scale drawing. This scale could beexpressed fractionally as 1/1, or it could be givenas 1 in. = 1 in.Suppose that instead of making a full-scaledrawing, you decided to make a half-scaledrawing. You should then draw a 3-in. by 6-in.rectangle on the paper. This scale could be2-16
Integrated Publishing, Inc. - A (SDVOSB) Service Disabled Veteran Owned Small Business