Scale Development

Engineering Drawing II Features of a Good Graphical Presentation

How can you make your presentation perfect? Each presentation should have a cover page. The presentations should have unique titles. The figures should be properly labeled and appropriate titles should be provided. Dimensions should be clearly given with proper units.

How can you make your presentation perfect? (Contd.)

If scaling is done, the RF or scale factor or scale should be provided. In a location map or plan, directions should be indicated. Hatchings, different types of lines (hidden lines, broken lines etc.) should be appropriately used. If any symbol is used, the meaning of the symbol should be mentioned.

How can you make your presentation perfect? (Contd.)

There should be no double lines or extended lines. The required object must be given the topmost priority. The presentation must be checked repeatedly before submission to ensure that all the criteria are fulfilled. The most important is to keep the presentation neat & simple.

Engineering Drawing II Scale Development

Representative Fraction Representative fraction (RF) is the ratio of the length in scale to the actual length.

Mathematically, RF = Length in scale ÷ Actual Length

Why RFs are used? To present a figure of a relatively larger object in a small sheet of paper.

To present a figure of a relatively smaller object in a relatively larger sheet of paper.

Examples of Uses of RFs In drawing maps In drawing road locations In drawing house plans In presenting enlarged figures of microscopic objects; i.e. bacterial cells, amoebas etc.

Significance of RF values RF greater than 1 Used in presenting enlarged figures of relatively small objects. RF less than 1 Used in presenting reduced figures of relatively larger objects.

Different Types of Scales Plain Scale Used where the required scale divisions are relatively big. Not preferred where accuracy is of prime importance. Easier to use for its simplicity.

Diagonal Scale Used where the required scale divisions are relatively small. Provides much accuracy. Complex to construct and use.

Construction of Plain Scales Example 1 The distance between Dhaka and Feni is 175 km. A passenger train covers the distance in 5 hours and 50 minutes. Construct a plain scale to measure time up to a single minute and corresponding distance. RF is 1/200000. Also indicate the distance covered by the train in 28 minutes.

Construction of Plain Scales (Contd.) Step 1: Check the RF to select unit Here, RF = 1/200000 i.e. 1 cm = 200000 cm or, 1 m = 200000 m or, 1 ft = 200000 ft etc. Our objective is to develop a 6” or 15 cm scale for a fair presentation. Here the distance is given in km. So we can take 1 km = 200000 km. But we can not draw a scale with such a big unit in a small sheet of paper. If we look for a smaller unit, we can take 1 cm = 200000 cm (2 km) easily and a 15 cm scale can show a 30 km distance. Moreover, km and cm can be related easily.

Construction of Plain Scales (contd.) Step 2: Solve the problem In these sort of problem, relationship between distance and time needs to be presented in a scale. Therefore, calculation of velocity is required. Velocity = Distance ÷ Time = 175 km ÷ 350 minutes = ½ km/minute So, the train travels 1 km in 2 minute.

Construction of Plain Scales (contd.) Step 3: Decide on Scale Type & Divisions Now, you may take ….. 2 major divisions of 7.5 cm (15 km in 30 minutes) or, 3 major divisions of 5 cm (10km in 20 minutes) or, 5 major divisions of 3 cm (6km in 12 minutes) or, 6 major divisions of 2.5 cm (5km in 10 minutes) As you need to show the distance covered by the train up to a single minute, it is better to select the number of major divisions to limit the fractioning of a division to minimum for simplicity & neatness. The last choice is ideal and can be easily drawn.

Construction of Plain Scales (contd.) Step 4: Draw the Scale

Construction of Diagonal Scales Example 1 Construct a scale of RF=1/2000 to read 300 meters to 1 meter. Also show 135 meters in the scale. Step 1: Check the RF to select unit As you need to measure the distance in meter unit, you can take 1 cm = 2000 cm (i.e. 1 cm = 20 m) In that case 300 meters will be 15 cm in our scale. We know that, RF = Length in scale ÷ Actual Length or, Length in scale = RF X Actual Length. So, the scale length for your actual 300 m will be, = (1/2000) X (300X100) = 15 cm

Construction of Diagonal Scales (contd.)

Step 2: Solve the problem

In these problem, there is nothing extra to calculate. You have completed the necessary calculations in the previous step. Step 3: Decide on Scale Type & Divisions This is an important part. You may take …. 2 major divisions of 7.5 cm (150m) or, 3 major divisions of 5 cm (100m) or, 5 major divisions of 3 cm (60m) or, 6 major divisions of 2.5 cm (50m) If you choose the last option to construct a scale with smaller divisions and try to show 1 m, then you need to divide a 2.5 cm major division into 50 minor divisions !!! ABSOLUTELY IMPOSSIBLE ! ! ! In such cases, you need to construct a diagonal scale.

Construction of Diagonal Scales (contd.) Step 4: Calculate to Draw Diagonal Scale If you draw a scale with 3 major division, then your one major division represents 100 m and the minor division have to show 1 m. Now 100 ÷ 1 = 100. You need to draw a diagonal grid and the (no. of rows X no. of columns) has to be 100. You may take … 10 rows & 10 columns or, 4 rows & 25 columns or, 25 rows & 4 columns or, 2 rows & 50 columns or, 50 rows & 2 columns or, 100 rows & 1 column or, 1 row & 100 columns (It will be a plain scale !!!) You should choose the first option, because it is not easy to divide your 5 cm major division into 25 or 50 or 100 minor divisions !!!

Construction of Diagonal Scales (contd.) Step 5: Draw the Scale

Construction of Diagonal Scales (contd.) Example 1 A car runs at 40 miles/hour. Construct a diagonal scale with RF=1/1584000 to read directly the position of the showing time interval of 3 minutes. Also show the distance traveled by the car in 1 hour & 42 minutes in the scale.

Construction of Diagonal Scales (contd.) Step 1: Check the RF to select unit Here, RF = 1/1584000 i.e. 1 cm = 1584000 cm or, 1 m = 1584000 m or, 1 ft = 1584000 ft or, 1 inch = 1584000 inch etc. Our objective is to develop a 6” or 15 cm scale for a fair presentation. Here the velocity is given in mph. So we may take 1 ft = 1584000 ft. But we can not draw a scale with such a big unit in a small sheet of paper. If we look for a smaller unit, can take 1 inch = 1584000 inch easily.

Construction of Diagonal Scales (contd.) Step 2: Solve the problem Now let us check the length of scale that represents 40 miles (the distance traveled by the car in an hour) according to RF. 40 miles = 40 X (1760 X 3 X12) inch = 2534400 “ According to RF, Actual 40 miles is (2534400 ÷ 1584000) or 1.6”. So in the scale, 1.6” is the distance traveled by the car in 1 hour (i.e. actually 40 miles). If you draw a 6.4” (1.6”X4) scale, it will show the distance covered by the car in 4 hours. The scale will be around 6” also and it will be fair enough !!!

Construction of Diagonal Scales (contd.) Step 4: Calculate to Draw Diagonal Scale Now your scale’s each major division represents the distance covered by the car in 1 hour and you need to divide it into it’s minor divisions to show the distance traveled by the car per 3 minutes. So,… 60 ÷ 3 =20 You need to draw a diagonal grid and the (no. of rows X no. of columns) has to be 20. You may take … 4 rows & 5 columns or, 5 rows & 4 columns or, 2 rows & 10 columns or, 10 rows & 2 columns or, 20 rows & 1 column or, 1 row & 20 columns (It will be a plain scale !!!) You should choose the second option, because it is relatively easy to divide your 1.6” major division into 4 minor divisions of 0.4”.

Construction of Diagonal Scales (contd.) Step 5: Draw the Scale