Guidelines for 06-91-321 Lab # 3 on Sep. 24, 2014 (CEI ,room no. 1181) Overview: During Lab #3 the students will develop a detailed technical drawing of a plastic toy block, starting with making hand sketches, measuring geometric features using common metrological tools, and creating a CAD drawing. Objectives: – To familiarize yourself with the use of a vernier caliper and micrometer. “ To refresh and apply your technical drawing skills. “ To consider the manufacturing processes involved in the creation of a simple toy. Experimental Procedure: Step 1: Calipers and Micrometers – Read the attached material regarding the use of the vernier caliper and the micrometer. “ Sketch the given piece on a paper sheet. “ Measure the dimensions of the given piece with both vernier calipers and/or micrometers. “ Record your measurements on the sketch. Make sure to measure internal geometries and thicknesses. Your drawing should fully describe the entire part. Step 2: Creating a CAD Drawing – Convert your hand sketch into a scale CAD drawing. Step 3: Questions 1. Speculate as to what manufacturing techniques are used to produce the Lego block. Make reasonable guesses and explain your reasoning. 2. Notice that the block has internal ribs. (Did you include them in your drawing?) What is the purpose of these ribs? Why are they located where they are? 3. Estimate the volume of material used to manufacture the Duplo block. How did you come to your estimate? 4. Is the part perfectly symmetrical? Are the thicknesses consistent across surfaces? 5. The interior of the part has markings. How were these markings created? 6. Comment on which block features and dimensions may be critical from a manufacturing point of view (hypothesize on how the blocks are made and what would make a block easy to make). 7. Comment about what block features and dimensions are critical from the functional point of view (e.g., interaction when used with other blocks) Lab Report – Write a lab report using standard format (Intro, Objective, Materials, Procedure, etc.) “ Answer the questions from Steps 1 and 3 and include your initial block sketch and resulting CAD drawing. “ Report Due Date: Wednesday, October 1, 2014 Hints: – Try to complete as much of the lab work as possible during the lab period as you will not be able to take the part home with you. Appendix I: The Vernier Caliper The Vernier Caliper is a precision instrument that can be used to measure internal and external distances extremely accurately. The example shown below is a manual caliper. Measurements are interpreted from the scale by the user. This is more difficult than using a digital vernier caliper which has an LCD digital display on which the reading appears. The manual version has both an imperial and metric scale. Manually operated vernier calipers can still be bought and remain popular because they are much cheaper than the digital version. Also, the digital version requires a small battery whereas the manual version does not need any power source. HOW TO READ A MEASUREMENT FROM THE SCALES EXAMPLE 1: The external measurement (diameter) of a steel hexagonal nut is measured using a vernier caliper, metric scale “ see the figure below -. A. The main metric scale is read first and this shows that there are 2 whole divisions and 4 decimal divisions “ see the left hand side red line -. Therefore, the first part of the answer is 2.4; B. On the moving jaw, the’ hundredths of mm’ scale is then read. Only one division on the main metric scale lines up with a division on the hundredths scale below it, whilst others do not “ see the right hand side red line on the moving jaw -. The division is then recorded as the hundreds decimal “ e.g. in the example below, the 7 th whole division on the hundredths scales – lines up exactly with a division on the metric scale above; C. The final answer is 2 + .4 + .07 = 2.47 cm. Animated demo: ADDITIONAL EXAMPLE: An enlarged view of a caliper in the figure below shows it has a resolution or precision of 0.02 mm “ the distance between two consecutive lines on the moving jaw -. The reading is 3.58 mm. The 3 mm is read off from the upper (fixed) data scale. The 0.58 mm is obtained from the lower (sliding) indicating scale at the point of closest alignment between the two scales. The superimposed red markings show where the readings are taken. Note: In this photograph, parallax error makes it unclear whether the right value is 0.58 mm or 0.60 mm Appendix II: The Micrometer The micrometer is a precision measuring instrument, used by engineers. Each revolution of the ratchet moves the spindle face 0.5mm towards the anvil face. The object to be measured is placed between the anvil face and the spindle face. The ratchet is turned clockwise until the object is ˜trapped’ between these two surfaces and the ratchet makes a ˜clicking’ noise. This means that the ratchet cannot be tightened any more and the measurement can be read. EXAMPLE MEASURE READINGS Using the first example seen below: 1. Read the scale on the sleeve; the example clearly shows12 mm divisions. 2. Still reading the scale on the sleeve, a further ½ mm (0.5) measurement can be seen on the bottom half of the scale. The measurement now reads 12.5mm. 3. Finally, the thimble scale shows 16 full divisions (these are hundredths of a mm). The final measurement is 12.5mm + 0.16mm = 12.66 READING AN IMPERIAL MICROMETER: Sleeve: The Micrometer sleeve is divided into 10 equal parts, each of these parts is equal to .100³ (1 tenth of an inch). Each of these 10 parts is divided into 4 equal parts. Each of these 4 subdivisions is equal to .025³ or one 40th of an inch. More simply, the line on the sleeve marked œ1³ represents .100³, the line marked œ2³ represents .200³ and so forth “ see the figure below with two examples. Thimble: The thimble is divided into twenty-five equal parts, each of these parts is equal to .001³ and, one complete rotation of the thimble coincides with the smallest division (.025³) on the sleeve. Example measurements. Taking a Reading on a .001³ Micrometer: To read either a .001³ or .0001³ micrometer, you place the material to be measured between the anvil and spindle, and then turn the ratchet until the spindle closes down and stops moving. Then you read the markings on the sleeve and thimble. In the case of a .0001³ micrometer you would then read the markings on the vernier scale to get the .0001³ measurement. In our example below, as we take our measurement, we fill in a box for each reading. We suggest you try this method while you are learning to measure with outside micrometers. Soon enough, you will become very fast reading these micrometers. 1. Read the sleeve: a. In the above figure, when tightening down the thimble on our material, it stopped at a point to the right of œ2³ on the sleeve, this indicates .200³. We wrote this in row (1) in the table below. b. There is one line visible between the graduation numbered œ2³ on the sleeve and the edge of the thimble, this indicates .025³. We wrote this in row (2) in the table below. 2. Read the thimble: The graduation numbered œ1³ on the thimble coincides with the center line of the sleeve. This indicates .001³. We wrote this in (3) in the table below. 3. Add it all up: Now just add all the numbers together to determine the thickness of your material. (1) Reading on the Sleeve .200³ (2) No. of lines between œ2³ and the edge of the thimble .025³ (3) Thimble line corresponding to the center line of the sleeve .001³ TOTAL READING .226³ Taking a Reading on a .0001³ Micrometer: To read to one ten-thousandth requires an additional scale called the œVernier scale. The vernier consists of ten divisions, marked on the sleeve, each graduation of the vernier scale on the sleeve, represents .0001³. 1. Read the sleeve: Follow the same instructions as step 1 above. 2. Read the Thimble: Follow the same instructions as step 2 above. 3. Read the vernier: Each graduation of the vernier scale on the sleeve measures .0001³ (or one ten-thousandth of an inch). To read the vernier, find the graduation on the vernier scale which lines up with with a graduation on the thimble and read the number off the vernier scale. In the above figure, the vernier graduation numbered œ2³ lines up exactly with a thimble line (number œ6³), therefore you read the vernier line œ2³ which indicates .0002³. 4. Add it all up: Now just add all the numbers together to determine the thickness of your material. (1) Reading on the Sleeve .200³ (2) No. of lines between œ2³ and the edge of the thimble .025³ (3) Thimble has passed .001³ line on the Sleeve .001³ (4) Vernier line that coincides exactly with a Thimble Line .0002³ 6 TOTAL READING .2262³ Appendix III: Other Measuring Equipment (Examples) Runout Gage Digital Depth Gage Laser Level Gear Tooth Micrometer Coordinate Measuring Machine Profilometer Appendix IV: Technical Drawing Basics Introduction One of the best ways to communicate one’s ideas is through some form of picture or drawing. This is especially true for the engineer. The purpose of this guide is to give you the basics of engineering sketching and drawing. We will treat œsketching and œdrawing as one. œSketching generally means freehand drawing. œDrawing usually means using drawing instruments, from compasses to computers to bring precision to the drawings. This is just an introduction. Don’t worry about understanding every detail right now “ just get a general feel for the language of graphics. We hope you like the object in Figure 1, because you’ll be seeing a lot of it. Before we get started on any technical drawings, let’s get a good look at this strange block from several angles. Figure 1 “ A Machined Block Figure 2 “ An Isometric Drawing Isometric Drawing The representation of the object in figure 2 is called an isometric drawing. This is one of a family of three-dimensional views called pictorial drawings. In an isometric drawing, the object’s vertical lines are drawn vertically, and the horizontal lines in the width and depth planes are shown at 30 degrees to the horizontal. When drawn under these guidelines, the lines parallel to these three axes are at their true (scale) lengths. Lines that are not parallel to these axes will not be of their true length. Any engineering drawing should show everything: a complete understanding of the object should be possible from the drawing. If the isometric drawing can show all details and all dimensions on one drawing, it is ideal. One can pack a great deal of information into an isometric drawing. However, if the object in figure 2 had a hole on the back side, it would not be visible using a single isometric drawing. In order to get a more complete view of the object, an orthographic projection may be used. Orthographic or Multi-view Drawing Imagine that you have an object suspended by transparent threads inside a glass box, as in figure 3. Figure 3 “ The block suspended in a glass box Figure 4 “ The creation of an orthographic multi-view drawing Then draw the object on each of three faces as seen from that direction. Unfold the box (figure 4) and you have the three views. We call this an œorthographic or œmulti-view drawing. Figure 5 shows how the three views appear on a piece of paper after unfolding the box. Figure 5 “ A multi-view drawing and its explanation Which views should one choose for a multi-view drawing? “ The views that reveal every detail about the object. Three views are not always necessary; we need only as many views as are required to describe the object fully. For example, some objects need only two views, while others need four. The circular object in figure 6 requires only two views. Figure 6 “ An object needing only two orthogonal views Dimensioning Figure 7 “ An isometric view with dimensions Figure 8 “ An isometric drawing that does not show all details We have œdimensioned the object in the isometric drawing in figure 7. As a general guideline to dimensioning, try to think that you would make an object and dimension it in the most useful way. Put in exactly as many dimensions as are necessary for the craftsperson to make it -no more, no less. Do not put in redundant dimensions. Not only will these clutter the drawing, but if œtolerances or accuracy levels have been included, the redundant dimensions often lead to conflicts when the tolerance allowances can be added in different ways. Repeatedly measuring from one point to another will lead to inaccuracies. It is often better to measure from one end to various points. This gives the dimensions a reference standard. It is helpful to choose the placement of the dimension in the order in which a machinist would create the part. This convention may take some experience. Sectioning There are many times when the interior details of an object cannot be seen from the outside (figure 8). We can get around this by pretending to cut the object on a plane and showing the œsectional view. The sectional view is applicable to objects like engine blocks, where the interior details are intricate and would be very difficult to understand through the use of œhidden lines (hidden lines are, by convention, dotted) on an orthographic or isometric drawing. Imagine slicing the object in the middle (figure 9): Figure 9 “ œSectioning an object Figure 10 “ Sectioning the object from figure 8 Take away the front half (figure 10) and what you have is a full section view (figure 11). Figure 11 “ Sectioned isometric and orthogonal views The cross-section looks like figure 11 when it is viewed from straight ahead. Cross-Sectional Views A cross-sectional view portrays a cut-away portion of the object and is another way to show hidden components in a device. Imagine a plane that cuts vertically through the center of the pillow block as shown in figure 15. Then imagine removing the material from the front of this plane, as shown in figure 16. Figure 15 “ Pillow Block Figure 16 “ Pillow Block This is how the remaining rear section would look. Diagonal lines (cross-hatches) show regions where materials have been cut by the cutting plane. Figure 17 “ Section œA-A Figure 18 “ The top œoutside view of the bearing This cross-sectional view (section A-A, figure 17), one that is orthogonal to the viewing direction, shows the relationships of lengths and diameters better. These drawings are easier to make than isometric drawings. Seasoned engineers can interpret orthogonal drawings without needing an isometric drawing, but this takes a bit of practice. The top œoutside view of the bearing is shown in figure 18. It is an orthogonal (perpendicular) projection. Notice the direction of the arrows for the œA-A cutting plane. Half-Sections A half-section is a view of an object showing one-half of the view in section, as in figure 19 and 20. Figure 19 “ Full and sectioned isometric views Figure 20 “ Front view and half section The diagonal lines on the section drawing are used to indicate the area that has been theoretically cut. These lines are called section lining or cross-hatching. The lines are thin and are usually drawn at a 45-degree angle to the major outline of the object. The spacing between lines should be uniform. A second, rarer, use of cross-hatching is to indicate the material of the object. One form of cross-hatching may be used for cast iron, another for bronze, and so forth. More usually, the type of material is indicated elsewhere on the drawing, making the use of different types of cross-hatching unnecessary. 13 Figure 21 “ Half section without hidden lines Usually hidden (dotted) lines are not used on the cross-section unless they are needed for dimensioning purposes. Also, some hidden lines on the non-sectioned part of the drawings are not needed (figure 12) since they become redundant information and may clutter the drawing. Sectioning Objects with Holes, Ribs, Etc. The cross-section on the right of figure 22 is technically correct. However, the convention in a drawing is to show the view on the left as the preferred method for sectioning this type of object. Figure 22 “ Cross section Dimensioning The purpose of dimensioning is to provide a clear and complete description of an object. A complete set of dimensions will permit only one interpretation needed to construct the part. Dimensioning should follow these guidelines. 1. Accuracy: correct values must be given. 2. Clearness: dimensions must be placed in appropriate positions. 3. Completeness: nothing must be left out, and nothing duplicated. 4. Readability: the appropriate line quality must be used for legibility. The Basics: Definitions and Dimensions The dimension line is a thin line, broken in the middle to allow the placement of the dimension value, with arrowheads at each end (figure 23). Figure 23 “ Dimensioned Drawing Figure 24 “ Example drawing with a leader An arrowhead is approximately 3 mm long and 1 mm wide. That is, the length is roughly three times the width. An extension line extends a line on the object to the dimension line. The first dimension line should be approximately 12 mm (0.6 in) from the object. Extension lines begin 1.5 mm from the object and extend 3 mm from the last dimension line. A leader is a thin line used to connect a dimension with a particular area (figure 24). A leader may also be used to indicate a note or comment about a specific area. When there is limited space, a heavy black dot may be substituted for the arrows, as in figure 23. Also in this drawing, two holes are identical, allowing the œ2x notation to be used and the dimension to point to only one of the circles. Where to Put Dimensions The dimensions should be placed on the face that describes the feature most clearly. Examples of appropriate and inappropriate placing of dimensions are shown in figure 25. Figure 25 “ Example of appropriate and inappropriate dimensioning In order to get the feel of what dimensioning is all about, we can start with a simple rectangular block. With this simple object, only three dimensions are needed to describe it completely (figure 26). There is little choice on where to put its dimensions. Figure 26 “ Simple Object Figure 27 “ Surface datum example We have to make some choices when we dimension a block with a notch or cutout (figure 27). It is usually best to dimension from a common line or surface. This can be called the datum line of surface. This eliminates the addition of measurement or machining inaccuracies that would come from œchain or œseries dimensioning. Notice how the dimensions originate on the datum surfaces. We chose one datum surface in figure 27, and another in figure 28. As long as we are consistent, it makes no difference. (We are just showing the top view). Figure 28 “ Surface datum example Figure 29 “ Example of a dimensioned hole In figure 29 we have shown a hole that we have chosen to dimension on the left side of the object. The Ø stands for œdiameter. When the left side of the block is œradiuses as in figure 30, we break our rule that we should not duplicate dimensions. The total length is known because the radius of the curve on the left side is given. Then, for clarity, we add the overall length of 60 and we note that it is a reference (REF) dimension. This means that it is not really required. Figure 30 “ Example of a directly dimensioned hole Figure 31 “ Example of a directly dimensioned hole Somewhere on the paper, usually the bottom, there should be placed information on what measuring system is being used (e.g. inches and millimeters) and also the scale of the drawing. This drawing is symmetric about the horizontal centerline. Centerlines (chain-dotted) are used for symmetric objects, and also for the center of circles and holes. We can dimension directly to the centerline, as in figure 31. In some cases this method can be clearer than just dimensioning between surfaces. References Lego mfg: Lego mfg.: Lego mfg.: Technical Drawing Basics Reading a Vernier The Vernier Caliper and Micrometer Micrometers, verniers and lathes Vernier Scale Micrometer