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1 Rhino Tutorial Mag Wheel 2/3 on Sat Sep 25, 2010 9:52 pm

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Page 2 - In which we'll model the inner hub of the Mag Wheel.
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Step 20 Now to start preparing the spoke to flow into the central hub. Draw two straight lines that intersect just within the inner edge of the spoke as close to the centre as you can make it. Extrude these into surfaces that intersect the spoke surface.
Logic This step is necessary as we will be building the section between the spokes next. This is the next component that I have broken the model down into. I find it easier to visualize the profile of the hub between the spokes than trying to build the whole hub and make it match the spoke ends at the same time.
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Step 21 Trim the corners of the spoke surface, using the Edit>Trim {Trim} command. Use one of the surfaces from step 20 as the cutting object and select all the surfaces of the spoke at the corner to trim. Note there may be several surfaces, and it's very important you get them all. Delete the trimming surfaces when you're done.
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Step 22 We've done all the work we need to do on the spoke/rim assembly. Select all the objects that make up the spoke/rim assembly and use the Transform>Array>Polar Array {ArrayPolar} command. For the centre of the array pick the point at world centre (where the green and red lines meet) in the top viewport. Snap will help to make this accurate. For number of elements type in 5 and for angle accept the default of 360. Hit enter. Ta da! We now have something that looks suspiciously like a mag wheel. OK, not quite, we're missing the hub.
Logic - See how it's easy to take advantage of the radial symmetry of this model, and make just 1/5 of it. We could have made the whole rim, then made a single spoke and polar arrayed it, but we would have then had to do all the fillets for each one individually.
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Step 23 Now to work on the centre hub, and introduce a whole new set of commands! In the front or right viewport, draw 4 single segment lines from the central Z axis (green line) to the points of two adjacent spokes. To aid in this, first hide any objects that are in the way (select the object and use the Hide button on the top toolbar; you can make them come back by right-clicking the same button). Turn snap on, click the centre line, then turn snap off, turn Osnap End on and click the point of one of the spokes. Repeat for the other 3 points. It is VERY important to make sure these lines are parrallel to the horizontal axis. You may have to turn on control points, turn off snap, and drag the central points up or down while using shift to make sure they are not at an angle.
Logic The reason for this last point is so that when we go to polar arrya the small section we are working on, all the new sections will line up without visible seams or creases. Again, I find it easier to work on the section between the spokes points to make the central hub.
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Step 24 We also need to duplicate the curves that make up the inside edge of two of the spokes points. To do this select Curve>From Objects>Duplicate Edge {DupEdge}. Select all segments that make up the edges of the spoke end within the angle made by the lines drawn in step 23 (could be 3 or 4 segments each).
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Step 25 Use Edit>Join {Join} and select all the curves on one side of the angle (see image). Hit enter. Now repeat this step for the other side. Now you should have only two separate curves making up this angle.
Logic When we go to make a surface from these curves, it will now match exactly to the inside trimmed edge of the spokes.

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Step 26 We now need to define the other edge of the wedge we're making of the central hub. Turn on Planar mode (click the word "Planar" on the bottom of the screen). This allow you to place points in the same plane as the first point you place. Turn on Osnap Near, and begin drawing a Control Point Curve in the top viewport. Start the first point on the middle of the curve of the open end of one of the spokes, the second point near world centre, and the third point on the middle of the curve of the open end of the other spoke (I hope that made some sort of sense). When looking at the curve in any other viewport it should look like a straight line, parrallel with the horizontal axis. If it isn't, fix it. Also, turn contol points on, and in the top viewport adjust the postion of the middle point to adjust the curvature of the arc. It should look like it's a smooth continuation of both spoke's curves.
Logic We want the spokes to flow smoothly into the hub and into each other. The combination of curves we have just created should accomplish this, as each line is a smooth continuation of some aspect of the spoke.
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Step 27 OK, here's a new one. Select the Surface>From Curve Network {NetworkSrf} command. Select each of the three curves (I don't know if the order matters, but I did: angle edge, centre curve, angle edge) and hit enter. Look at what happens! The illustrated dialog box appears and various letter appear next to the curves. I have to admit, I really don't know much about what each of these mean, but there is a little "Help" button at the bottom right that might set things straight. I just hit OK and it built exactly the surface I was hoping for (see the second image). Pretty slick!
Logic OK, not much logic here, just an explanation of the process I went through. I knew the shape I wanted, I just didn't know how to get it. From the curves drawn in step 26, I first tried a 2 Rail Sweep, using the angle curves as the rails and the end points and curve as the profiles. This didn't get the desired result. I tried lofting the curves together. Again no such luck. I tried a rail revolve using the curve as the rail. No go. I also tried surface from edge curves. This got me a little closer, but still not good enough. Finally the surface from curve network command worked! So the moral is, keep trying different ways! Something is likely to work.
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Step 28 Select this new surface and choose Transform>Array>Polar {ArrayPolar}. Click the world centre point as the centre of the array, number of elements is 5 and accept the default angle of 360 degrees. Cool! Looks like some sort of shooting star! Note how the centre of the star hub is nice and smooth, and you can't tell it's made up of 5 different objects. Also note how nicely all the spoke seem to flow into the hub. Looks like the logic is working...... Smile
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Step 29 OK, so I' m no automotive engineer. It looks like I have no way of attaching the wheel to a car, and my central hub certainly isn't big enough to put bolts through. I'm going to have to add another structure, which, for lack of a better name, will call the inner rim. To give myself some sort of structure to work in I'll draw a 72 degree arc, centred at world centre and extending from the middle of a spoke to the middle of an adjacent spoke and residing at about the distance away from the centre I want to make the inner rim extend. Do this on the Guides layer you created earlier.
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Step 30 Also in the guide layer, create a circle that roughly matches the curvature of the central hub. Turn Osnap Centre on and draw a second concentric circle larger than the first. These will be guides in the design of the various parts of the inner rim.
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Step 31 Now make a new layer, call it Inner Rim and make it active. Create a control point curve with Osnap End on. Snap the first and last points to the ends of the arc and distribute the other points as indicated in the image above. Try to match the curvature of the larger circle. If necessary, turn on control points for this curve and drag them around to refine the shape.
Logic This is going to be the edge profile of the inner rim. I will also be polar arraying this so I want it to fill exactly a 72 degree arc and have a nice, smooth curvature.
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Step 32 Turn off control points for the curve. In one of the side viewports drag this curve up (Ortho on) to where the bottom of the inner hub will be (lower curve). Copy and Paste the curve. Drag the curve up again (Ortho on) to align with the bottom of the spokes. Turn on control points and, with Ortho on, drag the controls points up or down to make the curve line up with the curvature of the bottom of the spoke as it approaches the hub. When doing this slect matching points and move them together, i.e. The first and last points, the second and second last points, etc. This now defines the top and bottom edge profile of the inner rim.
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Step 33 Note that often I will turn off certain layers to better see what I'm doing. Just right click the layers panel at the bottom of the screen and set whatever layer you don't want to see to Off. Use the same steps to turn it back on. I want to draw a profile curve at the end of the two curves from the previous step, but the angle from world centre to the end of the curves is not an orthogonal view. This will make it difficult to draw this curve with any accuracy. To help, we'll set up a new construction plane at this angle. Right click on the word "Front" in the front viewport. Chose set c-plane form the list and choose 3-point as the option. In the top viewport click at world centre and click at the end of the arc. In the right viewport place the third point anywhere up on the green (Z-axis) line. Now back in the front viewport rotate the view with the right mouse button so it appears you are looking straight at the grid (the squares should be, well, square). Now, in this viewport, draw the profile of the inner rim. I used the straight ployline tool to draw the bottom, side and top ridge, and a control point curve dor the top section. Make sure to snap the ends of the lines to the vertical green line at world centre and the overlapping end point of the two curves to each other (Osnap End). Use Edit>Join {Join} to turn these into a single curve.
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Step 34 Another way to control the curvature of lines is to draw straight ones like we've done here then fillet them. Do this by choosing Curve>Fillet {Fillet}. Type J to make the curves join the fillets and then type a number for the radius (0.2 to 0.3 worked with my curves and scale). Click the curve on each side of the sharp corner and it will round by the desired amount. Repeat this command (just hit enter or right click to repeat the last command) to fillet all the upper corners. If this does not work (and it does fail sometimes for no apparent reason), try turning control points on for the cuve and deleteing the one closest to the corner. This may round it.
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Step 35 The next version of Rhino will likely have the command I would really like to use here: 2 Rail revolve. Since this version doesn't have it, we will just use >Surface>Rail Revolve {RailRevolve}. Select the curve we just drew for the profile curve and the upper curvy line from step 32 as the rail. The axis can be easily defined by turning Osnap End on and snapping the axis to the endpoints of the profile curve. The wavy bottom of the new surface was not desired, and would not have occured if a 2 Rail Revolve command was available. However, it won't really be visible in the final model so we'll leave it. It could be trimmed flat or booleaned off later if we really felt it necessary. Right click on the Front viewports name and choose Set View>Front to reset the view.
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Step 36 Let's add a little more surface detail to this segment of the inner rim. Use the Solid>Ellipsoid>From Centre {Ellipsoid} command, setting the centre point to the centre of the smaller circle (use Osnap Centre). With Ortho on and in top viewport, make the first two axes extend to the edge of the circle guide. Move to the front or side viewport and set the third point so half the depth of the ellipse is equal to just a little less than the full depth of the wedge we just created (see image above).
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Step 37 Move the ellipsoid off to the side so you can see what you're doing. Now use the Transform>Copy {Copy} command to make a copy of the ellipsoid. Turn on Osnap Centre then select the centre of the ellipse as the point to copy from. Now turn on Osnap On Surface from Tools>Object Snap>On Surface {OnSrf}. In the perspective viewport click the top surface of hte wedge to select it. Turn off Osnap Centre if it's still on. And now move the copy on the surface so that the new copy is aligned vertically with the circle guide. Click. Hit Esc to exit the copy command. Now delete the original ellipsoid.
Logic That seems like an extra step to go through to move the ellipsoid to the surface, but the OnSrf command will only work when Rhino thinks you want to set a point. By copying we got to work with a point, despite the fact we were really moving around a surface. Whatever works......
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Step 38 Right now part of the ellipsoid (the left edge) is dipping into the high side of the wedge. I want the ellipsoid to lie along the slope of the surface it is on. To do this turn on Osnap Centre, and select the Transform>Rotate {Rotate} command. Click near the edge of the ellipsiod to snap to the centre. Turn Osnap Centre off. Hold down the shift key and select a point near the edge of the ellipsoid to define the axis of rotation. Now just tilt the ellipsoid until it essentially lines up with the surface. Check it's appearance by shading hte perspective viewport and rotating around to see that the top edge is fully visible.
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Step 39 Now we'll subtract the ellipsoid from the wedge. Select Solid>Difference {BooleanDifference} and click the top outside edge of the wedge, enter, then click the ellipsoid and hit enter. Hiopefully it worked and the outer surface of the ellipsoid and the inner surface of the wedge disappeared leaving you with the object in the image above. If it didn't work try again, selecting the objects in a different order, or, as a last result try Union instead of Difference (sometimes it works backwards).
Logic Adding surface detail is a good way to make a model look more complex, and believable. Boolean operations are often useful to make shapes that would be difficult to model with NURBS curves alone.
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Step 40 The hollow we just made is perfect to set the stud and nut in. We'll make the nut first, then we'll make the hole for the rim to mount on the car through. First off, we should make a new layer and name it "Nut" or something similar. Make this new layer active, and turn off the Inner Rim layer to make it easier to see what we're doing. Select the polygon tool or choose Curve>Polygon>Centre, Radius {Polygon}. Type 6 to set the number of sides to 6. Turn Osnap Centre on, and set the first point to the centre of the circle guide. Turn off OSnap Centre. To numerically set the size of the nut, type in a value equal to the radius of the nut (half of 7/8", the size of a nut on a car). Sinc emy scale is 1 square to 1/2", I typed 0.875. Click and the hexagon is the right size.
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Step 41 Now we'll make the hexagon into a solid. Choose the Solid>Extrude Planar Curve {Extrude} command. Drag upwards in either the front or side viewport and set the height to be somewhere around 1 inch (in your scale).
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Step 42 With Osnap Centre back on again, create a sphere using the Solid>Sphere>Centre, Radius {Sphere} command. Set the first point to the centre of the circle guide. Drag outwards (probably have to turn off Osnal Centre) unti the edge is just inside the hexagon.
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Step 43 Copy and paste the sphere. Now, with Ortho on, move the copy vertically until it is centered on the opposite end of the hexagonal solid. Now select Solid>Union {BooleanUnion} and select each of these three solids. We now have a single solid.
Logic This is a very simple nut. If we were going to see it up close, I would use a cylinder to hollow out the bottom, fillet all the edges slightly, and maybe even add threads to the inside. However, we need not go into that detail here.
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Step 44 We just need a hole for the stud to go through the inner rim. Make a cone using Solid>Cone {Cone}. Using Osnap Centre, start the base of the cone centred on the circle guide. Make the radius just bigger than the hexagon. Drag down in the front or side viewport, with Ortho on, so that the edges of the cone just contact the lower sphere.
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Step 45 Now would be a good time to turn on the Inner Rim layer. Select both the nut and the cone from step 44. With Ortho on, raise them up so that the base of the cone is completely protruding through the wedge structure from step 35. Check to make sure by shading the perspective viewport.
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Step 46 Cut the hole in the inner rim by selecting Solid>Difference {BooleanDifference}. Click the surface of the Inner Rim, enter, click the cone, enter. The hole should be cut.
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Step 47 One last time, let's do the Polar Array! Select the nut and the Inner Rim surface. Choose Transform>Array>Polar {ArrayPolar} and choose the world centre (Snap helps in accurately clicking) as the centre of the array. Number is 5 and dergees is 360. Turn on all the layers and check out the Mag Wheel!
A little cleanup is in order. Select all the Curves (Edit>Select>Curves) and delete them. Also choose Edit>Join and go around the main rim/centre hub/ spoke assembly, selecting every surface. Hit enter to join these all together. Do this as well for the inner rim surfaces. You should end up with 7 distinct objects, the rim with spokes and hub, the inner rim, and 5 nuts.
Whew! It's not bad as is, but some details are still in order.
We'll do these on Page 3............]

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