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1 Rhino Tutorial Rhino Mini Tutorial on bending objects on Sat Sep 25, 2010 10:02 pm

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Rhino has the ability to distort all types of objects, including NURBS models created within Rhino or Polygon Meshes either created within Rhino or imported models. This was a project inspired by the Renderosity Rhino3D Forum's Challenge of the Month for December 2000. The challenge was to create a portable music player. The MP3 player I chose to model has a belt clip on the back which posed a few problems when it came to bending it to it's final shape. I though that the process behind determining the best way to achieve the final result would be worthwhile of a short tutorial. This page has many graphics, so be warned, it may take a while to load.
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This is the final shape of the belt clip we are aiming to achieve.
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Step 1 - The profile of the belt clip, laid flat is drawn out in the top viewport. I usually draw profiles using the Polyline tool. Arcs are drawn with the arc tool, then all segments are joined together and the corners filletted using the Curve>Fillet command.
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Step 2 - This profile curve is then extruded using the Solid>Extrude Planar Curve command. The edges were rounded slightly using the Solid>Fillet Edge command. Other pieces (the screw head, the small rounded area around it, and the tooth) were added after this step. I won't go into detail on those here as that isn't the purpose of this tutorial. Save what you have here as you will need to get back to this step.
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Step 3 - If we just try to bend this object, we will run into difficulties. First off, we can't actually bend this object because it is a polysurface. To allow is to bend it, we must first explode it into individual surfaces using the Edit>Explode command. Now we can attempt to bend all the surfaces at the same time. Try it, using the Transform>Bend command. Select all the objects (just select everything) to be bent. Choose the Bend command, then set the starting point at the right hand edge of the object, the end point at the left hand end of the object and move the cursor toward the center of the radius around which you want to bend the object. Notice here how the edges bend but the two surfaces do not. The reason for this is that the edges have several control points while the surface has only points at the corners.
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Step 4 - In theory if we increase the number of control points we should be able to bend the object just fine. Select just the top and bottom surfaces and use the Surface>Edit Tools>Rebuild command. Here I increased the U and V counts to 30 each. New problem - the surfaces suddenly became rectangles when I did that. The re-building did increase the number of control points, however it also deleted the trims that were automatically created when the curve was extruded. Since I had done a bit of work booleaning the tooth and rounding the edges, I decided I didn't want to go back and do those steps over again in re-trimming it. This did have potential to work as now I would have had a smooth bend with all those control points. Oh well, back to the drawing board.
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Step 5 - Well, another option is to go back to the structure we had in step 2 (I hope you saved it....) and go straight to meshing it. Converting the NURBS structure to a polygon mesh will give us the same end result as re-building it; lots of control points over the entire structure. We lose some of the options we could use on NURBS surfaces once we mesh it, but if the structure is essentially how we want it, then that is no problem. Select the object then go to the Tools>Polygon Mesh>From NURBS Object command. Select the Detailed Controls button from the dialog that appears. In the detailed options dialog you can adjust several values. First make sure Weld is not ticked. Welded meshes often have severe artifacts when you try to UV map textures on them. Next adjust the Min Edge Length value to the smallest value you think will give you the detail you want. On areas that have small radius curves you would want to be sure you have several segments over the curve. This is all in reference to the scale you drew the object in in the first place and relates directly to the grid scale. I figure my tightest curve has a raduis of about 0.1 units therefore a Min Edge Length value of 0.02 will give about 5 segments over the curve. This may be a little much and you could probably get away with a value up to about half the radius ( 0.05). If you hit the preview button now you will see a mesh with small faces arrayed along the areas where there are tight curves and large faces over the centre of the flat surfaces. This is still not ideal for bending as those large faces will only bend where they join other faces (faces always remain flat in a polygon mesh). Since we want a rather tight curve this will still lead to facetting around the bend.
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Step 6 - By setting the Max Edge Length parameter we can control the maximum size of the faces. You can see here how setting a value of 2 results in the flat surfaces being divided up into faces measuring 2 grid units per side. These smaller faces will now enable us to make a smooth, tight bend. Note that the smaller the faces, the smoother and more accurately the mesh will represent the NURBS model, however, the file size of the model will increase proportionately.
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Step 7 - Now we can try bending the mesh. Select the mesh then choose the Transform>Bend command. Set the starting point at the far right end of the object, the end point at the far left end and move the cursor towards the inner radius of the curve you want. Notice how, when using the default settings for the Bend command, the entire object bends uniformly about the centre of the radius. We only want to bend the centre of the object.
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Step 8 - How about if we set the start point to the area of the object where we would like the curve to start? Still everthing from the start point to the end point (far left end of line) bends uniformly. We would like only a small section in the centre to bend. If you want to try making a real mess, set the end point to just past the area you want to bend. You'll find that the area past the end point stays in place while the area between the bend and the stationary piece gets horribly distorted. Not good.
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Step 9 - There's an easy fix to all this. Select the whole object, use the Transform>Bend command and set your start and end points as illustrated above (note: this is the same as step Cool. Now type S on the command line and hit enter. This keeps everthing past the bend radius straight and in tangent to the radius. Move the cursor around and see how the radius and angle of the end piece is manipulated.
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Step 10 - We still want to make one more bend to our object so it actually resembles the belt clip we're modeling. If we use exactly the same command and just set the start point near where we want the curve to begin we will run into a new problem. See how the other end of the object bends as well. This is because the radius of the bend overlaps this area and the entire object is still selected. If we could just select the portion we wanted to bend all would be fine.
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Step 11 - Luckily we can select just that portion! With the whole object selected, hit the button that turns Control Points on (or type PtOn at the control line). Wow! Look at all those densly packed control points! Since we're not terribly worried about accuracy here, just use the cursour to drag a selection marquee around the control points from the end of the last bend to the end of the object (highlighted here in yellow). Now choose the Transform>Bend command and set your start and end points as indicated. You probably won;t have to type S again on the command line as Rhino tends to remember what you did last. Now bend it so the curve is over a very short area and the end of the belt clip is in line with the back surface of the beginning of the belt clip. Remember to turn off all the contol points when you're done.
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This is how my final belt clip looks when rendered (in Bryce 4). Hopefully this has been a good overview of the Bend command and the role of meshing your NURBS object when you need to increase the control point count uniformly over your object. The same logic can apply to any of the Transform menu commands.
Have fun and play safe!]

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