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Creating A 3D Sopwith Pup, Part one: The Engine - Jeff Matheson - Page 3

Author: Jeff Matheson
Date Published: 2007-11-02
Contact: artkings[at]highconceptmedia[dot]com
Author Website: http://www.artkings.highconceptmedia.com
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As I mentioned in the introduction, this LeRhone engine exhausts directly to the outside, without any sort of modern exhaust system. The exhaust valve is simply exposed at the top of the cylinder, under a small structure that supports the valve and valve spring. This structure is basically round, with a curved edge, a hollow center, and a small window at the back for better air flow.

Using two cylinder primitives, one inside the other, I use the ProBoolean Compound Object to Subtract the inner cylinder from the outer one, creating the hollow inner space (SCREENSHOT). To create the outer curved edge, I place a much larger cylinder over the port, using part of its outer curve to define the new edge. I also create a ChamferBox that will be used to cut the hole at the back of the exhaust port housing. Adding the new large cylinder to the previous ProBoolean object (choosing the Intersection option this time) and then Subtracting the ChamferBox results in the final shape we need (SCREENSHOT).

The final step for the exhaust port housing is to make it appear to be an integral part of the cylinder top, based on the photo reference. A simple box primitive is altered to form a smoothed tapered wing on one side of the port, blending it with the nearest fin. Use the Mirror command to create one for the other side (SCREENSHOT).


When I first reviewed my reference photos, the valve springs on the engine seemed a bit light-weight to me (by modern standards) - not really up to the job of closing some fairly heavy valves. I wondered if they were stock or non-functioning - until I realized that due to the engine's maximum speed of 1200 RPM and a sizable amount of centrifugal force caused by the spinning engine, the valve springs had a reasonably easy time of it.



To generate the valve springs, I used the Helix object (under Splines), specifying 8 full turns and a constant radius. I matched the diameter of the rendered spline to my photographic reference (make sure to turn on "Enable In Viewport" so that you can see the actual size). Once the helical spline was the proper size and position, I converted it into an Editable Poly (this prevents the spline from disappearing if I export the model to another program that may not render the spline in the same way). The valve spring seat and valve spring retainer are simply made from three flat cylinders at either end of the spring. The cotter pin is a Line spline, converted into an Editable Poly and then stretched (scaled) sideways to make it look like flat wire.

By lathing another Line spline, the valve itself is formed. This is the same technique used to form the cylinder fins in the beginning of this tutorial. Often, this the best way to create an object that is radially symmetric (one that has a circular cross-section), especially if the object has complex curves.

With one valve assembly complete, I clone it to create the valve assembly for the intake side. Please note that the valves are not exactly parallel to the centerline of the cylinder, but are set at a slight angle. The final step is to use a Sphere primitive to create the round inset where the valve head sits. This is done by creating a ProBoolean compound object from the cylinder head top and Subtracting the Sphere from it (SCREENSHOT).


Now that we have the valves in place, we need something to open them, and that is where the rocker arm comes in. Activated by a cam in the crankcase, via a pushrod, the rocker arm opens the valves as the engine spins around. The rocker arm is a narrow V-shaped piece, with one arm slightly longer than the other, and has rounded edges. It is connected to the pushrod via a shaft and linkage that pivot inside a ball-bearing housing. To model the rocker arm, I begin with a Line spline, then use the Bevel object modifier to give it depth. By using a 3 level bevel, it is fairly easy to include a rounded/chamfered edge to the piece (SCREENSHOT). Note that the ends of the rocker arm should be directly above the ends of the valve stems.



Earlier I said that lathing a spline was a good way to create round objects - in spite of that statement, I did not do it that way here (though I could have). Since the shaft housing should have the ends straight and of constant diameter, it was easier to create it from a ChamferCylinder, rather than try to draw a profile spline that was perfectly straight. I always try to use the simplest, easiest method that can give me the results I need.

After the ChamferCylinder was created and positioned, I set the number of height segments to 7, (which gave me more vertices to work with), and then I converted it into an Editable Poly. I wanted to create a center section that was a smaller diameter than the ends, with a strong taper between the sections. By entering vertex editing mode, I selected the center vertices, and using the Scale and Move tools, reshaped the cylinder to the size I wanted. To indent the round section at the front (which will form the ball bearing race), use polygon modeling mode to select the polygons that make up the round section (make sure to turn on "Ignore Backfacing" or you may select more polygons than desired) (SCREENSHOT). Use the move tool to push this section back into the piece (SCREENSHOT).


The ball bearings are simply small Geospheres. I use Geospheres rather than standard Spheres because they give smoother results with fewer polygons. Create the first one (don't worry too much about the size), and position it at the exact top center of the bearing race. Align it with the shaft housing, and then adjust the Geosphere's pivot point (choose "Affect Pivot Only" from the Heirarchy tab) and align the pivot point (NOT the geosphere) with the center line of the shaft housing. Choose the rotation tool, and turn off "Affect Pivot Only". Now the Geosphere should rotate around the center line of the shaft housing, and not its own center. We want to have 18 ball bearings, so they need to be 20 degrees apart (20x18=360).Turn on the "Angle Snap Toggle" for the rotation tool, hold down the "shift" key, and rotate to 20 degrees to clone the first ball bearing. Choose Instance in the Clone dialog box and specify 17 copies. Now you should have 18 ball bearings evenly arranged around the bearing race. Adjust the size of the first ball bearing until all the bearings barely touch. Add a small flat cylinder to hold them in place (SCREENSHOT).

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