I import pictures and drawings into my CAD program (SolidWorks) and scale them to my working scale, this includes rotating them to establish some horizontal and vertical reference points. Most photos and drawings are not very good representations of a part as there is parallax and vanishing points and focal distances that distort the resulting 2D image. But I am not interested so much in just capturing an outline, but understanding the relationship of surfaces. For example the outside edges of the gear tower that contains the gears to the cam shaft have a negative inclusive angle, that is the sides are not parallel and get closer together towards the top; I have this angle shown as 3 degrees below. I’ll take many photos and drawings and attempt to ascertain this angle. There were variations between engines and drawings so I am a able to take a little liberty in establishing an angle that works for my design, looks representative to the eye and is true to the information I have at hand. But I don’t agonize over getting it “perfect”.
Source: Offenhauser 110 Engine
Serial number 214 Built early 1947
I have a spline estimating the curve shown in the highlighted red circle above. I don’t like splines, I try to reduce complex curves to a series of tangent arcs, but sometimes splines provide the best fit. When using splines I try to reduce the number of points to the minimum, maintain tangent entry and exit points and have some mathematical relationship between the points. Not that engineers design this way, but the eye is very sensitive to minor visual imperfections. Below is another try at estimating this curve using just two tangent arcs (highlighted in red). This is the simplest and I think the best.
In the picture below the included angle of the gear tower is about 6 degrees. Also the two arcs represent the side curve pretty well.
Dated 1935 – Source Gordon Eliot White’s “Offenhauser the Legendary Racing Engine and the Men who Built it” (as all following diagrams)
In the real Midget Offy, the shape of the gear tower was dictated by the size of the gears it was housing and these were as varied as were customers for the Offy.
Drawing from 1953
1965 Drake Engineering Drawing
So what do you think? I am leaning toward going with 6 degrees. OK, that is one dimension, now on to the next. I hope this provides a little insight into my process for developing a CAD model without having the factory drawing set (wouldn’t that be great).
I smell turkey cooking in the kitchen and my wife is called for me to get the extra leaves for the dining room table as company is due soon. So this is going to be it for today, Merry Christmas/Happy Holidays everyone.