Quote:
Originally Posted by Varn
I tried making a 31' arc for the top of my minivan (as described in a previous thread as a 5.64x ht). Found that it was very hard to define the correct shape. First tried string. I was fighting string stretch and gravity. My second attempt was to use a steel tape measure. Not much better. My attempts have been thwarted to get any kind of accurate shape.
A different approach is required. I could calculate the saggita of the chord that I wanted and then flex a 1/4" x 6' rod to give me that much sag.It would give me an approximation of the curve at least of the central regions of the flexed rod. The sag formula is s=r-sqrt(r^2 - l^2).
One method of construction would involve flexing 1/4" steel rods into the shape and then welding them together to form a removable frame work that could be covered with dacron fabric like a light airplane fuselage
The curve of a 2 foot long piece only has about 3/4" of saggita.
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For decades now,I've been using 1" Schedule-40 PVC pipe for a french curve with which to project a curve during fabrication.
After establishing a few bulkhead elevation references from the 'Template' I lay out the pipe,ballasting the front end down to the vehicle and the pipe creates a very smooth curvelinear edge from which infinite location vectors can be directly measured.
I use it for top and sides.
Stainless steel is superior,as it has no internal structural memory,but kinda price prohibitive.
Also,here at the copy center,I'll use the copy machines to do scale-up photo-enlargements with registration marks which can be taped together to create large scale higher resolution drawings from which to capture dimensions.It's dirt simple!And cheap!
Long ago,automakers would have full-scale blueprints from which to work.If you've seen the movie 'Tucker, The Man and His Dream,' you've seen this.