23 Nov 2008

This explains the thinking behind how the tubing drawings are evolving.

Since the basic chassis is complete it’s time to start a redraw to: eliminate inaccurate nodes, remove overlapping tubes, double-check that nodes are symmetrical, create individual tube assemblies, and start dimensioning everything. The overall drawing of the bottom tubes is complete though not the individual tube drawings, which brings up an issue that’s been nagging me for a while – how to draw the tube ends.

It would be great to have an exact drawing of each tube showing the precise shape of each end; cut them per the drawing and it all “just fits.” However, cut tolerances and welding distortion change the shape of the chassis, unless of course the chassis is assembled on a 10,000 lb welding jig. Since we don’t have that we tack-weld the chassis, clamp it together the best we can, and weld it up. This often results in hearing a loud “bong!” as the tube that’s being welded causes a tack-welded tube elsewhere in the chassis to let go due to the tremendous heat distortion. Once that happens it means the chassis has changed shape and exact-sized tubes aren’t going to fit. Kimini was welded on a fairly heavy jig which included 4″ I-beams (H-beams to you Brits!) Even then the chassis accumulated about 1/4″ of heat distortion, bent up at the ends like a banana. Part of the trouble is that after the lower chassis tubes are welded on the top sides, the chassis has to be unclamped from the table and turned over to weld the bottom. Having precise drawings does little good if a tube that’s¬† exactly 10.000″ long in reality having to fit into a space that might be 9.912″ long. This is one problem.

Then there’s how to convey to builders the shape of each tube ends; square tubes are easy to show on drawings but round tubes are much harder. Some chassis plans deal with this by showing tubes with the ends “unwrapped”. The idea is that a full-size paper template is cut out, wrapped around the tube, marked, then the tube cut along the line, and presto, perfection. However, I’m skeptical; there’s always some cutting error, which if the chassis is started at one end and built toward the other, can accumulate into a surprising amount, and then there’s material thickness. Wrapping a paper guide around a tube gives the correct line to cut, if the wall thickness is infinitely thin, which it isn’t, or if the tube junction will be 90 degrees, which it seldom is. What happens is that facing inside surfaces of tube junctions don’t fit up because the ID of the tube prevents the edges from properly mating up, and the problem becomes worse the more acute the angle.

Because of the above, tubes will be dimensioned but not unwrapped. Square tubes will have basic dimensions while round tubes, thought they’re very hard to show numerically, are trivial to fabricate if a tubing notcher is used. Since inexpensive¬†tubing notchers are now on the market it’s reasonable to require one. With a notcher, only the length of the tube and angle of the cuts are needed; the exact path of the cut on the ends of the tube becomes moot <em>and</em> the notcher deals with the wall thickness issue.

Somewhat related is the issue of customized tubes; no two Midlana cars are going to be the same due to different drivetrains, seat choice, pedal placement, engine cover design, etc. This will be dealt with on the drawings by colored tubes where placement depends on the parts or style the builder uses. For example, on the drawing of the bottom tubes, there are two diagonals stiffening the bottom of the engine bay. While they will be dimensioned they will only correctly fit a Honda K24 drivetrain. For other drivetrains they’ll have to be different in order to miss the oil pan and transmission bell-housing.

Brackets are missing from the drawings for now since it’s too early to know where exactly they go. They’ll be added in due time though for the most part, it’s pretty obvious when a bracket is needed when the time comes.