Worked on CAD most of the day cleaning up the chassis, still not done. After getting tired of that went out and layed out the bottom chassis tubes on the table; if you look closely you can just see the lines. With it actually taking shape it’s another vote for a large table. The difuser behind the engine and the radiator support at the front will both make the frame about two feet longer. Looks like cutting steel will start tomorrow…
Earlier Cooper came in to check why I spend so much time staring at the dumb computer. He’s perfectly happy walking around the house with a towel like this, looking like the Emperor in Star Wars. With his short hair he likes the warmth on cold days.
Phil pointed out a couple things about tube chassis construction that I was aware of but hadn’t really thought through. He stressed trying not to have a bunch of tubes coming together into one node. Technically it’s the right way to do it but causes a number of fabrication problems. The cuts on each tube can end up being very hard to fabricate, including being impossible to install without cutting off one fishmouthed end. Many tubes in one spot results in such a large mass of metal that a lot of heat must be pumped in to weld all the tubes which leads to heat warpage. Also, it can be near impossible to reach in between all the tubes and weld every inch of every joint. It’s easy to skip the hard-to-reach ones, which start corroding from the inside out since it’s open to oxygen and moisture (never mind rain water wicking inside.) Seems like good advice so I’ll see what I can do, space the tubes slightly to allow getting the torch in between and to simplify the tubing cuts. In many cases it doesn’t take much to greatly simplify the end-cut, sometimes just moving the end of the tube 1/4″ is enough to change a 3D cut to a single-plane cut.
Ordered some Christmas presents for myself. Google Sketchup works pretty well for creating the chassis but Google knows what they’re doing. Their free version only exports JPEG drawings which is very limiting. No .EPS format for the book, no .DXF for sending to metal fabricators, or .PDFs, or .TIFFs. Fine, they win; Sketchup Pro was purchased which has all the export formats.
Back when building Kimini, there were times it was nearly impossible to use the TIG welder foot pedal when welding in awkward positions. A finger-operated current controller fixes that. Since moving a finger to change current will likely move the torch head slightly it’s not a perfect solution, but better than the antics of struggling with the foot pedal.
Replaced the leather cover for the TIG torch hoses; it was worn through and contrary to my cheapskate nature it was replaced rather than waiting for a hose to spring a leak. Also ordered bandsaw blades for the weenie-but-faithful Harbor Freight bandsaw; we’ll see if it lasts another build. They have a larger $600 saw but as long as this one keeps running there’s no reason to consider it, plus it would consume sacred floor space.
Added a tubing rack under the build table. Keeps everything clean and visible, off the floor, easy to check stock, and saves floor space. After that, dismantled the wood mockup(!) and cleaned off the table in anticipation of starting for real. To be honest, things are moving a lot faster than expected. With the Mini it was nearly five years before the steel chassis was started.
Coming up in three weeks is the appointment with the DMV, to see if a coveted SB100 exemption number is in my future. It’s kind of a big deal; if I get one it’s a huge relief as it means there’s no concern about getting the car registered, smogged, and provides incentive to have it rolling in 2009. If it doesn’t happen it’s not terrible, but it does drag out the fear of the unknown another year and there would be no chance of driving it in 2009. It would give time to concentrate on the construction chapters of the book though it would be nice to know how it handles before that. If there’s anything dire that needs changing it would be nice to have the right stuff in the book the first time rather than reworking chapters.
I’m just holding my breath until then. The real fear is that this law is too good to last; at any time they might pull the plug which would make getting the car on the road much more difficult. Of course, with California’s $15 billion deficit, maybe the DMV will continue taking our money for this service. Have to keep a positive mental attitude.
The tubing is here, bent tubes, too! A big thanks goes out to Phil Burke and everyone at BurkeBuilt Fabricators for helping me get the tubes bent. Lots of places can bend 0.125″ wall tubing but not many can do 1.5″ x 0.095″ wall. Unfortunately I had the wrong bend radius on my drawing but they fixed it, which means some tube drawings have to change… which is another reason the tubing drawing issue bugs me. Anyhow, different businesses use different bend radii in their benders, making the main hoop and windscreen slightly different shapes and affecting tubes mating to them.
Received some good comments on what information to provide for tube fabrication. Of course the suggestions range from, “they don’t need anything”, to, “give them everything and let them pick what they need.” The answer’s somewhere in the middle; the trick being to not spend a lot of time providing info builders don’t need or can’t use.
Since Burke deals with this stuff all the time I asked his opinion on how much information to include, noting that a builder is typically one guy in a garage using a grinder or tubing notcher to do the ends. He had a good example: a simple angle cut on the end of a round tube. At shallow angles, the tip of the tube on the drawing will be quite long and sharp. However, in the real world it won’t be nearly that long because it’s not possible to weld such thin metal; it’ll either melt back or get ground off during deburring. His point is that measuring to the tip is only useful as a guide of how long to cut the tube before notching it. Another example for this same tube: the open end of the tube is an ellipse, so the centerline length of the tube is measured to the center of the ellipse. But how is a builder going to do that? It’s very difficult if not impossible before it’s cut and dang hard to do even after.
I know I keep beating this topic to death but when people who know this stuff say don’t bother with detailed drawings it makes an impression. Maybe it’s my excuse why tubing detail will be minimal on unimportant diagonal tubes but more detailed on important corner tubes. As was pointed out, probably the most important drawing is the one showing where the suspension pickup points go; everything else is just a big bracket to keep them in place.
A shelf will be added under the build table to store the tubing up off the floor. Learned that lesson the hard way last time; the tubing rusted quickly from the moisture coming up through the concrete.
The final chassis is progressing though the round tube drawings bother me. As mentioned before, it’s not clear the best way to convey how to cut the ends – or even how much to try and make the drawings precise. The overall length is good, and the angles of the end cuts, but I’m not so sure about index angles, which I think will be rather useless to a typical garage builder. If a drawing specifies that the miters are offset at each end by, say, 27.3 degrees, how is a typical builder going to make use of that information, really?
Curious about build accuracy, I asked FSAE teams how they make use of laser-cut tubes – just what does someone do once they have perfectly cut tubes? The theory is that with them accurately cut, the chassis becomes “self-jigging.” Teams confirmed that while having the tubes pre-cut is a tremendous time-saver, that’s all it saves. They said a builder cannot just weld laser-cut tubes together and expect the chassis to be square; it’s just not going to happen without a big heavy fixture to combat weld distortion. The reason I mention this is that just because tubes are accurately cut has little to do with them fitting due to tolerances. What good are tube drawings with 0.01″ resolution when the tubes are cut and installed into a chassis with a realistic accuracy of >0.125″?
If someone hands me accurate chassis drawings, what would I do with them? I’d make sure a few major tubes are accurately placed and not sweat the rest. About 90% of chassis tube placement just don’t matter much; the various minor tubes just keep the big ones in place and the inboard suspension points where they’re supposed to be. All the tubes in between, eh. That’s how I intend on building the chassis myself, using the drawings to double-check overall dimensions, but other than gross lengths and angles, they’re a guide only. That’s all they can ever be due to weld distortion making accurate drawings fairly pointless.
This isn’t anything new. The Gibb’s book (“Build your own sports car”) supplies complete drawings of the chassis accurate to 1mm (0.040″). Yet right up front he warns: ” Precise dimensions are provided for all components, but in practice allowance must be made for welded joints and distortion. It is advisable to cut tubes – and check and trim as necessary – as the chassis is built up, rather than cutting all tubes before work starts.” So there it is from another source, having drawings with high resolution aren’t of much use other than a reference.
The final chassis is taking shape, right side. It’s accurate as opposed to the “pretty good” original, at left, ensuring the final drawings are right – and the chassis symmetrical. The bottom tubes and bent tubes each have a dimensioned drawing which is why tubing is being purchased. The drawings will be kept ahead of fabrication to make sure each tube has a drawing to keep the chassis honest. This way when the chassis is done there is no doubt that it’s a faithful copy and nothing will have to be remeasured, edited, or rebuilt. Of course, only after the first trackday event will it be known whether the chassis needs any final tweaks. I’m trying hard to do all the work up front so that it “just works.” Kimini was pretty good right out of the box so we’ll see if it can be done again.
A lot of progress is expected over the next four weeks 🙂
Speaking of Kimini, at the last second the buyer asked that a color copy of the Kimini book and spare windshields be included. Since he’d already paid for the car I wasn’t concerned. However, after a couple months and a few polite reminders, well, so be it. Someday he’s going to need to know something about the car and I’ll be happy to explain, right after he pays the bill.
With tube drawings starting an incentive seems in order. All the tube lengths are known – though not yet formally drawn up – so the numbers were totaled and tubing has been ordered! Since the main hoop and windscreen tube drawings are done they’ll get bent at the same time the tubing is picked up. All this happens next week, so between continuing with tube drawings and having the tubing on-hand, there’ll be plenty to do over the holiday break. First thing will be cleaning off the build table! Happy now?