Parts of Parts of Parts of Parts

Fuel Tank Shennanigans

So I am deep in to the fuel tank project, having in the last episode placed the bodywork and determined that my tank design will fit within the boundaries of the car  

The fuel tank design is basically complete. 

But there are a lot of qualifiers and caveats buried in that 'basically.'

So these are the parts and their fitment. This .jpg was derived from Solidworks files so I do have the CAD work basically done.

One thing that turned out to be less done than I thought: I designed the oval fuel tank flange by making a few measurements of the screw hole pattern on the Miata fuel pump/filter/sender assembly and made my model assuming that the screw hole pattern would be symmetrical and sensible. As a last minute precaution before uploading the CAD file to send-cut-send for laser cutting, I printed out the screw pattern and compared it to the Miata assembly and whoah, I'm glad I did because this screw pattern makes no sense at all and my symmetric drawing would not have fit. Not have come close to fitting.

So I began an iterative empirical engineering process, which is an advanced technique that the uninitiated might mistake for good old trial and error.

Line up the paper print out (stiffned with card stock) with the OEM Mazda assembly. Trace the holes.

Take the traced hole pattern back to the CAD model and make corrections accordingly. Repeat until all hair on the entire body is torn out. The picture above will be (I hope) the last set of corrections. They're very close at this point. In the earlier iterations the corrections were dramatically larger.

I am currently taking a break from this repetition to update my blog. I have the day off today working to get the CAD files corrected and uploaded to Send-Cut-Send.

There were some other annoying corrections resulting from an annoying update to Solidworks. In previous versions, when you exported your model to the .dxf file used by the machines, and you wanted bend lines in the .dxf file, you simply clicked "include bend lines" and the .dxf file would then be bestowed with bend lines. Now the Solidworks people have built in the ability to use different bend line maps. So if you haven't uploaded a bend line map and linked it to your CAD model, there won't be any bend lines despite clicking the "include bend lines" box. 

Solidworks has so many irritating "you're about to make a mistake!" pop-ups, one would think that "you're asking for bend lines but you have no bend line map!" might be one of them. But, no: this is one mistake that Solidworks is happy to just sit back and watch the user make.

Solidworks is an endlessly powerful and endlessly annoying bit of software. It can do amazing things, but then makes some simple things vastly more complicated than they need to be. Solidworks can be had in two packages: 1) insanely expensive, and 2) not very expensive, but comes with the notorious "How dare you try to use our precious software for your pointless futile project! You deserve to fail and be subjected to our mocking laughter!" customer service package. 

Solidworks is made by D'Assault (the French fighter jet people) and there is probably some stereotype about French culture that could be applied here.

D'Assault makes very attractive-looking fighter aircraft (above, right). They also make my head explode. Once I got so frustrated at an unnecessarily complex and annoying Solidworks "update" that among the shrapnel of my exploding head there appeared a graphic representation of my frustration (above left). I also created a Facebook support group for people who are suffering from Acute Solidworks-Induced Cranial Explosion Syndrome.

Update: Took a break from blogging to proceed with the next iteration of the hole template process. Yeah, that wasn't the final go-round. Needs another one.

Leveling Up

Another slow-moving and intermittently frustrating process under the heading of "make a fuel tank" has been trying to level up my aluminum welding skills. Fortunately I have on-call assistance from Pops Racer who is a very good welder and metalsmith, and also had some help from the world renowned guru Pops goes to when he needs help or advice. My guru's guru is named Jerry and he is rightly famous in Indianapolis automotive circles. Probably his best known project was this:

This one-off Bugatti was originally done with magnesium bodywork which had basically disintegrated with age - so the owner had a literally priceless one of a kind chassis with no bodywork. Jerry recreated all the bodywork seen above. That's all hand shaped metal and there are welds in it that are literally invisible, even up close. Welding magnesium at all is the feat of a master. Leaving no visible evidence of the weld is magic. Jerry is a wizard.

Jerry welds aluminum fuel tanks with oxy-fuel which has become somewhat of a lost art. The welding torch Pops gave me was made for welding aluminum on aircraft. When I took it to the local welding store to find new hose for it, Welding Guy marveled at how tiny the torch was, and when I told him it was for gas welding aluminum, he said with great confidence that aluminum can't be welded with gas. When Jerry says gas welding aluminum is rapidly becoming a lost art, he isn't exaggerating.

Oxy-fuel (gas) welding aluminum is really not a job for the casual hobbyist. Welds like this picture are what convinced me to use TIG for making the fuel tank.

Gas welding aluminum is very tricky. The metal has to be immaculately clean. You have to use flux on the metal and on the filler rod. You have to use special goggles that are no longer made. You have to use exactly the correct alloy of the material and the filler rod. With Jerry's encouragement and advice I spent about two weeks practicing gas welding aluminum and achieved the above. Which admittedly is not pretty, but you should see what I was doing two weeks ago. At this point I can make short stretches of effective weld but I still get burn-through every once in a while. It's just so touchy to control the heat. Gas welding aluminum doesn't produce the pretty fillet like TIG welding but that's true even of people who are good at this. Gas welded aluminum joints can be gone over with a planishing hammer and made invisible or nearly so.

After a hefty amount of practice I went back to Pops Racer's shop and tried TIG welding a few scraps together and produced the best TIG welds I have ever done. Pops said trying to learn aluminum gas welding would improve all my welding skills and he was right. Besides being much easier for someone of my modest skills, TIG has additional advantages of being able to use different alloys and thicknesses. I think with another four to six weeks of practice I could get my gas aluminum welding up to an acceptable level but my aluminum TIG welding is serviceable now. 

TIG it is.

It's now the Friday of the week in which Tuesday was my day off for finishing and uploading my CAD files. I uploaded them the first time on Wednesday, but the friendly people at send-cut-send had a fairly large number of suggested corrections to make the cuts and bends more accurate. Real engineers know the best practices for CAD already. I'm self-taught so sometimes I do things that work on my computer screen but might not work in the laser cutter or the bending brake. So now after two days of transforming my Strange CAD Practices into Best CAD Practices, all the parts are uploaded, approved, and in process. Expected ship date Feb 21 (although I don't know if that still holds after two days of format corrections).

Tangentially Related to Fuel Tank

There have also been a couple of odd jobs clarified by mounting the bodywork and the cardboard fuel tank model, so I've been chasing some of those down.

I'm stubbornly determined to put the battery in the trunk (boot, in the UK) for various ill-defined and possibly delusional reasons. Batteries probably last longer if they're kept out of the engine bay heat. Miata people profess weight distribution advantages, but I am not particularly concerned with whatever nano-difference that might make to the handling. I certainly do like a tidy and not-crowded engine bay. And of course I'm stubborn. 

So, in the trunk it goes.

The full size glass pack Miata battery doesn't fit in the gap to the right or left of the fuel tank (outlined by the hole in the wooden trunk floor above left). I was reluctant to move the battery rearward as it would compromise the already limited trunk space. Since the whole custom fuel tank rigamarole above was driven largely by wanting more useable trunk space than the vastly easier and cheaper Miata tank would allow, it made no sense to throw a big battery down right in the middle of the prime real estate I am taking such great pains to preserve. Especially since I was also determined that any lead-acid battery in the car should be installed inside an even larger ABS container to prevent caustic slime leaking out and dissolving all my hard work.

Enter the Lithium-ion battery: Smaller! Lighter! Less likely to leak toxic goo! Some automotive lithium batteries are insanely expensive and some are prone to spontaneous pyrotechnics; some folks use batteries made for RC vehicles... which they seem to get away with... mostly... although that could be survivor bias. Antigravity Batteries in California appears to be the best choice for reasonably affordable lithium batteries that are actually designed for vehicular use and thus much less likely to provide an unplanned fireworks show.

'Battery fire:' an exciting new term in the automotive lexicon

I also learned more about fuel tank venting. The OEM Miata setup vents the fuel tank into a charcoal cannister that then vents into the intake manifold. This isn't really a problem, but it does clutter up the engine bay and could possibly cause confusion for my aftermarket ECU. Also it can make the engine bay reek of fuel vapors and thus diminish the driver's and passenger's motoring experience. 'Cannister delete' is one of the most popular Miata mods. But then where do you vent the fuel tank? 

Well, obviously you drill and tap the lid of an aluminum catch tank. Then fill the tank with activated charcoal. Then add a PCV vent filter and a fire extinguisher bracket, and mount the whole thing at the very back end of the car. It's no so much "Cannister Delete" as "Replace Cannister with a Much More Elaborate Cannister."

That's all for this installment. I will be doing odd jobs for a few days until my fuel tank parts arrive, after which I will be welding. 

For quite some time, probably.

Postscript

The Replicar project has been bought out by a different kit car company in the UK: Quantum Sports Cars. They make a traditional-looking Super 7 type, but it's a stainless steel monocoque rather than a space frame. They also have a more modern-era take on the Super 7, a dune buggy, and a project underway that I will be interested to see.

Nothing against Andy at ANC (the previous Replicar supplier), but Quantum is a bigger operation with more resources. They are adding more parts and services including a bespoke Replicar fuel tank (argh, too late), wheel well liners (not too late for those), and some refinements to the dash and bodywork. I expect this to be a net plus.