Etched trucks – the design process

A reader just asked with regards to my trucks,

“I’m most interested in your design process, how you first determined you could etch the truck sideframes in such a way several folds would complete the process, and then how you engineered the design. I’ll check out your blog and if I’m still confused I’ll email you again.”

To tell the truth, I thought about these trucks for years before coming up with this approach. I tried casting them in resin, but the sections are too small to cast reliably, and who knows what the performance would be like if you could get them to come out of the mold? I really don’t recall how I came up with the idea of etching them, but it was probably from reading too many British magazines.

I think the design probably started with thinking about how to keep the two axle holes the right distance apart. So, I might have started with a plate with the two axle holes etched in, planning to solder the arch bars to this plate, and then file away the excess once everything was solid. From there, it’s not a long way before you realize that the arch bars can also be etched and folded onto the plate accordian style.

The tricky bit, which stumped me for ages, was how to attach them to the bolster. In Proto:87, it is especially useful to have this all square. I toyed around with various ideas involving posts and holes in the bolster, much like some of the equalizing delrin trucks on the market. But, I could never come up with a satisfactory way of isolating the two sides. Finally, I realized that I could use the bolster much as the prototype did.

Once I’d figured it all out in my mind (I inherited very good spatial visualization from my grandfather), I drew it up in QuickCad. I didn’t know exactly how big to make the half-etched gaps for right angle and 180 degree bends, but I surmised that it must be something like the distance around an appropriate arc of radius equal to the 3/4 the thickness of the material. My laser printer only does 600 dpi, and so, that calculation is probably way more accurate than my laser printer is able to resolve anyway. It happens that the folds do come out crisply, so I guess I wasn’t too far wrong.

I then scaled up the design I forget how many times, and transfered the design to foam core for testing. I scaled it up sufficiently so the foam core was the right thickness to represent the final metal. This foam core mockup rattled around my basement for ages, but it seems to be gone now, otherwise I would show it to you. The mockup worked, and so, I started working in actual metal.

Now that I’ve assembled two pairs of trucks, I plan to completely revise the design before I make any more. The original design called for a half-etched bottom chord, which turned out to be too flimsy due to my unpredictable etching process. On the second iteration, I therefore changed the design to make the bottom chord full thickness; this was not a complete revision, and it turned out less satisfactory. The other failing is that the top two arch bars are only connected to one another at one end. This is a result of them being slightly different shapes. However, the difference is not noticeable at all, and so, I think I will connect them at both ends, and dramatically simplify the soldering process.

The rest, as they say, is history. Except in this case, it really is history because I’ve written much of it down somewhere.

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