Swapfest Revisited

Tuesday, October 20th, 2009 @ 10:39 | MIT & Boston, Project Build Reports, Stuff

SWAPFEST!! (capitalization and emphasis added) is the monthly flea market that MIT hosts for area radio enthusiasts, electronics tinkerers, cruftseekers, and frightened but daring onlookers. It’s more or less a hybridized radio hamfest. Because it occurs in the equivalent of my back yard, I try to attend every month.

Occasionally I fail because it involves waking up during the daytime, but historically speaking, Swapfest has been a boon for my cruft-hoarding tendencies. It has turned up stuff as weird as the alleged gyroscope gymbal out of a B-52, to mutant semiconductors to literal dozens of old 5.25″ full-height hard drives.

The season runs from April to October, because nobody wants to push their cruft while it’s snowing outside. The first and last Swapfests always seem to produce a more eclectic selection. I figure this is because nobody, in addition, wants to drag all their crap back home and store it all winter, so they dispose of it in October. Likewise, April is the time when everybody brings all the crap they collected all winter.

Following this pattern, I abandoned all inhibitions for this past Swapfest and actually… you know, brought some money. Apparently, the incremental gain in awesome parts when you go from “free” to “cheap” is nontrivial. This resulted in the best Swapfest hual in recent memory. And it even includes stuff I can actually use. Imagine that.

Naturally, I sprung for more mutant semiconductors first.

I buy the guy out of these every time I see him. They are the same ginormoFET modules I use on LOLrioKart. I wish they were higher voltage, but regardless, you just can’t beat 200-300 solid amps, 2 milliohms, and 100 volts for $7.

The downside is the incredible 0.1uF gate capacitance, and the lack of a real datasheet. The part number apparently only exists in the archives of shady Far Eastern electronic component dealers.

I decided to invest in some dial meters for Segfault. The variety of things measured ranged from the relatively tame “PERCENT OF RATED CURRENT” to … leak rate. How do you measure leak rate? Dribbles per minute?

Ultimately, no matter how weird the scale, these things tend to be voltmeters in disguise. Finding a straight up ammeter (voltmeter with a scaled reading) was actually pretty difficult considering most of the meters read kilovolts or milliamps (or milliRems per hour). I finally wound up with a 0-50 amp scale meter. There was also an oddball that showed both positive and negative amps. The weird thing, however, was that it required a voltage just to read zero. I thought it was a dial zeroing issue, but nope… the little adjustment screw doesn’t crank that far. Maybe it will be handy some day to read regenerative braking current.

You’d figure an easier way to do this is to center the scale at zero, like the +/- degrees meter. I liked the degree-o-meter  because the scale is very similar to the numbers I want to pursue for Segfault. 15 degrees forward tilt gives a comfortable torque margin for the motors to recover if it becomes necessary (The calculated maximum steady state tilt angle is about 30 degrees, which is substantial). If I ever get to 45 degrees, I’m probably on the way to a face/ass/extremity-plant already.

Speaking of meters, here’s a 750 amps DC meter. With the associated shunt, a very precise 3 and a half pound block of brass. Ammeter for LOLrioKart to go along the amp-volt-microns meter?

While on the topic of mutant components, here’s the functional component inside a variac. To me, this looks like a meaty inductor to use for a high-power boost converter. It weighs about 10 pounds and is 8 inches across.

Using an LCR meter, I found that the inductance of this toroid changes significantly with frequency. At 1kHz, the inductance was around 170 milliHenries. This fell to 60 at 10kHz, but was pretty damn close to 1 full H at 100Hz.

Considering the thing was designed to operate at 60hz, I’m not too surprised. If nothing else, I now have yet another door decoration.

Enough EE shenanigans. This is what happens when you let Charles near a bin of unsorted endmills.

He takes 20 minutes and multiple rusty-tool lacerations to the hand to mine out all the carbide cutters.

You can easily distinguish a carbide endmill from a steel one just by weight. The final tally is one 0.5″, one 5/8″, one 3/4″, and a huge 7/8″… carbide tipped cutter? I didn’t know carbide tipped endmills existed,  but… well, now I know. This looks like it dates from before people figured out how to make whole cutters from carbide.

A carbide spotting drill also falls into the mix. The whole load cost me a few Mountain Dews Equivalent (MDe, a new unit of currency used only in my universe).

The most exciting and highly valued find of the day was this Tapmatic self-reversing tapping attachment for your choice of spindle machinery. They drive a tap through a clutch and automatically pop into reverse, backing the tap out, if you start lifting the spindle up. The reaction arm stays stationary and hopefully braced against something to allow for the reversal.

I investigated these things a long time ago to use the underlying mechanism in a stored-energy flipper ‘bot design, but could not understand what went on in them at all, because smart people designed it.

I still don’t know, but now I have an excuse to powertap things smaller than 1/4″-20.

Now my goal is to find a use for everything before April!



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  • Überclocker 5: Finishing Up The Everything Else
  • Überclocker 5.0: In Which I Actually Have to Build the Bot, Not Just Talk About It
  • Überclocker 5.0: The Big Post of Designy-Stuff
  • The Overhaul of the Future Begins Now: Überclocker 5.0 (Also, Welcome Back to Robots)
  • Operation RESTORING BROWN Part 7: The Epilogue; or, Dragon Con 2019
  • Operation RESTORING BROWN Episode VI: Return of the Van Lights; the Conclusion
  • Late Stage #PostmodernRobotics: Welcome to Your Waifu is Trash, the Robot Dumpster Fire

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