Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
Our snowthrower rests the entire weight of the front end on a pair of skid shoes, which erode against the asphalt driveway. Replacements cost nigh onto eleven bucks each, which activates my cheapskate gene.
Worn OEM skid shoes
You can see from the markings that the slots are about twice as long as they need to be, so I figured I could replace them with some random angle iron. Might not last as long, but far less expensive.
Bedframe skid shoe
Having a nearly infinite supply of bedframe steel in the heap, I chopped off two suitable lengths, poked 3/8″ holes into the appropriate spots, then milled short slots to get some adjustability.
Bedframe steel is about the nastiest stuff you (well, I) can still machine: high carbon, fine blue-hot chips, and hard edges. It might actually be better-suited for skid shoes than the soft steel OEM parts.
They’re not pretty, but the driveway hasn’t complained yet.
The only real problem is that those sharp corners snag on the edges of what we loosely term “the lawn”. I should apply the smoke wrench, miter the corners, and bend the edges upward. If I’m going to all that trouble, I should also hitch up the buzz box and wave some hardfacing ‘trodes over the bottom.
But that’s in the nature of fine tuning and sounds a lot like work.
This is truly embarassing: I managed to leave a steel rule (not a ruler in the shop) atop a sploosh of ferric chloride for far too long. I eventually noticed the corrosion creeping around the edges.
Top corrosion
The bottom was hideous.
Bottom corrosion
So I sprayed it down with TopSaver, applied fine sandpaper, applied a Scotchbrite pad, and it came out surprisingly well.
After treatment
The ferric chloride, of course, came from a circuit board etching project. How you’re supposed to prevent that is to cover everything for about six feet around the spot marked X, but I don’t do that nearly as often as I should.
Mostly I lay a sheet of packing paper atop the workbench and whisk it into the trash when I’m done, but this time I’d left it in place because my resistance soldering gizmo wound up anchoring the far end. Soooo, a drop or two soaked into the paper and of course the ruler wound up exactly atop that spot.
The stuff is murder on stainless steel sinks, too…
MTD used the same design for the gasoline tank caps on our leaf shredder and snow thrower: an aluminum cone (which evidently serves to keep splashes away from the tank vent) mounted on a heat-staked plastic rod molded into the cap. It’s straightforward, but a bit suboptimal for high-vibration yard gadgets.
The aluminum cone eventually worries its way through the plastic post and falls into the tank, taking the heat-formed button from the post along with it. Trust me on this, fishing those things out of the tank is an exquisite little inconvenience.
4-40 screw post – inside4-40 screw post – exterior
The fix is straightforward.
Chop off the remains of the post, drill a snug 4-40 tapping hole straight through the cap, and tap it accordingly. Secure the cone to the screw with a nut tightened against the head, run the screw through the cap, run a pair of nuts onto it, trim to length, then jam the nuts together so the cone is about where it started out. Loctite on the nuts is a Good Thing, but I don’t know how it feels about gasoline immersion.
The snowblower cone is getting wobbly; I must make a preemptive strike on it to avoid fishing the debris out of the tank.
I dropped in to mooch some female header strips from my buddy Eks (which is not nearly as obscene as it sounds) and got the story behind this innocent-seeming 2.2 megohm carbon-composition resistor.
It seems he was debugging a defunct tube-based audio amplifier. He’d probed everything and discovered that the grid bias on one of the tubes was totally wrong, which caused protracted headscratching over the associated circuitry.
Now, in semiconductor work, a 2.2 meg resistor is an open circuit compared to the other circuit impedances. In fact, you can use pretty nearly any resistor with green or blue in the third band as a standoff in Manhattan-style construction in place of those small insulated pads.
Megohm-value resistors are actually useful in tube circuitry; you’ll see plenty of green and blue bands sprinkled around those sockets. Although we didn’t get into details, I suspect this one was part of a grid-leak bias circuit that holds the grid voltage just a bit below the cathode; the bias comes from the few electrons that whack into the grid wires rather than passing through, so the total DC current is in the microamp range.
After more headscratching, Eks yanked this resistor, measured it, and found it was a completely open circuit. A 2.2 meg resistor isn’t all that much different from an open circuit (it’s hard to tell the difference with an in-circuit measurement) when used in a transistor circuit, but the difference separates correct function from failure for a tube amp.
Eks swapped in a new resisistor and the amp worked fine. Case closed!
The digital multimeter in my desk drawer tops out at 2000 kΩ, which shows you just how much demand there is for high-value resistors these days…
I needed a brass tube with a lengthwise slit to serve as an electrostatic shield around a ferrite bar antenna. There are many wrong ways to do this, all of which produce terrible results, pose a serious risk of personal injury, or both. I say that with some confidence, having tried some of them over the years.
Here’s one right way: fill the tube with Wood’s Metal, thus turning it into a solid rod, then cut the slit with a slitting saw.
Wood’s Metal is a moderately toxic alloy that melts in hot water, which turns casting into a simple workbench operation. You might not want to cast it in the kitchen, but that’s your call. Clean up the scraps, wash the counter even though you used newspaper, wash your hands, and don’t suck your thumb.
As shown, I just poured the molten metal into the brass tube atop a steel block, broke off whatever seeped out, and remelted the scraps. Turns out I had just barely enough for the job.
Slitting brass tubing – overview
My buddy Eks gave me a stack of slitting saws a while ago and I modified a standard Sherline holder to fit them. Turns out there’s just barely enough room for everything within the mill’s working envelope; the saws are a bit over 3 inches in diameter.
So I cut the back of the tubing, making the pictures somewhat disorienting.
The tubing fit neatly into an old V-block (evidently homebrewed by a better machinist than I), held down by ordinary Sherline clamps on perilously long studs screwed into the tooling plate. The saw had just enough reach to clear the rather broad V-block’s shoulder.
The tubing is 0.630 OD with a 15-mil wall and the saw blade is pretty nearly 32 mils thick. I touched off Z=0.331 (630/2 + 32/2) with the blade atop the tubing, then jogged away to Y=+1 and drove down to Z=0 to cut exactly through the middle of the tube.
Slit 0.015 inch deep
The V-block is aligned with the front of the table, but I did a bit of nudging to persuade it into final alignment. Of course, the saw wasn’t quite centered on the holder, so a blade or three tinged on the tubing when I did a Y=0 trial pass at low RPM.
For lack of anything smarter, I cut at 500 RPM and fed at 5 inch/min. That’s painfully slow, but correspondingly boring… remember, in machine shop work, boring is good.
I did five passes: one trial at Y=0, three cuts at 5-mil steps, and a cleanup cut. The picture shows the 15-mil pass left a very thin web at the far end. A final 2-mil cut removed that web, leaving only a few burrs. You could do it in one pass, but I wanted to minimize the depth-of-cut into the Wood’s Metal.
Unclamp, discover that the cast metal rod slides right out, touch up the edges with a file, and it’s all good. A lovely slit, perfectly aligned, without bent metal or bloodshed.
As a bonus, I get a nice Wood’s Metal ingot out of the operation. The line along the rod is just barely perceptible with a fingernail; it’s more of a polished line than an actual cut.
Slit tube with Wood's Metal ingot
Turns out the shield works a bit too well: it cuts out the WWVB signal, too. I think the tubing is too close a fit to the ferrite rod and detunes the winding. More experimentation is in order…
We’re scanning a bunch of really old photographs to assemble a book of memories for Mary’s father. Most of the images are about what you’d expect for old photos: bad exposures, poor focus, and scratched emulsion. There’s not much you can do to save ’em, but one image really surprised me.
Scanned at 600 dpi with the black and white points set to maximize the dynamic range, we got this image (reduced resolution for display here).
Original
The original image is somewhat brighter than that: there’s a figure visible in the upper-left, but you can’t see much more.
After dramatically adjusting the gamma and switching to grayscale mode, her father popped out of the shadows.
Gamma = 3
Now, it’s not a great picture, but it’s one of the few we have from that era… and it’s a much better picture than no picture at all!
A similar trick can recover dull gray snow pictures, as mentioned there.
The Smell of Molten Projects in the Morning 