Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
Tag: Improvements
Making the world a better place, one piece at a time
After drilling that PCB, I noticed that the Z axis saddle locking lever (which also functions as the backlash adjustment) had come loose. It turns out that if you don’t tighten the thumbscrew or it works loose, then the locking lever can turn with the leadscrew and, at the very top of the Z axis travel, can walk off the leadscrew thread.
A snippet of rectangular brass tubing epoxied to the top of the Z axis saddle solves that problem by removing 3/32 inch of precious travel. A slip of brown waxed paper (yes, harvested from the new Y axis leadscrew wrapper) kept the epoxy off the dovetail.
Just for consistency, I removed 0.09 inch from the Z axis home offset, but that really won’t make any difference.
HOME_OFFSET = 6.84
HOME = 6.5
Now, I’d put the switch in that position because the saddle jams against the preload nut exactly at the end of the switch button travel. Now I can crush the switch by manually running the Z axis beyond its Home position …
While I had the tooling plate off, I cleaned the crud out of the tapped holes and ran a handful of 1/4 inch stainless steel 10-32 setscrews just below the surface:
Sherline tooling plate with setscrews
They’re pretty much invisible, of course, but they’re all present. FWIW, you need a 3/32 inch hex wrench for 10-32 setscrews.
In the event that I gouge the aluminum surface (you can see the odd ding and blind hole) through a setscrew, I’ll regret doing this. Not having to remove the plate to dig swarf out of the last clamping hole after carefully aligning a part seems like a win.
Reassembling the mill provided an opportunity to move the Y axis Home switch from the rear of the axis to the front. The key discovery happened during the teardown: I can get the saddle off the Y axis dovetail by removing the gib, without sliding it off the front, which means a front switch can remain firmly glued in place.
A few random hunks of steel and a wire nut held the switch in position while the epoxy cured:
Mounting Y axis home switch
The switch actuator bottoms out with the saddle just touching the preload nut, so the saddle can’t dislodge the switch: the switch trips just before the saddle hits the nut, at which point all motion stops and the motor stalls.
Moving the switch means I can remove all the gimcrackery that poked the rear switch with the tooling plate in place; I was never happy with that setup. I also removed the small block that trapped the rear end of the Y leadscrew, under the assumption that, as I haven’t yet dropped anything on the leadscrew, I probably won’t. That adds about 1/4 inch to the maximum travel and allows the tooling plate to whack into the column.
The switch wire runs along the stepper cable, a tidy technique that hasn’t introduced any glitches into the shared Home signal from the X axis drivers:
Sherline mill – X and Y axis home switches
The Y axis now seeks the Home switch in the positive Y direction, so that stanza in Sherline.ini looks like this:
The stereo zoom microscope over the electronics bench lives on the end of long support arm that tends to be just slightly wobbly. Part of the problem is that the far end is anchored on the sponge-backed laminate flooring I put atop the bench, but it’d be slightly wobbly even with a firm base on the plywood bench top.
So I prop up the microscope with a machinist’s jack and it’s all stable & good.
This one happens to be from an ancient Starret 190 set that I accumulated along with some other tooling, but any of the cheap imitations would work just as well.
The two bubble level vials help get the microscope axis exactly perpendicular to the bench surface, which makes the difference between good overall focus and a blurred image with a single line in focus. Here the jack is vertical and the microscope is tilted slightly toward the edge of the bench; the jack has a pivot below its knurled top plate.
There ought to be a survey marker pin at the front corner of the lot where it’d come in handy for locating the edge of the yet-to-be-contracted driveway paving, but if it’s there it’s been pushed below ground level. So I mooched a homebrew metal detector based on the Elenco K-26 PCB…
K-26 Metal Detector PCB
The kit included 45 feet of 22 AWG enamel wire that should have become a 5 inch diameter coil with 30 turns, but the as-built detector had a coil wrapped around a 1 foot diameter cardboard form. The coil inductance sets the oscillation frequency, which turned out to be around 300 kHz: far below the nominal 1000 kHz. So I wound 40 turns of 22 AWG magnet wire around an old CD-ROM spindle case (which is, quite coincidentally, just over 5 inches in diameter), and taped it atop the cardboard form.
The datasheet recommends a nonmetallic handle, so I swapped in a plastic umbrella support for the original metal mop (?) handle.
Rewound homebrew metal detector
The K-26 schematic looks like a common-base Colpitts oscillator, with only the most utterly absolutely vital essential components:
K-26 Schematic
In round numbers, the oscillation frequency varies inversely with the number of turns:
F = 1/(2π√(LC)) (for a simple tank)
L = stuff × N2 (stuff = various constants & sizes)
F = stuff / N
The rewound coil oscillated at 350 kHz, so I spilled off a few turns at a time to produce these results and a tangle of wire on the floor:
L – µH
Freq – kHz
330
350
186
535
107
711
65
840
42
1140
For the record, the coil in the photo corresponds to the last line and has 12 turns.
Contrary to what the instructions imply, trimpot P1 does not adjust the oscillation frequency. It tweaks the transistor bias for best oscillation, so it’s more of an amplitude control than anything else. I adjusted P1 while watching an oscilloscope connected across the negative battery terminal and the emitter of Q1, but you could probably use a small sniffer loop to good effect.
It draws about 2 mA, so the battery should last quite a while; labeling the switch positions should help a lot.
The oscillator produces an unmodulated carrier, so I tuned a Kenwood TH-F6A HT in LSB mode for maximum squeal. Any variation in L changes the carrier frequency and thus the pitch of the demodulated audio; an earbud just barely in one ear makes this almost tolerable.
As you should expect from the picture, that metal detector lashup is mightily microphonic, to the extent that touching a blade of grass wobbles the audio pitch and bumping the cardboard plate against an object can detune the whole affair. A bit more attention to rigid coil mounting would certainly help, but this isn’t the most stable of designs to begin with and I doubt anything will help very much at all.
The coil can detect a chunk of rebar sticking out of the ground at a range of maybe half a foot, but it’s not clear how well it will cope with buried treasures (like, oh, let’s say a survey marker pin). In any event, I must mow the grass down there before going prospecting.
Putting that battery into the Dell 8100 laptop produced the dreaded blinky light of doom, so it has been on the shelf for maybe half a year. Having gutted the cells from the case, the next step was to discharge the cells completely, thereby producing the lower four curves in this plot:
Dell 8100 Laptop Battery Cells
I arbitrarily labeled the cell pairs 1 through 4. Pair 1 has the lowest remaining charge and the other three seem very closely matched.
I recharged the four cell pairs one-at-a-time from a bench power supply set to 4.2 V. Each pair started charging at about 2 A, somewhat lower than the pack’s 3.5 A limit, so the supply’s 3 A current limit didn’t come into play. You probably don’t want to do this at home, but …
The usual charge regime for lithium cells terminates when the charging current at 4.2 V drops below 3% of the rated current (other sources say 10%, take your pick). The pack’s dataplate sayeth the charging current = 3.5 A, so the termination current = 100 mA. I picked 3% of the initial 2 A current = 60 mA and stopped the charge there, so I think the cells were about as charged as they were ever going to get.
As nearly as I can tell, increasing the voltage enough to charge at a current-limited 3.5 A (a bit beyond my bench supply’s upper limit, but let’s pretend), then reducing the voltage to 4.2 V as the current drops would be perfectly OK and in accordance with accepted practice, but I’m not that interested in a faster charge.
Unlike the other three pairs, Pair 1 quickly became warm and I stopped the charge. Warming is not a nominal outcome of charging lithium-based cells, so those were most likely the cells that caused the PCB to pull the plug on the pack. The other pairs remained cool during the entire charge cycle, the way they’re supposed to behave.
However, even with that limited charge, Pack 1 had about the same capacity as the (presumably) fully charged Pack 2, showing that the cells get most of their charge early in the cycle. Pairs 3 and 4 had more capacity, but they’re not in the best of health.
The blue curve in this graph shows the discharge curve for the 1.1 A·h Canon NB-5L battery (actually, a cell) that came with the SX230HS camera:
Canon NB-5L – first tests
Notice that it remains above 3.4 V until it produces 1.1 A·h at 500 mA, which is roughly its rated capacity. The other traces come from those crap eBay NB-5L batteries.
The two best pairs of Dell cells can each produce about 1.3 A·h at 1 A before dropping below 3.4 V (the cursor & box mark that voltage in the top graph), so they’re in rather bad shape. Strapping the best two pairs together would give a hulking lump with perhaps three times the life of the minuscule NB-5L battery, so I think that’s probably not worth the effort.
Particularly when one can get a prismatic 3.7 V 5 A·h battery for about $30 delivered, complete with protective PCB and pigtail leads…
While doing something else, I rediscovered the fact that common 5 gallon plastic bucket lids have an O-ring gasket that seals against the top of the bucket. Some seals are hollow tubes, some are solid rods:
5 gallon can lid gaskets
The white O-ring has about the right consistency to serve as a quilting pin cap, along the lines of those 3D printed and silicone rubber filled cylinders. Although the rubber / plastic stuff isn’t quite as squishy as silicone snot, it holds the pin point firmly without much of a push.
Chopping the O-ring into 10 mm sections produced another small box of prototypes:
Lid gaskets as pin caps
Garden planting season remains in full effect, shoving all quilting projects to the back burner and delaying the evaluation phase of the project…
The Smell of Molten Projects in the Morning 