The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Author: Ed

  • Re-rebuilding a Recumbent Antenna Mount

    Antenna Mount
    Antenna Mount

    Quite a while ago, I built this slab mount to hold an amateur radio antenna on our daughter’s Tour Easy. It worked fine until the bike blew over and whacked the antenna whip against something solid, at which point the mast cracked.

    The antenna screws into an ordinary panel-mount UHF connector secured to the bottom of the slab, with a hole through the slab just large enough to accept the antenna mast. That put all the mechanical stress on the slab, not the connector.

    Modified antenna mounting plate
    Modified antenna mounting plate

    Alas, the new antenna had a slightly different mast outside diameter, so I machined a new adapter to clamp the connector atop the slab. The antenna screws down into the adapter against a brass washer, again keeping the strain on the fitting.

    I recently found the commercial mobile antenna cable that I’d been meaning to use on her bike, which required Yet Another Modification to that slab. It turns out that the UHF connector on the cable expects to be secured to sheet metal found in a car body, rather than a half-inch aluminum plate: the threads aren’t long enough!

    So I machined circular recesses on the top and bottom to hold the mounting nut and washer, respectively, with 2 mm of aluminum remaining in the middle of the slab.

    Milling top recess
    Milling top recess

    The recesses are just fractionally larger than the nut & washer, so most of the stress gets transmitted directly to the slab. Even in the high-vibration bicycle environment, I think there’s enough meat in there to prevent fatigue fractures.

    Milling bottom recess
    Milling bottom recess

    I recycled a G-Code routine I’d written to chew out circular recesses. It does a bit of gratuitous (for this application, anyway) spiraling in toward the center, but got the job done without my having to think too much.

    The bottom view shows the washer in action. The recess is deep enough that the cable just barely clears the slab.

    Modified mounting plate - bottom
    Modified mounting plate – bottom

    The top view shows the recessed mounting nut. The nut has an O-ring around the connector threads, but the water will probably drain out through the four through-holes left over from the old panel-mount connector.

    Modified mounting plate
    Modified mounting plate

    I turned the top nut down as far as I could with a wrench & (ugh) needle-nose pliers, then tightened the bottom nut about 1/3 turns with a wrench.

    You’re not supposed to notice the crispy edges on the PVC bushing holding the reflector to the antenna mast. The high setting on that heat gun is a real toaster…

    The G-Code is over there.

  • Padded Spring Clamp Tips

    Padded Spring Clamp Tips
    Padded Spring Clamp Tips

    The OEM padding on the ends of my spring clamps wore off long ago; it was some sort of entirely-too-soft dipped plastic.

    Some large-diameter heatshrink tubing seems like it ought to be a good replacement… the tips are a bit floppy, but maybe that’s not entirely a Bad Thing.

    We shall see…

  • NiMH Battery Pack Status

    Here’s the status of the AA NiMH packs I’ve been using with the radios on our bikes, plus three packs I made up last year and have been keeping on the desk to measure their long-term storage characteristics. Click for more detail.

    Bike Radio Pack Status - 2010-03
    Bike Radio Pack Status – 2010-03

    The “Tenergy 09 x” packs are new & unused with, frankly, disappointing capacity of about half their 2.6 Ah rating. That’s not much better than the used Tenergy packs (T9x and RTU x), which is either a Good Thing (they have good long-term stability) or a Bad Thing (they’re grossly over-rated to begin with).

    The two Duracell packs are far better than any of the Tenergy packs.

    The three 6-cell packs along the bottom are fading fast.

    The previous test runs are there, albeit with a 1 A discharge.

    This season I’ll use some Li-Ion packs that weigh twice as much with three times the capacity… plus a built-in charge gauge, pessimistic though it may be.

  • Opening a Quartz Crystal Can: Effects Thereof

    A comment on yesterday’s post about quartz crystal measurements prompted me to destroy a crystal in the name of science…

    The question is, what effect does exposing a crystal to the air have on its performance? I would have sworn it would never work right again, because it’s normally running in an inert atmosphere and maybe a partial vacuum. One measurement being worth a kilo-opinion, here’s what happened.

    I picked random crystal from the bottom of the crystal box, based on it having a solder seal that I could dismantle without deploying an abrasive cutoff wheel or writing some G-Code to slice the can off with a slitting saw. The crystal was labeled HCI-1800 18.000 MHz and probably older than most of the folks who will eventually read this… younger than some of us, though.

    The overall response, measured in the same fixture as shown yesterday (click the pix for more detail):

    HCI-1800 18 MHz - Baseline Overview
    HCI-1800 18 MHz – Baseline Overview

    The center frequency is 18.0050 MHz (at this rather broad span) and it has some ugly spurs out there to the right.

    A closeup of the series-resonant peak:

    HCI-1800 18 MHz - Baseline BW
    HCI-1800 18 MHz – Baseline BW

    The bandwidth is 1.50 kHz at 17.99950 MHz at this span.

    Naked HCI-1800 18.0 MHz Crystal
    Naked HCI-1800 18.0 MHz Crystal

    Then I applied a soldering iron around the seal and yanked the case off. I think that didn’t involve whacking the crystal with the case en passant, but I can’t be sure. In any event, it looks undamaged and seems to operate properly.

    A pair of spring clips attach to the electrodes and hold the quartz disk in position. They’re just the cutest little things and quite unlike the other holders I’ve seen. I think the solder blobs fasten the spring ends together and don’t bond to the electrodes, but what do I know?

    HCI-1800 Crystal Overview
    HCI-1800 Crystal Overview

    The quartz disk has a few small chips near the edge:

    HCI-1800 Crystal Edge Chips
    HCI-1800 Crystal Edge Chips

    I think those are Inherent Vice… simply because:

    1. They’re not in a position where I could have whacked the disk and
    2. I doubt I could whack it that delicately

    Anyhow, with the can off, here’s what the series resonant peak looks like:

    HCI-1800 18 MHz - Opened BW
    HCI-1800 18 MHz – Opened BW

    The resonant frequency is now 17.99968, 180 Hz higher, which may be due to instability in the HP8591 spectrum analyzer’s not-stabilized-for-ten-hours ovenized oscillator. The bandwidth is 1.55 kHz, 50 Hz wider, although I think that’s one resolution quantum of difference.

    Here are the two bandwidth traces overlaid.

    HCI-1800 18 MHz - Overlaid BW
    HCI-1800 18 MHz – Overlaid BW

    The peak has been centered in both, so you can’t tell they’re slightly different. The interesting point is the difference in the slope to the low-frequency side of the peak, which is slightly higher for the open-case condition. Seeing as how the missing case completely changes the usual stray capacitance situation, I’m not surprised.

    Anyhow, I admit to being surprised: there’s not that much difference after opening the case. I’ll put the naked crystal in a small container in a nominally safe place for a while, then retest it to see what’s happening.

    Memo to Self: A “safe place” is nowhere near the Electronics Workbench!

    Here are some other naked crystals:

    Naked Crystals
    Naked Crystals

    Notice the tarnished (presumably) silver electrodes on the crystal in the lower left. That one’s been sitting on my monitor and in other hazardous locations for a few years. I can’t find these anywhere right now, but if they turn up I’ll test them, too.

  • Crystal Properties

    Spent some Quality Shop Time measuring an assortment of crystals, some data from which will make up a Circuit Cellar column.

    And the raw numbers will come in handy one of these days, so here they are…

    12 MHz Asst HC-49/U Co+Cc/2 2Cc Fs BW Rs
    ECS 1 4.85 1.47 12.000162 787.5 40.1
    ECS 2 4.50 1.42 12.000150 725.0 40.1
    ECS 3 4.70 1.42 12.000325 1100.0 50.0 Rs out of range
    HC1 4 4.34 1.11 11.999000 600.0 36.0
    HC1 5 4.24 1.06 12.000137 537.5 36.9
    Sentry 6 5.14 0.96 12.000250 625.0 31.8 many spurs
    11.0592 MHz HC-49/U
    1 4.90 1.42 11.059275 562.5 9.3
    2 4.99 1.46 11.059112 575.0 14.7
    3 4.87 1.42 11.059275 512.5 9.6
    4 4.87 1.41 11.059125 550.0 10.0
    5 4.29 1.43 11.058935 750.0 18.1
    6 4.93 1.47 11.059000 537.5 10.7
    7 4.95 1.47 11.059200 525.0 8.1
    8 5.03 1.45 11.059037 575.0 11.1
    10 MHz HC-49/U spur +150 kHz
    1 2.57 1.36 9.997888 200.0 14.2
    2 2.61 1.30 9.997738 225.0 16.7
    3 2.75 1.30 9.997788 225.0 20.0
    4 2.67 1.30 9.997750 225.0 16.1
    5 2.75 1.26 9.997725 250.0 22.7
    6 2.69 1.27 9.997788 225.0 21.0
    7 2.69 1.26 9.997825 212.5 16.5 Circuit Cellar example
    8 2.69 1.22 9.997832 212.5 18.4
    9 2.72 1.24 9.997788 250.0 23.4
    10 2.68 1.20 9.997738 225.0 18.0
    18.43 MHz HC-49/US many spurs +10 +76 kHz
    1 3.86 1.33 18.432425 1.56 24.1
    2 3.79 1.22 18.432987 1.21 10.9
    3 3.93 1.39 18.432050 2.44 46.3
    4 3.97 1.40 18.431175 1.90 27.7
    5 3.89 1.33 18.431888 2.11 32.2
    6 3.92 1.39 18.430888 1.39 16.5
    7 3.99 1.35 18.431500 1.36 11.8
    8 3.97 1.35 18.431675 2.18 38.4
    9 3.95 1.31 18.430512 1.30 10.1
    10 4.04 1.50 18.431427 1.36 11.8

    The 18.43 MHz crystals are in the short /US cans with surprisingly high stray capacitance. Their bandwidths are in kHz and all over the map, as are the series resistances. Weird. Bad crystals? Bad technique?

    Capacitance measured with that fixture.

    Frequency & bandwidth from HP8591 spectrum analyzer with a fixture similar to the K8IQY design; the bandwidths seem to come in 12.5 Hz increments despite a (very narrow) 2 kHz span. The general process is there. Resistance measured from a cermet trimpot using a multimeter good for 0.1 Ω around 10 Ω.

    Crystal Test Fixture
    Crystal Test Fixture

    Useful equations, with column headings in boldface:

    • Lead-to-can capacitance for each lead: Cc = 2Cc / 2
    • Lead-to-lead capacitance: Co = Co+Cc/2 – Cc/2
    • Circuit Q: Q = Fs/BW
    • Circuit resistance: R = Rs + 25 (assuming 4:1 transformers)
    • Reactance XL = XC at series resonance: X = Q R
    • Motional inductance: Lm = X / (2 π Fs)
    • Motional capacitance: Cm = 1 / (2 π Fs X)
    • Parallel resonance Fp = Fs √(1 + (Cm / Co))

    More equations there.

    Memo to Self: Zero the capacitance fixture before critical measurements!

  • Just Another Bicycle Trip

    Mary & I did the weekly grocery run today, with a few add-on errands.

    I’m (finally) shipping the Totally Featureless Clock to my friend and hauling a bag of shredded leaves (the first of a dozen) with which Mary mulches the plants in her remote garden plot. We dropped off the leaves and some garden gate fencing (from her bike), then continued on for groceries.

    Trailer with Package and Shredded Leaves
    Trailer with Package and Shredded Leaves

    Mary returned to the garden to spend the afternoon coaxing the plants to grow nicely, while I hauled the TFC (and the groceries) to the UPS inlet.

    Trailer with Groceries and Package
    Trailer with Groceries and Package

    And then I hauled the groceries home. Most of the four bags of chow fit in the trailer, with squishable fruit & veggies in the bike panniers. A whopping 13 miles, all told, but a good time was had by all.

    The trouble with bicycles is that they have approximately the cargo capacity of your car’s glove box. Panniers help, but for bulk capacity you need a trailer. Think of it this way: these days, a good trailer costs maybe three or four tanks of gasoline.

    If you keep coming up with reasons why you can’t get your butt on your bike and “I can’t haul X!” is one reason, a trailer might be the answer for reasonable values of X. It’s no good for plywood sheets and water heaters, but I’ve hauled plenty of other X that would ordinarily call for a car trip.

    It’s an old B.O.B Yak. Works fine, tracks well, doesn’t wobble, carries more than you think possible.. Just do it!

    We each put about 2000 miles a year on our bikes, most of it on errands just like this. That’s not many miles by bicycle fanatic standards, but we do lots of other stuff in addition to biking…

    Search the blog for “trailer” and you’ll find a few other hints & tips.

  • Sherline CNC Mill: Defining Home Switches

    Having mounted & wired the switches, the next step involves defining the homing sequence & configuration for each axis. All this goes in Sherline.ini and is adapted from the doc there.

    The travel limits are somewhat empirical and I think the Y axis will require some adjustment due to the tooling plate switch extender gadget.

    The HOME_SEARCH_VEL values may be a bit too high, given the rather lethargic 5.0 in/sec^2 acceleration I’m using for X & Y, with just 3.0 for Z. I’ve heard the occasional thwack as the switch trips, so maybe 20 mils of overtravel isn’t quite enough.

    For the X-Axis:

    [AXIS_0]
... snippage ...
MIN_LIMIT = -1.0
MAX_LIMIT = 9.5
HOME_IS_SHARED = 1
HOME_SEQUENCE = 2
HOME_SEARCH_VEL = 4.75
HOME_LATCH_VEL = 0.016
HOME_FINAL_VEL = 0.25
HOME_OFFSET = 9.1
HOME = 4.5

    For the Y-axis:

    [AXIS_1]
... snippage ...
MIN_LIMIT = -0.5
MAX_LIMIT = 4.95
HOME_IS_SHARED = 1
HOME_SEQUENCE = 1
HOME_SEARCH_VEL = -4.75
HOME_LATCH_VEL = -0.016
HOME_FINAL_VEL = 0.25
HOME_OFFSET = 0.0
HOME = 4.5

    For the Z-axis:

    [AXIS_2]
MIN_LIMIT = -0.1
MAX_LIMIT = 6.9
HOME_IS_SHARED = 1
HOME_SEQUENCE = 0
HOME_SEARCH_VEL = 0.333
HOME_LATCH_VEL = 0.016
HOME_FINAL_VEL = 0.25
HOME_OFFSET = 6.9
HOME = 6.5

    The A axis doesn’t get a home switch because I can’t imagine needing one for a rotary table:

    [AXIS_3]
... snippage ...
MIN_LIMIT = -9999.0
MAX_LIMIT = 9999.0
HOME_SEARCH_VEL = 0
HOME_LATCH_VEL = 0
HOME = 0.0