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.

Category: Machine Shop

Mechanical widgetry

Homebrew V-750 Dosimeter Charger Pedestal: Insulator & Light Pipe

Contact Detail – Bottom View

The cylindrical center of the pedestal must conduct light into the dosimeter, conduct a positive charge into the contact pin, and push that pin hard enough to make contact inside the dosimeter.

The general notion is to turn an acrylic rod to a slip fit inside an 11/32″ telescoping brass tube, glue a wider acrylic disk to the bottom to take up the spring pressure, and run a 4-40 machine screw right down the axis to carry the current. There’s also a screw in the side to prevent the shaft from rotating.

Although the bulk of screw and solderless lug looks like it should block much of the central shaft’s view of the LED in the base, enough light gets around to illuminate the dosimeter’s scale. The acrylic doesn’t need an optically perfect finish, either, as diffuse light works fine.

Turning contact base ring

Turning central contact post

I used a hole saw to extract a disk from a piece of acrylic that used to contain one of those crappy desk clocks they give out as awards when money’s too tight to mention. The diameter should be a bit larger than the EMT’s ID so you can turn it to a slip fit.

Chuck the disk up reasonably square and drill out the center to a bit over 11/32″, so the tubing will bear against the rod rather than the base.

The acrylic rod has two slip fits: into the brass tubing and into the disk. Neither will be particularly fussy, so don’t lose much sleep over perfection here. Apply some good solvent adhesive to the rod’s large end and slide it into the disk. Pause for a day while it cures: it’s a big joint.

After it’s cured, chuck up the rod and turn the disk so it’s nice & square & neatly finished.

You’ll need two more disks: one for the pedestal base and another to act as a collet. I made them from 3/8″ polycarbonate sheet, of which I have what may turn out to be a lifetime supply. The base disk will be another slip fit in the EMT, the collet must match the actual OD you just turned on the contact disk. Saw a slot in the collet disk to convert it into a (crude) collet.

Drilling 4-40 clearance hole

The original V-750 pedestal has a nice stamped rod running the length of the central post, but I figured a long 4-40 screw would work as well. However, there’s no reason to thread the entire length of the post, so drill out a 4-40 clearance hole from the disk to within about 1 cm of the other end. This is where the collet disk comes in handy; you can see the saw slit at the bottom, between two jaws.

Take the rod out out and thread the end.

Drilling rotation stop hole

The last step is drilling & tapping a 4-40 hole in the side for the rotation stop screw. This will fit into a corresponding slot in the EMT shell to prevent you from twisting the contact wire off.

Put everything together, with a dab of cyanoacrylate adhesive to keep the brass tubing in place, and you’re done with this part.

2009-05-08
Homebrew V-750 Dosimeter Charger Pedestal: Overview
Although my V-750 dosimeter charger cleaned up reasonably well, I wanted to see if I could build a high-voltage supply from more-or-less contemporary parts to charge the dosimeters. The circuit is easy enough, but the charging pedestal that connects to the dosimeter turned into an interesting shop project.

V-742 dosimeter charging contact

Pencil-style electrometer radiation dosimeters, like the V-742 shown here, have a charging contact pin embedded in a transparent plastic (?) end cap recessed in the bottom. Inside the dosimeter a mighty spring (or, perhaps, the plastic cap itself) holds the pin outward so that it does not make electrical contact with the gold-coated quartz fiber in normal use.

This baffled me at first, because I did not understand why the charge didn’t just leak off the fiber through the charging pin. In order to dump charge onto the fiber, you must first press the pin inward by about 1 mm against the internal spring: no pressure, no contact, no charge.

Duh…

The dosimeter’s innards must be kept scrupulously clean and full of dry air. After you pull the pin out to admire it, the dosimeter won’t hold a charge ever again. I yanked the pin out of a dosimeter that simply didn’t work and, after a bit of fiddling, the dosimeter can now be set to zero, but the charge leaks off in a matter of hours rather than days.

Charging contact pedestal

The V-750 charging pedestal has an outer sleeve (the negative contact) and a central pin (the positive contact) that fit neatly into the end of the dosimeter. The pin stands about 2 mm proud of the plastic insulator that pipes light into the dosimeter to illuminate the scale. The sleeve, insulator, and pin move as a single unit: the dosimeter presses them down into the V-750 against two stacked springs.

A 1-lb spring holds the insulator in place by pressing the whole cylinder outward against its shoulder. The charger turns on when the dosimeter reaches that spring’s limit of travel at about 1 mm, but it’s not firm enough to press the dosimeter pin into contact with the quartz fiber. That’s the position you use to read the dosimeter: the light is on, but the fiber won’t move yet.

In order to charge the fiber, the dosimeter must move down an additional 3 mm against an 8-lb leaf spring until it seats against the pedestal’s threaded shell. Holding the dosimeter steady against that pressure while twiddling the voltage knob to adjust the dosimeter fiber to the zero point of the scale is more challenging than you might expect: grab it in your fist and hold on tight. It’s a good idea to wear glasses, as the dosimeter optics provides maybe 5 mm worth of eye relief: you can easily poke yourself in the eye with the fool thing if your grip loosens.

So, basically, a new charging pedestal must include a shell that meets the dosimeter’s body and a central shaft consisting of a sliding outer sleeve, a transparent insulator, and a central pin. The shaft must be pushed against the dosimeter by a really stiff spring to close the charging contact.

Not-quite-as-built cross section sketch

Finished charging pedestal

The overall plan looked something like this, at least before I started cutting metal…

What changed:
- a larger spring surrounds the LED
- no need for the weak spring
- no switch: the voltage-adjust knob has one
- a single slot in the side to prevent rotation
- screws, not solder, holding bolt to EMT shell
- no sleeve inside the bolt: it’s a copper bolt
But, all in all, it worked out OK.

Charging pedestal components

Here’s what the final result looked like, all spread out so you can see the innards…

The next few posts will show various bits & pieces, with notes & asides.
2009-05-07
Laser Pointer Annoyances

Laser pointer battery contact

Maybe it’s just me, but all of the laser pointers I’ve bought, even the relatively spendy ones, have crappy switches and unstable battery contacts.

For example, this is the business end of a $12 (!) pen-style pointer. The battery contact was off-center and poorly secured; I pried the white plastic retainer out, bashed the spring into submission, and replaced the retainer with a length of heat-shrink tubing. It wasn’t pretty.

This pointer has an actual mechanical switch module inside, with a clicky mechanism actuated by the external button. Cheaper pointers seem to rely on bare PCB contacts bridged by the button’s base. Ugh.

Laser pointer battery orientation: positive DOWN

Memo to Self: The AAA cells fit into the housing with the positive terminal away from the laser head. The white plastic plug has a molded cross that could be mistaken for a + symbol, but it’s not.

2009-04-30
Tour Easy: Fitting Novara Transfer Bike Panniers

Mary recently replaced her well-worn REI packs with a pair of Novara Transfer panniers, chosen because they’re just about the biggest packs available without insanely specialized world-touring features. They seem rather less rugged than the older ones, so it’s not clear how long they’ll last.

They fit her Tour Easy recumbent fairly well, but there’s always a bit of adjustment required.

Ramp on front edge of lower clamp rail

She hauls tools and clothing and veggies to & from her gardens, food from the grocery store, and the Token Windows Laptop to presentations. She brings the packs inside, rather than leave them on the bike, so they get mounted & dismounted for every ride.

The packs hang from the top bar of the rear rack, with a sliding clamp near the bottom of the pack that engages the rack’s vertical strut. I adjusted the clamp to the proper fore-and-aft position, but we found that the front end of the rail holding the clamp jammed against the seat support strut. That’s not a problem found on a diamond-frame bike.

The top picture shows the solution: Mr Pack, meet Mr Belt Sander. A ramp chewed onto the front end of the rail lets it slide neatly over the strut and all is well. The only trick was to avoid sanding through the pack fabric: the line perpendicular to the rail is sanding dust, not a gouge!

Acorn nut caps inside pack

Each top rack hanger mounts to the plastic pack frame with three bolts covered by plastic acorn nuts on the inside; the acorns cover actual metal nuts, so it’s a lot more secure than it looks. Three more bolts secure the bottom rail to the frame, with three more acorns poking into the pack, for a total of nine acorn nuts.

Most folks carry clothing and suchlike in their packs, so the 10 mm bump at each acorn presents no problem. Unfortunately, those things look like a nasty bruising hazard for soft veggies and groceries.

Top hanger pad – outside view

I sliced up some closed-cell foam packing material (everybody saves some of that stuff, right?), punched holes at the appropriate locations, and tucked the pads over the acorns. An inner fabric layer covering the frame and nuts should hold the pads in place.

Bottom pads with hole punch

It’s not clear the bottom pads will stay in position, but I wanted to try this without adhesives, mostly because I doubt any adhesive can secure polyethylene foam to whatever plastic the pack frame is made from or coated with. Perhaps double-sided foam tape will work?

Top pad – with tools

So far, the early reviews are good …

2009-04-27
Zero-dollar Power Screwdriver Repair

I’m in the midst of cleaning up the shop after a winter of avoiding the too-cold basement. The best way I’ve found to pull this off is to pick up each object, do whatever’s needed to put it away, and move to the next object. Trying to be clever leads to paralysis, so I devote a few days to fixing up gadgets and putting tools back in their places. After a while, it gets to be rather soothing.

Broken wire in power screwdriver

Some months ago I snagged a power screwdriver from a discard pile; while it didn’t work, un-bending the battery pack connector solved that. It runs from a quartet of AA cells, which means I can use alkalines and it’ll always be ready to go. It’s not a high-torque unit, so I’m using it for case screws and similar easy tasks.

But it quickly became intermittent and finally would turn only clockwise. Onto the to-do heap it went…

Power screwdrivers consist of a battery, a motor with a planetary gear reduction transmission, and a cross-wired DPDT switch in between. Not much can go wrong and, if it turns at all, most likely the problem has something to do with the switch or wiring.

Opened it up, pulled out the motor, and, lo and behold, one of the wires has broken off the switch. As nearly as I can tell, pushing the switch that-a-way forced the solder tab down on the wire and made the connection, pushing it the other way pulled the tab off the wire.

While I had the hood up, I replaced the wires with slightly thicker and longer ones. Soldered everything back together, mushed the grease blobs back into the planetary gearing, and it works like a champ…

Now, fairly obviously, there’s absolutely no economic sense to this sort of thing, given that the driver probably cost under ten bucks, but I just can’t stand to see a perfectly good gadget wind up in the trash.

I’d love to do this sort of thing for a living, if only I could figure out how to avoid going broke while doing so. Maybe I can get me some of that my economic stimulus money that’s sloshing around these days?

2009-04-22
Sunglasses Repair: New Hinge Holes

Milling to remaining hinge plate

With the epoxy cured overnight, I fired up the Sherline CNC mill to poke screw holes in the brass hinge splice.

The first step was to mill a flat-bottomed hole in the lower surface of the thin brass to expose the threaded hole in the remaining hinge plate. I crunched the end of the frame in a machinist’s clamp, then grabbed that in the Sherline milling machine vise; the frame is upside-down in the picture.

The brass stock was 0.015 inches, so I milled downward 0.020 inches to get through the epoxy. I’d love to say that worked perfectly, but I had to fiddle around a bit and eventually put a slight divot in the hinge plate.

That alignment was by pure eyeballometric guesstimation, but poking a small epoxy disk out of the threaded hole revealed that the 2 mm milled hole was centered on the hinge hole. Pretty close. Kinda-sorta. Good enough for my purposes, anyway.

Laser alignment to hinge hole

I aligned the spindle to the actual hinge hole with my laser aligner, a process that turned out to be surprisingly easy: note where the red dot vanishes on each side of the hole, split the difference, repeat for the Y axis, and you’re done.

Through-drilling top hole

With the spindle centered, I ran a #60 drill through the threaded hole (which it just barely cleared) and poked a hole in the thicker top plate (which is on the bottom here, remember). The packing under the hinge is a cut-up credit card; a handy source of thin sheets of stiff plastic.

Then I flipped the frame over and drilled out the top hole with a #54 drill to clear the threads on a 00-90 machine screw. I’d like to say I did a precision alignment job, but what I actually did was chuck that little bitty drill up in my big drill press, run it on the slowest spindle speed (maybe 400 rpm), brace my arms on the table, and feed the frame onto the drill by hand. Works perfectly… if only because I’m enlarging the hole by, what, 7 thou on each side.

Finished hinge – top view

A bit of filing cleaned up the drill chaff inside the hinge so I could mount the earpiece on the frame and screw it in place. I don’t have a 00-90 tap and wouldn’t use it in a titanium frame anyway, so you can tell this isn’t going to have a happy outcome, but, by and large, the undoubtedly metric threads in the frame did a pretty good job of re-forming the 00-90 brass threads. Ugly, but serviceable.

Some Dremel-tool work with an itsy grinding wheel on the flexy shaft eroded the back side of the U-shaped brass and new hinge plate to clear the earpiece; I think it only took half a dozen trial fittings & tiny grindings before the earpiece folded properly.

A dab of low-strength purple Loctite in the threads and I’d say that screw is in there for life!

Finished hinge – side view

Finished hinge – bottom view

Then I cleaned it up with a miniature wire wheel and, hey, it’s got a certain geeky charm, doesn’t it?

I have my doubts about how well the epoxy affixes itself to the brass, so I suspect I’ll be drilling a hole or two to mechanically lock it in place with some urethane adhesive when it falls off.

If the remaining hinge plate fractures, however, then the frame is toast.

Until I get around to having the optical shop dye up another pair, these should suffice for my simple needs.

Trivia:

The plastic film on the lenses comes from a big roll of the stuff they use to protect CRT monitors in shipping. Works great for shop projects and, back in the day, I used it when I was hauling monitors around. I think it’d suck the front right off an LCD panel, so I haven’t used much of it lately.

If you’re following the pictures, you’ll notice that the dsc* numbering series resets right in the middle of the story. That’s where my Sony camera gagged while writing an image and explains why I don’t have pix of the first drilling steps.

The color balance is weird on the milling machine pix because the shop lights are much cooler than the warm compact fluorescent bulb hovering over the table.

2009-04-18
Sunglasses Repair: Half a Hinge Is Better Than None

Broken hinge and brass shim stock repair parts

Some years ago I managed to talk one of the local optical shops into stripping the anti-reflective coating off my second-oldest pair of glasses, dunking the lenses in the gray dye pot, then re-coating them. I got a fine set of variable-bifocal sunglasses for 75 bucks; that’s why it took some persuasion. The near vision lens is a bit under my current prescription, but it’s good enough for driving and biking.

All good things must come to an end: the right-side hinge broke. The bad news is that it’s a titanium frame (can’t be brazed, at least by me) and the whole affair is old enough that it’s not worth sinking a bunch of money into a new frame. This year calls for new glasses anyway, so with any luck they’ll be able to do it again.

The good news: I can fix this thing with JB Weld epoxy and a few brass bits. So off to the Basement Laboratory, Machine Shop Division, we go…

Resistance soldering brass parts

Fortunately, the unthreaded top hinge plate broke off, leaving the threaded bottom plate intact. The plan: replace the plate with a suitable bit of brass shim stock, solder it to a U-shaped sheet of brass, epoxy the thing to the temple, drill a hole through the new plate, and run a screw into the threaded plate.

The top picture shows the broken hinge and the tediously cut-and-filed brass parts. The tab on the end of the flat plate fits around the remaining part of the hinge, the upper part is flush against the frame, and most of the plate will be trimmed off.

Rough-filed hinge splice

After demonstrating that my smallest torch can satisfactorily melt tiny bits of brass shim stock, I (tediously) re-cut and re-fit another set of parts, then deployed the resistance soldering gadget I built a while back (and wrote up for Circuit Cellar, Feb / Apr / Jun 2008) and silver-soldered the bits together. I must describe that thing here one of these days; it’s built around a rewired kilowatt-class microwave oven transformer with triac pulse-duty-cycle switching to control the heat.

A bit of diagonal cutter and file work produced a U-shaped channel that exactly fit over the remaining hinge. The rounded end (in the rear) is too wide, but that’ll get trimmed to fit when it’s in place.

Aligning earpiece and frame for epoxy

Wisely is it written that you cannot have too many clamps, which is what I used to build a fixture and align the earpiece with the lens frame to epoxy the hinge splice in place. As is always the case, nothing is square, plumb, true, or parallel. Fortunately, the glasses weigh basically zilch, so after I get the pieces aligned, they won’t shift out of place.

That done, I worked some JB Weld epoxy into the hinge stub’s crevices, then slipped the splice into place. A small blue clamp applied a bit of pressure to make the friction fit marginally more secure…

Final clamping overview

The trick here is to leave the plastic lid with the rest of the mixed epoxy sit on the workbench; if the epoxy in the lid isn’t cured, then there’s no point in moving the glasses and breaking the bond.

Clamping with epoxy applied

Then I let the epoxy cure overnight… when the story continues.

2009-04-17