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

Camera LCD Sunshade & Magnifier: Part 2
Lens end of viewer

With the bottle formed & trimmed to shape, it’s time to mount the lens. This view shows the final result, with the camera body angled upward.

The general idea is that the bottle cap already attaches securely to the bottle, so I can just cut a rectangular hole in the lid, make it just slightly smaller than the lens, and affix the lens inside with the planar surface facing outward.

Two motivations for making the hole slightly smaller than the lens:
- The lens has rounded corners, as it was cut from a 38 mm diameter round lens
- It won’t stick out, get bumped, and fall off
Lens opening cut in bottle cap

The first step was, of course, to make a fixture: a sacrificial wood block with a raised section that fits snugly inside the cap. I found a nice maple disk in the scrap bin, chucked it in the lathe, and turned a section to fit. I don’t have a dust extraction system, so I did this one-handed with another on the shop vac to suck up the swarf. Yuch, wood is dusty!

That simplified clamping the rather slippery lid in place. It’s probably polyethylene that would slide away under heavy cutting loads, but with a 2 mm end mill that wasn’t a problem. The origin is at the center of the cap, directly atop the convenient injection-molding sprue button.

The lens is 34.4×22.1 mm, so I cut a 32×20 mm opening using manual CNC. Given a 1 mm cutter radius, the G-Code looked something like this:
```
#1=[20/2-1]
#2=[32/2-1]
g0 x[0-#1] y[0-#2]
g1 z-2 f100
x#1
y#2
x[0-#1]
y[0-#2]
g0 z30
```
That’s from memory, so it might not work quite right. Basically, store the key variables in parameters and use those instead of mistyping a digit somewhere.

The opening even has nicely rounded 1-mm radius corners from the 2 mm cutter…

Cutting acrylic lens holder

I added a sheet of acrylic inside the lid to hold the lens in position and provide a more glue-attractive surface. The lens opening here was a slip-fit for the lens: 34.5×22.2 mm. The G-Code looks pretty much the same:
```
#1=[22.2/2-1]
#2=[34.5/2-1]
g0 x[0-#1] y[0-#2]
g1 z-2 f100
x#1
y#2
x[0-#1]
y[0-#2]
g0 z10
```
Trimming outside of acrylic lens holder

The wood disk even had a convenient hole in the middle, making it easy to re-clamp the acrylic from the center with a stack of washers. The laser aligner made alignment easy: make the nut finger-tight, put the spot on the left edge near the front, jog to the rear, twist to split the difference, iterate a few times, then snug down the nut.

Then the origin is halfway between the edges. Knowing the opening size, find one edge and touch off by half that amount.

The cardboard lid liner was 43 mm in diameter, so I figured that would work for the acrylic sheet. Circular interpolation makes getting a precise diameter trivially easy, after you remember that this is outside milling so you must add the cutter radius:
```
#1=[43/2+1]
g0 x[0-#1] y0
g1 z-2 f100
g2 i#1
g1 z30
```
Finished cap with lens

What’s not shown there is the blob of acrylic that welded itself to the cutter because I was taking picures rather than dribbling water on the workpiece to keep it cool. I hate it when that happens.

But everything pretty much worked out. The holder was a snap fit inside the cap, just like it was supposed to be.

I glue the lens to the acrylic holder with silicone snot (aka “adhesive” or “caulk”), let it cure overnight, snapped the cap on the bottle, and iterated once to get the lens properly aligned with the opening (the acrylic sheet rotates freely inside the cap).

Viewer attached to camera

The end result is, admittedly, ugly on a stick, but the first reports from the user community are encouraging!

We may add a dark cloth ruffle around the bottle cap as an eye shade and eyeglass protector, but that’s in the nature of fine tuning.
2009-05-14
Camera LCD Sunshade & Magnifier: Part 1

Viewer attached to camera

Mary take her gardening pictures with our Sony DSC-F505V camera, which has one compelling advantage for the job: the lens and body pivot, so you can take pix at odd angles without groveling in the dirt or hovering over the camera staring downward. Alas, it lacks an optical viewfinder, which means she does a lot of outdoor close-up photography peering into a washed-out LCD panel in full sun. Worse, she’s far-sighted and can’t see fine details without her reading glasses or bifocals, so it’s really hard to get proper focus.

Something must be done!

The general notion is to put an opaque shield around the LCD with a lens that magnifies the viewfinder. If you happen to have perfect near vision, the lens is optional and you can probably use one of the commercial sunshades that attach with hook-and-loop strips. That isn’t going to work for us.

With inspiration from that project, I retired to the Basement Laboratory. [Update: a somewhat less intricate do-it-yourself project starting with a slide viewer. I suspect it works better for normal-looking cameras, not this one.]

Raw material: rectangular lens and opaque bottle

Rummaging in the Bottle Supply turned up a dark brown plastic bottle made from PETE, the same stuff that makes soda bottles, with a black plastic snap-cap lid. PETE has a glass transition temperature around 75C, which means you can reshape it with a heat gun (not, alas, a hair dryer). Actually, I found two bottles, so I have a backup.

A bit of soaking in water, followed by a generous application of xylene, got rid of the label & adhesive residue. You can get xylene in small quantities as Goof-Off adhesive remover or just buy a quart at your local big-box home-repair store. Do the xylene part outdoors and don’t toss the rags in the trash until they’re dry.

Further rummaging in the Lens Supply turned up a 34.4×22.1 mm plano-convex rectangular lens with perhaps a 100 mm focal length. Haven’t a clue where it came from, but perhaps from the Surplus Shed optical supply shop. Pretty nearly any lens with those general specs will work, so use what you have. You do have a box of lenses, don’t you?

Putting the flat side of the lens close to my (distance-corrected, I’m nearsighted) eyeball and looking through it at the LCD from about 75-125 mmm produces a very nicely enlarged, distortion-free image. This will work!

Bottle cutting and forming

The bottle is much thicker than a soda bottle, but easily cut with a razor knife and a bit of care. I removed the bottom and measured the ID as 68 mm. The circumference is, obviously, 214 mm, which is a key dimension: it must fit around the LCD’s perimeter with a bit to spare.

I made a wood mould block that’s sized and shaped roughly like the back of the camera around the LCD: Mr Block, meet Mr Belt Sander. This avoids the prospect of melting the camera with the heat gun, as it’s largely plastic, too.

The block is 52×57 mm, for a perimeter of 218 mm, and a totally non-critical 38 mm tall (it came from a 2×4″ chunk of lumber). The pyramidal section acts as a forcing cone to persuade the bottle to stretch around the slightly larger block and become nicely rectangular as it does.

Wood forming block

Position the bottle over the block, apply the heat gun all around, and ram the bottle downward as it softens. Eventually the bottle will eat the block, even though it’s not completely happy about doing so, at which point you can concentrate on heating each side separately. The bottle will settle down and stretch neatly around the block, giving it a rectangular base with a smooth transition from the round top. The cut edge tends to curl outward in the middle of each flat side, so don’t overheat it.

Cut the corners back so there’s about one focal length from the cap to the cut, then heat the side flaps (the shape is rectangular: get this right!) and bend them back. I flattened them against the bench to remove the curve. The top and bottom flaps will fit over the top and bottom of the camera and hold the whole affair in place.

Trim the side flaps to a few mm, as they’ll just form a light shield, and shape them to clear the controls as needed. Form the top & bottom flaps to fit snugly around the camera and trim to fit; they cover up the buttons just under the camera’s LCD, but those aren’t used in normal operation.

I used plain old electrical tape to hold the bottle in place, as anything thicker will interfere with the lens rotation. If you have a box-shaped camera, hook-and-loop may be your friend.

That was enough for one day. Tomorrow: a bit of CNC work on the lid…

2009-05-13
Homebrew V-750 Dosimeter Charger Pedestal: LED Holder, Spring, and Assembly
Drilling mounting holes in base ring

The charger pedestal includes an LED to light up the dosimeter’s graticule. I seated a 10 mm white LED into a polycarbonate ring that also serves as the base for the stiff spring that presses the contact assembly against the dosimeter’s internal spring.

I made the base while I was doing the lathe work for the contact assembly, then grabbed it in the Sherline mill’s 3-jaw chuck to drill the 4-40 holes with a touch of manual CNC.

As before, I manually tapped the holes, but it’s a lot easier with each hole at the right location and pointed in the right direction!

Step bit making an annular ring

I described the step-drilling that produced the correct hole and shoulder sizes there. That won’t work every time, but in this situation it was just about perfect.

The LED power wires pass through the central hole in the ring. I used a blob of hot-melt glue to hold the LED in place; epoxy would be more in keeping with the nuclear weapons theme, but HMG is just fine with me.

There’s another hole just to one side of the LED, more or less centered between the mounting screws, that passes the wire from the dosimeter charging contact out of the pedestal. This wire starts at the center of the top, passes inside the spring, and must not be pinched along the way.

LED in base with spring positioning ring

I added an aluminum cylinder as a positive stop to prevent the dosimeter contact assembly from getting pushed too far into the pedestal. The length matches up with the anti-rotation slot in the EMT: the screw doesn’t quite hit the top or the bottom of the slot.

A wrap of green electrical tape around the outside made the cylinder a slip fit inside the EMT shell. It shouldn’t move at all.

The cylinder also holds the spring in place so it can’t rub against the charging wire, but I’m pretty sure that isn’t necessary.

The spring comes from my parts heap. It must provide a bit under 8 pounds of force to activate the dosimeter charging spring with about 3 mm of travel. I picked the length of the EMT shell to preload the spring to make the answer come out right, which also affects the length of the aluminum cylinder.

The spring OD must fit into the EMT and the ID must clear the 10 mm LED and charging wire in the base. Your mileage will most certainly vary.

Charging pedestal components

Assembly is straightforward, but goes much more easily with three hands.
- Screw the panel mount bolt into place
- Attach the charging wire to the central contact & remove the anti-rotation screw
- Slide the central contact in place, reinstall the screw through the slot
- Slide the spring & aluminum cylinder in place, wire in the middle
- Pass the wire through the LED base ring
- Press the base assembly into position and hold while installing the screws
Finished charging pedestal

The charger I built turns the LED and charger power on with a push-to-activate digital encoder knob, so there’s no need for the 1 lb spring & switch found in the V-750 charger.

To read the dosimeter, just hold it loosely atop the pedestal, push the twiddle knob down, and the LED comes on.

To zero the dosimeter, press it firmly and twiddle the knob for zero!

I’ll describe the charger circuity at some point; it’s detailed in my Circuit Cellar column in the August 2009 issue.

V-742 Dosimeter set to Zero
2009-05-11
Homebrew V-750 Dosimeter Charger Pedestal: Outer Shell

Finished charging pedestal

This is a chunk of EMT (Electrical Metallic Tubing) with 4-40 clearance holes that attach it to the panel mounting bolt, hold the base disk in place, and keep the central contact assembly from rotating. The overall view gives you a good idea what’s involved.

The nominal EMT size is 3/4″, which (of course) means the ID is about 0.8″ and the OD is a bit over 0.9″. There’s a weld seam running the length of the tube that I cleaned up on the lathe, so the actual ID is slightly enlarged. While it’s in the lathe, face off both ends to whatever length suits the spring you’ll eventually use.

There’s nothing tricky about this, other than getting the three holes on each end lined up properly with their mating parts. Once again, manual CNC comes in handy: grab it in the 3-jaw on the rotary table, use G81 to drill the hole and G0 A120 and G0 A240 to index the locations. Make sure you retract the drill bit far enough to clear the chuck jaws!

The two sets of holes need not be perfectly aligned with each other.

Milling rotation stop slot in shell

The photo shows that I milled the rotation stop slot after drilling the holes. It’d be easier to do that without removing the cylinder from the chuck, but this was one of those incremental designs where I was checking the fit as I built it.

The slot should be long enough to allow the contact assembly to slide almost completely into the pedestal. That prevents you from crunching the dosimeter’s innards when you’re pressing it down on the spring.

The clearance from tool holder to chuck isn’t all that large; you might want to put the slot at the far end of the cylinder… but then I’d have to conjure up a pipe center for the Sherline tailstock and figure out how to mount it high enough to match the rotary table’s axis.

2009-05-10
Homebrew V-750 Dosimeter Charger Pedestal: Panel Mount

Copper 1/2"-20 bolt with thinned steel nut

The original V-750 pedestal is a threaded bushing around the cylinder that contacts the dosimeter. Hard to make from scratch, but it’s basically a bolt with hole in the middle. I can do that…

A foray into the parts heap produced a copper bolt threaded 1/2″-20 and a matching steel nut. I bandsawed the nut in half, doing a surprisingly good job of cutting it parallel to the surfaces, and filed off the obvious blems. The thin washer fit a 7/16″ bolt until I filed the hole out; the OD is a bit undersized for a 1/2″ head and looks much better in this application.

I grabbed the bolt threads in the lathe and turned down the head for a slip fit in the EMT. Turns out the head wasn’t exactly concentric with the threads, but now the rounded-off hexagon tips are. Drill out the middle for a slip fit around the 11/32″ brass tubing, break the edges, and it’s all good.

Drilling EMT mounting holes in bolt head

The bolt threads need to be barely long enough to go through the aluminum box I’ll eventually mount this thing in and pass through the nut, so I sawed the bolt off to 3/8″, more or less, and cleaned up the end in the lathe.

I thought about soldering the bolt to the EMT shell, but, fortunately, came to my senses before doing any damage. Instead, I drilled & tapped three 4-40 holes in the head that will match with similar clearance holes in the EMT. This is the sort of thing that works really well with “manual” CNC: get the first side lined up, then just type G0 A120 and you’re at the next face. A manual G83 peck drill cycle pokes the hole exactly where it’s needed.

Manual tapping, a bit more edge breaking, some cleanup, and the thing looks pretty good.

Pedestal mount – oblique view

Pedestal mount – top view

2009-05-09
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