# Archive for category Oddities

### Monthly Image: I’m So Poughkeepsie

Posted by Ed in Oddities, Photography & Images on 2017-11-15

This example of the City of Poughkeepsie’s branding seems poorly thought out:

Maybe not quite as bad as the “Too Cool to Do Drugs” pencil, but …

Selah.

### Nothing Lasts: Cranberry Harvester Corrosion

Posted by Ed in Machine Shop, Oddities on 2017-11-11

Cranberries grow best in acidic conditions, as shown by the conditions inside an antique cranberry harvester:

Admittedly, it’s been sitting untended for many years, but the worst corrosion formed along the midline of the machine, eating the conveyor housing, drive shafts, and support struts.

I managed to go all this time without realizing cranberry plants are evergreens.

### Battleship Wiring

Posted by Ed in Oddities, Photography & Images on 2017-11-05

A view from behind one of the switch panels in the 16 inch gun control compartment, deep inside USS Massachusetts BB-59:

Although the doc isn’t contemporaneous with Big Mamie, the Navy can still teach you to lace your cables like your life depends on it (starting on page 2-94, 157 of 1412).

We explored the interior for several hours, all the way to the lower Turret 2 barbette:

Each 16 inch projectile weighs 2700 pounds, with 800 shells distributed around three turrets. Looking at the drawings doesn’t make up for seeing the machinery.

The Massachusetts did shore bombardment during the Solomon Island campaign, where my father was assigned to guard a forward observer targeting Japanese redoubts and caves. He said the first rounds went over the far horizon, the second group landed short in the valley, and, from then on, the observer called out coordinates, walked the impact points down the valley, and wiped out each target in succession. BB-59 may not have been on the other end of those trajectories, but he said the Navy saved them plenty of trouble and inconvenience …

Capital ships became obsolete during the Battle of Midway, but battleships remain impressive hunks of engineering.

### Failed Compact Fluorescent Bulb

An overhead light in the Basement Laboratory went dark:

One end of the twisty tube got really *really* hot as it failed!

The Lab didn’t smell of electrical death, so the bulb must have failed while I was elsewhere. Metal enclosures with actual UL ratings suddenly seem like a Good Idea …

### Two-Wheel Plotter

Posted by Ed in Electronics Workbench, Machine Shop, Oddities, Science on 2017-10-30

While contemplating all the hocus-pocus and precision alignment involved in the DIY plotter project, it occurred to me you could conjure a plotter from a pair of steppers, two disks, a lifting mechanism, and not much else. The general idea resembles an Rθ plotter, with the paper glued to a turntable for the “theta” motion, but with the “radius” motion produced by pen(s) on another turntable:

The big circle is the turntable with radius R1, which might be a touch over 4.5 inches to fit an 8.5 inch octagon cut from ordinary Letter paper. The arc with radius R2 over on the right shows the pen path from the turntable’s center to its perimeter, centered at (R1/2,-R1) for convenience.

The grid paper represents the overall Cartesian grid containing the XY points you’d like to plot, like, for example, point Pxy in the upper right corner. The object of the game is to figure out how to rotate the turntable and pen holder to put Pxy directly under the pen at Ixy over near the right side, after which one might make a dot by lowering the pen. Drawing a continuous figure requires making very small motions between closely spaced points, using something like Bresenham’s line algorithm to generate the incremental coordinates or, for parametric curves like the SuperFormula, choosing a small parameter step size.

After flailing around for a while, I realized this requires finding the intersections of two circles after some coordinate transformations.

The offset between the two centers is (ΔX,ΔY) and the distance is R2 = sqrt(ΔX² + ΔY²). The angle between the +X axis and the pen wheel is α = atan2(ΔY,ΔX), which will be negative for this layout.

Start by transforming Pxy to polar coordinates PRθ, which produces the circle containing both Pxy and Ixy. A pen positioned at radius R from the center of the turntable will trace that circle and Ixy sits at the intersection of that circle with the pen rotating around its wheel.

The small rectangle with sides a and b has R as its diagonal, which means a² + b² = R² and the pointy angle γ = atan a/b.

The large triangle below that has base (R2 – a), height b, and hypotenuse R2, so (R2 – a)² + b² = R2².

Some plug-and-chug action produces a quadratic equation that you can solve for a as shown, solve for b using the first equation, find γ from atan a/b, then subtract γ from θ to get β, the angle spearing point Ixy. You can convert Rβ back to the original grid coordinates with the usual x = R cos β and y = R sin β.

Rotate the turntable by (θ – β) to put Pxy on the arc of the pen at Ixy.

The angle δ lies between the center-to-center line and Ixy. Knowing all the sides of that triangle, find δ = arccos (R2 – a) / R2 and turn the pen wheel by δ to put the pen at Ixy.

Lower the pen to make a dot.

Done!

Some marginal thinking …

I’m sure there’s a fancy way to do this with, surely, matrices or quaternions, but I can handle trig.

You could drive the steppers with a Marlin / RAMPS controller mapping between angles and linear G-Code coordinates, perhaps by choosing suitable steps-per-unit values to make the degrees (or some convenient decimal multiple / fraction thereof) correspond directly to linear distances.

You could generate points from an equation in, say, Python on a Raspberry Pi, apply all the transformations, convert the angles to G-Code, and fire them at a Marlin controller over USB.

Applying 16:1 microstepping to a stock 200 step/rev motor gives 0.113°/step, so at a 5 inch radius each step covers 0.01 inch. However, not all microsteps are moved equally and I expect the absolute per-step accuracy would be somewhere between OK and marginal. Most likely, given the application, even marginal accuracy wouldn’t matter in the least.

The pen wheel uses only 60-ish degrees of the motor’s rotation, but you could mount four-ish pens around a complete wheel, apply suitable pen lift-and-lower action and get multicolor plots.

You could gear down the steppers to get more steps per turntable revolution and *way* more steps per pen arc, perhaps using cheap & readily available RepRap printer GT2 pulleys / belts / shafts / bearings from the usual eBay sellers. A 16 tooth motor pulley driving a 60 tooth turntable pulley would improve the resolution by a factor of 3.75: more microsteps per commanded motion should make the actual motion come out better.

Tucking the paper atop the turntable and under the pen wheel could be a challenge. Perhaps mounting the whole pen assembly on a tilting plate would help?

Make all the workings visible FTW!

Some doodles leading up to the top diagram, complete with Bad Ideas and goofs …

Centering the pen wheel at a corner makes R2 = R1 * sqrt(2), which seems attractive, but seems overly large in retrospect:

Centering the pen wheel at (-R1,R1/2) with a radius of R1 obviously doesn’t work out, because the arc doesn’t reach the turntable pivot, so you can’t draw anything close to the center. At least I got to work out some step sizes.

A first attempt at coordinate transformation went nowhere:

After perusing the geometric / triangle solution, this came closer:

### Honeybee for Supper

We often have supper on the patio, with a fly swatter at the ready, but honeybees get special treatment:

She surveyed both our plates, landed on my cooked squash, and probed into the crevices as she would to extract nectar from a flower. The weather has been dry for the last few days and we think she was looking for anything providing a bit of moisture.

I splashed some water on the table and plopped that part of the squash nearby, in the hopes she’d find what she needs. We’ll never know the end of the story.

### Pure Matte Black the Hard Way

Posted by Ed in Machine Shop, Oddities, Photography & Images, Science on 2017-10-09

The best way to get a pure, non-reflective black uses optics, not pigments:

The shiny steel blades reflect light just fine, but the reflections have no way back out of the gap between adjacent edges: the angle of reflection always points away from you and the incoming light.

I always admire the blackness when I open that box.

Yes, I’m a member of the Society of the Easily Amused.

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