Geared 7-Segment Display – Part 2, More Gears

While time at home is scarce this week, I’ve stolen a couple moments late night to continue working on the design of the geared 7-segment display, including finishing the modelling of the 7 drive gears.

Each drive gear is based on a 30-tooth, involute gear of module 4. Each segment of 3 adjacent teeth represents a single transition of a segment (or lack there of) – if there are teeth in a segment, the associated arm gear will rotate, changing a segment from active to inactive for a given transition or vice versa.

As a side note, each gear has a even number of teeth remaining, and each segment makes an even number of transitions as the display makes a full cycle of ten digits. If a segment made an odd number of transitions, it would start the next “cycle” in a different state than on the previous cycle, causing the numbers to “look” different on each time a given number came up, which is clearly wrong. This served as a useful sanity check as I was working through each gear in turn.

Here is the mechanism in its current Fusion360 form (support plates, arms and mounts, and a drive mechanism yet to come):

 

The A, D, and G arm gears lie on the vertical axis of the mechanism. The A and G arm gears, as noted in my previous post, are currently intended to be co-axial, the shaft of the A segment being a small hollow tube which completely surrounds the shaft of the G segment. Of all the details in this mechanism, this one seems the most fiddly at the moment, since any tolerance issues are going to compound on each other.

The B, C, E, and F arm gears lie at ±50° from the vertical axis, which is just about as close to the vertical axis as they can be and still have their arms clear the axes of the A/D/G segments.

In contrast to what I said a the end of my last post, I’m thinking I’ll print each of the gears individually and then mount them on the center axle with spacer washers. The whole-gear-assembly-as-barrel has one fatal flaw: printability. That’s a lot of overhanging teeth to worry about. That said, the print-individual-gears approach means needing to worry about registering adjacent gears to each other, but that seems like a solvable problem.

Looking down the road, here’s a quick Vectorworks sketch of how close adjacent digit displays could be. It seems I could squeeze them to about 175mm (~7 inch) centers.

 

Currently, the plan is to build one digit and evaluate… but the only thing better than N mechanisms is N+1 mechanisms…

Geared 7-Segment Display – Part 1

Over the weekend, I got started on a project I’ve been musing about for a few months – making a mechanical 7-segment display, using gears to move individual segments in and out of the display area via rotation of a central shaft or belt.

The inspiration for this idea is undoubtedly Arthur Ganson’s mechanical sculpture Gary’s Yellow Chair at the MIT Museum in Cambridge, video of which periodically makes the rounds on Reddit. In it, a bicycle-chain drive six separate sprockets, each of which moves a long rod to which is connected one sixth of a chair. Each time the sprockets make a full rotation, their connected arms point toward a central point and the fragments of the chair briefly assemble into a whole (if tiny) yellow chair. Then the pieces split apart again, sent on another rotation by the action of the driving chain.

In this vein, my goal is to create a series of seven moving arms, each with a segment of a 7-segment display on it. A central shaft will drift seven attached gears, each with teeth placed and left out at specific intervals. These seven drive gears will turn seven arm gears, which in turn attach via shafts to long, thin (metal?) arms at the front of the device. The spacing of the teeth on the drive gears will ensure that each of the arm gears turns at the appropriate time to move the segments in and out of the display area. Each time the arm gear needs to move an arm in or out of the way, the drive gear will cause its paired arm gear

Here is a quick drafting of how I currently think this project will be laid out. The grey circles indicate the base circle of each gear, while the concentric circles are the pitch and addendum (i.e. maximum extent) circles.  The green segments indicate when a given physical segment is in its “displayed” position, while blue indicates where that segment will be when “not displayed”. The dotted lines around each segment indicate its travel, and are useful that none of the arms sweep through another segment’s shaft. In section, you can see that the segments are going to be situated on 3 different front-to-back planes to avoid collisions between arms and shafts. You can also see the concentric relationship between the top segment and the center segment.

It turns out, fitting 7 arm gears around what is essentially one central drive gear shaft is tricky, especially to do so in such a way that none of the arms contact each others’ shafts as they rotate. To accommodate this, I currently have the top segment operated with a hollow shaft, and the shaft for the center segment runs through this hollow shaft to protrude out the top. We’ll see how that goes.

Here is a quick sketch of the digits on a typical 7-segment display as it moves through the digits 0 through 9. The small red marks in between each digital denote which segments change between digits.

Which leads us to the following chart of which segments need to move between which digits.  Note that the horizontal axis is for “moving to this digit,” so that an X in the “7” column, for example, means that that segment needs to change when moving from a 6 to a 7.

After some preliminary work in Fusion360, I did a couple preliminary test prints, both of one of the “30-tooth” drive gears and some of the 6-tooth pinion gears.  (Since the total number of possible necessary transitions is 10, and each transition only needs to turn the arm gears ½ a rotation, the arm gears have 1/5 as many teeth as the drive gears.) You can also see one of the 2mm spacer washes I whipped up, which I think will be unnecessary (see below).

With the slight creep and elephant’s-foot that my printer makes, I think I will need to depth these a little further apart than the idealized spacing – even when the teeth are not engaged, the tips of the pinon teeth drag a bit on the drive gear. Even another .2 or .4 millimeters would help here.

It occurs to me at this point that there’s no reason for the central gears to all be separate assemblies and prints – they’re all meant to rotate in lockstep, so there’s no reason not to print them as one large barrel with protruding teeth at 7 depths. That will be a necessary future improvement. Of course, the supports, axel holes, and whatever I’m doing for that hollow shaft are also future problems to be solved.

 

3D Printing on Fabric – Dragon Scales

In our upcoming production of Macbeth at work, one of the many spooky ingredients that the Three Witches drop into their wondrous cauldron is "scale of dragon." To that end, I'm helping our props department out by printing some dragon scales onto some toule. The design is essentially some raised, horned scales repeated in a hexagonal pattern.

The print is in silver eSUN PLA+, which strikes a nice balance between the darker, dull tone of most grey filaments, and the super shiny "metallic" filaments. eSUN PLA+ has become my defacto standard in the past couple months, after I used 4kg or so to complete a project printing portable 

So far, I've had three failed prints with this file and technique. The first was my own fault: after pausing the print between the second and third layers to insert to toule, I hit "Stop Print" instead of "Resume Print." Arg. The second and third failures, though, are a mystery. They appear to have stopped extruding following printing the bottom layers before starting on the infill and walls. I don't know why exactly this happened, as I started both prints late at night after work before heading to bed. But both times, I came out in the morning to find the print head at the final Z height I would have expected had the print completed... but with only a small bottom layer of rectangles trapping the fabric.

I'm currently printing the pattern again, re-sliced. Here's hoping!

Update after printing: close, but no cigar.

Re-slicing the file alleviated the "no extrusion after the bottom layers" problem, so I can only assume it was some odd setting in Cura that was to blame. I'd been trying out a "pause at given height" plug-in script, but a little more googling has lead to believe that this not usable in its current form with the Duplicator i3. Perhaps that was the cause.

This time, the error was my fault. It seems that one of the binder clips holding the toule to the heated bed was contacting the frame of the printer at one end of its Y travel. This only turned out to be an issue for the top %30 of the print, when the tip of the "frontmost" scale leaned far enough in the Y+ direction to cause the binder clip to contact the frame. Thus, everything above this level shows layer-shifts every two or three layers as the clip hits the frame and causes the y-stepper to skip a step.

Back again later with smaller binder clips!

Designing 3-Way Initial Blocks [Video]

In my experiments  with Fusion360 recently and casting around for inspiration, I stumbled across my old copy of Godel, Escher, Bach: An Eternal Golden Braid, which has a curious cover design consisting of two objects that cast shadows of three different letters along three different axes. In the following video, I look at the process of designing one of these shapes with arbitrary letters for 3D printing.

 

Faceted Lampshade from Clear PLA

I’ve had a clear, “low poly” angular sculpture in clear PLA sitting on my self for awhile now. Earlier this week, I turned it into a lamp shade.

I built the original model in Blender along the lines of this video from Maker’s Muse. The gist of the process is:

  • Start with a basic solid, like a sphere or a cube.
  • Decimate the solid (reduce the number of triangles that make up the shape) to a very very low level, perhaps down to 20 or 30 polygons.
  • Stretch and place the vertices of this blocky solid until satisfied.

In this case, I had ideas about turning this shape into a lighting table topper, with an Arduino and LEDs underneath the open base. But other projects popped up, and this open-base blocky pyramid has sat dormant for several months. This week, I finally found a use.

In my living room, I have a tall two-socket light fixture standing in the corner. One socket stands at the top of the lamp, with a broad frosted dish shade, and there’s a small flexible arm coming off the side of the lamp which lost its shade long ago. My partner and I both enjoy using this flexible arm as a re-positionable reading lamp, but that lack of a shade means it’s been pretty glare-y.

By trimming the top off of the clear pyramid, I converted the Agrocrag into a lampshade.

The shade is trapped in place by the bulb itself. Of course, an LED bulb is a must, since the heat of an incandescent (or even CFL) light bulb will melt or deform the shade.

Just a little project-reuse for the home!

Hello, World! Again!

I must have started half a dozen blogs in my life. I still maintain a separate one for Ham Radio at KK9JEF.wordpress.com. I’ll have to see about transferring those posts to this site.

So why another new blog? Two reasons:

  • My blog at KK9JEF.wordpress.com was specifically for my Ham Radio projects and learnings. As my hobbies expand into other areas, I thought it might finally be time to have a general-purpose personal hobbyist blog.
  • I’ve always struggled to find the balance between blog posts that catalog a completed project in whole. For example, my entire K3NG Morse-Code Keyer Project was wrapped into one big post, but I split up my Beach 40 Build into multiple small sections. Neither is perfect – the “blog as you build” method leaves a somewhat scattered trail for those who come after and want to replicate what you did (which is sometimes you in the future), but the “wait until it’s done” method means a lot of the process is undocumented.

To that end, this new site has both BLOG and PROJECTS sections – the blog can be for more single-shot observations, and the projects subpages can contain coherent descriptions of where a particular build or project is at.

It’s worth a shot!