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Special Content: Repraps for Education

This is part of a series of posts about starting and facilitating a project-based 3D printer club at a local elementary school, with the ultimate goal of replicating the program at schools everywhere. We'll be posting as many details as possible, including lesson plans and supporting materials. For more information about the entire project, including a listing of posts related to it, please visit the 3D Printer Club for Schools project page. 

Here’s a quick update on some of the projects we’ve been working on this year:

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Greetings Maker Faire North Carolina Attendees! It was fun speaking with many of you today. Here is a collection of links to bots and technologies that I spoke about:

StickBot (the simple 6-legged walking robot)

SphereBot (the bot that draws onto ping pong balls, eggs, etc)

Other links:

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To install the heated print bed for my reprap, I placed the PCB on top of the plywood top print plate (which is slightly larger than the PCB), clamped it in place, and then used the holes in the PCB to drill out mating holes in the top plate. I put a washer on a 16mm M3 screw, and inserted it up through the print plate, added another washer, and a M3 nut to hold it securely in place. (I actually used SAE washers, as you can probably see from the photos, and that’s okay, too.)

Next I took my PCB heatbed to Lowes and had them cut a piece of glass to the exact same size. (While I was at Lowes, I picked up some Frost King 2″ insulated pipe wrap.) The glass is about 2.2mm thick, and it was only a few dollars. I cleaned up the edges of the cut glass with some fine grit sandpaper to make it a bit safer, then I put a small piece of Kapton tape (polyimide) on each corner of the glass to provide a little protection from the screws. Using the same tape, I secured the glass to the top of the PCB.

I covered the bottom of the PCB with the Frost King insulated pipe wrap to help keep the heat going up. I had to cut out notches to make room for the screws that would hold it in place (at the corners) as well as the tops of the screws that fastened the print top plate to the print bottom plate. I added a couple bits of extra pipe wrap to the center to help support the PCB and keep it from sagging.

The PCB/glass assembly rests solidly atop the four screws — there is no “play” at all. Here are a couple of pictures. (Click for larger view.)

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I built a framework for robot wheels a while back using OpenSCAD and used it to create a few different wheel styles. I recently decided to combine them all into one massively configurable wheel model, and add a number of new features as well. The result is an OpenSCAD file with 46 parameters that provides a limitless set of combinations and wheel designs. I call it One Wheel To Rule Them All.

It includes twelve tread patterns (all configurable in often surprising ways), eight core spoke patterns (also highly customizable), configurable support for o-rings, bands, and even optical encoder timing slots (directional and non-directional), and a lot more. Plus, I’m still adding features as I think of them.

First, here are a few images of the types of the various basic elements, as well as a few variations that show the flexibility of the designs. For example, as shown in these images, the Spiral style can be used to create a variety of interesting designs that you might not think of when you think “spiral.” After the images you’ll find full details on the parameters.

It’s important to note that you can configure this wheel to such an extend that it may not be printable at home with extruded plastic printers. In these cases, services such as Shapeways could come in handy.

The source file is available at http://www.thingiverse.com/thing:21486, or on github at http://github.com/alexfranke/Highly-Configurable-Wheel.

Tire Parameters

Often wheels are built around the tires. In this section, specify the properties of the tires you’re using, and this will define the diameter of the wheel. If you’re using o-rings, the tireCSDiameter should be the cross-section diameter of the o-ring, or if you’re using some other flat tire material (such as rubber bands), jsut specify the its thickness. If you’re not using any tire at all, set the tireCSDiameter to zero.

  • wheelWidth: The width (or thickness) of the the wheel
  • tireCSDiameter: Cross-sectional diameter (CS) — How thick is the tire rubber?
  • tireID: Internal diameter (ID) — How wide is the inside opening?
  • tireStretch: Circumferential stretch percentage (usually 1 + 0-5%) — How much to you want to stretch it to get it on?

Rim properties

The rim sits at at the outside of the spokes and supports the tires or added treads. Installed tires (such as o-rings, rubber bands, etc) are set into grooves carved out of therim, while trads are added onto it. Keep this in mind when you’re using tires — as an example, the rim height should not be smaller than the radius of o-ring tires.

The rim also supports rotary encoder timing holes for wheel feedback. Use the padding parameters to adjust the location of those holes. See the compiler output for helpful information about the distance indicated by each timing hole. Directional timing holes will produce a second set of holes that are 90 degrees out of phase with the first. This allows you to stack sensors at the same location over the wheel instead of trying to position them along the circumference. Directional timing holes essentially double the resolution. You can also double resolution by looking for both rising and falling edges.

  • rimHeight: The height of the rim portion of the wheel
  • timingHoles: The number of timing holes to carve into the rim
  • timingHoleInPad: The inside padding for the timing holes
  • timingHoleMidPad: The middle padding if direction timing holes is selected
  • timingHoleOutPad: The outside padding for the timing holes
  • directional: A directional encoder renders two sets of slots, 90 deg out of phase

Tread Parameters

In this section, specify the properties of the tire tread you want to render. If you’re using a wheel (e.g. o-ring, rubber bands, etc), then use either the “o-rings” or “slots” settings, which will cut a groove (or grooves) in the wheel rim to fit the tires. The othertreat styles will render a tread pattern protruding out from the tire surface by the amount you specify in third part of “knobSize”.

Imagine the tire is mounted on a robot and facing straight at you. The “knobSize” parameter defines the size and shape of knobs in an [x,y,z] format, where x goes across the rim, y goes up and down along the perimeter of the wheel, and z protrudes out from the wheel toward you.

The “staggerOffset” parameter allows you to stagger knobs across the tire by an amount you specify. Set this to zero if you want all the knobs lined up along the perimeter and aligned with the edges of the rim.

“numberOfKnobs” specifies how many knobs there are across the tire, and “lineThickness” specifies how thick the lines are from “drawn” tire styles, such as “x”, “cross”, and “zigX”. You can use these pameters together in creative ways — for example to extend a single tread profile across the width of the tire, or to create a contiguous zig-zag.

Finally, “radialTreadSets” defines how many sets of treads are rendered around the wheel. Each set contains two rows in order to create the staggered effect.

Tread styles are:

    • none: No tread is rendered
    • cross: Each knob is the shape of a plus sign with the specified lineThickness
    • o-rings: Grooves are cut into the rim to accept o-ring tires
    • squares: Each knob is a rectangle, whose size is specified by knobSize
    • spheres: Each knob is a smooth bump, whose size is specified by knobSize
    • cylindersX: Each knob is a cylindrical shape running across the wheel, whose size is specified by knobSize
    • cylindersY: Each knob is a cylindrical shape running along the perimiter of the wheel, whose size is specified by knobSize
    • cylindersZ: Each knob is a cylindrical shape protruding from the surface of the wheel, whose size is specified by knobSize
    • spikes: Each knob is a cone or spike protruding from the surface of the wheel, whose size is specified by knobSize
    • slots: Grooves are cut into the rim to accept flat tires, defined by numberOfKnobs (number of grooves), the first and third numbers in knobSize to define the width of the slots and the depth, and spaceBetweenTires for the distance between the tires and also from the outside edges to the first slots.
    • x: Each knob is in the shape of an “x” protruding from the surface of the wheel, whose size is specified by knobSize
    • zigX: Each knob is in the shape of a zig-zag protruding from the surface of the wheel, whose size is specified by knobSize
    • v: Each knob is in the shape of a “v” protruding from the surface of the wheel, whose size is specified by knobSize
  • treadStyle: none, cross, o-rings, squares, spheres, cylindersX, cylindersY, cylindersZ, spikes, slots, x, zigX, v
  • knobSize: The size of each knob [across wheel, along the perimeter, prodruding]
  • radialTreadSets: How many sets of treads to render around the wheel (2 rows per set).
  • numberOfKnobs: The number of knobs to render per row.
  • staggerOffset: A distance to offset the staggered rows.
  • lineThickness: The line thickness for “drawn” styles, such as “x” and “zigX”
  • maxTires: For o-rings, the maximum number of tires per wheel
  • spaceBetweenTires: For o-rings, the space between each tire, if there are more than one

Spoke-related Parameters

This section is used to define the spoke style of the wheel. Some of the properties are only applicable to certain wheel types, and these properties can be used together in creative ways to create a wide range of tire designs.

The “proportion” property affects how some spokes are rendered. The first number is the proportion of the design from the center of the wheel to the inside of the rim, and the second number is the proportion of the width inside of the wheel. For example, to create spokes that are roughly in the shape of a “U”, you can use a “circle” style, and set the proportion to [1.5, 1.0], for cirle spokes that are 150% as long as the distance from the center to the inside of the rim, 100% as wide.

Use spokeInset to specify the inner and outer inset of the spoke area from the inner and outer faces of the wheel. You can use a negative number to make the spoke area stick out further than than the rim. The hub position will be based on the inner surface resulting from this inset.

The spoke styles are:

    • biohazard: A biohazard logo-inspired design. Set numberOfSpokes to 3 to mimic the logo.
    • circle: Spokes in a circlar or oval form, defined by spokeWidth and proportion.
    • circlefit: The maximum number of circles that will fit between the center and the rim, with a set of smaller outer circles specified by outerHoleDiameter.
    • diamond: Spokes in the shape of a diamond (rhombus), defined by spokeWidth and proportion.
    • fill: Fills in the spoke area with a solid cylinder.
    • line: Straight line spokes, like you would see on a typical wagon wheel.
    • none: Leaves the spoke area empty and does not make for a very useful wheel.
    • rectangle: Spokes in the shape of a rectangle, defined by spokeWidth and proportion.
    • spiral: Spokes in the shape of a semicircle, defined by curvature, reverse, spokeWidth.
  • spokeStyle: none, biohazard, circle, circlefit, diamond, line, rectangle, spiral, fill
  • spokeInset: The [inner,outer] inset of the spoke area from the surface
  • numberOfSpokes: Number of “spokes.” Set this to three if you’re doing the biohazard design
  • spokeWidth: This is how wide each spoke is.
  • proportion: proportion to rim, proportion of width
  • curvature: For “spiral”, this is how curvey the spokes are. >0, but
  • reverse: For “spiral”, setting this to “true” reverses the direction of the spirals
  • outerHoleDiameter: For “circlefit”, the diameter of the outer holes, or zero for non
  • concavity: Concavity distance of spoke area for [inside, outside] of wheel

Hub Parameters

These properties define the hub — or how the wheel connects to the motor. The default values for the captive nut are precise for a M3 nut and will make the nut a very tight (if not impossible) fit. I prefer this because it allows you to “melt” the nut into place with a soldering iron. However, if you don’t have a solder iron or prefer a looser fit, then just adjust the nut diameter and thickness. (M3 hardware is, by default, set to 3mm screw diameter, 5.4mm nut diameter, and 2.3mm nut thickness.) Similarly, the holes for the motor shaft and grub screw are also precise. This allows the holes to be drilled out for a more precise fit. Again, you can adjust these to suit your needs.

The hubZOffset can be used to “sink” the hub into the wheel, and it defaults to half the wheel thickness. For example, when the hubHeight is 10 and the hubZOffset is -2, then the hub will protrude 8mm from the wheel, but the shaft hole will be 10mm deep. The set screw will still be positioned in the middle of the exposed vertical height, and the fillet/chamfer will also be rendered in the correct position. This property is also useful if you want to poke a hole entirely through the wheel. (e.g. If the wheel is 6mm thick, set the hub height to 16 and the hubZOffset to -6, and you’ll get a hub that protrudes 10mm from the wheel surface with a hole that extends all the way through the wheel.)

To mount a servo motor, set includeHub to false, set shaftDiameter so that the hole will accommodate the servo horn screw and any bit that protrudes from the top of the servo horn. Then set the servoHoleDiameter to the size of your mounting hardware, and set servoHoleDistance1 and servoHoleDistance2 to the total distance between mounting holes on your servo (not the distance from the center). These sets of mounting holes will be rendered at 90 degree angles from one another. If you only want one set of holes, set one of the values to zero. Adjust the angle of all the holes to avoid openings in your wheel design if necessary using servoArmRotation.

Use innerCircleDiameter to specify a solid inner circle to use as a base for the hub. This can be useful if you need a a solid surface for servo mounting hardware or for the base hub fillet/chamfer.

Use outerNutTrap to create a nut or bolt head trap on the outside (bottom) of the hub area. Used in conjunction with shaftDiameter and false for includeHub, this will create a wheel that can drive a bolt much like the large gear on Wade’s Extruder. (This feature is inspired by that design.)

Use servoNutTrap to create nut traps for bolts used to mount the wheel onto servo arms. This feature was suggested by AUGuru.

  • includeHub: Set to false to remove the hub and only include the shaft diameter hole.
  • hubDiameter: The diameter of the hub portion of the wheel
  • hubHeight: The total height of the hub
  • hubZOffset: The Z position of the hub, negative numbers from the surface of the wheel
  • shaftDiameter: The diameter of the motor shaft
  • innerCircleDiameter: The diameter of the solid inner circle under the hub, or zero for none.
  • setScrewCount: The number of set screws/nuts to render, spaced evenly around the shaft
  • setScrewDiameter: The diameter of the set screw. 3 is the default for an M3 screw.
  • setScrewNutDiameter: The “diameter” of the captive nut, from flat to flat (the “in-diameter”)
  • setScrewNutThickness: The thickness of the captive nut
  • baseFilletRadius: The radius of the fillet (rounded part) between the hub and wheel.
  • topFilletRadius: The radius of the fillet (rounded part) at the top of the hub.
  • chamferOnly: Set to true to use chamfers (straight 45-degree angles) instead of fillets.
  • servoHoleDiameter: The diameter of servo arm hounting holes, or zero if no holes
  • servoHoleDistance1: Distance across servo horn from hole to hole (0 to ignore)
  • servoHoleDistance2: Distance across servo horn from hole to hole, rotated 90 degrees (0 to ignore)
  • servoArmRotation: The total rotation of all servo holes
  • servoNutTrap: Size [indiameter, depth] of servo arm captive nut, or 0 (any) for none.
  • outerNutTrap: Size [indiameter, depth] of a captive nut, or 0 (any) for none.

Quality Parameters

  • $fn: Default quality for most circle parts.
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OpenSCAD apparently doesn’t have an easy way to make an arc or wedge, or any way to do a partial rotate_extrude. This presented a problem when I started to design my Parametric Encoder Wheel, because in order for it to look nice, it would need to have lots of extruded arcs or partial rotate extrudes in order to make the holes.

Encoder Wheel, OpenSCAD Arc

Creating an Arc with OpenSCAD

Here’s how I solved the problem. Currently this will render slots up to 180 degrees around, but extending it should be pretty easy. If you’re doing a lot of these in your model, be sure to use render(), or badness will occur.


/* 
 * Excerpt from... 
 * 
 * Parametric Encoder Wheel 
 *
 * by Alex Franke (codecreations), March 2012
 * http://www.theFrankes.com
 * 
 * Licenced under Creative Commons Attribution - Non-Commercial - Share Alike 3.0 
*/

module arc( height, depth, radius, degrees ) {
	// This dies a horible death if it's not rendered here 
	// -- sucks up all memory and spins out of control 
	render() {
		difference() {
			// Outer ring
			rotate_extrude($fn = 100)
				translate([radius - height, 0, 0])
					square([height,depth]);
		
			// Cut half off
			translate([0,-(radius+1),-.5]) 
				cube ([radius+1,(radius+1)*2,depth+1]);
		
			// Cover the other half as necessary
			rotate([0,0,180-degrees])
			translate([0,-(radius+1),-.5]) 
				cube ([radius+1,(radius+1)*2,depth+1]);
		
		}
	}
}

Creative Commons License
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.

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Yeah, free. If you’re starting a 3D printer project (such as RepRap) at your public K-12 school in the United States for the benefit of the students, then we’ll give you a free Sanguinololu v1.3a PCB to help you out (while supplies last). They typically sell for about $12-14. We won’t even charge you for shipping.

What is a 3D printer? Check out the video at the bottom of the page for an introduction.

Not a school but want one anyway? I’m selling some of them to raise funds for a printer for a local school. Just select “None – Buy one” for affiliation below, and I’ll send you the details. They’re $11, shipping included within the USA.

Sanguinololu 1.3a PCB

Sanguinololu 1.3a PCB

We don’t see any reason why kids shouldn’t have access 3D printing technology in their problem solving toolkit, and soldering up the electronics is all part of the fun. A club with only six kids raising $50 each could be well on its way to building and operating a 3D printer for its school, and while the electrionics can typically amount to 1/3 or more of the final build price, hopefully a free PCB will help ease the pain a bit.

Are you interested? Submit the form below for instructions on how to get your free Sanguinololu 1.3a PCB.

Name (required)
Email (required)
School (required)
Affiliation (required)
Description of Your Project (required)

What is a 3D Printer?

Here’s an introduction to the technology:

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The shipping “estimator” at Mouser.com is a little vague, so I thought I’d post the results of a recent order. In general, you’re not going to know the shipping cost or the weight of your order until after it’s shipped.

My recent order totaled $189.12 and had 33 line items, including all kids of components (resistors, capacitors, ICs, connectors, crystals, buttons, crimp terminals, etc), in quantities ranging from 2 to 250, primarily for assembling four Sanguinololu circuit boards (with some exceptions, plus some other stuff).

Selecting residential delivery via USPS to North Carolina, USA, the Mouser shipping estimator listed $6.95 for 1 pound and $9.60 for 2 pounds. I guess that the order wouldn’t weigh any more than 2 pounds. The actual invoiced shipping cost turned out to be $7.74.

Although the order was prepared the previous night, I didn’t remember to actually place it until just after 4:00 EST on January 25, 2012 — so I had just missed the same-day shipping cut-off time. The order shipped on January 26, and the credit charge (for the exact invoice amount) was posted on January 27.

The delivery tracking updates were as follows:

  • Out for Delivery, January 28, 2012, 9:55 am, CHAPEL HILL, NC 27514
  • Sorting Complete, January 28, 2012, 9:45 am, CHAPEL HILL, NC 27514
  • Arrival at Post Office, January 28, 2012, 5:08 am, CHAPEL HILL, NC 27514
  • Electronic Shipping Info Received, January 27, 2012
  • Depart USPS Sort Facility, January 27, 2012, FORT WORTH, TX 76161
  • Processed at USPS Origin Sort Facility, January 26, 2012, 8:18 pm, FORT WORTH, TX 76161
  • Accepted at USPS Origin Sort Facility, January 26, 2012, 7:03 pm, MANSFIELD, TX 76063
One final note: Recently I’ve noticed that there are a number of different ways people pronounce Mouser. Recently I’ve heard Mouse-er (rhymes with house-er), Moze-ure (rhymes with rose-ure), and Mau-zer (rhymes with cows-er). So how do you pronounce Mouser? It’s pronounced Mau-zer (rhymes with cows-er), but I still like to think of it as Mouse-er.
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I’ve set out to build a Prusa Mendel for $300. So far I think I’m doing pretty well. The top part is what I’ve already picked up, and the bottom part is what I have yet to purchase — that’s where I still have some flexibility in cost. The first column is the percentage of total cost for that part. Shipping charges for a group of items from the same supplier are listed with the first item in that group. [Note: This chart has been updated many times as I build, and as because I’m already up and running with my reprap 3D printer, there’s no longer anything left to purchase.]

Where I discovered mistakes, I’ve corrected them. For example, I actually ordered nylock #6 nuts instead of regular ones. In cases like these, I’ve just corrected the prices and pretended like I never incurred the cost of the wrong product.

There’s one important point I’d like to make out here: I was patient, I waited for good deals (and occasionally got exceptional ones), and I arranged some group buys for bulk discounts. Often if you agree to purchase larger quantities of a product, you can secure a discounted rate, and so that’s what I did where I could. You might not find all of the same deals I did, but you may get pretty close if you are patient and work with your suppliers.

I’ve added a table below that breaks out the electronics order, including what I had on hand, what I bought from ebay, and what I didn’t actually need. (Welcome Hack A Day readers!)

Finally, be sure to scroll to the right in the spreadsheet views for important notes or shortcuts on most line items.

Electronics

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It’s not easy to find concrete examples of shipping costs for McMaster-Carr online, and they don’t tell you what an order is going to cost to ship until after it’s already shipped — and that can be a little scary especially if you’re ordering odd sized parts, even if they only charge their shipping cost. So here’s a real example of shipping costs for a small order with some long parts to NC, USA. The order was placed online and shipped the same day from their Atlanta, GA, USA location. It was ordered on December 29, and received in North Carolina on January 3.

For a reprap build, I ordered these items, which include three 3-foot steel rods, 200 nuts, 292 washers, and 200 machine screws. Shipping was only $6.52 for everything. That’s not bad at all…

Description Ordered Unit Price Total
95462A030 Zinc-Plated Grade 5 Steel Hex Nut, 5/16″-18 Thread Size, 1/2″ Width, 17/64″ Height, Packs of 100 1

Pack
4.56

Per Pack
4.56
90126A509 Zinc-Plated Steel Type A SAE Flat Washer, No. 6 Screw Size, 3/8″ OD, .03″-.07″ Thick, Packs of 100 1

Pack
1.27

Per Pack
1.27
90272A153 Zinc-Plated Steel Pan Head Phillips Machine Screw, 6-32 Thread, 1″ Length, Packs of 100 1

Pack
3.16

Per Pack
3.16
90631A007 Zinc-Plated Grade 2 Steel Nylon-Insert Hex Locknut, 6-32 Thread Size, 5/16″ Width, 11/64″ Height, Packs of 100 1

Pack
2.48

Per Pack
2.48
92005A120 Metric Pan Head Phillips Machine Screw, Zinc-Plated Steel, M3 Size, 10MM Length, .5MM Pitch, Packs of 100 1

Pack
2.30

Per Pack
2.30
8890K41 W1 Tool Steel Rod, .3125″ Diameter, Trade Size 5/16″, 3′ Length 3

Each
4.24

Each
12.72
90126A030 Zinc-Plated Steel Type A SAE Flat Washer, 5/16″ Screw Size, 11/16″ OD, .05″-.08″ Thick, Packs of 192 1

Pack
4.50

Per Pack
4.50
Merchandise 30.99
Shipping 6.52
Total $37.51
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Well, I discovered that Popsicle is a brand name, so I’m officially renaming this little guy “StickBot” so the Popsicle police don’t come after me. To be clear, this little bot has nothing to do with Popsicle brand ice pops, and never did. My appologies Popsicle; I hope you still let me eat your ice pops because life just would not be the same without them.

Now that I’ve come clean, here’s a video that show how to assemble StickBot’s right eye. It’s an analog 555 timer-based 1Hz oscillator that controls the right/left PWM servo signals generated by the left eye.

This is the latest version, powered directly by four 1.5V LR61 batteries (similar to AAAA batteries, and often found inside 9V batteries). As a result, it does away with the voltage regulator.

Details for the previous version can be found here: StickBot V2.0 - Untethered!. The original tethered version can be found here: StickBot: A Simple 6-Legged Walker. I’ll try to get updated schematics and more assembly videos up soon.

I’ve had a few people ask me if I have a kit for this little critter. I’m working on one geared toward kids — it’s a fun project to do with kids, and it seems like they really learn a lot from it.

The parts for this eye include:

  • 555 Timer IC
  • 1k ohm resistor
  • 330k ohm resistor
  • 2.2uF capacitor
  • About 9 inches of Cat 5 network cable, phone cord, or other similar wire
  • Some electrical tape, solder, and soldering iron
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