Mary has been quilting up a storm lately and is growing dissatisfied with the special safety pins she’s been using to hold the layers together. Long straight pins are ideal, except that maneuvering a large quilt through her sewing machine resembles stuffing a porcupine into a keyhole. A commercial solution costs nearly half a buck per pin, which seems unreasonably spendy for something you need by the hundreds.
We kicked around some finger- and quilt-friendly dimensions and I cobbled up a solid model:
Which turned into an array of small octagons that won’t roll off the table:
We figured 25 would be enough to decide if this is workable and whether the dimensions fit fingers, pins, and quilts.
Filling them with silicone rubber required one squirt each:
The trick with the silicone rubber is to cut the snout so it fits flat on the cylinder top. Put the cylinders on a piece of non-stick paper (I used the back of the carrier for some double-sided tape, but wax paper would be better), hold one with tweezers, squirt in enough rubber to fill the cylinder solidly from bottom to top, then slide the snout sideways to smooth the surface.
Wait for a day, pop them off, and remove any drool:
It’s garden planting time right now, so it’ll take a while before I tweak the design and run off the next batch.
I don’t know how to compute an actual cost for each of those things. I regard the entire Thing-O-Matic as fully depreciated and pretty much a sunk cost, which means the expense boils down to the incremental cost of plastic and silicone. All the Quality Shop Time is, of course, free… and maybe even therapeutic.
The (trivially simple) OpenSCAD source code:
// Quilting pin caps
// Ed Nisley KE4ZNU April 2012
//- Extrusion parameters must match reality!
// Print with +1 shells and 3 solid layers
ThreadThick = 0.25;
ThreadWidth = 2.0 * ThreadThick;
HoleWindage = 0.2;
function IntegerMultiple(Size,Unit) = Unit * ceil(Size / Unit);
Protrusion = 0.1; // make holes end cleanly
//----------------------
// Dimensions
ID = 5.0;
OD = ID + 2*ThreadWidth;
Length = 5.0;
Sides = 8;
//----------------------
// Useful routines
module PolyCyl(Dia,Height,ForceSides=0) { // based on nophead's polyholes
Sides = (ForceSides != 0) ? ForceSides : (ceil(Dia) + 2);
FixDia = Dia / cos(180/Sides);
cylinder(r=(FixDia + HoleWindage)/2,
h=Height,
$fn=Sides);
}
module ShowPegGrid(Space = 10.0,Size = 1.0) {
Range = floor(50 / Space);
for (x=[-Range:Range])
for (y=[-Range:Range])
translate([x*Space,y*Space,Size/2])
%cube(Size,center=true);
}
//----------------------
// Build them!
ShowPegGrid();
rotate(180/Sides) {
difference() {
PolyCyl(OD,Length,8);
translate([0,0,-Protrusion])
PolyCyl(ID,(Length + 2*Protrusion),8);
}
}
The Smell of Molten Projects in the Morning 
If I remember correctly silicone needs air to cure. Is there a concern that the silicone in the center won’t cure, or is it narrow enough that it will cure from both ends into the middle?
They must cure from the ends, but they’re only 5 mm long: about the same size as the bead you’d lay down to seal flashing. After a few days I poked one with a pin and it put up a pretty stiff resistance, so I declared them cured and scraped ’em off the paper.
I considered adding some crosswise holes around the middle to provide air access and lock the silicone in place, but figured I’d try a simple cylinder first and see how it worked. Turns out that the stuff bonds so firmly to the ridges on the ID that it’s not going anywhere!
That’s why some is labeled RTV (Room Temperature Vulcanizing). Exposure to the atmosphere sets up a chain reaction curing the rubber. This is different than glue drying which is largely a solvent evaporation. Thus, this is why once a tube is cut open, either use it that day or expect it to be cured the next time you try. http://en.wikipedia.org/wiki/RTV_silicone
I’m using silicone caulk, not the two-part RTV stuff… but it’s the same notion and, for sure, every tube is a race between using it up and throwing it out.
I used two-part silicone, in a job a long time ago, and it’s great stuff. Around here, it’d age out much too quickly. Or maybe I should just use it in more projects? [grin]
Reading all your Thing-o-Matic solutions to problems, the saying “if you have a hammer, every problem tends to look like a nail” comes to mind…. :-)
Have you considered sticks of hotglue, cutting them in slices (small cylinders)? Might do the job too.
If you can’t find sticks of the right diameter, it should be straightforward to heat them and cast them into a piece of (silicone) tube. Let it cool and slice it with a sharp knife?
Not sure if the hotglue can stand being pricked dozens of time without losing its functionality. Another option might be to use the right diameter hose and fill it with silicone. Let cure, slice it…. and use it?
But then again, that wouldn’t involve quality shop time with a 3-axis CNC machine and wouldn’t help to convince the wife that all that fancy machinery in the basement and the time you spend with it can actually be useful. :-D
Sometimes there are not just technical constraints on how to go about solving a problem….
Absolutely!
Remember, however, that I originally got the TOM because I already made stuff that it can produce, so it’s just a faster and easier way to get what I need. Compare the machined and printed versions of the APRS interface cases…
I hadn’t thought of hot melt glue, but it’s much stiffer than silicone rubber. I couldn’t poke a pin more than about a millimeter into a stick, which isn’t far enough to have it stay put. Conversely, pins slide smoothly into silicone rubber and it grips them firmly.
That’s not a problem: even by the most conservative guesstimate, I’ve paid for the shop many times over just in avoided repair costs! Besides, this way Mary always knows exactly where I am and what I’m doing… [grin]
And actually, going back to my earlier comment, here is a better explanation of why RTV Silicone is a good choice in this application and Hot Glue is not. http://en.wikipedia.org/wiki/Vulcanization
Basically, Hot glue is http://en.wikipedia.org/wiki/Thermoplastic process, while RTV Silicone is vulcanization (hence the name). The bonds formed between the long polymer chains is what allows a needle to go in multiple times and still retain the “friction” from the shape of the Silicone.
Squirt silicone into a large diameter drinking straw to fill it. Allow to cure. Slice into chunks.
The “squirt” part would be tricky, as it’s really viscous. I could see pushing maybe 20 mm or so, then it’d ooze out around the nozzle.
Ah! A 3D-printed fitting between tube and straw! I get it! [grin]