3D Printed Safety Interlock for Model Rocketry

I have a four position launch controller and stand for launching model rockets, which I have written about here.  It’s been a few years since I’ve had an opportunity to use it, but it looks like I might get to have some launches again this year.  I’m hoping, anyway.  So I pulled it off the shelf to check it out.

The controller was designed to use a variety of UPS batteries, that is, sealed lead-acid units with quick-disconnect terminals.  I’ve been pulling those batteries out of UPS units in a used condition, which means they don’t have a great shelf life; as a consequence, I’ve been obliged to take spares with me to every launch.  When I saw this model by fdavies on Thingiverse, I had an idea… why not upgrade the controller to use 20V Max batteries from Black & Decker?  Of course, I remodeled fdavies’ design so I could print it the other way around and not need supports, and also to fit my project better.

I printed out the parts and made and installed the electrical connectors, cutting strips from the cheap case metal of old computers (mild steel, perfect for my purposes, and I have a nibbler tool and snips to cut it with) and using JB Weld to complete that part.  This last week I finally got around to installing it in the launch controller, and after changing the protective resistors to 1K Ohm units due to the increase in voltage I was ready to test it.

But, no joy.  It would show good continuity but would not fire the igniter.  It took me quite a bit of poking around with my meter to figure out that the safety interlock key wasn’t making consistent contact.

Here’s my solution:

The metal bits are the original safety key contacts; the key itself is cut from an old antenna wire guide I had lying around.  The key contacts are, as you can see, held by a single screw each; screwed into the original controller (as shown in the upper left of the picture below) they had worked pretty well, for a while.  Due to wear, they had begun moving around, twisting on the screw and making intermittent contact.

I made new leaves (blades? dunno) for the controller with a second screw to stabilize them, but I didn’t like that solution either as it was tricky to bend them just right so they would be close enough for the roughly 4mm diameter key to make contact, but far enough apart not to touch without it.  So then I designed a barrier setup as shown in the first picture (the red part).  Side walls to prevent twisting, and a slotted 3mm thick barrier between the blades to prevent them from touching.

That was cool and all, but then I got fancy and designed the faceplate shown in the first picture.  It involves a filament change in the middle of printing, which I had never done before, but I followed the directions and it sort of worked.

Sort of, because my Ender 5 Plus has a Micro Swiss Direct Drive Extruder installed; the 300mm unload and reload distances had never been changed, but 80 or 85mm is more correct.  The 300mm unload wasn’t an actual problem, just annoying; the 300mm load, on the other hand, chewed on the filament and pushed out a big blob from the nozzle.  I cleared it away (with the head in flight, so to speak) as much as I could but I got a blob right on the first letter of the message.  The faceplate on the left in the first pic is the result after I cleaned as much of the blob away with a knife after printing.

So I figured out how to do the M603 command to change the distances, choosing 80mm after measuring around 71 mm of manual retraction to clear the filament; for some reason it seems I need more like 85mm but 80 worked for my purposes this one time.  I also manually edited the gcode for the faceplate, after reslicing it and using the preview mode of Ultimaker Cura to show me the printing order.  The gcode file has comments where each phase of printing begins, and I noticed that the layer began with an outside perimeter followed by an inside perimeter.  Watching the preview I saw that the outside was the first two words, and then the inside was center of the keyhole.  So I moved the M600 reload command to just before that inside perimeter, figuring that if there was a blob I could just drill it out.

Anyway, the faceplate on the right is the unmodified result of those steps.  I think a thicker font would look better but this will do.

Now to put it all together…

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