Replacement base for a smoke alarm

A smoke alarm in our house is held onto its base by two thin tabs of plastic that eventually broke. I found a design on Thingiverse by buwprinter (from Germany). It was obviously not made for my model - there are zillions out there - but I thought it might work. It was provided as an STL file, not a modifiable design. MatterControl has a scale feature, but it's an overall resize not intended for specific accurate needs. So I had to learn a couple of things in order to resize it accurately in Fusion 360:

• How to import a file.
• How to scale a mesh design.

The overall shape and appearance of the base is not that important. What matters is the spacing of the locking tabs that hold the alarm in place. I measured that carefully with calipers and then used the scale ratio feature and hoped for the best.

Also, this was the first time I was switching my printer from a high-temperature plastic (PETG) to a lower temp (PLA). I had heard that this could be an issue, so recently I bought some cleaning filament. It's an odd size, so you don't feed it through with the motor, you push it by hand. I was surprised how much I had to push through before the result was fully clear with no trace of green from the PETG.

I noticed that the PLA was not extruding straight out but rather curling up as if it were sticking on one side. I had to scrape the outsize of the nozzle to get rid of some scorched debris, and also tried some advice about pushing and pulling filament getting out a few times until getting a nice clean "cone" on the end. Seems to be printing OK now.

The diameter of the locking tabs came out perfect, but each tab was a bit too long for the slots. I ground off about 1/8" from each with my Dremel tool and then it locked on just fine. Here's the final result:

Jack-o-lanterns

Just for fun since I have orange and green PLA on hand. An LED tea light fits inside. I downloaded this from Thingiverse, provided by mb20music.

Lamp repair - again - with metal and PET-G

There was just too much stress on the plastic threaded tube, and the beefier version broke in the same place again. That vertical tube needs to be metal, so I shopped on line and found some 6" long nipples of the same diameter and thread as the original one. That meant that the "base" could be simplified. I decided to stop using a captive metal nut but to build threads right into it. That meant no more support to trim out!

About that time I saw an ad for PET-G filament, describing it as just a tough as ABS but without the fume problem. It is translucent, and comes in some nice rich colors. I ordered some red and green, thinking I might make some Christmas ornaments. PET-G requires hotter extruder and bed temperatures, and writers said it can be finicky.

I read up on how to reduce stringing and - I guess you could call it "globbing" - where oozing plastic goes where it's not supposed to. That led me to reading about calibrating my printer's feed speed. Huh? I didn't know I needed to do that. There's a process for marking a measured length of filament and telling the software to extrude that length, and then checking the accuracy. Mine was off quite a bit, feeding 92.5mm when 100mm was called for. I'm not sure what effects that may have had on my printing so far, but I went through the steps and corrected it through a steps-per-mm setting in the firmware. Test prints gave good results with the PET-G.

The part wasn't quite working, though. There was no good way to keep the top wedge in place in the tube while turning the screw to pull it down. Back to the electronic drawing board. I realized I needed a way for the base to keep the side wedges in place, and for them in turn to keep the top wedge in place. For that to work and still allow the side wedges to slide out and up/down, I designed a tab-and-slot arrangement:

Two wedges sit on the base with the tabs in the slots, free to move but not rotate. Then the top wedge fits between the side wedges, free to move down on the screw but not free to rotate. As long as I hold the base in place and tighten the screw, it should all stay aligned as it expands.

Only problem is, when I created threads of the right size with Fusion 360, the part was too tight on the screw. I fought this quite a while before figuring out I could expand the threads slightly. With the help of a video from Autodesk or someone, I learned to push back three parts of the four faces that make up threads. A little trial end error led me to expand by 0.6mm, which makes for a smooth-turning thread with no slop.

Here is the final assembly ready to insert into the lamp tube. The "top wedge" on the left side is threaded, and the "base" on the right is not, it sits against the bottom of the tube and the nipple pulls down through it. The wedges sit loosely between the two. The nut on the right is just to hold it all together temporarily. It worked great. Holding the base stationary and turning the nipple (with two nuts locked on the threads) locked it in place with no trouble at all. Then I removed the nuts, put the big lamp base and washer and nuts on the nipple, and tightened it all as much as I dared. It feels very secure.

I initially printed some of the parts at 99% infill to make them as tough as possible. But I needed to do a couple trial prints, and went to 10% for speed. Well, the PET-G is *very* tough. I bent and pushed on it and it's quite rigid. So I used the final test print at 10%.

Another great thing about PET-G: if I let it cool to room temperature, it self-detatches from the bed! It just pops right off. PLA never does that. Yet during the print it sticks really well even with a light, leftover amount of hairspray. No curling, no coming loose. Really nice.

So I've put the lamp back in service again - hopefully for the last time.

Second try at the pole lamp connector

Just as with software development, I can't always anticipate everything and turn out a perfect design the first time. Fortunately the low cost of 3D printing materials make it feasible to try several versions as I refine a design. I'll be honest on this blog about what works and what doesn't.

The first version of this repair insert didn't work because there was no good way to fasten the PVC pipe into the pole. Glue didn't set up around the shims. Because the first inch or so at the bottom of the pole is a smaller diameter than the rest, I can't just insert a solid pipe or something - anything that can get past that first constriction will slop around above it.

I was working on ideas based on a "toggle bolt" concept, when I found on Thingiverse someone's design for an expanding part kind of like what bicycle handlebars sometimes use to connect to their stem: wedges that move outward as a screw pulls them together. One problem with that idea is that it goes off center as it expands, which would not be good for a pole that needs to stand up straight. Also the center screw needs to be hollow to allow the lamp cord to pass through. So I got to work on my own design.

In this pic the black part is what sits at the bottom of the pole. I added a flange below the pole, sitting on the top of the base, to give it some more stability, and since that's going to be visible I printed it in black. It still has a place to hold the nut for the metal nipple that extends into the base. That's what takes a lot of the stress, so I didn't want to make that out of PLA, I'll continue to use the original metal nipple. Now the stem that goes upward is threaded so it can engage the wedge-shaped nut. As that is tightened, it will pull the two-sided wedge down between the two loose wedges, forcing them apart, gripping the inside of the tube.

The threads on the two plastic parts were quite tight and needed some cleaning out with the metal nipple and a nut, and soap to smooth the way. That's OK, I don't want it to slop around.

Not shown is a slot (kerf) in the bottom of the flange in case I needed to turn it with a tool, but I never used it.

Inserting the wedge into the pole.

Slipping the loose wedges in at the same time. I needed to expand the wedge a little at a time as it went in, because there was no way to hold the main wedge/nut once it was in. Carefully adding friction  seemed to work.

Unfortunately after I added the base (which is about 5 pounds) and stood it up, it soon broke as I tried to move it around on the floor. Back to the drawing board! The design seemed sound, but the parts must have had some weak spots.

The threaded tube broke right above where it connects to the main insert above the nut. I identified two aspects that I could improve:

1. Looking at it through a magnifying glass, I realized that the threaded tube was infilled, i.e. not solid plastic. That's how most 3D parts are printed by default, and I had set it for 60% infill (or 40% empty space). For the next iteration I made the whole black part 100% solid. I also added a chamfer around the base, to give it a little buttress in case it wanted to flex. That meant the side wedges needed a little more clearance, so I expanded their center cutout a little bit.

2. I speculated that the wedges were not seated down on the top of the black cylinder. If so, that would allow the leverage of the pole to flex the threaded tube from side to side. If the wedges were to seat down firmly on the cylinder, it should be rigid and resist that bending moment. If the point of the wedge-nut bottomed out before the wedges were tight, it would not be as strong as it could be. So I truncated the point of the wedge - it's noticeably shorter and rounder in this pic. By ensuring it would force the wedges outward, and by starting with it screwed down a little further, I hoped to ensure they would seat all the way down.

That all went really well. I inserted and tightened it by hand as much as I possibly could. It seemed to get really tight and never approached stripping the plastic threads - they're quite long and tight, so that's not really much of a problem.

Here's the repaired lamp standing upright. It wobbles just a little bit - that's nearly six feet of leverage on the bottom joint. I speculate that if someone were to drag it sideways by grabbing the top of the pole, it could still break the threaded part - it is hollow after all, to allow for the cord. But as long as we lift it up vertically if we need to move it, I think it will be OK.