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.