Wednesday, October 25, 2017

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.

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