One of the weaknesses of FDM printers (the filament extruding type that most of us use) is that the objects they create are not isotropic; they are not equally strong in all directions. The bonds between the individual extruded filaments are not as strong as the filaments themselves. In the case of our beloved Open Alpha cups, the bosses that the baffle screws thread into are created by essentially layering concentric rings of filament on top of each other (when the cups are printed flat). Since the bond between these layers of rings are weaker than the rings themselves, they are prone to twisting off when the baffle screws are tightened.
I generally have pretty good finesse when it comes to fasteners, but after breaking the bosses off two sets of cups, I set out to find a solution. One possibility would be to print the cups vertically, but that creates a whole ‘nother set of headaches in getting a smooth finish on the outside and on the sealing surface. Another solution would be to print the cups using an SLA printer, which creates an isotropic print. Unfortunately, this is significantly more expensive (in my case 5x more).
For those looking for a cheap, relatively foolproof way of not having the bosses break off the cups, here is what worked for me:
1) I modified the 3D models for the cup to make them significantly stronger (and easier to seal). STL files are linked below.
2) I had these files printed in Z-HIPS, a material that has significantly higher tensile strength that ABS or Z-ABS. Z-HIPS is for Zortrax printers, but I’m sure there are other similar materials available for other models. I also had them printed at 100 microns resolution so the fine details around the bosses would be better formed.
3) I took better care to make sure the bosses weren’t damaged during finishing. (Acetone doesn’t work on Z-HIPS, but I think on the previous cups I had probably deformed the bosses a little during the acetone treatment.)
4) I purchased a #54 drill bit (1.4mm) and a pin vise to ream out the screw holes and make sure they were the right size for the Fostex baffle screws to thread into. (The pin vise can also be used with a 1mm bit to ream out the bass vent holes to a perfect, consistent size on both cups.)
5) I made sure the acoustic foam lining the cups didn't go so far up the side of the cup that it required additional force to compress it.
6) Before assembling the parts, I made sure there were no burrs, debris, or stray filaments on top of the bosses, in the screw holes of the cups, or in the screw holes of the baffle.
Maybe all of these measures were not necessary; perhaps if I had been more careful when finishing the parts and preparing them for assembly, they wouldn’t have broken. However, I did not want to suffer the crushing disappointment of having the cups come apart during final assembly again.
(Almost) Foolproof Cups
Another thing that scares me about the original design of the cups is how narrow the sealing surface gets around the bosses. No doubt this is perfectly fine for a production headphone that is being put together by an experienced assembler, sealed with silicone, and rarely, if ever, being opened again. However, as MrSpeakers noted in his original instructions, many of us will want to use a thin closed cell gasket to seal between the cups and baffle, allowing for easier disassembly for subsequent internal tuning. For those of us cutting that gasket out by hand, it seemed devilishly difficult to make sure it sealed in those narrow areas around the bosses.
For the following STL files, I hope this will make both problems considerably more foolproof. I reinforced the area around the bosses so that 1) the sealing surface is considerably wider, without interfering with the baffle, and 2) the bosses are tied in to the side of the cup, so they cannot twist off so easily. (Detail images of the changes are below.)
The files above are for the original Hirose jack. I also created a set of these modified for the 2.5mm panel mount jacks from Vetco. Check the dimensions before ordering, but I’m pretty sure they would also work for the 3.5mm jacks from Vetco or Parts Express.
I stink at soldering and I hate doing it, so I chose these jacks primarily because the nut goes on the outside of the cup. Once they are soldered to the driver, you can easily disassemble the headphones (even completely remove the cups) without additional soldering. It also gives you a number of different options for pre-made cabling: Anything that works for Oppo PM-1/2, Nighthawks, or HD700s will work with these jacks. There is even a $15 replacement cable for some of the old entry level Senns that works great for a start.
(Note that the jack is not recessed, just the nut that holds it on. It’s positioned at the outside edge of the cup so that a narrow or stepped plug is not required.)
More Detail
For those who may be thinking that this wider sealing surface and reinforced boss must cause some problem with the baffle, here are some close-ups of the shapes I added (shown in red), along with a transparent cut-away of the baffle. As you can see it does not interfere with the fit of the baffle. (And yes, It would have been better to put fillets in there to make it even stronger, but I just didn’t have time for that. Did I mention that I’m doing this in Tinkercad, a free online modeling tool designed for kids?)
