Broken by Design was my second choice title for this instructable. Garmin has managed to design a quarter turn attachment, which simply does not stand up to the rigours of normal use. If you sniff around on the net, you will quickly find out that a) this problem is old, b) common, and c) Garmin shows no sign of dealing with it. I suspect they couldn't care less, since the tab probably breaks only after the end of the guarantee period.
I will not earn any points for originality or novelty for this instructable. There are already a couple of video instructions out there. I could not find any instructable, however, so here we go.
Step 1: Buy a Repair Kit
One of the nice things of being a late adopter, is that the early adopters will have found simple solutions before you are hit by one of the common problems. In the case of the broken Garmin quarter-turn attachement tabs, there are at least two third party repair kits available:
The aluminium “Dog Ears” used in this instructable, and the somewhat less imaginatively named, plastic tab repair kit from RaceWare. I didn't know about the kit from RaceWare until after I had ordered my dog ears, so no fancy thinking went into my selection.
Step 2: Remove Tab Remains
Since there is no room for any tab remains inside the aluminium cap with the new taps, the remains will have to be removed. I suppose you can use whatever tool you feel comfortable with. I started out with a sharp hobby knife, but found the plastic a bit resilient. I therefore continued with a hand-held milling tool, but switched to a file for the end game, for added control.
Step 3: Optional: Tab Tuning
I tried to attach the replacement tabs thing to one of my Garmin mounts, but found that I needed to apply quite some torque to initiate the locking quarter turn. I am not sure, but I attribute this to the squareness of the new tabs.
First I thought about bevels to the offending edges with my humble 3-axis manual milling equipment. After almost eyeballing myself to death in my efforts to navigate the milling drill into position, I wisened up and used a sharp knife instead. As suspected, the aluminium alloy was nothing out of the ordinary, and didn't put up much of a fight.
Step 4: Roughen the Surface
The surface to be glued is roughened by sanding.
On second thought, I maybe should have roughened the inside of the aluminium part too…
Step 5: Align and Make Screw Holes
The aluminium cup has a flat edge. It is there to help alignement, and this will work like a dream on Garmin 510:s. On the slightly bigger 800-models, the serial number is located farther away from the quarter turn attachement foot, which will make the eyeballing slightly more challenging.
I tried to fill the gap with a file with parallell sides to facilitate the sighting. Another possibility is to spy for angle errors, using the screw holes on the case.
The screws have sharp tips, and will dig into the plastic when some pressure is applied to the screwdriver. They don't go very deep into the plastic, and you run the risk of stripping the impromptu threads in the plastic, if the screws are tightened beyond 1/4 turn after the head has become flush with the top of the aluminium cup surface. The official repair instructions are very clear about this.
Step 6: Clean Surfaces
The screws are removed, and all parts to be glued are cleaned with ethanol, for instance.
Step 7: Glue
The official instructions recommend epoxy or Loctite 425. Since I happened to have about a kilogram of Nils Malmgrenlamination epoxy 275 at home, I picked that. The epoxy was mixed with some collidial silica to make it thick enough to stay in place.
As you can see, I was quite liberal along the outer edge. I think this part will contribute the most to the joint's strength. I also figured that glue added outside the edge of the flat “plateau”, is unlikely to spread to the beeper holes at the centre of the foot.
Excess glue was removed directly.
At the time of writing, the glue hadn't hardened yet, so I cannot tell you if the repair was a success or not. Stay tuned. A final step will be added after the first trip.
Step 8: Addendum
It seems that my Fingerspitzengefühl failed me slightly when I mixed the epoxy. Instead of curing to a hard, solid final state, the end result felt more like old bicycle brake pads. This doesn't seem to be a problem, though. I used the GPS today, and it worked OK.
Unlocking it from the support takes more of a twisting effort than the original foot, which may indicate that the notches in the new foot catch too well on the locking edges of the support. These edges may wear out with time, since the aluminium of the new foot is much harder. Replacing the support is much easier than replacing the foot, and is available in many designs and colours, so that's a compromise I can live with.