3d Printed Door Handle Extension in Fusion 360

An elderly neighbor couple has venetian blinds covering their glass door. The lever handle of the lockset collides with the blinds making it difficult for them to open the door. After watching them spend two years trying to find a solution from big-box retailers and handymen with no success, I got an opportunity to do a quick design in Fusion 360 and test whether a 3d printed extension tube was durable enough for actual daily use.

Step 1: Taking Measurements

I had thought about a 3d printed solution before, but I took no action as they searched for a pre-made solution. Then one day they asked me to change the batteries in the electronic portion of the lock. While I was doing that I also took off the lever and grabbed some quick measurements with a cheap ruler they had in the house. I used a sharpie to jot down the dimensions on a piece of floral notepad.

I went back to the shop, opened up Fusion 360, and designed a shortened version of the extension to test the basic fit. I wasn't sure the "free advertisement" ruler would give me accurate dimensions and I worried about the tolerances for real world use. But the design seemed simple enough. It's just a sleeve with matching male and female ends. Still, I hate to waste time and plastic printing a full sized object when I can print 10% of it to test the fit of critical parts.

The next time I visited the neighbors, I took the test print of the extension and my calipers to get some more accurate measurements. Surprisingly, the dimensions from the rough test print were "close enough" to work. A little loose in some places, a little tight in others, but not too bad.

Even though it worked okay, I still took new measurements with the calipers and these definitely helped me dial in the fit even more. A millimeter isn't much, but it makes the difference between a snug fit and a sloppy fit, so using calipers was a good idea.

Additionally, I had not noticed some subtle sloping and fillets. I had only taken one measurement of each feature "assuming" it would be consistent on all areas. This time I took more extensive measurements, not just one, and made a note of where the shapes changed. So the partial print wasn't perfect, but using it to test my measurements and design really helped get the fit correct before I wasted a lot of plastic and time.

I used the new, more accurate dimensions and printed out a full version of the tube. There are still some spots that look odd to me, like the location of the set-screw hole right at the end of the raised ridge, but the extension works and I stopped designing.

Step 2: Design, Print, Test

The design process was simple - measure the existing shape and add a longer tube in the middle. The Fusion 360 sketch is really just a set of concentric circles with a few lines for the raised ridges.

I drew the sketch to exactly match the dimensions I had measured. I extruded the circles, moved the extruded bodies so they stacked correctly, then extruded the ridges/slots for joining/cutting , then cut the holes for the set-screws and combined all the parts into a single body. It was a few minutes work at most.

But the lockset is made out of metal. So when I started looking at the dimensions I realized the 1mm thick walls on the female end would not be strong enough on a plastic 3d print. I thickened walls wherever I could, about 300% all around. Since Fusion is timeline based, all I had to do was update the original sketch and the 3d objects updated automatically.

The thickening does give a slightly bulging look to the print, but it also makes it look stronger - certainly good enough to use. I even thought about adding a bell or flare to the thin walled area of the female socket, or perhaps using an ovoid shape. But I didn't because I didn't want to force an aesthetic change onto the elderly couple.

When I printed the extension I upped the number of walls, used every interlocking and overlapping slicer setting I know about to increase layer adhesion, and printed with a high infill percentage.

Then I negated all the work by becoming impatient and printing quickly with thick layers. I really expected to be re-working the design and printing several more iterations and thought I would have several more chances to dial in the print for maximum strength. However, this first full test print turned out to be the "final" print.

Now I have a well measured object with a high percentage 3d mesh infill and overlapped walls printed with basic low-cost PLA with layers that barely adhere. You can see the ridges on the walls and the speed was set so high you can sense the "squaring" of the curves. That's fine for a first test fitting but not the final print.

Usually, I can overcome cracking issues by correctly orienting the print on the bed. But with this project there didn't appear to be any good, or best way, to orient the print. There are forces acting to split the body on its longest axis, to sheer the tube around the circumference and even pulling straight up between the layers. It was going to be weak in some way no matter what, so I just printed it vertically.

I took the extension tube back to the neighbors and installed it. The tube worked great but I know it has some problems that still need to be solved.

Step 3: Conclusions

Both the design and the printed object were successful, but only for a limited test. I only left the extension on the door for a short while. The elderly neighbors were kind enough to let me use their front door as a test case, but they weren't ready to commit to an extended research program.

Having me take apart their door lock several times in one week was enough for the neighbors. I was told not to do any more designs or tests or improvements. See, what I think of as "The iterative design process" the neighbors think of as "He doesn't really know what he's doing." There's not much chance that I will get to test the two to five more prototypes that I had planned.

The quickly printed, cheap PLA extension tube did survive the summer with no failure or even minor cracking, so I deem that a success. However, I took it off before the winter set in. The neighbors' house is very susceptible to both moisture and frost movement. That door in particularly is prone to binding due to heave and wracking. This winter has been bad for that, with a hard kick and the occasional shoulder needed to open the door during single digit temperatures.

I'm glad I took the extension tube off because it would have never survived the first cold snap. But the blinds stay open most of the winter for more sunlight, so having the extension installed is also not as important. Besides, I think they have decided to buy that pre-made metal extension instead.

With simultaneous forces applied in tension, twisting and bending I worried about the extension snapping off. The length of the tube increase those forces. It was fine in summer, but in winter, when two hands and full body weight is required, the tube would have failed.

I knew this when I designed it, and even posted in a forum asking for tips on increasing the strength. Several people had wonderful suggestions - everything from wrapping it and resin coating it, to metal rod or sleeve inserts, to flooding the infill regions with resin. Machining or casting a version in metal was also suggested. Of course the best solution is probably to purchase the metal version that's already being manufactured.

In the end, I consider this project a success. It isn't ready for commercial distribution, but it worked as intended and it solved a real life problem that numerous others had failed to solve. And the process led me (thanks to other community members) to an even better solution. Plus, I got some milk and cookies for my work... that makes any project a winner.