My motivation was to have a quick alternative solution for the Google Expeditions project - where anywhere between 5 to 30 phones along with Google Cardboards are used in a Classroom to take kids on virtual field trips. The logistics of carrying and charging that many phones is harder than it might seem.
I started with a goal of designing a holder for 5 phones. My goals in this design were:
> keep the phones from banging into each other
> make it easy to insert and remove the phones
> have a method to keep the phones in place during transit
> expose the usb/power port for easy charging
> have a handle to carry the phones
> have room for air circulation to avoid over heating
> work with Nexus 5 phones to start (later Asus ZenPhone)
I originally started thinking of a design which would allow for expansion - allowing multiple holding racks to be connected into larger racks, but soon realized that I should focus on getting a solid design done and later figure out that feature.
Starting the Design
With the Nexus 5 phone dimensions taken - I created a simple rectangular box with 2mm of added material on all sides. I decided that the holder should allow for about one-third of the phone to stick out, so it is easy to grab to remove - so I reduced the height of the rectangle by 1/3 (about 25mm).
Having 5 of these alongside each other in my modeling tool (123D Design), I could clearly see that I should almost completely remove the inner walls between phones so that there was plenty of airflow and much less material needed in the print. I left about 5mm of separator wall on the bottom between each phone and about 10mm of separator at each end. This would form slots which each phone would easily slip in and out of.
The outer walls were still solid, and I wanted to give more airflow there and reduce the material needed. This is where I had my most difficult challenge in actual printing.
I created several "holes" through the outer walls, with rounded tops to allow for better printing without supports. When printed, the "stanchions" which formed between those holes kept printing too weak and breaking during prints.
This kept happening almost regardless of how thick I made them. I experimented several times, and lots of fails and broken 10-hour prints later, I discovered a hole design which didn't produce the problematic stanchions - triangular holes (see pictures later in post).
With the 5 holders together now in what looked like a "rack", there were 3 important adjustments.
Avoiding Errant Button Pressing: The side of the phones contain power and/or volume buttons, and with the phone on its side, I did not want the buttons being pressed by the weight against the holder. To avoid this, I cut holes along the bottom of every phone slot, leaving plenty of material to hold the phone, but with enough of a hole to allow the buttons to never hit anything and press by mistake.
Room for Charging Cable: With the phone on its side, the charging port at the end needed to be accessible, so I cut simple grooves about 12mm deep into both sides, so the phone charging cable could be inserted from either end.
Handle and Phone "Seatbelt" : I knew I'd need a way to carry this thing - a handle of sorts - and a way to secure the phones in their slots so they wouldn't dump out if it tipped over. I created simple handles on each side which would be used for either an elastic strap (seatbelt) and for a Grip that I would design.
The Grip (Handle):
With the handle holds on either side of the phone rack, I designed a simple handle that would be printed on it's side so that it had better strength (things printed along the x and y axis of the printer are stronger than things printed in layers up the z-axis). It needed two hooks to grab onto the handle holds and it needed both a cross bar to hold the phones and a grip for a person to carry it by. I decided to use the slight flexibility in the final PLA plastic part to my favor - so that the handle has to be stretched slightly to fit it into the holds, which would avoid it coming loose while carrying.
From 5 to 10 phones - and Nexus 5 to Asus ZenPhone
With the 5 phone holder finally printing successfully, I decided (with a strong push from my friend who is running the Google Expeditions Pioneer program) to try a 10-phone holder. It was not too hard to cut up my 5-phone model to extract and then duplicate one of the inside phone slots another 5 times. The harder part was stretching the model to hold a bigger phone - the Asus ZenPhone.
Once I had the new measurements, I realized that it would be easier to start the model again with a single phone slot and then duplicate it 9 more times. I eventually got this right - and while it's too hard to describe in words, perhaps I'll make a video of that process eventually. The challenge was more one of measurement discipline than the rote work of adjusting the design. It took 3 tries to get right, as I first forgot to adjust the length of the phone slot, then I forgot to adjust the height of the handle to accept a wider phone.
I finally got this right, and after a 13 hour, 50 minute print on my Lulzbot TAZ4, I had a great 5 phone holder. I simultaneously printed the handle on my Polar3D - and the two parts fit happily ever after :)
I got a chance to test my phone holder in the field at the edcampNJ event on November 21, where we demonstrated Google Expeditions for about 100 teachers! The phone holders (both the 5 and 10 phone versions) worked flawlessley.
The one weakness I'm seeing in this design are the separator parts between phones where the charging cables come in - they are weak and starting to give. I expect they will all eventually break off, but I also don't think they are completely necessary to the design. Always looking to improve, I'll tweak that in the next version.