What holds your phone in the car?
You know those cell phone holders that clip to your car’s vent? Well, I’m not a big fan of them and I have a strong feeling a mean pothole in the road could easily snap the vent blades. Afterall, I do have the tendency to drive my car to places it was not meant to go.
I really want a phone holder in my car and after searching, most of what is on the market hasn’t caught my attention. I guess you can say, I’m sort of bitter about that. Then one day I REALLY noticed this space as an opportunity to take matters into my own hands:
There is a mystery slot on my car’s dashboard! A friend pointed out that it seemed to fit a credit card perfectly, and it did!! That’s when we realized it is meant for garage door RFID cards. Well, I don’t have one and I’m not going to let this sit there collecting dust. Let’s put it to good use.
Designing the Mechanism
I want a create cellphone holder that will allow different size phones to fit, both horizontally and vertically. There must be clearance for the charging plug in both positions and both arms should move in synch. The holder must snap in place inside the slot to handle a 2 lb force from pulling it out so it doesn’t just slide off with phone’s weight when I accelerate.
Using springs would be fun, but what good is it to have a Stratasys Fortus 250mc printer if I’m not exploiting its capabilities? This thing will be completely 3D printed!
Let’s Get Started
For the first SOLIDWORKS concept, I’ll have two-piece assembly arms to stay seated in the slot. I am hoping their deflection will help keep the phone snug. A gear between the arms will drive them in sync. During this process, I kept being told that printing this small was just going to fail, time to prove them wrong! The backside is three credit cards thick with the phone holding geometry on one side:
- The sliding arms have an initial clearance of 0.005 into the slide in the slots, this may change later.
- The sliding arms have a slit to glue to the back snap in case pegs fail.
- The vertical back plate has the least layer adhesion due to print orientation, it must withstand speed bump shock.
- The gear has an initial clearance of 0.005 in all around the load bearing face.
- The gear will stay in place by a snap fit from the back with a 0.003 in clearance, requiring no glue.
- The two-piece arm assembly snaps around the slot to guide and withstand speed bumps taken at unsafe speeds.
- Rack and pinion teeth clearance set to 0.005 in. I have my doubts about this geometry’s resolution.
I took the liberty to cut some pretty gears on its back for a little panache, plus model material IS a bit pricey. The piece will be printed with ABS-P430 at 0.010in layer height. Off to the printer!!
Partial Success!!! The arms and bottom lip hold the phone snug while driving but the design proved inferior to the forces exerted by one speed bump. Probably because I decided to skip the glue to hold the sliding arm peg.
You can see the pin that broke off in the picture below.
Let’s Review Improvement Needs
- I forgot to take the print layer height into account when I set the minimum distance between guiding slots. If I want the printer to give me accurate geometry, every dimension should be a multiple of 0.01in from the bottom of the part. I had to do a little sanding for the top sliding slot for a good sliding fit.
- The bottom sliding arm (no sanding required) was a little wobbly, so I changed the clearance from 0.005in to 0.0025in for the next print.
- The sliding arm back pins were too weak for stress but great for positioning if only I had glued it… I exchanged the pins for robust squares and kept the sliding arm slits in case I do need glue.
- Even though the sliding arm worked well I added guide rails to the bottom and top of the main body to lessen stresses on the pin due to the gear pressure angles.
- The gear and back snap assembly actually works pretty well. No wobble, no glue, smooth motion. We have confirmed a Toolbox ANSI Inch Spur Gear with diametral pitch of 40 and 16 teeth will print and mesh perfectly with the equivalent Toolbox Rack Spur at 0.005in clearance.
- I do need to add locking tabs on the sides to keep the phone holder from sliding out from the dashboard slot, there is a new scratch to remind me of it.
In retrospect, the first design did not do much for “wanting a strong phone holder” but it did wonders for small part 3D Printing. The new revision will do the job. Back to the modeling board to click away by findings.
The Killer Outcome
Here is it is: The Car Phone Holder 2.0
It’s a mean, thick, orange machine. I actually kept the gear and back snap parts from the previous iteration. It survived the speed bump that killed version 1.0 and quite a bit more. You can see the locking tabs sticking out from the sides of the main body to keep it in place, sliding out is no longer a worry. The added top and bottom guides reduced overall flexure so I only need to pull one arm for both to open smoothly. I had to clean up some of the layer seams for smooth motion. This one, I do like.
The clearances I used for each of the contact points are as shown in the Section view below:
3D printing small parts ended up being a breeze. And since pictures are worth a thousand words here is a short video of this device and a link of the file in case you have a mysterious slot in your car as well: **link for Car phone holder**