How about a design challenge

Despite putting an entry into Ender5r’s awesome FreeCAD Challenge tread, I never got around to actually printing one. If you haven’t read that thread, you might want to scan it quickly. I was about to print a seat back bag hook yesterday while I had PETG loaded in my printer when I ran across another design that I really liked on Thingiverse. I decided to mash-up the elements I liked in that design with the idea from the FreeCAD challenge of incorporating a gap between the hooks to make it easy to grab the bag with a finger.

So here are the elements of this design challenge:

  • Use any parametric CAD application to design a seat back bag hook [LIST]
  • It should mount to a car's headrest post and hold a bag suspended behind the seat back. [LIST]
  • Optionally, it can be spun towards the front for holding a bag suspended over the head rest's seat.
  • The design should prioritize the ease of grabbing the bag with one hand. (See Ender5r's post here for an example.)
  • There should be no sharp corners or edges that could cause injury to a flailing hand.
  • When printed by FDM printer (you pick the model and slicer settings), it should be able to carry a 38 lb bag in a moving car without breaking or failing to hold the bag. (That weight came from the first Google result of a search for "average weight of a grocery bag") [/LIST]
  • Make the design parametric so that it accommodates the measured parameters from a variety of cars:
    • The caliper measurement of the headrest post diameter.
    • The measured distance from the post axis to the edge of the seat back.
    • Size of the finger gap (or other hand dimensions if your design accommodates one-handed operation without the use of a finger gap.)
    • Width of the bag handle.
    • Any other parameters needed to make your design fit different cars, people, and bags.
  • Imbue a measure of style, form, or flair into your design. [/LIST] Geit will hand over his winnings in the FreeCAD Challenge to the winner of this challenge, LOL.

    Of course I made my own design, but I will wait till tomorrow to share it. We wouldn’t want to taint the imaginations of any other participants, now would we.

    Some example measured dimensions if you want to test your design:

    • 2019 Subaru Outback [LIST]
    • headrest post diameter: 12.76 mm
    • post axis center to seat back edge: 58 mm
  • 2007 Honda Fit
    • headrest post diameter: 10.0 mm
    • post axis center to seat back edge: 54 mm
  • 2011 Toyota RAV4
    • headrest post diameter: 11.94 mm
    • post axis center to seat back edge: 62 mm
  • Finger width: 20 mm
  • Bag handle width (bunched up): 10 mm [/LIST]
  • Here is the design that I saw on Thingiverse that I mashed-up with the idea of having a finger gap: Towball Headrest Hook / Car Bag Hook / Seatback Trash by qurvento - Thingiverse

    Here’s the child the two ideas produce I think:

    [ATTACH=JSON]{“data-align”:“none”,“data-size”:“medium”,“data-attachmentid”:7496}[/ATTACH] [ATTACH=JSON]{“data-align”:“none”,“data-size”:“medium”,“data-attachmentid”:7497}[/ATTACH]
    [ATTACH=JSON]{“data-align”:“none”,“data-size”:“medium”,“data-attachmentid”:7495}[/ATTACH] [ATTACH=JSON]{“data-align”:“none”,“data-size”:“medium”,“data-attachmentid”:7494}[/ATTACH]

    The FreeCAD screenshot shows the parameters the model can accommodate:


    I took the time to make sure all the parameters work well. The buildplate_factor lets you decide how much flat surface there should be on the bottom of the model.

    Anyway, I’m printing them up and we will see how they do in practice.

    Nicely done my friend. It’s good to see designs inspire other designs. The one I designed kind of looks like legs with feet. Your looks a little like some other body parts :wink:

    Yeah, I was afraid that would be the first comment I would draw. ? LOL, oh well. But so far the design is working well in our cars.

    OpenSCAD–wow! Watched Dr.VAX’s latest video on OpenSCAD and really enjoyed getting to know it.

    I hadn’t done anything with it before Dr.VAX’s introduction, but it was surprisingly easy reproduce my seat back hook design in OpenSCAD. I do have a lot of coding background, but even given that I was surprised at how much of a pleasure OpenSCAD is.

    OpenSCAD does not have loft function, not a native one at least, but the hull function did the job just as nicely. I’ve attached both the FreeCAD and OpenSCAD source documents if anyone is curious to see them (both in the same zip).

    Thanks Thingiverse for being broken so that Dr.VAX had to teach us how to use OpenSCAD.

    [ATTACH=JSON]{“data-align”:“none”,“data-size”:“large”,“data-attachmentid”:7557}[/ATTACH] (129 KB)

    I, too, found openSCAD interesting. Like you, I have decades of programming experience. I was getting along really well with it until I found it doesn’t really do filleting. You can use the Minkowski function to carve fillets onto objects, but it’s tricky, and r-e-a-l-l-y slow. So, I stopped using OS.

    I agree it would be nice if fillet worked in FreeCAD as easily as it seems to work in Fusion 360. Then there’s OpenSCAD where it isn’t even a native function. But I like both of the tools enough that I’ll find ways to design in them without cutting my fingers on their printouts. There are multiple ways keep the sharp edges out of one’s designs.

    I hope to do some videos in the future about when to use a specific CAD program and how to combine them together. For example, it is painful to add text to a model in FreeCAD. So why not create the model in FreeCAD, save it as an STL file, and use TinkerCAD to add text. In my woodshop, I have both power and hand saws and use both. No reason in CAD we should limit ourselves to one CAD program.