A simple clamp for holding a flag on a bike trailer, designed in CAD and printed on a 3D printer.
Step 1: The Problem
I like pulling the kids in a bike trailer, but the old trailer I bought didn't come with a flag. I'm sure you are an excellent driver, but have you seen how the rest of them drive? So I went to the local bike shop and bought the trailer flag they had.
The flag pole seated in a metal bracket, which seated on the axle between the hub and the frame of the trailer. The metal bracket lasted about two months before it got squished and bent and loose.
Step 2: The Solution
Attaching the flag onto the trailer frame doesn't have to be complicated, but there'a a rapid prototyping machine (a 3D printer) at work, and if you have to ask why, dude, you'll never understand.
The rapid prototyper we have is a Stratasys Prodigy Plus; an FDM 3D printer. The Prodigy Plus seems to have been discontinued, of the current lineup it's probably closest to the FDM 200mc.
The Prodigy Plus takes CAD models as input and prints the parts out of ABS plastic. The resolution is a bit better than 0.010 inches in all three dimensions, though structures of that size are very weak. It prints about 1 cubic inch per hour, depending on settings & geometry. Overhanging geometries are supported by printing a light-weight, water-soluble scaffolding (the prototyper's CAM software adds this automatically).
Step 3: The Design, Making of the CAD Model
The frame of the bike trailer is aluminum tubing, 1" od. It's straight where I want to put the clamp, which makes things easy.
The flag pole is some kind of plastic, .25" od.
I want a sheet of rubber wrapped around the trailer frame, to give the clamp something squishy and high-friction to clamp down on. The old dead innertubes you get for free from the bike shop are perfect for this. The one I had on hand measured about 0.075" for two thicknesses.
I drew this design in Solidworks (running in Windows XP, in a QEMU virtual machine on my Linux laptop). Anyone know of a good CAD program that runs on Linux? None of the ones I've tried compares to Solidworks.
The big hole is oversized by 0.075" to make room for the rubber.
The little flag-pole-hole if oversized by .060" (0.030" all around the flag pole) to make it easier to put the flag in and take it out. The flag pole will slide in freely and just rest there, held by nothing but gravity. When I need to stow the trailer (in the back of the family minivan, say), it pulls out to lay flat out of the way against the wall.
The four screw holes are 0.177", which Machinery's Handbook says is the proper free-fit clearance hole for #8 screws. I dont trust the FDM'ed ABS to hold threads, so I opted for through-holes and nylon locking nuts.
The .SLDPRT files are the Solidworks native files, analogous to source code, suitable for editing. The .STL files are simple geometric descriptions, analogous to object code.
Step 4: Printing!
The rapid prototyper comes with a CAM program called Insight. Insight takes .STL solid models for input and drives the 3d printer. Insight lets you orient the model, then slices it into horizontal slices 0.010" thick. Each slice is treated as two-dimensional. The Insight software generates toolpaths: low-level instructions instructing the printer how to realize the model. On each slice it first draws the perimeters as smooth curves, then rasters the interior in a cross-hatch pattern. Then it lowers the workpiece by 0.010 and prints the next layer.
I printed both halves of the clamp in a single job, by positioning them next to each other. Altogether it used 3.91 cubic inches of plastic and 0.56 cubic inches of support material. It took 4:30 (four and a half hours) to print. The folks who run the machine shop charge 6 $/in3, so this is a $27 clamp (plus a buck or two for the fasteners).
Insight automatically prints scaffolding to support overhanging structures. The scaffolding material is similar to caramelized sugar. It's supposed to dissolve in hot water in an ultrasonic cleaner, but both the ultrasonic cleaners here are broken right now. I ended up soaking the parts in boiling water, then scraping the support material off with a screwdriver. The support material came off pretty well except inside the flag-pole-hole, where a little stuck to the edges (not enough to interfere with the mating). The ABS took this abuse without problems.
Step 5: Putting It on the Trailer
- the two parts from the 3d printer
- some rubber (inner tube ftw!)
- #8-32 stainless hardware: 4x 2" button-head screws, 4x nylon locking nuts