Most families nowadays would have at least one member who wears spectacles. These correctional eye fixtures compensate for our short sight, long sight or other optical deficiencies by using lenses that bend the light in the required fashion.
The weak spots!
These things need to be light and strong. Real strong and flexible when they need to be worn by kids! So we had bought specs last year for my daughter but as luck would have it, she managed to have its leg broken at its weakest spot, the hinge where it gets attached to the main eye-piece holder.
None of the eye-wear stores here replace just the legs of course, which would have been neat! They require us to buy a new frame and maybe a new pair of lenses as well!
The design process
The one good thing about this whole covid quarantine episode was that it forced us to look for alternatives. So, we thought, could we 3D print a replacement frame entirely? Well, there were a few designs available on the internet for this but the one drawback was that I could not get the lens out of the frame without damaging one or both and it would require significant precision to engineer the 3d frame perfectly around the contours of the existing lens design. So that idea was dropped.
The next thing was to see if we could just replace the broken leg. Well, it turns out that the screws used in spectacles are super-tiny sub-millimeter types and the hinges used are super tiny as well! Well, the desktop FDM printer that we had (a Creality 10S Pro) was certainly not designed for that size or precision. So that would not work out either.
That’s when we realized that maybe we could build around the existing eye-frame and just enlarge the hinge to be large enough to take common M2x6mm screws that were easily available off Amazon!
Many design iterations later, (it was hard trying to get the the dimensions as small as possible, getting the print direction right, getting the layers to just hold themselves together and getting the material right), we reached a decent point.
The design is two parts. One part is a small hinge attachment that can wrap on top of the existing L curve on the main spectacle frame where the normal spectacle hinge would normally end-up. The other part is the spectacle leg itself. It was easy to customize the length of the stem as per the position of my daughter’s ear :)
The following is the render of the final design on FreeCAD:
So the prints that worked ok were based on white PETG filament, 0.20 mm layer height, 230° C extruder temp and 70° C heated-bed. Didn’t take more than 30 mins to print the entire set out. Although, did have to print them separately, with a 3 mm raft and with carefully selected supports to avoid stringing.
The nice thing about it is that, if a part breaks again (which it would, in the hands of a child), we can just print a new part to replace just that piece in a few minutes! :)
What could be better:
- Maybe the carbon-fiber filaments would fare better for this application? Haven’t tried them out yet.
- Better color, of course! Just used the one we had in stock. Plus, my daughter could still paint over it.
- Anneal it to make it set harder. Will experiment that. Have read that table salt stuffing+ kitchen oven works wonders with this.
Try printing them, or just be inspired :)
Please get the STL files from our Github page:
PS: Remember to flip them along the right axis to print the other pair!