Living Hinges: An exploration in acrylic and plywood


#1

Last year, after I finished what I could designing the matchbox, I was thinking about another project that would be practical and showcase the laser’s precision. I have this roll stamp case on my desk and while I don’t use stamps too often, the case does make a difference for keeping the stamps from getting messed up.

I had several different ideas and iterations, but never came up with a suitable design. I had to wait until I got the material to do this project. Since the case would be small, it needed not to be bulky or from thick stuff. I had thought about two boxes that fit inside each other. That would work very well and would be easy to do, but I thought that it needed to be slim and light as a desk or drawer item that didn’t look clunky.

The Inkscape extension elliptical box generator gave me some ideas and @m_raynsford’s flex box had potential. The 1/8" acrylic is pretty thin, so the case would be small. So how small could I make an acrylic round box or a flex box? What is the tradeoff between flexibility around a small radius and strength to hold up in the tabs and in the many kerfs.

Here are some of the acrylic tests:

Pretty tight radius. Not strong enough. I think that was around a millimeter in between kerfs.

A little wider in the kerf for strength but not flexible enough to make an inch and half box.

1.4mm spacing seemed to be about right for strength and curvature at least.

However, no matter what I tried, I couldn’t quite get the tradeoff correct without breaking the tabs or breaking the sheet.

The ends really looked nice. But the tabs kept breaking off. It’s hard to assemble a living hinge this small without breaking something. Also, I hadn’t tackled the removable lid yet.

I tried the OpenSCAD flex box a little. Varying the relief cuts. This was a promising direction, but I couldn’t quite see how I would make an easily removable lid. More tests.

Kerfs to wide apart to make the bend. Also, didn’t help to have pre-scored acrylic. Just using up the scraps.

The OpenSCAD file works great. With a little fiddling, I am sure I would be able to use the 1/8" proofgrade acrylic to make a box. Just wouldn’t work for a stamp box.

So back to wood and using scraps of Proofgrade walnut (and some scraps of maple)

Here is the detritus.

This is a cherry box that worked out well when I was testing the elliptical box maker, but I had to think about a hinge and a closer. Additional hardware was something I wanted to avoid. I was determined to make a design that didn’t require anything beyond assembly.

So using the the design with the 120 degree angle lid, I thought about putting matching slots so that the top would slide into the bottom in matching engraved slots. That took a lot of tests to get the engraving depth correct and exactly 1/2 (as well as I could) on the top and bottom side pieces. It also took a little mental gymnastics to think about orientation and rotation with the engraving slots needing to come together. Easy enough in 3D land, but I’m 2D Inkscape. So you have to match and flip the top and make it as symmetric as possible.

In the meanwhile, I wrestled with the round sheets that would wrap the box. Once again, adjusting for flexibility and strength. Those tabs are pretty small. They kept popping off and the sheets kept breaking. It’s pretty tough to get the bigger piece all the way around. Couple that with making sure I had to orient the sides correctly. It holds together and assembles without breaking IF, and ONLY IF you get it right the first time. I kept having to take it apart and reassemble because I didn’t get the matches and sides right. More room for errors and damage the more you take it apart and reassemble.

Getting closer, but hard not to break the sheets or the tabs.

In this picture is a slot that I hand carved because I forgot to set it for 3 passes and instead it did one. Not deep enough. Arg! Note the tabs are a bit mangled. This walnut plywood seems to have an MDF core. It’s very consistent, but doesn’t hold small tabs like this well unless you are very careful. Which is hard because you really have to wrestle with the covering to make it go all the way around.

Getting closer. Challenging to tweak the matching engraves and cuts to perfectly go together like an inlay.

Always something wrong. Still can’t get all the parts together without breaking something or having the slot off just a little.

These weren’t too bad, but not perfect.

So I gave up on the walnut for the time being. The tabs are just a little too small for this type of plywood to reliably assemble without breaking something. Once it goes together, it turns out pretty well, but I’ll have to rethink this because I’m so far unsuccessful in getting a perfect assembly.

Here is the piece in cherry. This side is good, but with a scratch from a blade where I was cleaning out the soot in the engrave and slipped.

Pulling it apart to fit without breaking it is hard. I keep yanking too hard and forcing it. The hinge will work but it has to be finessed in. Interesting enough. I’ve learned that the hinge can be more flexible with the top to the inside rather than the bottom. The wider kerf at the top allows for compression more.

So that’s it. I’ll try again to get it together without breaking, but for now I’ll leave it as is.


Living Hinge D&D Boxes
Lap and Dovetail joints
Weekly Highlights for the Week Ending April 15th, 2017
Proofgrade Plywood: Maple, Cherry, and Walnut Roll Stamp Box
Wedding present for my brother-in-laws wife-to-be
#2

Incredible! Thank you.


#3

What about some small magnets to hold your box together? Also you might be able to use a rod through the middle. :yum:


#4

Would getting the ply wet help with this at all? I don’t know how the filler will react but I’ve seen some pretty incredible bends accomplished with wet wood, might help


#5

Love this! Practical and pretty at the same time! :grinning:


#6

This has so much great information. I’ve been hoping to see someone work with living hinges, and this is exactly what I was looking for. Even though you had issues, seeing those issues and getting an idea of the tolerances and limitations helps more than just seeing a pretty, finished product. Thank you!


#7

One thing you can try with the acrylic is annealing it before you start flexing it. Pop it into a 170F oven for an hour and let cool.


#8

That is a great idea. I had forgotten about annealing. I would really like this in acrylic and will have to try that!


#9

This is a wonderful write up! It covers so many of the questions that I have about this sort of project (and it also affirms that I am not yet ready to attempt a design at this level! :wink: ). Thank you for sharing your experiments in such detail.


#10

Wow, what a great body of work! Thanks for going into it in such detail and for sharing the design with us. I really like the shape of it; I wonder if Delrin/acetal would give you the strength you need. It would look terrific in black or white as well!


#11

Trying to think about my acetal stock. I think I only have a 1/4" sheet of it since I was thinking of stamps and embosses. Will look through it again, but that’s a good idea.


#12

A tremendous amount of work!! Thanks for sharing your trials with us!
I don’t know if this would work, but I’ll shoot it out there anyway. I hope I can describe this so its understood correctly…
If you make two live hinges, attach one of them into half of the sides into a groove , make a groove that is above the first hinge and let the second hinge act as a sliding door so that you can replace the stamps when empty. So one of the live hinges slides over the other to create a door. This eliminates any metal hinge problems.
Does this make sense?


#13

Makes perfect sense. It would function as a roll top desk works or the cover over an organ keyboard. Might need to figure out a way to keep the smaller hinge at a curve by gluing it or shaping it.


#14

Exactly! Glad I was able to explain it OK.


#15

@marmak3261, I greatly admire your patience and dedication. And I’m so grateful for your willingness to share with all of us. You’re doing amazing work, and it benefits all of us here. Thank you so much!


#16

Do try it, it made a big difference on my stacking boxes.


#17

One thing that jumped out at me is that the cuts in the hinge are extending into the side tabs. I don’t see how this is needed and it would make the tabs much stronger.


#18

thanks for sharing your progress! What a lot of effort to get things just right! You are awesome


#19

Thanks for sharing all your work and learnings!

Some of those shots looks like a couple of spirographs got in a serious fight.


#20

This is the way the extension drew them so I left them. I can try it with the cuts not going into the tabs. Great suggestion.