# 3D Printed Knife Cover

Eating at a picnic spot or wilderness campsite is great, but prep can be tricky.
You still want decent kitchen utensils, but for knives there’s a downside to having something sharp and pointy rattling around in your belongings!

When you have a 3D printer, problems like this have an easy solution…

# Springless Reversible Mechanical Logic

Logic gates! The building blocks of computing as we know it. What if we ditch the electrons?

I want some mechanical logic for an upcoming project, so I did some research…

# Research

There are several examples of mechanical logic out there on the internet:

Keshav Saharia’s Lego LogicPush-pull, needs a spring return.
Randomwraith’s Lego LogicPush-pull with internal rotating elements, reversible.
Mechalogic’s Logic ElementsPush-pull, AND & NOT reversible, OR needs a spring return.
Xiaoji Chen’s Linkage ComputerRotating logic, reversible, some mechanical issues with parallelogram weakness.
Spillerrec’s Lego Logic GatesPush-pull, very compact, needs a spring return.
He identifies the problem with needing springs, but doesn’t have a fix. Mentions how AND/OR gates can be reversed to become OR/AND gates.
Zeroumus’s AND GatePush-pull, needs a spring return.
KNEX XOR GatePush-pull, needs a spring return.
Carolin Liebl and Lisa Hopf’s NAND GatePush-pull, reversible, complex.

There are many more ideas out there, even water pipe logic, and of course the early mechanical computers Digi-Comp I and Digi-Comp II.

For my project, I have the following requirements;

• Push-pull logic
• Output movement range must equal input movement range
• No springs/elastic bands, and no forces for inputs to overcome
• No gears!
• Easily chain-able, and simple enough to implement a score of them.

This disqualifies most of the designs I found, particularly the ‘no springs’ requirement.

# AND Gate

Mechalogic’s AND gate design uses an elastic band, but doesn’t appear to need it, so it serves as a good starting point:

It’s time consuming to tweak parameters in the physical world, so I wrote a small simulation: (click to see in action)

Success! Meets all the requirements, and only needs three parts.

The geometry is important – the ratio will determine the output’s position when only one of the inputs is asserted, according to;

• distance between the inputs (red),
• length of the input links (green),
• length of the output link (teal)
• input displacement

Now, can an OR gate be designed with the same success?

# OR Gate

Most OR gates look like this; the output (on the left of this picture) has a flat plate, and either input will push the flat forward. Unfortunately the design needs that elastic band to return the output to zero.

Inspired by some earlier attempts to couple an AND and a backwards OR gate together, I tried flipping the AND gate around; (click to see in action)

Success! Oddly enough, an OR gate is just an AND gate connected up backwards.
Even the linkage lengths are the same!
Best of all, the OR gate is reversible like the AND gate – no springs and no force required.

# AND Then?

As the graphs show, output displacement equals input displacement, so these designs should be chainable. Let’s have a look; (click to see in action)

Success!

Now I just have to build something complex with them…

All the simulations are viewable here.
The code they’re written with is available on GitHub;
https://github.com/MrTrick/logic_gates

# Finished: Display Case Doors

IKEA, your ‘professional assembly service’ sucks. My grandmother’s display case doors have scraped since the day it was installed, and the repeated twisting caused the glue to fail on one of them.

• Glued back together the broken door.
• Planed a few millimetres off the bottom of both doors.
• Smooth, silent doors that no longer stick.
• Happy grandmother!

# Finished: Bubble Wrap Dispenser

This one slipped through the cracks – a Bubble Wrap Dispenser!

Mostly pretty simple to make;

• The box is a gift box from the \$2 shop, with a slot cut in the lid and fancy tape applied.
• The logos are some quick work in Inkscape, printed and cut from glossy paper.
• The bubble wrap…

…was perforated on the laser cutter! I made a dashed line file, and cut the ~5m length of bubble wrap with littls perforations – just pull and tear, and you get a square of bubble wrap.
Worked great, and was really well received. ðŸ™‚

# Finished: Garden Box Cat-Proofing

Our cats love munching on greenery. OUR greenery!
This should keep them away, but still allow the sun and rain in.

Time will tell…

# Neuschwanstein Castle Pop-up

Just finished putting making a nice birthday card for my grandmother.

Not sure if I’m willing to deface it with my horrible but well-wishing handwriting, though! Maybe on a post-it note?