*video of it working in step 7

This is my final project for ME 130 at UC Berkeley. I worked on it with a group, and although my focus was on designing the housing I helped with other things too.

Usually a busy person’s morning routine involves cracking a few eggs for breakfast. Sometimes, one might even use many more eggs for baking too. However, cooking with eggs can easily get messy. For these reasons, we created an egg cracker that will allow someone to quickly and cleanly crack multiple eggs for whatever they may be cooking. By creating a motorized egg cracker we provide a way for someone to easily load eggs into the machine and have the contents be dropped into a bowl for easy use. This device would be very helpful for people who lack fine motor skills due to a physical or mental disability.

Step 1: Design

Our initial idea was to use gravity to our advantage by dropping the egg on a split blade and then pulling the two halves apart. We prototyped this with a metal blade but the egg would rotate while falling, making it really hard to get a good drop. We deemed this method to unreliable and moved on.

We thought about some ways to drive a blade into the egg, and then another link to separate the blades. This would mean another motor, and the headache of linking them together.

Our final mechanism was heavily inspired by this design. The design seems to be inspired by the motion our hands go though when we manually crack an egg. The benefit of this mechanism is using one motor to both drive the egg into the blade and split the shell to dispense the the egg yolk and whites. The plunger applies a distributed force on one side of the egg as it drives it into the blades while the two arms pull the shells apart.

Step 2: Math (Force Analysis)

The purpose of this math is to determine the max dyad length we can use before stalling the motor. The gist is to use force analysis and the max torque output of the motor.

Assumptions

About 5lbs (2.27kg)** of force required to crack an egg
Input angle of the dyad link = 15 degrees
Peak torque delivered through motor = 20 kg*mm
Peak torque delivered through motor = 20 kg*mm

F₃₄ = F_torque

Math:

F_torque = 2.27 kg (49.07mm/12.7mm) = 8.77 kgF
Maximum dyad link length = Peak torque/ F_torque = 2.28 mm

Step 3: Prototype

We first made a prototype of the mechanism links that was driven by hand. This was to ensure we had the correct lengths, and that our method of using nuts and bolts would work. The prototype actually did NOT work that well until we made a few revisions to the blade geometry.

Step 4: Modify Design

Once we had our core mechanism working, we started the design of the housing. This was a ridiculously complicated CAD model that took days spread out over weeks to complete, with multiple versions. The idea was to separate the electronics from the cracking mechanism as cleanly as possible. When I was happy with the design I moved on to fabrication, which took a long time as well. There were quite a few times where I had to go back and tweak the CAD and re-print parts in order to get it working.

Step 5: Fabricate

The 3d printing is rather easy; print the parts and clean them up with pliers, flush cutters, sandpaper, etc.

I printed most of the parts on my 3d printer at home. The main box didn't fit on my print bed so I sent it to our schools 3d printer. When printing a large part like this, it is also really helpful to have a dual extruder printer running PLA in one and support in the other, which is the setup we have at school.

Step 6: Assemble

Assembling the 3d printed parts isn't too hard, that being said I do have a few tips and tricks for you. Firstly, use tweezers and long needle-nose pliers to reach into the back of the box. Test with regular nuts and bolts, but for final assembly use nuts, washers, bolts, and thread lock.

The wiring and coding on the other hand, is a bit trickier.

Step 7: Test

Our first couple tests revealed problems, so we went back and changed things like the motor, dyad length, code, etc.

Step 8: Done!

Our egg cracker works! There are a couple issues we could fix if it was going to market, but would require re-printing the main frame which would be slow and expensive. We hope you enjoyed reading about this process and if you have any questions please drop them in the comments below!