The project brief

Now we start the thing you came for. The next six chapters design, print, test, and improve a portable lemon squeezer. This chapter writes the brief: a plain statement of what we are making, who it is for, and how we will know it is good. A brief is the most useful habit you can borrow from professional makers. It is cheap to write, it stops you halfway through from quietly drifting into a different project, and it gives you something to measure your result against instead of just shrugging and saying "good enough."

What we are making, in one sentence

A small, refillable-free, hand-powered lemon squeezer that lives in a lunch box and lets you squeeze fresh lemon over food, away from a kitchen.

That sentence already rules things out. It is not an electric juicer. It is not a full-size kitchen press. It is not a bottle of pre-squeezed juice. It is a tool you carry.

Who it is for and the situation it is for

Picture the actual moment of use, because the moment is what we design for:

• Lunch at a desk or on a bench. You have a salad, some fish, or a bowl of something, and half a lemon you brought from home.
• No sink, no big cutting board, maybe just a napkin and the lunch box lid.
• You want juice on the food, and you do not want seeds in it or pulp all over your fingers.

Designing for that moment, rather than for "squeezing lemons" in the abstract, points us at small size, easy cleaning, and seed catching as the things that matter most.

Requirements

A good brief separates what the design must do from what would be nice, so that when something has to give (and on a first print, something always gives), you sacrifice a nice-to-have and not a must.

Must have:

• Fits in a lunch box. Target footprint under about 80 mm across and 60 mm tall, so it tucks into a corner next to a sandwich.
• Squeezes a normal supermarket lemon half with hand force only, no tools, no electricity.
• Catches seeds and most pulp, so what reaches the food is juice.
• Pours or drips the juice where you aim it.
• Comes apart enough to rinse clean, because a juicer you cannot clean is a juicer you stop using.
• Prints on a beginner FDM printer with no exotic settings.

Nice to have:

• Squeezes a whole lemon's worth (both halves) without emptying in between.
• A pour spout rather than just a drippy rim.
• Looks tidy enough that you are happy to set it on the table.
• One single printed piece, or as few pieces as possible.

Explicit non-goals (saying what you will not do is part of a brief):

• Not trying to extract every last drop. A bit of waste is fine.
• Not handling limes, oranges, and grapefruit in one tool. Lemon first; we note later how to resize for other fruit.
• Not food-rated for commercial sale. This is a personal-use object, and Chapter 14 is honest about exactly what that means.

Constraints we did not choose but have to respect

Three hard limits shape every decision from here:

Don't be confused. A requirement is something the finished object must do for the user (catch seeds). A constraint is a limit reality imposes that you cannot negotiate (hand force is only so strong). Requirements are your goals; constraints are the walls of the room you get to design in.

How we will measure success

Vague goals give vague results. Before we design anything, we write down numbers and checks we can actually test in Chapter 15:

• Juice yield. Weigh the lemon half before and after. Getting roughly 15 ml or more from a medium half (a realistic target; see the geometry math in Chapter 13) counts as working.
• Seed catching. Squeeze a seedy half and count how many seeds end up in the juice. Zero or one is a pass.
• Mess. Can you do it over a lunch box without juice running down your hand? Yes or no.
• Cleaning. Can you rinse it clean under a tap in under a minute, with no pulp trapped where you cannot reach? Yes or no.
• Size. Does it actually fit your lunch box? Put it in and see.

Write these on a sticky note. When version 1 comes off the printer, you will not wonder whether it is good; you will run five quick checks and know exactly what to fix.

The plan from here

• Chapter 12: how real lemon squeezers work, so we borrow the right mechanism instead of guessing.
• Chapter 13: turning the brief into actual dimensions and a 3D model.
• Chapter 14: the food-safety questions, which shape the material and finishing choices.
• Getting it printed: every route from your file to a finished part, home or outsourced, with the steps for each.
• Chapter 15: printing it and running the five checks above.
• Chapter 16: reading the results and making version 2.

👉 With the brief in hand, let us look at how lemon squeezers actually work.