December EDU Project: Hot Tub + Hour Of Code

As the chilly winter weather sets in across the nation, we thought this would be a welcome subject for this month's Thornburg EDU Project. At the end of this month's EDU Project, we've got a challenge in conjunction with the #hourofcode initiative, so be sure to check that out as well! Now, on to Dr. Thornburg's lesson!


Hot tubs are relaxing! They come in a wide variety of models, but the main concepts are the same. Unlike swimming pools, hot tub users generally just sit and relax in warm water with small jets blowing bubbles and water streams on your back. After a half-hour, you emerge from the tub well relaxed with aches in your joints pretty well dissipated. The therapeutic value of hot tubs is why they are in training rooms for athletic teams, but a lot of people have installed hot tubs in their homes.

Hot Tub Image Source: Morguefile

Hot Tub Image Source: Morguefile

This project involves the design and building of a hot tub suitable for inclusion in a doll house. This project involves a lot of mathematics, and even explores the concept of water pressure. While it doesn't have any jets in it, you can add those if you want, and make a nice place for your fingers to relax, since our model is pretty small. Normally, hot tubs are about 2.5 meters across. Ours has an outside dimension of only 13 cm.

As with the other projects we've explored so far, this one will be done using Tinkercad.

Step-By-Step Design

1. The first step is to put the ruler on the workplane, and drag a Polygon from the Basic Shapes area of the screen onto the workplane. The shape of this polygonal prism is hexagonal ― just what we want.

2. Our next step is to resize this shape to represent the interior of the tub. The original dimension is 20 mm corner to corner, and 17.32 mm across the faces. The goal is to maintain the aspect ratio and resize the hexagon to 120 mm corner to corner. The face to face size will be 120 x 17.32 ÷ 20 which is 103.92 mm. Next, set the height to 40 mm.

3. Next, we'll make the outer part of the tub. Select the hexagonal prism, copy it, and paste it. To keep from being confused later, make the new shape a different color.

4. Move the interior piece up by 5 mm. You can pull it up with the black arrow, or enter 5 mm in the base position field on the left of the screen, just below the height setting.

5. Now it is time to change the size of the outer piece. Select it, make the corner to corner dimension 130 mm, and adjust the face to face dimension proportionately. In this case, the size will be 130 x 103.92 ÷ 120 which is 112.58 mm.

6. Now we need to align the two pieces so they are centered. Select both pieces, choose the align tool and click on the center button for both the X- and Y-axes. This might be easier for you to see if you use the Top view of the workplace.

7. When you are done, the pieces are centered like this.

8. Next, select the interior piece and choose the Hole tool to the right of the workplace.

9. Select both pieces, and click on the Group button to the top right of the workplace. This gives us the finished view of the tub itself.

10. Now we need to add seats to the tub. Drag a cylinder to the interior of the tub.

11. Change the seat size to 30 mm in diameter, with a 20 mm height. Finally, raise the seat 4 mm from the base. As before, this kind of movement can be done with the black arrow, or by entering 4 mm into the field below the height field.

12. Now move the seat to one of the corners so it makes a tight fit.

13. Copy the seat, and paste five more of them to the interior of the tub. Position them to the remaining corners, and you're done.

14. If you choose the Home view of the workplane, you can see a nice view of the tub. Export everything to an STL file and you're ready to print!

Printing & Assembly

Print a copy of your tub and check to make sure there are no holes in your model from which water can leak.

Things To Do & Notice

Calculate the volume of your tub. Here are some hints: A hexagon is made from six equilateral triangles. In our case, these triangles are 65 mm on each side. The tub is 35 mm deep, and you need to subtract the volume of the six cylindrical seats, each of which has a diameter of 30 mm and a height on 19 mm.
Using a measuring cup, fill your tub with water and compare the volume you found with the volume you calculated. If the numbers are different, how do you account for this?
Our tub has a wall thickness of 5 mm. Water exerts pressure on the walls of its container. Since the density of water is one gram per cm3, what is the pressure inside a full tub in newtons (N) per cm2? Where is this pressure the highest? Where is it the lowest. How thin do you think you can make the tub walls and still safely hold a tub full of water?

#HourOfCode Challenge

From December 5th to the 11th, multiple companies, schools, and organizations are participating in the Hour Of Code program to encourage students to learn and experiment with code. We'd like to get in on the action with this monthly lesson. Can you reconstruct this month's EDU lesson using the code based CAD program, BlocksCAD? If you send us a screenshot of your code solution with an image of your final model, and we will put your name in the hopper to win a 1kg roll of PLA filament in the color of your choosing! We will select a winner from the submissions on January 1st, so you have nearly a whole month to fit in an hour of code! Best of luck everyone!