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NORTH AMERICA ONLY - LULZBOT TAZ
LulzBot
Castle Crushing Catapults
2023-03-09 | By Lulzbot
License: See Original Project 3D Printer 3D Printing
Courtesy of Lulzbot
Guide by Jason Erdreich
While the technology may be ancient, the real-world concepts and lessons that can be learned through designing and building catapults is still very much alive today! Through this hands-on design challenge, students will learn how catapults have evolved over time as they design and build their own modern-day projectile throwing devices under real-world specifications and constraints.
Introduction
Lesson Overview:
Through this lesson, students will adventure into the science behind catapults of centuries old, and modern day. Through hands-on activities, students will create their very own catapult prototypes that combine simple machines and physics to make a projectile move.
By utilizing an engineering design process, students will gain real-world problem-solving skills as they research, brainstorm, design, build, test, evaluate, and reflect upon their prototype solutions. We will discover that there is no one answer to any problem as we research the variety of mechanisms that have been used overtime. We will also discover that making mistakes does not mean we failed, but that we are learning new ways to improve our creations.
This lesson offers opportunities to modify and adapt to suit the needs of a wide range of learners in both age and prior experiences. Additionally, the lesson can be combined with a variety of materials or production techniques to suit available resources in your classroom. Through safe prototyping techniques, students will learn how to blend computer aided design software and rapid prototyping through 3D printing with other manufacturing and prototyping techniques as they fabricate their catapult creations that must meet the specifications and constraints of this real-world design challenge!
Utilizing An Engineering Design Process:
An Engineering Design Process, or design loop, is a method used by scientists, designers, and engineers to develop solutions to our everyday problems. Through a design loop, students will develop skills in problem solving as they brainstorm solutions and work to create a prototype through hands-on activities.
Design loops come in many shapes and sizes, but none are ever truly ending. The “last” step of any design loop is redesign, or reflection, where we look at what we’ve learned in our developed prototype to improve upon its design. Not being afraid of failure is a powerful concept that leads to greater success and implementation of problem solving.
Lesson Objectives:
- Students will be able to identify the differences between various forms of catapults and explain the physics behind how they work
- Students will be able to identify simple machines and how they function
- Students will be able to determine the effectiveness of a product or system through experimentation and analyzing collected data
- Students will utilize an engineering design process to develop their own solutions to a real-world problem
- Students will utilize computer aided design (CAD) software to create a 3D model that can be produced on a 3D printer
- Students will understand how 3D printers work and how they are used in an industrial setting
- Students will be able to safely apply prototyping techniques to construct designed solutions to real-world problems
Materials:
This is a list of materials each student will need to complete this lesson:
- Computer or tablet with Internet access
- Computer Aided Design (CAD) software
- 3D Printer and Filament
- Castle Challenge Blocks / Tape Measure for Testing / Evaluation
- Paper, pencils
- Rubber bands, string, weights, and ping pong balls
- Assorted non-3D printed materials to include in prototyping (optional)
- Click here for sample models shown throughout this lesson
Example Trebuchet prototype counterweight system
Printed on a Lulzbot TAZ SideKick 747
Modifications:
In addition to this lesson plan, see our One Page Brief [PDF] that can be used to guide students through the lesson. Additional examples as to how this lesson could be modified are:
- Additional tools and materials to construct prototypes such as cardboard, popsicle sticks, Styrofoam, or hot glue to combine with 3D printed parts
- Catapults come in many different shapes and sizes, constrain students’ proportions to fit within the limits of your classroom
- Metal washers or pennies and make excellent weights for trebuchets and are easily accessible
- A variety of projectiles could be used to add a level of discovery and experimentation such as ping pong balls, practice wiffle golf balls, or foam projectiles. Alternatively, students may even design and fabricate their own projectiles
- The testing criteria can be modified to use cups, tape lines, targets, or whatever else creates an opportunity to allow students to test their catapult’s precision, accuracy, and power in a real-world setting
Considerations:
Based upon the age of your students, introduce the concepts of machines, forces, medieval times, precision, and accuracy using terms and concepts familiar to their prior experiences and needs. When working with catapults, there is a level of danger in that students are creating a projectile throwing device. Proper care and instruction must be taken to create a safe testing environment where students are informed of the danger and how to use their prototypes safely.
Proper safety procedures should be introduced to students when working in any makerspace or lab environment. When students are around machines such as 3D printers, or using tools to cut or glue materials, students must be informed of potential hazards and taught how to use these resources safely. For reference, see the safety resources offered by ITEEA.
Assessments:
Opportunities for formative assessments will take place through observations and discussions between students as they interact with the content in this lesson. For summative assessment, we recommend utilizing a rubric. Students’ success will not be based solely on the performance of their catapult, but the implementation of a problem-solving process to solve an open-ended problem. Example Rubric – PDF.
Step 1: Identify the Problem
The Evolution of Catapults
Catapults originated as weapons of defense to protect towns from raiding armies. Dating as far back as 400 BC, the first catapult originated in Greece and resembled a crossbow with a slingshot-like mechanism to fire arrows or rocks long distances. After some evolution and design innovation, this early catapult was widely known as the Ballista and was popular with both the ancient Greeks and Romans.
Torsion Machines, another type of catapult was also invented around 400 BC by the Romans as an alternative to the Ballista. Torsion Machines are often what we think of when we envision the classic catapult shape, with a frame and large basket or sling to propel projectiles towards an enemy. Torsion Machines take many forms and shapes, but all use a twisted rope to creation torsion, a force that is used to propel the projectiles forward.
A newer style catapult that has also proven to be popular over the years is known as a Trebuchet. Originating in Europe around 500 AD, the Trebuchet differs from both a Ballista and Torsion Machine in that no torsion or spring-like energy is used to propel a projectile forward. Instead, a counterweight is dropped from great heights to provide energy.
So then are catapults just extinct old machines of ancient wars? Not at all! While the design and general mechanics are the same, modern catapults are used in many applications for hobbies or work. Tinkerers and makers create catapults of all types and sizes to compete in an annual “Pumpkin Chunkin” competition every fall, and the United States Navy even uses a STEAM powered catapult to propel jets to very high speeds in very short distances for takeoff on Aircraft Carriers.
A torsion-type mechanism on a catapult prototype.
Printed on a Lulzbot Mini 2
How Do Catapults Work
So how do these catapults work? Well, all catapults, regardless of style are used to send a projectile across great distances, but they achieve that in a few different ways. For this challenge, we will be looking at torsion machines and trebuchets as we plan to create our real-world prototypes.
For both of these styles we need a frame to hold the catapult together and allow us to aim it at our target. We also need some type of basket that we can load our projectile into that is connected to a moving arm. From there, the catapults differ in parts and mechanisms.
For Torsion Machines, we will need a torsional spring to create the force we need to send our projectile forward. This spring could be a tightly wound rope, or rubberband, or combination of the two. This twisted rope also serves as the fulcrum, or pivot point, that our arm / basket pivots on. A key component of torsion machines is the crossbar, a beam that the arm hits during travel. This bar stops the arm and releases the projectile through a transfer of energy. The position of the crossbar will determine the angle your projectile releases from. Consider adding additional slingshot like rubber bands around the arm for more support and power, like a Ballista!
For a Trebuchet, the energy comes from dropping a counterweight. You will need an arm that is much longer than a torsion machine mounted at the top of your frame acting as a lever with a rotating fulcrum. At one end will be your basket, the other will have the counterweight. For an added amount of energy, consider adding a sling rather than a basket to create a second fulcrum, or add a slingshot to your counterweight for added moment in a hybrid-like design!
Identify the Problem
Engineers, unite! Using modern-day tools, resources, and manufacturing techniques, you are all challenged to bring the medieval technology of catapults into new light. Is there hope this ancient technology can live on?
Through a series of challenges, you must choose to build either a Ballista, Torsion Machine, or Trebuchet style catapult that can complete a series of real-world tasks. In these tasks, we will be testing your catapult’s precision, accuracy, and power. For our design challenge, you must abide to the following:
- You must create a catapult using only the materials provided
- You may research for existing solutions as inspiration and knowledge, but you must create a unique and original design
- Your catapult must be designed so that it can be operated safely during testing stages by passing a safety inspection
- Your 3D Model build volume may not exceed 36 in3
- You have 1 day to brainstorm, 3 days to build, and 1 day to test & evaluate your designed solutions
Step 2: Brainstorm Possible Solutions
Why Solutions and Not Solutions?
The second step of our Engineering Design Process is “Brainstorm Possible Solutions.” A key part of this step is solutions being plural, meaning more than one. Why do designers and engineers think of more than one way to solve a problem?
Brainstorming Our Solutions
As we work to think of different ways to solve this problem, there are a few things we can consider to assist in our design, such as learning from existing catapult and trebuchet designs. Take time to research existing catapults and think of how you may be able to recreate them using the given supplies and constraints. Remember, you must choose to create a ballista, torsion machine, or a trebuchet.
After researching existing catapults similar to the one you plan to create, begin to brainstorm different ways you could construct your own catapult prototype under the specifications and constraints of the challenge. Consider your testing criteria, how can you ensure your catapult will perform well in the precision, accuracy, and power challenges? What can you take from existing solutions to incorporate into your own small-scale prototype design?
Thumbnail sketches are a great way to think of many ideas quickly without getting caught up on the details. Once you’ve completed the thumbnail sketches, narrow your choices down as you create your final design.
For your final sketch, create a clear design that is neat and labeled. Consider drawing your design from multiple views (front, top, side, or isometric) to better portray your ideas.
Step 3: Develop a Prototype
What is 3D Printing?
Step 3 of the engineering design process is all about constructing our prototype solution! In this step, we are going to get hands-on with software and machinery to create our final design.
One of the key prototyping machines used by today’s professional designers, engineers, and scientists is a 3D printer. There are a lot of different types of 3D printers out there, but all 3D printers create physical objects you can touch, and hold based on a 3D design or model. Some 3D printers melt rolls of plastic into the model, while others use light to harden a liquid resin. There are even 3D printers that can print concrete, metal, or living cell tissue!
Lulzbot 3D printers use the fused deposition modeling process (FDM) that feeds and melts spools of plastic through a nozzle, kind of like glue traveling through a hot glue gun. The plastic is fed, or extruded, layer by layer to create the model designed in computer aided design (CAD) software. Once we design our catapult prototype models in CAD software, we will be able to send them to 3D printers to be manufactured!
Developing Out 3D Models
Now that we’ve brainstormed our catapult designs, it is time to begin to fabricate them! But before we can 3D print our parts; we need a 3D design. To create this, we will use computer aided design software, or CAD. There’s plenty of great free CAD programs out there, we recommend Tinkercad, FreeCAD, Fusion360, or OnShape for students.
All of our catapults, whether they be ballistas, torsion machines, or trebuchets, will have moving parts as we create a sliding groove or rotating fulcrum. This area of your design must be loose enough that movement can occur, but not so loose that your catapult performs inconsistently. It is important to have accurate measurements, or dimensions, when creating components that fit together.
In addition to having accurate dimensions, we must also create tolerances within our designs. Tolerances are added to the measurements you take to allow for some “wiggle room” between our parts and components. For example, if you wanted around 1/8” cylinder to rotate through a hole, we should not make the hole 1/8”. Instead, we would make the hole a little bigger to accommodate for shrinking during the printing process and to reduce friction during rotation. In general, adding 1/16” to our hole diameter (now 3/16”) would accommodate the rotation and acts a good general tolerance when working with PLA. Note, model shrinking and required tolerances can vary based on filament, printer, printing conditions, and print orientation.
We also want our models to be strong, avoiding thinner parts that are less than 1/8” in thickness should avoid snapping under load on our catapults. Orient your parts so they print with the 3D print layers and against the direction force will be applied. Understanding how FDM printers work allows students to design more efficiently as they use the layer direction to strengthen their models.
Other important factors to consider are your model’s overall size and any overhangs in your design. Any issues with these factors could render your design unprintable or cause it to fail during or after the manufacturing process. Provide examples of these design constraints to students, as well as ways to avoid them.
Checking dimensions for a prototype arm designed in the advanced Onshape CAD program
Printing!
Once students have completed their designs, it’s time to download and prepare them using Cura. Cura is not a CAD program in that it allows you to design your models. Instead, Cura “slices” models layer by layer to create a program file, or Gcode file, for the 3D printer to read. This Gcode file is a set of directions that the 3D printer follows as it prints your model.
In general, we recommend PLA filament for most classroom uses as it’s a safe plastic to print in schools and prints easily in nearly any setting. PLA works well for most applications, but if you need your prototypes to be flexible or exceptionally strong, consider looking into other materials that may better suit your needs. When printing your student’s models, we must also choose our print settings to best suit the needs of our models.
High Speed has a default layer height of 0.38mm and works best to create models that do not require lots of detail quickly.
Standard has a default layer height of 0.25mm and is the best compromise between speed and detail for parts that need to fit together but still be strong.
High Detail has a default layer height of 0.18mm and is best for printing detailed and delicate models slowly.
For the catapult models, the standard print setting will most likely work best as it will allow students’ tolerances to print accurately without taking too long to print. Additionally, if you students have any overhangs, you should use support material. Support material is automatically drawn by Cura and it fills any gaps or structural flaws. After the model is printed, support material can be carefully removed by peeling it off of the model.
Preparing a catapult prototype for 3D printing on a Lulzbot Mini 2 using Cura Lulzbot Edition
Constructing Our Prototypes
In the final part of this stage in the engineering design process, we must construct our prototypes after all parts have been 3D printed. Depending on available resources and the specifications and constraints of the challenge, this step may involve assembling 3D printed parts together, or gluing other materials like popsicle sticks, paper cups, and wooden dowels to parts that have been 3D printed. Time will vary based on how many materials and resources students have to build with.
For our catapults, time and care must be taken to ensure all prototypes are assembled safely and strongly. We recommend the teacher to create a safety inspection model that allows students to present their prototypes to the teacher prior to testing stages. Attaching rubber bands, tensioning string, or fixing weights to prototypes should be closely monitored and checked for potential failure or hazards.
Drilling holes or sanding parts may be needed based upon desired fit, or to compensate for measurements and tolerances used in the modeling stages. To reduce friction, sand parts that rotate among one another or use powdered graphite lubricants to aid in reducing binding.
Remember, proper safety procedures should be introduced to students when working in any makerspace or lab environment. When students are around machines such as 3D printers, or using tools to cut or glue materials, students must be informed of potential hazards and taught how to use these resources safely. For reference, see the safety resources offered by ITEEA.
Example catapult prototype made with a combination of materials in addition to 3D printed parts
Printed on a LulzBot Mini 2
Step 4: Testing and Evaluation
To test our catapult prototypes, we will be looking at three factors to consider how well they perform across two different tests.
A torsion machine style catapult is lined up with 3D printed blocks to test the precision and accuracy of the prototype in the Castle Challenge
Precision will allow us to determine how reliable, or consistent, your catapult is. This factor is important for your prototype to perform well over a long period of time. To determine precision, we will be using our Castle Challenge. In this challenge, students will fire five consecutive shots at a nine-block castle. Take note of where each shot hits the castle and rebuild the castle each time. If your catapult is precise, each shot should land in nearly the same location as shown in the graphic below.
Accuracy will allow us to determine how well tuned and easy to operate your catapult is. We will be testing accuracy simultaneously with precision in the Castle Challenge. During the five shots, take note of where the ball hits the castle each time while aiming for the center block. If your shots cluster around the center block, your catapult is accurate as shown in the graphic below.
Power will allow us to determine the strength and efficiency of your catapult. To determine power, we will be using the Distance Challenge. For this challenge, utilize a long tape measure to create a straight and incremented testing area. Take three shots to send a projectile as far as possible while recording the distance the projectile travels each time. Once all trials have taken place, find your average distance across all three trials. Then compare your average distance with your classmates to determine the difference in power among your prototypes solutions.
In addition to the Castle and Distance challenges, we recommend some friendly competition among groups to determine catapult champions! During the castle challenge, each block knocked to the ground can count as a point across all five trials. After the trials, total the number of points scored in addition to determining the level of precision and accuracy obtained. Points can also be awarded for performance in the distance challenge by assigning points based upon the distances traveled across the class averages.
The Castle Challenge allows us to determine if our prototypes are precise, accurate, or both by tracking where the projectile lands during each trial.
Step 5: Redesign
No design is perfect, nor is it ever truly finished. As new technology is developed improvements like cost, speed, performance, or aesthetics can always be made. Consider your findings from testing and evaluating your catapult prototypes. What worked well? What could be improved?
Which one of the challenges (precision, accuracy, and power) did your catapult perform well in? Did it perform better in one challenge more than the others? Why do you think this happened? When considering redesign, we must look at both the successes and failures of our prototypes. A failed design does not mean we failed; it means we have room to improve upon for the next prototype solution.
Create a sketch of an improved catapult design with changes you would make to improve the performance of your catapult. Your sketch should be neat and label the changes you are making to improve your solution’s performance. Include why you’ve chosen these changes and how you think they will improve your design.
Collaborate and Share What We Learned
Based on how the Test and Evaluate stage of the design process took place, there are many ways to allow students to collaborate and share what they learned through the challenge. In the competition model, high scores, records, or exceptional achievements can be acknowledged. Afterall, we’re always looking for reasons to 3D print a trophy!
Additionally, students must identify that their success is not solely based upon the points they gained in testing, but also the lesson and knowledge they learned. In this step, look at the unique features from each catapult prototype as a group to discuss the variety in design and ways to improve. Allow students to share how they would improve upon their own designs as they analyze and provide constructive feedback to their peers.
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