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 Reid Bailey
(434) 924-6352

Vern Johnson
(520) 297-3247



Catapult Variability Design Project

Your team has been assigned the task of designing a catapult capable of delivering a specific projectile to a prescribed target. You will be loaned a generic catapult and a projectile for this project. This is a simulated medieval catapult that you are to position on the floor and use to launch the projectile at a target on the floor at least 3 meters, but no more than 5 meters, away. The distance measurement is to be made from the front edge of the catapult base to the center of the target. You are not allowed to modify the catapult in any way other than varying the built-in parameters and the number of rubber bands. Also, you are to use the projectiles and rubber bands provided with the catapult. You will be allowed to specify the distance to the center of the target, but once specified, your projectile is to land within ? 17.5 cm (measured in the direction of the launch) of that distance.

Based on the catapult settings that you determine best, you are to conduct a catapult capability study to determine if your catapult is in statistical control, and if it operates within the specifications of ? 17.5 cm. To do this, you are to collect at least twenty-five subgroups of three measurements each to ascertain the capability of the catapult:

1. Plot these 25 points sequentially on a control chart.

Is the process in control?

Are all data points associated with common causes, or are some associated with special causes?

• If there are special cause data points outside the process control limits, and if you decide to recommend process improvement actions, should you address these causes prior to using the other control chart data to adjust the process, or should you use the other data to adjust the process before addressing the special causes?

2. Calculate Cp, Cpk and St. Then give a description of what they mean with respect to future improvement of the catapult to meet the designated need.

Since you probably have never designed a catapult, the following will be a useful guide:

Play with the catapult for a while to learn its capabilities. Examine the catapult and determine the variables (control factors) that can be adjusted to change the distance the projectile will be thrown. A sketch of the catapult indicating the variables will be very helpful. Determine what the noise factors are that have an impact on the distance, but that are not under your control.

Gather some data and brainstorm as a team to determine which three control factors are most important in making a projectile land on target (distance and accuracy). Fix all other variable factors at whatever positions you think best, but for each of the three most important control factors select two levels and run a half-factorial Design Of Experiments to determine which of the two levels of each factor results in an optimization of the performance index.



Do your measurements in meters or cm.

bullet While testing you may need some type of positioning and/or aiming device (a little tape on the floor is great)
bullet Pennies make great markers for where the ball hits (then measurements can be made after a set of shots).
bullet One of the important skills you will need to develop is how you let go of the launch arm with your hand as the projectile is sent skyward.


Project Related Links:


Why Control Chart Your Processes?

bullet Process Capability / Capability Indices
bullet Assessing Process Capability
bullet Process Capability and Product Design
bullet Process Capability Analysis
bullet See also the Catapult Project


Copyright 2006 - Vern Johnson & Reid Bailey- All Rights Reserved