Figure 2. NASA Engineers assemble the Mars Pathfinder lander and Sojourner rover. click for copyright

After scientists and engineers finish the design of a rover, the design plans are sent to the Advanced Manufacturing group at NASA, whose responsibility is to review the design plans and decide what can be made on-site and what should be outsourced (sent outside of NASA) to local manufacturers. Typically, standard parts are sent out to various machine shops for fabrication. When Spirit and Opportunity were built, approximately 80% of the parts were made outside of JPL. However, the rover parts that had a complex geometry were made at the high-precision fabrication shop in JPL. By keeping the complex parts in-house, the designers could closely communicate with the individual making the parts and make design changes when needed.

As expressed by Mike Mangano, a project element manager with NASA's Mars Exploration division, "You can do the design, you can put it on paper, but until you start to actually make it, you don't know what kinds of problems you're going to run into."

Figure 3. A dimensioned rectangle. click for copyright

The individuals who work in fabrication shops and make the metal parts are called machinists. When making parts for a rover, it is essential that a machinist manufactures or fabricates parts that exactly match the engineer's size specification. An engineer is able to communicate the exact part size that they desire by creating a drawing of that part and placing dimensions (sizes) on the drawing. A dimension, therefore, is a measurement between two points. A basic block with dimensions is shown in Figure 3. The size of a rover's parts is critical because all the parts must fit together precisely. Accuracy of the parts will help the rover stay alive on its long journey to Mars and when landing on the planet. Figure 4 shows machinists hard at work at NASA's Jet Propulsion Laboratory.

Figure 4. JPL machinists manufacturing parts for the 2003 Rovers. click for copyright

Some of the machines utilized to make rover parts at JPL's high-precision fabrication shop — and at fabrication shops outside of JPL — include milling machines, lathes and CNC (Computer Numerically Controlled) machining centers. A milling machine is a very versatile piece of equipment. A vertical milling machine uses rotary cutters that are vertically mounted to remove material from a basic shaped (cube, cylinder, etc.) work piece. The machine's table can move up/down, right/left and forward/backward.

Fun Fact: One machinist at JPL compared the assembled rover to a Russian nesting doll, a Matryoshka doll. Like a Matryoshka doll all of the rover's pieces fit together in a minute space. This creative assembly allows NASA to build and launch rather small spacecraft systems.

Figure 5. CNC Machining Center (right) and a Vertical Milling Machine (left). click for copyright

CNC machining centers (see Figure 5) are automated machines that allow machinists to quickly and accurately create parts with complex surfaces. The machining center can perform operations such as milling, drilling, boring and tapping. However, for parts to be created on a CNC machining center, the machine's computer must first be programmed. During the design phase of a project, engineers use computer-aided design (CAD) to create an exact picture of their model and its parts. A CAD program allows engineers to draw parts on a computer in 2-D or 3-D and then analyze the design and how parts fit together for assembly. Once a part has been drawn in CAD, a conversion program is used to convert the part's shape from CAD to fabrication instructions that the CNC machining center can understand. The CNC machining center will automatically change its tools to what is needed and then create the desired part by removing material from a basic shape (e.g., a cube).

A lathe (see Figure 6) is typically utilized to fabricate parts that are cylindrical in shape. Metal (or plastic) is held in a lathe horizontally, and material is removed to manufacture a desired shape. During operation, the cylindrical material is held in a chuck, or collet, and then rotated while a cutting tool travels left and right or forward and backward to remove material.

Figure 6. Left: A Monarch manual lathe. Right: A lathe cutting tool removing material from a cylindrical piece of material. click for copyright

A tolerance is a numeric range used to specify accuracy levels. Tolerances are typically specified on engineering drawings similar to the one shown in Figure 7. If the thickness dimension is examined one can see that the dimension is 0.33" and the tolerance is +0.05" and -0.05". What this means is that the thickness of the Bottle/Can Opener should have a maximum thickness of 0.38" and a minimum thickness of 0.28".

Figure 7. An engineering drawing showing dimensions and tolerances. click for copyright

So why do engineers need to use tolerances? One of the best examples of a company that relies on tolerances is LEGO™. If the cylindrical nub on top of a LEGO is too big, then an additional LEGO piece will not fit on top of the first piece. Furthermore, if the cylindrical nub on top of a LEGO is too small, then any LEGO piece that you attempt to affix to the top will not stay positioned. A simple way to illustrate the need for tolerances is by showing a peg and block assembly, as shown in Figure 8. If the peg is too large, the peg will not fit in the hole of the block. If the peg is too small, the peg will fall through the hole in the block and the two pieces will not fit together.

Figure 8. From left to right: an ideal 3-D peg and block drawing; a 2-D side view showing that the peg is too small for the hole; and another 2-D side view showing that the peg is too large for the hole. click for copyright

It is important that NASA and the aerospace industry maintain very tight tolerances on the parts that they manufacture to ensure spacecraft operation. As a result, NASA will specify that a part be fabricated within 0.001" (or 0.03 mm) of the specified dimension. These manufacturing specifications help to ensure that assembled space exploration equipment will fit together properly and be robust enough to complete the intended mission.

Each part of a rover needs to be properly fastened together before the rover is ready for launch. Furthermore, keeping the rover "alive" once it lands on Mars requires engineers to use extreme caution when putting each specific piece of the rover together. It would be terrible if the rover lost a wheel or robotic arm as it journeyed across relatively unknown Mars terrain! However, the pieces of the rover are like a complex 3-D puzzle. There are fragile pieces of instrumentation mixed in with lightweight metal parts, and each rover part has a proper place. If any piece is left out or put in the wrong place, the motors, controls, instruments or communication system may fail, leaving the rover stranded on Mars.

Figure 9. FIDO during field testing. click for copyright

Before sending the exploratory rovers Spirit and Opportunity to mars in January 2004, NASA's scientists and engineers prepared themselves for the intense operation by conducting field tests here on Earth. The prototype rover utilized for these field tests was named FIDO (see Figure 9).

The FIDO rover provided NASA the opportunity to practice the quick-paced exploratory mission that would be conducted by the Spirit and Opportunity rovers. During the field test, the mission team practiced navigating the Mars terrain with FIDO, and the engineers were, as a result, trained to make prompt decisions as a team.

Once a rover lands on Mars, the mission team has a very short period of time to review incoming data. Scientists need to quickly decide what they would like to investigate, while the engineers determine how the rover will reach each destination and if it is capable of reaching requested research locations.

Brainstorming: In small groups, have the students engage in open discussion. Remind students that in brainstorming, no idea or suggestion is "silly." All ideas should be respectfully heard. Encourage wild ideas and discourage criticism of any ideas. Ask the students:

• To brainstorm with a partner what steps are required for a Mars rover after the rover is designed and before it is launched. Students should discuss what each step entails.

Question/Answer: Ask the students and discuss as a class:

• Who is in charge of deciding what parts are made at NASA and what parts are made at other companies? (Answer: Advanced Manufacturing Engineers)
• What is the name of the individual who operates a vertical milling machine, a CNC machining center or a lathe to make metal parts? (Answer: a machinist)
• How can you best describe to a machinist the size that you want a part to be made? (Answer: use dimensions)
• What is another word you could use for fabricate? (Answer: manufacture)
• What does CNC stand for? (Answer: Computer Numerically Controlled)

Tolerating Tolerances: Break students into teams of 2 and have groups solve the following problems. Teams should discuss their answers with the entire class.

• If Part A has the dimension 0.50 inches and the tolerance is ±0.25 inches, what is the minimum and maximum size Part A can be? (Answer: 0.25 inches and 0.75 inches)
• If Part B has the dimension 0.75 inches and the tolerance is ±0.1 inches, what is the minimum and maximum size Part B can be? (Answer: 0.65 inches and 0.85 inches)
• Which part has a more precise tolerance? (Answer: Part B)

Flashcards: Each student on a team creates a flashcard with a question on one side and the answer on the other. If the team cannot agree on the answers, they should consult the teacher. Pass the flashcards to the next team. Each member of the team reads a flashcard, and everyone attempts to answer it. If they are right, they can pass on the card to the next team. If they feel they have another correct answer, they should write their answer on the back of the flashcard as an alternative. Once all teams have done all the flashcards, clarify any questions. Sample questions follow:

• Name the three machines that are used to manufacture parts for mars rovers. (Answer: A vertical milling machine, lathe and CNC machining center)
• Why is it important for NASA to have field tests? (Answer: so engineers and scientists can practice for their missions)