Case Study: CMM Guides Complex Mars Rover Project
July 29, 2010
When Andrew Tool (Plymouth, MN) got the call to quote on a Mars Science Laboratory (MSL) project, they were determined to succeed by taking advantage of their expertise. At Andrew Tool, coordinate measuring machines (CMMs) have been an integral part of their manufacturing processes for years, but they had never faced a project with such intricate measurements, tight tolerances, heat treatments and a very short time frame requirement.
Carl Zeiss (Maple Grove, MN) had proven to be a great resource at Superior Tool, Andrew’s sister company. “Our Prismo CMM at Superior Tool is the backbone of our quality department and my goal was to replicate and build on this capability at Andrew,” says Bruce Hanson, president and CEO. This goal and initial project discussions with the MSL top-tier contractor pointed Andrew Tool toward a Zeiss CMM as the best option.
Andrew Tool is a unique machine shop with more than 30 years of experience handling everything from simple geometries to the most complex. They are strongest when incorporating all of their areas of expertise: five-axis milling, electrical discharge machining (EDM) and grinding. With a 28-employee staff, they are small enough to adapt and respond to market demands, while taking advantage of new technology and methods. Their success also is dependent on their emphasis on fostering good relationships between the tool designer or engineer and the machinist. “They need each other, and our employees understand that it’s a team effort that links us to our customers,” says Hanson. The majority of Andrew Tool’s customers are found in the aerospace, defense, medical and micro electronic industries, all having parts that often require extremely tight tolerances.
This new, complex project from MSL required Andrew Tool to manufacture actuators (gearboxes) for the next Mars rover, Curiosity. This new rover will weigh more than 10,000 lbs., five times the weight of the current rovers, and carry more than 10 times the weight in scientific instruments compared to the current Spirit or Opportunity rovers.
Therefore, the propulsion system’s power and torque will be more robust, and the unit’s wheels considerably larger than previous designs. NASA engineers believe these changes will help prevent the problem that Spirit is encountering now: it is stuck in a sand pile.
New project challengeThere were many parameters that Andrew Tool had to adhere to in order to help NASA make their new actuator design a success. Many of the parts had very deep pockets (almost a 20:1 ratio), small radii added to the challenge, along with extremely tight tolerances, many of which are tied to different gear pitch diameters. The parts are labor intensive with thousands of points of data measured on individual parts.
The VascoMax material used for the actuator parts changes size slightly during heat treatment, and as a result, many of the part features were machined and inspected to process dimensions that allowed for this size change if the feature was not going to be final finished post heat treat.
Additionally, position tolerances of 0.0002 inch, geometric control of 0.00008 inch and size control within 0.0001 inch, even on relatively large (5-inch range) dimensions, added to the challenge. All of these factors coupled with a demanding 18-month timeline and AS 9100 certification requirements made it critical that Andrew Tool bolster its CMM capabilities for precision and speed.
Andrew Tool decided to purchase the Zeiss Accura with the Vast XT gold active scanning sensor. The Zeiss Accura RA was an affordable solution with the range they needed while the Vast technology and automatic stylus rack system increased flexibility and productivity when determining size, form and position. The Vast XT gold is an excellent feature for the complex and heavy stylus configurations required in measuring MSL’s actuators.
Of course with any new machine being added to the production process, there is always a learning curve. The first surprise they had was seeing how seemingly benign parameter setting changes could dramatically affect measurement results. The discipline of always verifying critical measurements with another method was crucial to the learning curve.
Brent Helgeson, metrology applications manager at Concept Machine (Minneapolis), was responsive in efficiently diagnosing the correlation problems enabling Andrew to quickly get up to speed with their new CMM. Additionally, probing strategy for some of the features and proper alignment sequences to achieve correct results were developed and refined to achieve consistently correct results.
“The Zeiss Accura helped us orchestrate the project by providing timely, accurate and understandable in-process reporting. It can’t be overstated how critical good inspection is for process development and setup,” says Bryant Broderick, quality control engineer at Andrew Tool.
The project included 14 different part numbers with quantities ranging from four to 12 in addition to set-up parts. Throughout all of the different processes including milling, gear cutting, heat treatment and stabilization, each part was successfully measured up to 60 times by the Zeiss CMM to ensure accuracy every step of the way.
The temperature compensation feature was particularly helpful because they were able to check parts right off the machine tool and relay the results to the toolmaker instead of having to wait for the part temperature to stabilize before measurement. With a coefficient of expansion of five and one half millionths of an inch per degree, a few degrees could mean the difference between pass or fail. They were amazed by how effortless the Accura made this whole process.
The robust reporting capability in Calypso is a great asset for the AS 9100 certification process required by NASA. This certification involved a lot of time on the program management side of things ensuring there was documentation for all of the critical paths of each part throughout the whole manufacturing process. All of this information was contained in a spreadsheet with all of the serialized parts to clarify where each part was at any point. The Calypso software documented the information electronically each time the part was measured. “Verbal information of part details means nothing; documented data is everything,” states Don Felix, director of sales and marketing.
Long-term Benefits“We couldn’t have been successful without Carl Zeiss,” says Felix. “The benefits Zeiss brings have been enormous and we see positive results every time we use the CMM.” Ongoing training continues to drive Andrew toward the goal of using the Accura to its full capacity.
Andrew Tool even acquired a new customer after another NASA supplier saw several of the MSL actuator parts. They were so impressed with the precision and complexity of the parts that they were instantly sold on Andrew Tool’s capabilities. This supplier also required AS 9100 certification and fortunately, Andrew Tool was well on their way to attaining this certification and ready to take on this new opportunity with enhanced confidence in data from their Zeiss CMM.
Carl Zeiss IMT