In the Center of Things
Introduced at Quality Expo Detroit in June, the CenterMax CMM from Carl Zeiss IMT Corp. (Minneapolis) is designed not just for the shop floor, but for the high-speed production line where most CMMs dare not go. While other CMM makers have offered machines dubbed shop-floor ready, the CenterMax combines features such as the patented temperature resistant frame and a new temperature accuracy specification that allows the CMM to be used as an inline production gage. The CMM can be placed in the production line and loaded from three sides.
"Historically, Zeiss built CMMs and sold them to quality assurance personnel who put them into gage labs that had a nice temperate environment," says John Plohetski, vice president of engineering for Zeiss. "But our customers' needs have changed. Now they need CMMs out in the manufacturing area to support production as closely as possible."
In the past several years, Zeiss developers adopted a machine-in-a-box approach, meaning that they enclosed their CMMs in environmentally sealed rooms that were placed on the shop floor. While effective, enclosures add cost, complexity and a larger footprint. The CenterMax, which has been under development for several years, eliminates the need for the enclosure.
The CenterMax can be on- and off-loaded from above and from both sides of the machine. It can be automatically loaded, which is a key to the CMM's direct support for production operations. "One of the objectives is to be an inline production gage," says Plohetski. "We have projects right now that load parts in a variety of ways, including having a robot pass the part over the top of the machine. Another project we have supports an assembly operation. That operation uses a conveyor that passes directly through the machine. The system clamps the part, measures it, unclamps it, and then the part rolls out the back of the machine."
The CenterMax is based on the Prismo, Zeiss' high-end CMM. Some components that carried over included the air bearings, drive system and the Vast scanning probe head.
It has a measuring volume of 35 by 47 by 27 inches. It can be equipped with a 16-position probe changer rack without reducing the measuring range. It is equipped with Calypso CAD-based software.
Work on the production line requires a robust machine that is able to withstand harsh environments with fluctuating temperatures.
The CenterMax frame is built like a milling machine. It is squat, with a small footprint, and is made from thermally stable and rigid materials. The frame includes mineral casts, which are made from polymer-based concrete, that look like four girders and run horizontally around the machine. Invar poles, or carriers as Zeiss calls them, support the bridge like the posts on a four-post bed. Invar is one of the world's most thermally stable metals, made from approximately 64% iron and 36% nickel, and several CMM manufacturers use the materials in their products (see "Invar Is 'In' in Dimensional Measuring Machines," Quality, August 2001, p. 26).
"The Invar material is important, especially with a raised bridge, because it is very rigid and thermally stable," says Plohetski. "It is the most cost effective and stable material that we can come up with."
The casts rest on the vibration dampening system that includes vibration dampers under the four corners of the machine. The dampers use compressed air, and the compressed air levels are monitored to detect any loss or increase in pressure. When detected, the system automatically compensates for the pressure change.
In addition, the CenterMax features bearing and scale covers. These covers were designed to protect the CMM against harsh environments. If any dust, oil or other contaminants were to get past the covers, there is an added layer of protection. "There is a labyrinth of sheet metal that is used around the guideways," says Plohetski. "The intersecting flanges of sheet metal block any contaminants away from the guideway."
The thermal stability of the CMM has allowed Zeiss to increase its temperature accuracy range. On earlier Zeiss models, accuracy is guaranteed in ranges from 18 to 28 C, which is already one of the widest accuracy ranges on the market. The CenterMax is guaranteed at a temperature range of 15 to 35 C.
CMMs are calibrated to work best at 20 C. Temperatures exceeding that level can cause structural changes to the machine components and adversely affect accuracy. Zeiss, like other CMM manufacturers, use temperature sensors on each of the guideways to track temperature changes. In addition, the CenterMax has a temperature probe that can be included in the tool changer that can measure the temperature of the part and compensate for temperatures above and below 20 C. When temperatures vary, the changes are run through an algorithm and the measurement results are compensated to that degree.
While most CMMs are accurate within the specified temperature range, the question is how accurate they really are within that range. Zeiss says it is the first to define the temperature accuracy specification that allows the user to know exactly how precise the machine is even as temperature changes. With the CenterMax, Zeiss has introduced Temperature Variable Accuracy (TVA) that shows how accurate the CMM is measuring up to 35 C. Knowing this reduces measurement uncertainty.
"On a standard machine, we provide the temperature spec in which we guarantee a certain measuring uncertainty," says Plohetski. "When we build a machine at a perfect 20 degrees, the machine only uses maybe 25% of the total tolerance range of error. When we go to the high end spec we are still going to be within that bandwidth, but we don't know exactly how accurate it is. With the TVA, you know exactly what you can expect in terms of measuring uncertainty, because you know exactly where it is going to be at a given temperature."
For more information on the CenterMax, contact:
Carl Zeiss IMT Corp.
6250 Sycamore Ln. N.
Maple Grove, MN 55369
FAX: (763) 533-0219
- A temperature resistant frame includes Invar carriers and mineral casts.
- The CMM's accuracy temperature range is 15 to 35 C.
- A Temperature Variable Accuracy feature shows how accurate the CMM is at various temperatures.
- The CMM can be placed in a production line and loaded by conveyor belt, material handling equipment and rotary tables from the top or front and back of machine.