Cutting Tool Engineering
April 2012 / Volume 64 / Issue 4

Looking for Mr. Goodpart

By CTE Staff

Diversification helps minimize the risk that a downturn at one customer or one industry will negatively impact a parts manufacturer’s business. For more than 78 years, Mitchel & Scott Machine Co. has thrived by machining components for a host of industries, including agriculture, construction, power generation and HVAC/R. It recently expanded its automotive industry operations.

That occurred when the Indianapolis company, which employs 200 people at its 200,000-sq.-ft. facility, won a contract to produce two parts for a Tier 1 auto supplier. The combined annual quantity is about 1.6 million, and the contract is for 5 years. Mitchel & Scott produces the parts on a ZPS 642 6-spindle, 42mm machine and a ZPS 867 8-spindle, 67mm machine.

Case Study - Mitchel 1&2-2.tif
Courtesy of Compass Automation

One of the two automated vision inspection and thread checker systems from Compass Automation that Mitchel & Scott Machine Co. purchased to provide 100 percent inspection of parts for a Tier 1 auto supplier.

The customer requires 100 percent inspection of internal thread pitch and depth and nine external dimensions on one part and 18 external dimensions on the other. The complexity of the inspection, coupled with short cycle times, made manual inspection impossible. “We knew we would need some sort of automated inspection and material handling equipment to keep up with the customer’s requirements,” said Patrick Mitchel, director of quality and engineering.

During an online search of companies that provide automation, gaging and vision inspection equipment, Mitchel found the YouTube site for Compass Automation, Elgin, Ill. “I saw videos of similar parts being inspected off the back of a CNC multispindle and knew they were the guys to do the job for us,” he said.

After correlation and accuracy studies showed that measurements from the vision inspection system agreed with results from Mitchel & Scott’s Zeiss coordinate measuring machine, the parts manufacturer ordered two vision inspection systems integrated with New Vista thread verification units. Robots are part of the inspection system, and Compass was able to reduce the system cost by more than $25,000 by using an existing Mitchel & Scott robot. According to Compass, the systems exceeded the quoted performance, providing 2µm repeatability and 4µm reproducibility and achieving a gage repeatability and reproducibility score of 7.4 percent.

A GR&R score indicates how much measurement variability is being contributed by the gage itself, Mitchel explained. “A GR&R under 10 percent is very acceptable to our customers,” he said. “A GR&R under 30 percent can be conditionally approved with the understanding that we continue to improve the gage to remove the measurement variability the gage is adding to part inspection.”

The next challenge was meeting the auto supplier’s preproduction date, which it moved up after the inspection systems were ordered. To satisfy the expedited schedule, Compass designed, built, programmed and integrated the first system in 13 weeks, 3 weeks faster than the initial quote, and installed the second one on schedule a month later.

The measurement operates seamlessly with the part manufacturing operation. Once a part is completed, a machine’s conveyor delivers it to an inspection system’s in-feed nest. For production on the ZPS 867 machine, the Fanuc robot first takes the part and locates a slot in the end of it using a table-mounted proximity sensor. The robot then moves the part to a thread checker to verify pitch and depth. If the part fails this check, the robot takes it to the system’s out-feed conveyor, which has three sections: thread failure, vision failure and good parts. Upon passing the thread check, the part goes to a blow-off station where excess cutting fluid is removed and mist is collected.

Next, the robot moves the part to the vision inspection station, which includes an Imperx 16-megapixel Bobcat camera, a custom hypercentric lens, an Opto-Engineering collimated illuminator and measurement software. At the station, a set of air slides clamp the part to measure an over-the-knurl diameter and the camera takes a picture. The software processes the image against the high- and low-limit requirements for the other 17 features. If all are acceptable, the robot takes the part to the good-part section of the conveyor. If it fails any length or diameter check, the part goes to the vision-fail section.

For the ZPS 642 machine’s inspection system, a thread checker is integrated into a fixture on the camera and a separate pick-and-place robot arm removes parts from the inspection system. A second robot is needed to keep pace with the 642’s faster cycle time. Therefore, while the pick-and-place arm removes the part from the vision station, the other robot gets the next one from the in-feed nest and senses the locating slot before fluid is blown off so the system can perform the vision check prior to the thread check. “One system checks the thread before the vision check and the other system checks the thread after the vision check,” Mitchel said, adding that both methods are effective.

The other inspection operations for parts produced on the ZPS 642 machine are similar to those for the parts produced on the ZPS 867 machine. Mitchel noted that both systems cycle about 2 seconds faster than the machines to maintain production flow.

Since they were installed last fall, Mitchel said the inspection systems have been performing as expected. “We had one issue, which I believe we caused, and Compass was able to talk us through it on the phone and get the system back up and running quickly.” CTE

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