Article # 3

Filtered for success, Published Januray 2001, Tooling & Production
Air dryer boosts CMM's productivity

Brackett & Cochran Manufacturing Inc. is a contract manufacturer that supplies individual component parts and subassemblies to a variety of industries. Much of the company's production is related to the healthcare industry, such as parts for hospital beds and operating room tables...

Recently , oil in the air supply to a CMM caused inaccurate measurements, resulting in scrapped parts. A check valve that passed CMM inspection, for example, later failed functional testing and had to be scrapped, indicating that the CMM measurements were inaccurate. Oil contamination also required the CMM to be taken out of service at least monthly to be thoroughly cleaned.

The CMM, a Brown & Sharpe Miroval model, is used to compare new parts and finished parts against customers' drawings. Manual measurements are also taken, but the CMM is preferred because it is faster and provides the ability to program the inspection process. For example, the QC department can program the CMM to guide the technician through an inspection process, displaying critical dimensions and allowable tolerances as he takes measurements. Another benefit of the CMM is that it can get accurate measurements on parts that are too complex to inspect adequately with manual tools.

Fair-operated CMM

The Microval CMM operated on air bearings that allow the operator to easily move the arm around the part being inspected. The bearings have 0.020" air holes so it is important that the compressed air supply is clean. At Brackett & Cochran, the air supply for the CMM is the same compressed air used in the rest of the facility. The Microval includes both a particulate filter and coalescing filter that are supposed to remove both oil and water particles from the air supply. At Brackett & Cochran, they controlled water effectively but did not prevent oil from reaching the CMM. The QC department tried installing another air drying filter system upstream of the CMM but it did not prevent oil contamination either.

When oil permeated the filters, the holes in the air bearings clogged. With less air passing through the bearings, the CMM became difficult to move. Because the operator must carefully move the probe of the CMM all over the part, the sluggish motion compromised accuracy. At time' parts that passed a CMM inspection failed functional testing, indicating that the measurements obtained with the CMM were wrong. The device was then taken apart and cleaned but this was not a simple process because all the tubing that supplies air to the bearings had to be cleaned as well. Old filters were replaced with new ones, costing $600 annually.

A different approach

One possible solution to this problem was replacing the compressed air system for the entire building. That would have been expensive and the CMM was the only device that required a clean air supply. Another option was presented by a salesperson from Whatman Inc., Tewksbury, MA, who came by to demonstrate a new type of compressed air dryer.......continued.

Comments:

This article appeared in the 2001 January issue of the Tooling and Production magazine. It is basically a real-life example of the problem addressed in article (1) and (2), where an inexpensive CMM can cause much unnecessary grief.