Rohr Industries Inc., Chula Vista, Calif., is expected to put two abrasive water jet machines into production later this year and a third is expected to be installed next year for cutting of titanium, Inconel and Hastalloys up to 3 inches thick in parts for jet engine shrouds.

A computer numerical control (cnc) machine for high production volume parts and a manual system for trimming contoured or uneven parts will be installed, Chuck Holland, project engineer for the Rohr abrasive water jet project, said during a technical seminar at the recent Fabtech conference here.

Most of Holland's remakrs centered on results of tests on the cnc machine in a research laboratory at Rohr. The company is using some of the results observed in those tests to design the third machine for which Holland hopes to secure an outside vendor to build to specification. The third system, he said, will be further automated with automatic loading and unloading.

The test machine was built by Flow Systems Inc. while Ingersoll-Rand Corp. also makes comparable machines (AMM/MN, Dec. 10). The cnc machines has Allen-Bradley 8200 controls. The cost of the machines are about $250,000 each, Holland said.

Rohr has been using standard shearing equipment as well as band saws and other techniques for cutting the titanium, Inconel and Hastalloy materials, Holland said. Laser cutting was considered but Holland said some of the parts in question require that their grains structure not be affected by the heat that a laser would generate.

A Camsco automatic nesting system originally purchased for cutting aluminum parts on a Trumpf router has been adapted for use on both the router and the abrasive water jet. That system uses computer-aided design data to determine how to cut the components to assure maximum material usage, Holland said.

Among the initial problems encountered with the laboratory system was heavy tile wear on the bottom of the water tank. Holland said this happened because the abrasives were hitting the tiles. As a solution, the machine being designed will have a mechanical metal "catcher" that will move under the water jet in synchronous action with the jet stream gantry to catch the abrasives before they hit the water tanks.

Rohr also found that while the cut finish appeared smooth, magnification of the edge showed the cut was not as smooth as needed in some applications. This could result in short fatigue life. The initial study, however, used 60 grit abrasives, the most commonly used type of abrasive for abrasive water jet cutting. By moving to 150 grit, a much finer abrasive, fatigue life was increased by 50 percent on Inconel and doubled on titanium, Holland said.

However, the finer grit gives a slower feed rate than the 60 grit. Holland said the solution is to have a machine that has two hoppers filled with the two types of abrasives. The 150 grit will be used on the parts whose fatigue life is critical and the coarser material will be used where speed is more important.

The abrasive water jet technique is being used for cutting titanium, Inconel and stainless steel up to 3 inches thick and aluminum up to 5.5 inches thick.

Because the machines have not been put into production, Holland said the productivity gains of the technique have not yet been determined. However, he noted that the experience of other companies indicates that production should be several times greater than in current systems.