Quality Waterjet Newsletter – 05/24/2005

Submerged Cutting or Not

The main design consideration of the earlier waterjet catcher tanks was to absorb the energy of the spent jet so that it did not cut through the tank. Scrap metals were usually placed at the bottom of the catcher tank to serve this purpose. These machines produce a tremendous amount of noise and dust during the cutting process. In some machines, steel balls contained in a basket were used to dissipate the residual energy of the jet. The steel ball design slightly reduces the noise and dust level, but is still not a satisfactory solution. In the 90s’, catcher tanks with adjustable-water-level design emerged. These catcher tanks have built-in air domes. When the air dome is filled with air, water is propelled out of the air dome, bringing the water level up quickly. When the air is let out, water withdraws back into the air dome, causing the water level to drop just as fast. These machines fully or partially submerge the part during cutting, which suppresses the noise and dust level significantly. The part can be exposed for inspection or removal when the cutting is done. This type of catcher tank design has become the standard of today’s machines.

According to a study by Munoz and Kain (*), the noise level of waterjet or abrasive waterjet cutting with an open tank can reach 120 dBA, which is way above the 90 dBA permissible noise level for an 8 hour exposure, set by OSHA. Fully submerged cutting can drop the noise level below 90 (or even 80) dBA. Partially submerged cutting, with bottom of the part being submerged, can reduce the noise level to below 95 dBA.

Obviously, fully submerged cutting is ideal. Not only does it bring the noise level down to a safe level, but it also eliminates the dust, which is very harmful to the machine motion equipment (guide ways, bearings, ball screws, etc). However, users choose partially submerged cutting for some understandable reasons. One of them is that the part may be too thick for being fully submerged. However this situation is rare. A common reason is that the operator has the need to monitor the cutting to avoid scrapping the part or even crashing the machine. The operator constantly watches/listens to the cutting and stops it if he/she notices a change in the cutting condition or a risk of collision. Cutting parts on warped sheet metals presents the risk of jamming the nozzle. Some small cut parts or scraps can pop up and get in the way of the nozzle, causing collision. Both situations need some sort of monitoring. In the case of partially submerged cutting, operator must wear eye and ear protection and machine motion equipment should be fully protected against water and grit. These situations indicate the need of an intelligent monitoring system to replace the need of human monitoring.

* Munoz, J. and Kain, I., “Abrasive Waterjet Cutting a Comparative Study between Open Catch Tank and Water Catcher Tank,” Proceedings of the 2001 WJTA American Waterjet Conference, Minneapolis, Minnesota, August 18-21, 2001.

Unusual Cut: Run(way) For Your Life!

Mike Gracey reports some unusual stories that he heard while interviewing some of the folks in the water blasting industry. Runway cleaning vehicles, pumps, nozzles and controls are usually safe because the equipment is secured and the cleaning heads are mounted. In spite of that, it can get strange out there some times.

Mike Woodward of Gardner-Denver related a story about a job at Hobby Airport in Houston, Texas when a 737 aircraft tried landing in the middle of their cleaning operation. It was RUN FOR YOUR LIVES because someone had forgotten to shutdown the runway they were working on.

Russell Reed of Reliable Pump, Inc. said that one of his customers told about when he was in the middle of a runway job, a plane headed directly at his truck. He jumped out of the truck and ran for the grass while the plane barely missed his equipment.

Runway cleaning often involves a 24-hour per day schedule including travel to the various airports around the country. Things can happen on the job site. One equipment operator reportedly fell asleep while cleaning a runway and wandered off of the runway taking out a few landing lights. Another time, a sweeper operator fell asleep and rammed into the back of the runway cleaning truck. Get out the No-Doze.

If you have some “Unusual Cuts”, send them to us for the newsletter.