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Corrosion repairs using the radial bristle disc
Mech, Summer, 2004 by Thomas Doughty
Editor's note: We ran a story on the testing and evaluation of radial bristle discs in the fall 2001 issue. Here's an update.
Abrasive wheels and flap brushes long have been used to remove small amounts of corrosion and paint from aluminum surfaces. However, these tools also can abrade the metal.
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The Naval Air Systems Command has authorized using the 3-inch radial bristle disc for removing small spots of corrosion from aluminum surfaces. It was viewed as a replacement for the flap brush and abrasive wheel. Approval for use on high-strength steel and titanium surfaces is pending until a formal IRAC is issued. The Lancers of VAQ-131 used the radial bristle discs during their 2002-2003 deployment to the Persian Gulf aboard USS Constellation (CV-64). AM3 Josh Lawson of VAQ-131's WC-12C said, "I did not like them at first, but then realized that I was trying to remove too much material. With some training and practice, I am now able to remove the corrosion very quickly with pinpoint accuracy. Using these discs, the Alodine Touch 'N Prep pen, and paint pens (called SemPens) allow us to get the job done in a hurry when we have to."
Two types of discs have been approved for fleet use. An orange disc with a grit equivalency of 400 has been designed for use on light surface-corrosion discrepancies (such as filiform). This disc also is ideal for edge feathering. A yellow disc with a grit equivalency of 360 is meant for heavier corrosion and light-to-moderate surface pitting. The yellow disc also can remove light amounts of sealant around access panels and similar structures. The working area of the disc is 0.75 inches. It stops working when the bristle fingers are worn down to the disc hub. At this point, you have to get a flesh disc.
The bristle disc has five key advantages:
* Saves time. You can get the job done in about half the time because the uniform concentration of abrasive minerals provides a high cut rate for a faster, more uniform finish.
* Protects the metal. Bristle discs will not abrade bare metal like the flap brushes can. The flexible bristles of the discs conform to the contours of the part, which reduces the amount of damage done and the rework required.
* Reduces surface temperature. The bristle disc produces a surface temperature of about 100 degrees Fahrenheit, while the abrasive wheels yield temperatures around 200 degrees F.
* Smaller treatment area. The bristle discs are more precise and do not remove excess paint.
* Easier to use. The bristle disc requires 1.5 pounds of pressure to be applied (via the grinder) in order to remove corrosion. Abrasive wheels require 12 pounds of pressure to accomplish the same task.
Bristle discs are not meant to be used inside fuel tanks, or on magnesium or composite surfaces.
AM3 Jason Clark, a member of the Lancers corrosion-control team, stated, "The biggest advantage of the bristle disc is the ability to remove small spots of corrosion with less effect on the area around it. We have had good results feathering the paint with the orange disc. The use of these discs cuts the prep time in half as you are dealing with an area of bare metal smaller than a dime. The bristle disc also removes more of the corrosion from around screws and rivets--corrosion that a flap brush would leave. This fact results in fewer repeat discrepancies." AM3 Joy Timpog said, "The discs really helped out when working discrepancies found in the flap wells and under the slats. The time allotted for a wing spread on the ship is minimal, and we really have to hustle. What used to take us four to five hours with a flap brush is now one-and-a-half to two hours."
AMCS Jay Shannon, the QA senior chief, is an advocate of bristle discs and has been instrumental in its evaluation. He tested the discs at VAQ-129. when assigned to COMVAQWING. They used the discs to prep the landing-gear area, around the center-section wing in the wheelwell area. to comply with AFB 548. The job took only two hours, or about half the time of a flap brush.
The bristle disc requires an air source of 90 to 110 pounds per square inch. a high-speed pneumatic die grinder rated from 22,000 to 25.000 rpm (not the 3.200 rpm with most available grinders), and a mandrel assembly. You have to operate the grinder at its maximum rated speed, but you only need to apply a small amount of pressure to strip a surface. If you apply too much pressure, the bristles will fold, and the tool won't work. You'll avoid the "valleys" that form with flap brushes and abrasive wheels.
Safety and health are big concerns. The radial bristle disc generates dust particles that may contain chromates from primers, so you must wear eye and respiratory protection when using the tool.
You will need a little training on using the discs. Naval Air Technical Data and Engineering Service Command (NATEC) martial technologist (MT) representatives at most Navy and Marine Corps activities provide hands-on training for this, too, as well as all other corrosion-relater materials and processes.
