Use a soft touch for a finer finish

Don’t let initial price scare you. Flap discs and wheels may be your best choice for finishing materials.

Sometimes, there seems to be a magic line drawn between the fabricating shop and the jobsite, and never shall workers from either side meet. However, it can pay to cross that line and take a peek over workers’ shoulders to see if you can pick up some tricks of the trade that could make your jobs go better or faster.

That’s definitely the case with abrasive flap wheels and discs, report abrasive experts.

“Before flap discs, if an operator wanted to remove a fair amount of stock such as a weld bead and refine a finish beyond what a rough bonded grinding wheel could provide, it was necessary to switch from a rough grinding wheel to a fiber disc to produce the desired finish. The advent of the flap disc made this two-step process a one-step process, saving the time and labor of switching wheels as well as outlasting conventional fiber discs to a ratio of about 20 to 1,” says Phil Benincaso, Pferd Inc. technical sales representative.

Plus, a bonded grinding wheel in the hands of an inexperienced operator can ruin a work piece. “A flap disc is much more forgiving,” says Leo Lavigne, product manager at United Abrasives/SAIT.

A big misconception about flap discs is that they are not as aggressive as rough grinding wheels. “In fact, they are significantly more aggressive and remove more material faster than rough (hard) grinding wheels. This is due to the orientation of abrasive grains on the flap disc. When produced, a positive electrostatic charge is applied to the grain and a negative charge is applied to the backing material. This pulls the grain onto the backing in a columnar fashion with its most pointed, sharp edge facing forward. Result: a much more aggressive grinding action compared to a hard bonded wheel where grain is poured into a mold and oriented in random fashion,” says Benincaso.

So why aren’t more of them on jobsites?

“It often comes down to cost vs. a grinding wheel,” says Roger Harbaugh, director of operations at Mercer Abrasives. “The flaps cost 75 percent more than bonded wheels, but are often as much as 80 percent more productive. Initial cost is the biggest downfall, even though faster and better finishing is possible with them.”

Flap wheels and discs are made with many segments of coated abrasive material that are mounted in a radius to a wheel or hub. Like their bonded abrasive cousins, the flap discs come in Type 27 depressed center flat and Type 29 conical configurations and can be mounted on angle grinders. Flap wheels with a straight or threaded quick-change shank can also be mounted on drills or stationary grinders.

“Type 27 flap discs achieve their maximum efficiency at working angles of zero to 15 degrees and are more suited to large, flat surfaces. Type 27C flap discs are more suited for contoured work pieces and are designed to work at a 15 to 30 degree angle of attack to the work piece,” says Benincaso.

Lavigne agrees. “A Type 27 style disc excels on flat surfaces and is an excellent choice for fine finishing. A Type 29 flap disc provides a larger point of contact that enhances its aggressiveness and speeds stock removal. The built-in angle of the Type 29 disc allows workers to use the product at a steeper angle when compared to the Type 27 style.”

Test on your jobs
There are many manufacturers of flap discs and wheels, and the best way to find what works for your operation is to test them.

“High-density flap discs like SAIT’s new Ovation flap disc are much more forgiving. There are more flaps packed into the disc, which provides a cushioning feel to the operator. This cushioning effect allows the product to conform to uneven surfaces and minimizes surface gouging, especially on critical parts and pieces. Because there are more flaps, these discs have a much longer life than regular density flap discs,” says Lavigne.

Harbaugh concurs. “Overall flap disc quality depends upon the quality of the backing and the number of flaps on the disc. For example, we have up to 138 flaps on Mercer Abrasive’s Jumbo line where most regular flap discs only have 80 or 90 flaps. The more flaps on a disc, the longer it lasts.”

The experts agree that not all flap discs are created equal. “Safety is a big factor with these products and the fiberglass backing is a key determinant of safety. It is also the most costly component of the wheel. In order to lower costs, manufacturers reduce the number of layers of fiberglass in the fabric, which compromises the integrity of the wheel,” says Lavigne.

Also, the quality of the abrasive material makes a huge difference when it comes to the construction of a flap disc. “Having a consistent product made with only quality raw materials enhances the performance significantly,” Lavigne says.

Use tips
In general, flap disc or wheel users should use one grit coarser than they would normally use with a resin fiber disc. “For example, if they would use an 80 grit abrasive on a job, then we would recommend a 60 grit in order to achieve a similar finish,” says Lavigne.

Users may think that flap discs are only for metal working and blending, but that’s not the case.

“That’s a common misconception about flap discs. With advances in abrasive technologies that offer stronger backings, better grains and stronger resin strength, these products can now do moderate grinding and finishing processes with one disc,” says Lavigne.

But that doesn’t mean these are a one-disc-for-all-jobs abrasive. “We often see users using flap discs in applications where they should not be used, like removing heavy welds. In these instances, the user would benefit from the speed and life of using a bonded grinding wheel and then finishing the weld with a flap disc,” says Lavigne.

Fabrication and welding shops have always used bonded abrasives to rough-grind welds, but flap discs are best for finish work.

“They blend the finish better before painting. The feel of a flap disc while grinding is more spongy because the flaps flex at an angle and float on the material. If you push hard, you remove material, but if you back off, you get more of a buffing action and a nicer finish,” says Harbaugh.

Grit selection depends upon the type of grinding you are doing, says Lavigne.

“What type of grinding needs to be done? If you are removing welds, rust or paint, 36 grit or 40 grit is a good starting point. If you are doing light stock removal and weld blending, choose 60 or 80 grit flap wheels or discs. If you are preparing a weld for paint or a final finish, 120 grit is the best choice,” says Lavigne.

“Aluminum oxide is the most common grit, but if you are working with aluminum or stainless steel, zirconium is the grain of choice due to its aggressive, long lasting nature,” says Harbaugh.

If the work has many sharp corners, the experts also suggest opting for bonded abrasives to do the work because the sharp corners can easily damage the flap wheel or disc and shorten its life.

Finally, Benincaso says it’s important to watch your speed when using these tools.

“Often, the user assumes that flap wheels are made to run as close to maximum speed as possible without going over the maximum rpm. Actually, the optimum operating speed is significantly less. The maximum rpm only indicates safe operating flap wheel speed. When flap wheels run too fast, centrifugal force makes the flaps very rigid and allows only the tips of the flaps to contact the work piece. When the flap wheel runs at the proper speed, the flaps have a chance ‘lay’ down on the work piece. This gives more surface area that contacts the work piece and provides optimal performance,” says Benincaso.

Published in the November/December 2006 issue of Contractor Tools and Supplies magazine.

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