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pfonline.com/experts MEDIA TYPE MEDIA LIFE CYCLES % BREAKDOWN Garnet 3-4 cycles 25 % Plastic Media 7-9 cycles 13 % Glass Beads 9-12 cycles 10 % Silicon Carbide 9-10 cycles 11 % Aluminum oxide 10-12 cycles 9 % Ceramic media 70-90 cycles 1.2 % Steel shot, grit 80-100 cycles 1.0 % Stainless shot, grit 90-180 cycles 0.7 % MECHANICAL FINISHING CLINIC PAT WENINO massfinishing@pfonline.com 42 NOVEMBER 2016 — PFonline.com THE VOICE OF FINISHING 80 YEARS Steel, Stainless Burnishings for Edge Dulling Q. We have a precision 5 x 6-inch stainless machined part with various small holes. We need to dull the many edges and brighten the part. We've tried vibratory mass finishing, but the medias wear down and lodge into the holes. What finishing system would you recommend? A. Steel ball burnish with stainless media in a bowl vibratory machine does both: dulls edges and brightens the part. Stainless media is a product that is cold-formed from wire, then hardened and polished and is available in many shapes and sizes. They have precise measurements and wear very slowly with service life up to 10,000 hours. The steel and stainless steel burnishing media weighs approximately 300 pounds per cubic foot. Machines have to be heavy-duty to handle the weight. The applications include burnishing, cleaning, peening for strength, and light deburring, without media lodging in the part. Your part must be tested in the stainless media to make sure it will do the job without lodging. Once a media is selected, it's important to know the tolerances of the media, which is ± 0.010. Add the media tolerance to the tolerances of your hole sizes to determine potential lodging problems. Often, if not calculated properly, the accumulation of these tolerances will disqualify a media that originally looks like it works in testing and fails after you buy the media. Only choose quality manufacturers of steel media. This assures tight tolerance media and systems that work. Pressure Blasting for Heavy Mill Scale Removal Q. We are blasting heavy mill scales off of medium-sized parts in our suction blast cabinet with 80-grit aluminum oxide. We want to continue to blast by hand in a cabinet but would like to be more efficient. Can you suggest a quicker process? A. Change your blast delivery system to a pressure pot blast cabinet to deliver the media at higher velocities, and also change your media to a heavier 36- to 60-grit aluminum oxide. Air blasting utilizes air compressor energy to deliver air/media mix at high velocities and large volumes, impacting the part being processed. There are two types of air blast media delivery systems: suction and pressure. Suction blasting uses a venturi principle of siphoning media from a hopper. The air jet is ½ the ID of the nozzle. As the air stream is passed through both, it creates a low pressure. This siphons the media from a hopper up and into the air stream. The media acceleration distance is approxi- mately 4 to 12 inches from the nozzle to the workpiece. The suction systems work well and can be blasted continuously as long as blasting media is in the hopper. Suction systems do not deliver media well at low pressures (5 to 15 psi), with limits on the length of the suction feed hose. Large, heavy blasting medias are difficult to convey into the air stream with suction blasting. Most industrial blast cabinets are suction systems and work with most medias. Pressure blasting utilizes various sizes of ASME-approved pressure vessels, called pressure blast pots that contain the media, and as it is energized with compressed air, it pressurizes the pot. When the air/media mix is released from the pot, it accelerates from the pot through at least 5 to 10 feet of hose and then even faster as it travels through the venturi of the nozzle. The acceleration rates of air/media mix are much higher (3-4 times) in pressure blasting than suction blasting. When the pressure pot is depleted of the media, it has to be depressurized in order to refill the pot. Pressure blasting is more productive than suction systems, can blast all medias regardless of weight or size and can deliver medias at low pressures. Recoverable Blasting Media Q. We blast with a No. 9 glass bead media in our blast cabinets, but are interested in what our hourly media cost would be and how cost compares to other medias? A. Here are estimates of recoverable media life cycles and cost per hour. There are many medias, but these are the most popular types for blasting in recoverable media systems. Note: Material blasted, blast pressure (psi), blast angle and nozzle distance from the part affect media life. Nozzle size will determine the media's hourly delivery rates. Media Cost Per Hour: Hourly media delivery in lbs. × % breakdown ×cost per pound = Media cost per hour. Media Cost Per Part: Hourly media delivery × % break- down × cost per pound × part blast cycle time (percent of hour) = Media cost to blast a part. Other factors in total blasting cost include: Air compressors or wheel motors usage, labor, blast system cost, and media disposal fee. When blasting heavy metals, a highly recyclable media is recommended to minimize disposal cost.