Dear Ken and List, I have been following this thread with some interest as some of the methods for checking this plate for cracks are ones that I use in the aviation industry. I use Magnaflux to check aircraft parts (engine, transmission, landing gear, etc) for defects. As I listened to the suggestion to Magnaflux a 6' grand plate I had to sit down and think, "with the equipment we have, how would I do that?" Let me comment on some of the problems and then make a few alternate suggestions. The limitation is the size of the magnetizing coil (24 inches) or the distance between the pressure heads (42 inches) on our machine. We magnetize parts by two different methods and a plate of this size is too big. I'm sure there are Magnaflux machines that are bigger and could handle it. I'm sure you could arrange an appointment with one but you might stagger at the cost. There are two other problems with this method: 1. We require all parts to be bare metal, the plate would have to be stripped. I suppose you could Magnaflux it with the paint on, but then I would not guarantee the results. 2. I don't know how I would demagnetize the plate. To demagnetize the plate properly it would have to fit through the coil. There are methods to do it after all, the US Navy demagnetizes a whole ship after construction before commissioning. I don't think Magnaflux should be the first line method for checking the plate. Let me suggest 3 other non destructive crack detection methods that we also use in our shop: 1. Visual inspection with a 10X magnifying glass and bright light (ie. a Maglite which can focus the light beam). This is always our first line of attack. Focus on obvious stress points, weak spots, where strength changes because the cast thickness changes or heads off in a new direction. God has made the human eye incredible just like our ears. We can detect a gaps as small as 1 millionth of an inch if under the right conditions! A lot of cracks are not going to escape a good visual inspection. Let me make an aside here. We place hard paints on aircraft parts for two reasons; for protection and to indicate cracks. Seldom do we find a crack in the metal part that did not first indicate it's presence in the paint. From what I have been reading on the list about what we paint our plates with, a crack should be observable with the above method by looking for it in the paint. CHEAPEST 2. Dye Penetrant inspection. This method uses a dye that gets absorbed into the unseeable crack and then indicates when the developer is applied. The plate is cleaned thoroughly, the dye is applied, the dye is cleaned off again (but the dye in the crack remains) and then the plate is developed. The developer is a dry powdery substance that is sprayed on the plate and now sucks the bright red dye into it's white powder. SMALL EXPENSE 3. Zyglo Penetrant inspection. This method is a little bit more elaborate as it requires the penetrant and a black light (ultraviolet) for detection. It uses at florescent chemical instead of dye and is basically the same method as number 2 above. KIND OF EXPENSIVE We use these 3 methods to guarantee aircraft parts as being free of defect upon which people are placing their lives every day. I know they work. A word of caution: If you ever find what you think is a crack with the above methods, don't stop there. Crack indications show up for a host of reasons but not all are cracks. If you find a crack, remove the plate paint by desolving the paint. Don't sand it as you can pack paint into a crack and cause it to stop indicating. Removing the paint with strippers, MEK (Methyl Ethyl Ketone) or other paint desolving product would be the way to go. Sorry this is so long. I hope it is helpful to some. Glen --------------------------------------------------------------- Glen and Ruth Deligdisch P.O. Box 248 Waxhaw, NC 28173 Tel: (704) 843-9089 E-mail: Glen_Deligdisch@SIL.ORG
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