I am a Graduate Chartered Engineer from the UK, and I will try to explain why these disks are mainly failing in fairly simple language. I studied Materials technology at University a long time ago, and although I work now in a very different area, I still remember some of this material!
The question is, how are flap disks supposed to work? Ideally as you use one, the abrasive on the edge of each flap should do the cutting, and as the abrasive becomes blunt it should either break to expose a new sharp edge or the flap should break off at the edge, in each case exposing new abrasive. This is not happening as in the comment “glazing”. This simply means that the abrasive is blunt and therefore no longer cutting.
There is another thing about conventional grinding wheels, and that is that a weaker bond wheel should be used for harder materials, thus the range of wheel “hardnesses” available. Also coarse wheels work best where large amounts of materials are to be removed, with consequent high grinding forces. Very high pressures are not really suitable for flap disks, so we must also take care that the grit size of the disk is not unreasonable for harder materials, and all of these are too coarse for bulk steel removal! A 40 or even 60 grit flap disk will behave much mor as I describe above as the smaller grains are weaker and break to become sharp much more easily, and have a lot less binding material allowing the flap edges to wear off as they should. A 40 grit wheel test would be interesting as it would probably last the test and consequently grind off more material. Finer wheels CAN give better surface finish, but this is not a necessary requirement as seen in surface grinding to a mirror finish with a 60 or 80 grit wheel.
Probably enough for now, questions and comments fine. David
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Good information that makes sense and was well presented.
Thanks Jimmy. Yes, I hope Jason reads it!