i use these drill bits a lot . home depot sells a flat file about eighit dollers here in texas ,when sharpening your drill bit keep the file level with your bit .when sharpning you must sharpen with same level as the bit . any diffrent and the bit well be worse. a little practice and you can master this. allso replace the self feeding worm. about ten dollers buys about five self feeding worms. around forty dollers a bit and you can this way.