FILENAMES: Drillbit-boring.jpg DESCRIPTION: This is a drawing showing the problems of using a drill bit as a boring bar. Posted by Wayne Cook . The following description was provided: ================================================================= >Wayne I am sure you are right about the problem being clearance . Maybe >I went wrong initially by thinking that a drill had clearance built in , >at least from the side . I did keep adjusting the clearance until I hit >upon what worked but honestly I can not see any difference in the last >grind and all the failed attempts . > I think that is the problem alright. Drills do not have clearance on the sides. They're ground cylindrical with the only clearance being the relief just behind that lip you see on the flute. That lip (I can't think of the proper name for it right now) is ground cylindrical. That's why a drill bit can't be used as a endmill. The problem gets worse when used as a boring bar. I'm sure you put the cutting edge on center. The cutting edge on a drill bit is actually above center by a little ways (usually at least the same amount as the web thickness). This is hard to explain with words. Ok. I'm going to make a little drawing and put it in the dropbox. It'll be under "Drillbit boring.jpg". This is a exaggeration but it shows it fairly clearly. Notice how the side of the bit is touching the hole well before the actually cutting edge. The red line indicates approximately where it needs to be ground. Note that you also need to grind the flutes on down the length of the drillbit to ensure they don't touch as well. Wayne Cook Shamrock, TX