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Run a Daewoo puma 250. using kenametal fixed index drills to drill holes. drills fine most of time but sometimes I think i'm not exactly drilling on center. leaves a nipple inside of hole. changing drill inserts often especially the inboard tool. what can be done to know that i'm exactly on center?? usually happens with smaller diameter holes such as .500 .625 .750 all the way to 1.0 the 1 inch diameter holes and up to 2.125 holes seem to drill alot better. ????
Is is possible that the drill's you are using, are not meant to do the complete hole, all at once. You may need to do these hole in two or three steps... so the final hole is more accurate.
I would research the companies documentation, and verify you are using them correctly. They may work, like your using them, but may not be meant to do so.
Years ago, when we ran the big spade drills on the Warner Swassy's... we would always step up, at least for the start of the hole. 2.5" drill, we would use a 7/8" pilot...
Also on very tight tolerance, we would go in with a boring bar after.
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Otherwise, cannot you measure the accuracy of your parts? The question seems funny, for that reason. Could not you measure the part at the bottom of the hole, and at the entrance, and see if it is wondering? As well the wall thickness, all the way around.
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If your machine is older, you may find you have to change your practices or routines, as the machine start to wear, and is not as accurate as it was when new. You may have something beyond the part that is giving you trouble, during heavy cuts.
To check turret to Chuck center, use a coaxial indicator.