Drill Regrinding or How to regrind drills by The Tap & Die Co :
- For unknown terms, please view our ‘Drills Terminology’ page, which is shown in our 'Technical Data' -
Most drill troubles arise from inaccurate pointing. Correct drill pointing will to a great extent eliminate drill breakage and in accurate holes. Drill pointing should be varied depending upon the materials to be drilled. General purpose drills, have a point angle of 118o included 12o to 15o clearance angle and with a chisel edge angle of from 125o to 135o
Most operators do not have the skills required for good hand pointing, The use of standard drill pointing machines is highly recommended. Drills are machine pointed at the factory to a 118o angle to ensure a correct and uniform point angle, equal cutting lips and correct chisel edge angle
For general purpose drilling a point angle of 118o included angle is recommended. The point angle may vary somewhat one way or the other, but the variation should be uniformin both cutting lips. A twist drill point grinding gauge should be used for checking the point angle of both cutting lips.
CAUTION: When high speed steel (HSS) drills are sharpened by dry grinding, care should be taken to prevent overheating. If the drill does get too hot, do not cool in water, but let it cool by itself in air. Sudden cooling of HSS drills in water or other cooling mediums will usually produce grinding cracks, which may result in chipping of the cutting lips or breaking of the drill point
CUTTING LIPS: The cutting lips must be of equal length. Even though the point angle is equal on both sides, if the cutting lips are not equal in length the chisel edge will not be central, and the drill will cut oversize. Ascale should be used to check the lengths of the cutting lips.
LIP CLEARANCE: Sufficient clearance behind the cutting lips must be provided so that the cutting edges can enter the work. Usually 9o to 15o lip clearance is sufficient for average work if the point is ground with the proper angle and the proper clearance, but without the proper contour back of the cutting edges, the cutting lip will be weakened.
The Tap and Die Co, London website has a lot of other very useful 'Technical’ Data & Information for you to view