On 28 Apr 2005 07:03:47 -0700, wrote:

I have one of those entry level drill doctors. SOmetimes, when I
sharpen large bits, the bit comes out with the point ground down. What
am I doing wrong and are these repairable?

Is there a better way to sharpen these?


Lots of good advice has been given on how to freehand sharpen
drill bit and plenty of people are well skilled in this art. Pretty
well all of it is directed towards duplicating the usual offset cone
geometry found on standard machine ground jobber drills. However, for
newcomers it is sometimes easier to learn to use use the 4 facet
drill grind.

Commercially this is only used on the more expensive special
purpose drills because it needs 2 extra grinds per drill point and is
only marginally better.However the beauty of the 4 facet system is
that it's much easier to describe and understand.

It's best carried out using a cup wheel.

For resharpening a blunt but OK drill, present one drill cutting
edge to the wheel.

Set the length of the edge horizontal and almost touching the
wheel surface

Tilt the top surface of the flute which forms the cutting edge
roughly 10 deg.

This is exactly the same operation as grinding a 10 deg front or
side clearance on a lathe tool but now only grind away as little as
possible - just deep enough to remove the blunt area.

Repeat the operation for the second drill flute taking care to
grind away the same amount so that the facets are symmetrical. If you
get this a bit wrong the drill will still cut well but will drill
oversize.

This completes the sharpening of the cutting edges but the
back edge of the flutes may foul the newly cut surfaces. You need an
additional grind to eliminate this.

The exact shape of this grind is not important because it
doesn't need to form part of the cutting action.The easiest method is
simply an exact repeat of the first sharpening operation but using 30
deg downward tilt. In an ideal world this grinding should continue
until the new facet overlaps the first facet halfway along the central
chisel edge. However this is not essential and, as long as the new
facet extends 1/3 or more round the periphery of the flute, clearance
will be established and the drill will cut just fine.

If you haven't got a cup wheel you can grind on the periphery
of a disk wheel but it's a bit more difficult to eyball the angles.

If the drill is damaged or mis shapen, use the 30/60/90deg
protracter from a childs geometry set. With the long hypotenuse edge
laid along the drill the shorter remaining edge shows the correct
angle to present to the wheel.

Jim