We use a Darex drill sharpener, the predecessor of the Drill Doctor. It was quite the invention, and is actually as good as they say so far as being handy enough to actually use on the spur of the moment.
The trick to sharpening drills is to not give them too much or too little clearance. Too much clearance usually can be suspected whenever you hear the drill bit make a snapping sound while drilling. Sometimes the snap is really due to the fracturing of the edge, but often it is due to hogging in and releasing in soft materials. Proper clearance helps control the feed in.
As a rule of thumb, I always check that the angle of the "chisel edge web" to the cutting lips is 135 degrees ( commonly referred to as a 45 degree angle). This tells you that the clearance is about right. Parabolic drills do not follow this rule, and I find them more difficult to get the proper clearance for. I guess it is because the cutting edge is curved, it makes it hard to tell what the average angle is.
One thing about the Darex drill sharpener, is that it is very convenient to change the clearance angle by a few degrees after you have made an initial sharpening. Just loosen the chuck, put the chuck and drill back into the setting fixture and advance or retard the chuck body a few degrees (on a scale) and reclamp. I do not know if the drill doctor allows this.
High helix drills position differently on the finger stop when chucking, so it is necessary to be able to allow for this (or else the clearance will be way out of wack), and the Drill sharpener does allow it.
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