A question I see a lot is in regards to tuning up auger bits, and getting them to cut well. A lot of old bits seem to give folks trouble, even though they appear to be sharp. If you have a bit the just won’t cut well, or won’t cut at all, here are a couple of things to check.An auger bit self feeds via the lead screw at its tip. There are coarse pitched lead screws for soft woods and fine pitched lead screws for hard woods. If the bit is properly sharpened and this lead screw is functioning properly (and isn’t too coarsely or finely threaded for the wood being bored), the bit should require no downward pressure at all to progress through the cut once the lead screw is engaged. In fact, putting more downward pressure on the brace will not make the bit cut any faster (this is a mistake we see a lot at the Hand Tool Olympics booth at Woodworking in America). Only turning the brace faster will make the bit cut faster. If the lead screw threads are damaged, it could cause the screw to clog with dust or chips and stop the screw from pulling the bit through. If the threads aren’t completely gone beyond repair, some light filing with a very small triangular saw file or a triangular needle file can smooth out any burrs in the threads and help the lead screw to self feed once again. So this should be the first thing to check if you have a bit that won’t pull itself through the wood. The next thing I recommend checking are the cutting spurs. These rounded spurs serve to score the outer perimeter of the hole before the cutting lips make contact with the wood. They need to be very sharp in order to bore a clean hole, but more importantly, they need to be sharpened correctly. They should only be sharpened on their inside edges. Wiping the burr off the outside with a very fine stone or sandpaper is OK, but you should NEVER file the outside of the spurs. Filing the outside edge will change the diameter of the hole that the bit cuts, making it smaller than the flutes above. This will cause the bit to jam in the hole as soon as the bit gets past the cutting spurs. If you are really bearing down on the bit and it still won’t cut, the lead screw and spurs are most likely not the culprits. Sure, the bit may be sharp, but if you are putting considerable weight on the bit and it still isn’t cutting well, it is probably not sharpened correctly. The problem is almost definitely the angle of the cutting lips. Just like a hand plane, an auger bit needs enough clearance behind the cutting edge to allow the cutting lips to do their job. If there is not sufficient clearance behind the cutting edge, the bit cannot cut correctly, no matter how much pressure is applied. On these old auger bits, improperly filing the cutting lips is probably the #1 reason that a bit doesn’t cut well.
When sharpening the cutting lips, it is extremely important to file ONLY the upper edge of the lips, as viewed when using the bit. Filing the bottom edge of the cutting lips, as viewed when using the bit, more often than not, results in reducing the clearance angle of the cutting edge, preventing the bit from cutting. This can be seen as a small secondary bevel on the bottom edge of the cutting lip. Think of it in terms of turning. When turning on the lathe, you want to ride the bevel of the tool to help prevent the tool from catching. In an auger, if the bottom of the cutting lip is filed and the angle is accidentally changed, the bit will basically ride the bevel, instead of cutting. If this has happened to the bit, the only way to fix it is to file the bottom of the cutting lip properly, removing the secondary bevel completely and restoring the bottom of the cutting edge to its original angle.
Nine times out of ten, when someone asks me why their auger bit won’t cut, this is the problem. So if you are having problems with bits that won’t cut, check the bottom of the cutting lip carefully for a secondary bevel. I’d be willing to bet that it’s the problem with your bits as well.