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.
Filing the Lead Screw
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.
File ONLY the Inside Edges of the Spurs
DO NOT File the Outside Edges of the Spurs
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.
File ONLY the Top Edges of the Cutting Lips
DO NOT File the Bottom Edges of the Cutting Lips
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.
Thanks Bob! Your list of DO NOT’s is pretty much my list of things I’ve done to try and sharpen one of my bits.
Have you got any advice on size or sources for auger files?
Somewhat unrelated, but is there something out there that’s appropriate for boring into end grain? I tried that with one of my auger bits and it was a miserable failure – I have a hunch that’s what ruined one of my bits (the lead screws don’t really work anymore).
I think The Best Things still has some auger bit files left. You might try them out. I heard a rumor that Nicholson was not making them anymore, so I’m not sure if they will be available in the future. A small needle file can be used too, I’d just grind the teeth off one edge to make it a safe edge. It’s not necessary, but nice to have.
Boring into end grain doesn’t really go well with any kind of bit. Forstner is probably the best bet, but even that doesn’t work real well. It shouldn’t ruin the lead screw though. It probably just clogged all up with chips and dust. Pick the wood out of the threads with an awl, needle, or whatever you can. Then look at the threads. If they are munged up, hit the with a triangular saw file. If they are just a little rusty, but not burred, then do what Josh says below to polish the threads. I did this in Part 4 of the sharpening podcasts with loose SiC grit. It works great to polish the threads and help prevent them from clogging. If the threads have a lot of bent metal and burrs though, you need to file that off first. The SiC grit or valve lapping compound will only polish what is there. It won’t fix damage.
The tip that had been handed to me – and I’ve seen it on Sawmillcreek as well – was to touch up the lead screw by drilling the brace just enough to bury the lead screw, back it out, and then drop a bit of valve grinding compound in the hole, and screwing in and out. This helps sharpen the leadscrew. Sometimes it takes a couple of holes worth to get it good.
A lot of times, especially with bits you find used, the problem with the leadscrew is not that it’s not sharp enough, but that it’s gummed up with pitch and woodbits. A little care to clean up the leadscrew really helps.
Yep, this works great to polish the threads and help keep them from clogging, as long as the threads aren’t damaged. It won’t repair damaged threads. If the threads are damaged, you need to file them first to remove the damage, then polish them with the valve lapping compound.
Bob,
Lie Nielsen has found an alternate supplier in India and sells auger bit files at a good price.
Question: The bits I find are for softwood. If you use them in hardwood with enough torque, will they function?
Look for Russell Jennings no 100 bits. The thread pitch of the set I have works well in hard and soft woods. If you are using Irwin bits, I’ve found their screws pitched too coarse for hard woods. They require more torque but even then the screw seems to strip the guide hole out more often because the coarse screw makes them more aggressive.
Bob,
thanks a bunch! This is timely for my ailing bits.
You’ve saved my life (well, my cutting edges), once more. Thank you!
I can’t get the correct bevel on mine. I am still doing something wrong. I can get it to start but can’t drill deeply. Is picture one have the correct bevel? I guess that’s my problem.
The bit pictured has the correct geometry. The bevel on the top of the bit (picture 4)really doesn’t matter all that much. The geometry of the bottom of the bit (picture 5) however is extremely important for proper function of the bit. Make sure that the bottom of the cutting lip (picture 5) has no secondary bevel at all. It should simply be one continuous surface starting at the cutting edge and spiraling all the way up the bit. If there is any hint of a secondary bevel on the bottom of the cutting lip, it will need to be filed away so that the surface is uniform without any secondary bevel.
However, if your bit starts to cut and then stops part of the way into the cut, I suspect the cutting lip is not the problem. More than likely, either the spurs are not sharp enough to score the outer edge of the hole well, or the lead screw is clogging up.
What kind of bit are you using (Irwin, Jennings, etc>)? What kind of wood are you boring? Are you boring through the grain (face or edge) or into the grain (end grain)? If you can, email me a picture of the cutting end of the bit so I can see what you are working with.
They are Jennings bits. This weekend I will give them a look over, especially the screw and cutting lip. I was just trying the bits in the face side of pine. Thanks for the help. Obviously I don’t know what I’m doing! Your blog has been helpful. How many people can I ask help on sharpening auger bits!
Hi Bob,
Thanks for the tips on sharpening auger bits, they’re a great help.
I have a couple that need tuning for use.
I prefer using auger bits with a brace, to driving flat bits using an electric drill.
Using an electric drill is always a lot slower noisier and most often overheats the bit ruining the temper (bluing it) and not holding its edge.
The old ways still work best.
Maurice.