Sharpening Jointer Knives on the Machine

Woodworkers discuss various ways to dress or sharpen jointer knives without removing them from the machine. August 1, 2011

It's been years since I sharpened jointer knives while the machine was running, and I can't remember the exact procedure. It involved dropping the outfeed table a tiny bit and running the machine with a sharpening stone held against a stop on the infeed table.

Anyone ever do this? What was the exact procedure? Yes, it's scary, and isn't for novices, but doing it once or twice between regular sharpenings keeps the blades honed.

Forum Responses
(Solid Wood Machining Forum)
From Dr. David Rankin, forum technical advisor:
From a safety point of view, do not attempt this unless you have a built in fixture for holding the stone.

From contributor A:
My 16" General jointer's manual describes this procedure. Just the thought of it terrifies me. Sounds like it would work, but I can imagine it working with the machine off by sliding the stone back and forth along each knife.

I've seen jointer knife grinders, and thought of using an offset router (adjustable speed) base with cup stone chucked in it to grind in the head. Anybody done that?

From contributor E:

I've read about it and tried it a couple times but never found it really helped. Maybe I just didn't do it the right way? Nowadays I just change the blades.

From contributor O:
Jointer knife honing/sharpening for cheapskates: Get a large granite tile or piece of glass. Glue 3M SC paper to it with grits from 180ish to 600ish. Attach a few small blocks to the top of knife with super glue, hot melt glue, whatever. Hone the bottom from 180 through 600. Break off blocks and hone the bevel on the 600 very carefully. This won't remove any large knicks, but will get you a razor sharp blade in about five minutes.

From contributor D:
I jointed the knives on a large old Northfield Patternmaker's jointer in the shop I worked in 35 years ago. It was a 16" wide head, and Northfield (?) had a fixture for the stone, with fine adjustments for adjusting the height. This would bring all the knives into concentricity, and make for cleaner cuts. The Northfield allowed for a skewed fence, and you could skew curly material on the jointer and get cleaner cuts with an effectively reduced cutting angle. It also had an in-head grinder that would sharpen the knives in the head, with the head fixed in place. This also registered off the outfeed table and had very fine height adjustment.

Beyond the caution inherent in jointing on the fly, this method will also increase the land of the knife so that each knife was hitting the wood on the heel of the knife. This dramatically increased noise levels to painful. I have significant hearing loss due to that particular machine. Feed pressure also increases to overcome the added force of the knives slapping the wood.

I am unaware of any real gain that the method provided. Seems like it was vaguely done to help prevent tearout, but I suspect it was done mostly for economy - a poor man's method of sharpening.

From contributor N:
I have done this before, but I didn't consider it to be sharpening, just making sure that all the knives were concentric, and maybe adding a micro-bevel to a knife that was slightly high. Some machines don't have a very good knife adjustment feature, or the knives slip a little when you tighten them down, and this procedure will ensure that one knife is not doing all the work, or a little bit higher than the rest. This procedure is a lot less dangerous if you are just barely touching the knives.

From contributor J:
I tried this once (just once). I securely fixed a large sharpening stone into a sliding wood frame clamped to the planer table. Next I switched on the machine and turned up the crank (ever so slowly).

Bang, boom! (Along with the sound of flying shrapnel.)

Well, the sharpening stone had come loose (just a bit) and rattled around under the knives. A big bite was now missing out of the stone and the knives. Needless to say... I don't recommend anyone trying anything quite so stupid as this.

From contributor M:
30 years ago when I bought my first new Powermatic jointer, it was actually included in the operating manual to touch up the knives like that. With the liability laws the way they are today, no manufacturer would ever offer that suggestion, plus it helped, but was not the end all of the problem.

With the aftermarket solutions like Esta knife holders, it's now very easy to always have sharp knives without the hassle of taking out your old knives and having them re-sharpened. That's what I ended up going to before running all insert heads, whether they were Byrd or Tersa.

From contributor D:
To take this a bit further, no matter how careful one is in setting knife heights, only one knife makes the actual finish cut - the surface that shows. My understanding is this holds for Teminus, Centro-fix, and segmented knife heads. Make a cut, find the mill marks, and do the math (number of knives, RPM, kcpi - knife cuts per inch, etc). Jointing on the fly will make all the knives concentric and then they will all cut equally and produce the final surface. This is how molders can run good surfaces at above 30 l/f per minute.
However, a jointer is not a finish machine in most uses, and doesn't need a fine finish since that will be made in further processing.

Contributor J, your story on the stone coming loose reminds me of the time I put the sanding drum - 1/4" shank , rubber cylinder and an abrasive sleeve - in a router - at 20k rpm. Seemed like a good idea at the time...

From contributor S:
I have heard of people brag of such things on moulders, jointing without a jointer. It involved attaching a jointing stone to a long stick, and slowly/carefully running the stone across the tip of the knife, back and forth.

I have heard of a lot of things, like the fellow who thought it would be a good idea to use a crow bar to dislodge a doubled up moulding from under the hold down of a Weinig H23 (or similar). Without turning the machine off. I guess it would have worked if the bottom head didn't grab the crow bar and just about rip his arm off.

The point is, jointer knives are usually $10 or $20 a set on eBay all day long. Hardly worth taking the risk.

From contributor G:
I always stone my jointer knives after replacement. But I pull belt cover off back of machine and pull belts by hand, so rotation is in opposite direction. Also, I do it with both beds level with top dead center of knives after replacement. This ensures concentricity and produces very small micro bevel and glass smooth cuts.

From contributor U:
Used to routinely grind jointer knives with the setup contributor A suggests (cup wheel in router with an extended base referenced to the out-feed table; you also need a reliable method for clamping the head). This worked fine for two or three times between blade replacements. As I recall, the jointer did eventually end up with some unattractive slip marks on the out-feed table.

These days I'm a big advocate of carbide knives (straight, not insert) in a jointer. You will be too the first time you realize you can joint a piece of plywood and not ruin your knives/set up. True, they aren't quite as sharp from the outset, but they do just fine and last many times longer. You can usually get one hone by hand with a medium diamond stone if you emphasize the flat face of the knife over the heel of the bevel.

I would be very reluctant to take a stationary stone to a running jointer in any set up configuration.

From contributor V:
I have a Powermatic 180 planer that has a jointing and grinding bar mounted over the top of the machine. The jointing fixture holds a fine, narrow stone over the cutting head, with a very fine screw to feed the stone down, and then a long fast screw feed to move the head over the length of the knives, all while the cutterhead is rotating. The sole point of this is to bring a fresh cutting edge to the knives, but that is not the end of the procedure. I take a diamond honing plate and make a few passes over the land left by the jointing head to refine the texture. Then the grinding head, an actual grinder with a 4" stone, is worked over each knife individually to grind the knife bevel back up very close to the new edge. The cutter head has fixed stops that position it for proper back grinding.

So the whole point of that is to restore the edge and retain both concentricity and balance (presuming the blades were carefully installed in balance originally), but not to simply sharpen. The very small land left makes it easier to hone with the diamond plate from time to time. But if the land is anything more than a hair's width there will be a substantial increase in compression of the wood fibres behind the cut, which does not make for a good surface.

The proper solution is not to cut corners. You can lightly hone the knives a few times while still mounted in the jointer, but to properly restore the blades they need to be done on a bar grinder. Get a good setting gauge for resetting them all to the same height to reinstall. When I first started out I used a 2' aluminum level off the outfeed table and listened to the whisper of the each knife against the bottom of the level as I set the height.

From contributor K:
I have jointed the knives on my Oliver 24" jointer with a jointing attachment. When you do this, you are making all of the knives the same height while exposing a new cutting edge. You want to take the least amount off of the knife. Your jointer will cut better and sound different because all the knives will cut equally. I would only do this if you have an attachment.

From contributor F:
As the owner of a sharpening/supply company, I've turned this thread over to OSHA, Comedy Central, and the "I'm too cheap to pay 80 cents an inch to sharpen my planer knives" police. Wow.

From contributor A:
It's not that we're too cheap to have them sharpened... We're just too lazy to take them out and spend the hour or better to reset the sharp ones. I unfortunately only have one set for my 16" jointer, and haven't felt the need to part with another $200 for a second set.

From the original questioner:
My question wasn't meant in any way to recommend this process to anyone, only to refresh my memory as to the procedure. I've done it before with satisfactory results, but yes, it was a bit hairy. If I do it, it's my responsibility. I've been a full-time professional woodworker since 1968 and I have never been injured woodworking because I have respect for the tools, processes, and forces involved in this work. No one should ever attempt anything involving power tools or sharps unless they have well-founded confidence and knowledge of the consequences of any mistakes.

The horror stories you hear from long-time cabinet/furniture makers aren't all apocryphal. Bad things happen to those who are too casual about the work. It requires a high degree of concentration, which is part of the reason why it appeals to us. The world goes away when we're completely involved in the art and craft.

If some woodworking procedure makes you uncomfortable, you can't go wrong by not attempting it. Every woodworker I've talked to who has been injured at work has told me that they had a feeling beforehand that it wasn't a good idea to try. My question has been answered - I replaced the knives instead of jointing them.

From contributor F:
I was just being sarcastic, but I'll submit that having an extra set of knives for your machine is better than pricing out an extra set of fingers. Gotta be careful.

From contributor W:

I have done this, but it has been a long time. I think the process takes years off the life expectancy of the operator. Now, when I replace knives, if I feel I need to adjust them, I set the beds so the highest knife just touches, then rotate by hand against a giant water stone.

But after that, I hand hone with a diamond hone fitted with a handle, maybe 4 ~ 5 times between removing them. With a good light, magnifying visor, and moving the outfeed back out of the way. I just sharpen until there is no shine on any knife. About 15 minutes will take care of 4 12" blades, and there is some clearance on the top secondary bevel.