Sharpening Jointer Knives.

Brent Dowell

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Spent some time getting my pm 54 6" jointer back in shape.

Pulled out the blades and decided to give sharpening them a shot.

So I measured the angle on the blades, and I'll be darned if they weren't 45 degrees. So I cut a little block of wood at 45 and used that as a guide to hone up the blades on my diamond stone until I could shave hairs with it. Did that to all three blades. Then the fun started.

Forgot to mark the TDC before I started sharpening, so put one bladed back in, guessed at TDC and used that as a mark to do the rest of them. Used one of those magnet jigs to set the blades. Then I used a magnetic dial indicator to check it all and set the outfeed table to just slightly (.001 ?) higher than the blades. Does that sound right?

Did some test planing and it seemed to work pretty darn well. Much easier to push the wood through the planer, and it had a much 'smoother' sound than the old dull blades...

Only took a few hours, but I should be back in business... :thumb:
 
A thou high isn't a bad starting point for the outfeed... to be twiddled with depending on how you use the jointer & what your results are. The slower (without ever stopping) you feed stock through, the nearer the outfeed can be to level with the highest knife. Too high & your workpiece will have a funny "tip up" near the leading end, too low & it'll go concave on you, and also have one snipe near the trailing end. If your one-thou spot is giving you good results, then it's right for how you use the jointer.

Sure sounds "softer" with nice sharp knives in it, doesn't it? :)

I use the "lift and drag" method to set my knives - it's reliable & quick.
 
That sound great Brent. It is always refreshing to use a tool right after a tune up. Brings back that "new tool smell" :thumb:. When I changed knives for my dad recently, I used Don's method except I used glass plate instead of plastic as that was what was handy. Very quick and reliable.
 
Thanks guys!

The plexy and plate method sounds very similar to the little magnetic jig I've got. It's basically 2 bars and 6 magnets. 4 magnets keep it flat on the outfeed and two of them bring the blade up to height.


Sure sounds "softer" with nice sharp knives in it, doesn't it? :)

It does have a much softer sound. Before when I would turn it on, without even running wood through it, it would sound like it was ripping the air. Now it's much more of a 'whirrrr'...
 
Did a little more searching. This article here describes pretty much what I did, using the same tool I used.

Jointer Knife Change Tutorial

I suppose if I sharpened or adjusted my jointer knives more than once every couple of years I'd get his down!

The only difference between what I did and the article is that before I adjusted my knife, height, I put my jack screws all the way to the bottom, and then after I had the knife height set, I snugged them up to the knives for support.
 
I can't add anything to setting up the knives, but I can add a cool jig that a guy named "Dizzy" shared (he has passed away since).

Dizzy's jointer/planer knife honing jig

dizzys_jig_1.JPG dizzys_jig_2.JPG dizzys_jig_4.JPG dizzys_jig_3.JPG

dizzys_jig_5.JPG dizzys_jig_6.JPG dizzys_jig_8.JPG dizzys_jig_7.JPG

Shown here are planer knives, but you can understand that jointer knives can also be done with a similar jig. :wave:
 
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Now thats a slick sharpening jig. :thumb::thumb:

If I can just figure out how to make it work for 3 blades, that would be killer.

I guess I could just order a new set of jointer knives. Once I get both sets worn down, I could then go and sharpen all 6 blades and then have 2 sets of sharp knives!
 
Now thats a slick sharpening jig. :thumb::thumb:

If I can just figure out how to make it work for 3 blades, that would be killer.

I guess I could just order a new set of jointer knives. Once I get both sets worn down, I could then go and sharpen all 6 blades and then have 2 sets of sharp knives!

Makin' it work for 3 knives ain't too hard... you mount all three on the same side, same orientation, like sheets of plywood leaning against a wall. On the other side you attach a brass rub strip (wears better than wood), which can be vertical or angled, it doesn't much matter. The brass rub strip should be slotted so it's got a little adjustment to it.

The only trick will be in getting the three knives lined up - if they're "holed", the holes won't be quite in alignment with each other so ya'd hafta' use an undersized screw to pass through all three holes. If they're not, it's easier... one clamp'll hole 'em all.

They'd all register against three little curbs in the body of the jig, all cut with the same height setting but stepped over 3/32" (for 1/8" knives) per pass to make the curbs exactly the same. Go a little further & add two setscrews to each curb for fine adjustment (in case the knives aren't perfectly identical).

EDIT: The classic way to do it, though, is to work two knives at a time till they're about half done, swap one out for the unworked knife, work 'em till the first is done & the last is half-done, then swap out the done one for the first one you took out - at which point you'll have two half-done knives again so you can finish the job.
 
My thinking about the honing angle is that it's less important than the height above the cutterhead. If the knife edges are close to the cutterhead it will limit the depth of cut, and if they're too high above they won't have enough support. Trial and error imo. I don't like to take heavy cuts on the jointer so I opt for support. The angle is important though...too small an angle and the edge is weakened, too large an angle and it hammers the chips off...but it's a range. Personally I tend toward a smaller angle since I think it makes a smoother cut, and I have a good setup for sharpening as it's needed.

The outfeed table ideally will be exactly even with the knives at TDC. This is difficult to achieve...I agree with Tim that a thousandths is acceptable. The infeed table is lowered to determine the depth of cut, and as the chips are cut off, the piece slides right onto the outfeed table with full support and no snipe on either end. Just as important is getting all 3 knives even with the outfeed table, since the highest one will be doing all the cutting. This is also difficult to achieve to perfection. Even a jig like the Jointer Pal may not keep the knives from shifting slightly as the gib bolts are tightened...rig up a dial indicator to check this...I think you will be surprised. Anyway, some deviation has to be acceptable or you will end up in the nut house.

As an aside, I bought this jig from Lee Valley a few years ago:
http://www.leevalley.com/wood/page.aspx?c=2&p=33002&cat=1,43072,43078
and never really used it...maybe a couple of times. My jointer now is a 12", and of course the jig does me no good for that. 20 bucks plus shipping and you can have it. I'll email you a pic if you want. Let me know.

Cheers.
 
My thinking about the honing angle is that it's less important than the height above the cutterhead. If the knife edges are close to the cutterhead it will limit the depth of cut, and if they're too high above they won't have enough support. Trial and error imo.

I hadn't even thought about that, but that makes a lot of sense. I'll have to check it out.

Even a jig like the Jointer Pal may not keep the knives from shifting slightly as the gib bolts are tightened...rig up a dial indicator to check this...I think you will be surprised. Anyway, some deviation has to be acceptable or you will end up in the nut house.

Good advice. I've got a dial indicator on one of those magnetic bases, but I did see a jig someguy rigged up with 2 dial indicators that could read from either end of the blade. Might have to put something like that together.

I'll admit to only checking one of the blades for height. Looks like I need to get out there and double check the rest before I run some good wood through the jointer.

My jointer now is a 12", and of course the jig does me no good for that.

Now that's a serious jointer! :thumb:
 
The outfeed table ideally will be exactly even with the knives at TDC.

I do have to take exception to that, Ed. It'd be true only if the cutterhead was of infinite size or revolved at infinite speed, or the stock was fed across the cutterhead infinitely slowly, so that the resulting jointed face was perfectly smooth with no "peaks and troughs". That can only happen in a perfect theoretical world, though. We live in a different world, one in which our jointer does leave troughs and peaks, tiny "waves" in the surface, because of the lag between knife cuts. The TROUGHS will be exactly (or very very nearly so) even with the edges of the knives at TDC, but the (upside-down) peaks won't be - they have a measurable height above the troughs.

It might be good if I posted an exaggerated sketch of the troughs & peaks to illustrate... but for the moment I'll hold to the statement that the outfeed table MUST be lower than the knives at TDC... by an amount predicated by the way the user uses the jointer - very fast stock feed over a small-diameter (1-7/8") two-knife cutterhead wants a much lower outfeed table than very slow stock feed over a large-diameter (3"-4") three-knife cutterhead.

EDIT: It'd be true, too, if the cutterhead contained an infinite number of cutting edges, but now I'm gnashing hairs. :)
 
Tim...

I understand the theory you're proposing. From a practical point of view the peaks and troughs can be ignored...they are virtually unmeasurable...nonexistent, actually, if the stock is fed at a proper rate. Given the high angular velocity of the cutterhead the knives follow each other into cutting position so quickly that there aren't any peaks. If you're experiencing this phenomonen it's due to something else. These are the possibilities that come to mind:

feeding stock too fast (this would be really fast...dangerously so);

insufficient down pressure holding the stock against the table;

dull knives (the knives at the beginning of the arc are attempting to push the workpiece up off the table...this effect is exacerbated by dull knives...sharp knives mostly just slice).


The geometry of a jointer is not complex:

infeed and outfeed tables coplanar;

infeed table can move up and down while remaining parallel to the outfeed table;

cutting circle exactly in line with the outfeed table at TDC.


It's simple, but something I've discovered over time is that it's easy to attribute a jointing problem to the wrong reason. It really has nothing to do with infinite anythings.
 
Not quite immeasurable... it's not hard at all, given a 1-7/8" three-knife cutterhead at 5000 RPM (83.3 RPS) to get scallops 1/8" wide with a fairly quick (still quite safe) feed rate. At 1/8" peak-to-peak, the depth of a trough is pretty nearly one thou. At that point, we're effectively linear so scallops 1/16" apart (half that quick feed rate) will be about half a thou deep.

Now, if one of the knives is a thousandth of an inch higher than either of the others, ya gotta' add that because troughs & peaks will now have different altitudes. The outfeed will need to be lower than the highest knife by the height error between the highest & lowest knife plus trough depth at a given feed rate.

Nobody sets their knives exactly even with each other - it can't be done, not exactly, and so everybody's outfeed will need to be slightly lower than the top of the highest knife at TDC.

Nitpicking, to be sure... but I do like to debate precision. :)

In the real world, if the jointer's working right don't mess around with the outfeed. If ya run into trouble that can be attributed to the outfeed being either too high or two low, adjust it a little & make another pass, repeating till the problem goes away... then stick with that.
 
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