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Woodworking Jigs & Fixtures

October 15, 2011

Planer / Jointer Knife Sharpening Jig

Having a low cost surface planer with two sets of HSS knives, I wanted a way to reliably sharpen the knives. This would save me time and money by avoiding the trips back and forth to the sharpening service, and I would be more likely to be using knives in the machine that are at peak sharpness.
Doing some research online, I found a jig design that looks like this:

 

The idea is that the blade will clamp into it and the arm and roller at the back would set the correct angle for sharpening. It is meant to use wet / dry sandpaper as the abrasive, and the sandpaper would be adhered to a very flat surface, such as a piece of glass or granite slab.
Having made the jig I tried it out and found that although it worked, it was rather awkward to use and the sheets of sandpaper wore out very quickly. It took quite a long time to get a knife in usable shape, that is if I was lucky enough not to accidentally flip the jig forward (not difficult to do when hand fatigue sets in) and the freshly sharpened edge gets blunted. Overall, not a very satisfactory method.
I gave it no more thought, just accepting the idea that the next time I need the knives done I'd either send them out or take another crack at doing it with that jig.

Then recently I watched this video:

Ingenious! I figured the jig that he is using for plane irons and chisels could be adapted for wider blades, such as my 12.5" planer knives.

The knife would have to be securely held and the holder would have to be perfectly straight (within reason), very stable and strong. I selected a piece of clear, straight grain maple and cut it to 1-1/2" x 1-1/2" x 12-1/2":

I cut a rabbet in it to support the blade and drilled 1/4" through holes for the clamping bolts.

The flat head bolts have a washer under their heads to put moderate pressure on the blade. This holds the blade firmly without distorting it:

I then cut another piece of maple for the swing arm. This is cut at 40 degrees on the end to orient the holding bar at the correct angle to allow the arm to be low to the work surface, nearly parallel. This is one difference from the jig in the video - I don't like how high the end of the arm is and how the guide rod that adjusts the sharpening angle can bend because of this extra height.

Like the original, I used an eye-bolt as the pivot point of the swing arm:

This slips over the 3/8" threaded rod bolted to the work surface. On that subject, the work surface is a piece of 5/8" melamine, 24" x 20". I used this because it is extremely flat and has a tough finish that's easy to wipe clean and limits moisture absorption.

The underside of the work surface where the guide rod bolts through:

 

I used a t-nut to screw the threaded rod in. A long 3/8" carriage or hex bolt could be used also, in place of the threaded rod.

To hold the stone, I cut pieces of maple 1-1/2" thick by 3-1/2" wide:

The notches are just slightly less than the thickness of the stone, to let the knife swing over without cutting into the blocks. The blocks also provide support, to stop the blade from being damaged by the edge of the stone if it swings too far.

 

The stone is a Norton crystolon-India combination, medium silicon carbide (grey side) for fast cutting on the hardened steel. The fine aluminum oxide (orange side) produces a keen edge and is hard wearing. It's a good idea to use a hard stone for this to prevent premature or uneven wearing. The notches in the wood allow the stone to be moved forward and back to ensure that all of the surface gets equal use.

I clipped the corners off the work surface, to lighten it slightly:

Here's the swing arm installed on the guide bar. Jam nuts adjust the sharpening angle. This mounting height was what I was after when I cut the arm at 40 degrees. With it this short, there is no chance of the guide bar bending. It also makes it more convenient to store, if there's not 15" of threaded rod sticking off.

The finished unit:

 

Ready for action. This is as finished as I'll make it - no sense dressing it up as it will get filthy before too long.
On using it for the first time, I found that it is a massive improvement over the other version (top of this page). Much less effort, faster cutting and nearly no chance of ruining the edge. I'm able to make an even, flat grind across the length of the blade.

Here I've spent about 20 minutes dressing this knife on the coarse stone:

This blade was badly damaged from my previous sharpening attempts and was in need of heavy grinding. Clear in this picture (above, right) is how I've changed the cutting angle slightly to more quickly recover the edge. Still, not enough to cause problems - maybe half a degree. With some of the old edge still visible, I have a bit more to do on the medium stone before I can hit it with the fine.
Once this sharpening angle is established, it'll take less time on subsequent sharpening sessions to get the edge back.

I made a short video of the jig in use:

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I expect this will prove to be a very useful addition to my shop, doubly so when I have jointer knives to sharpen as well. Fairly simple and straightforward to build, it actually took longer to write this article than to build the rig!

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