At my local MakerSpace we have an old (Harbor Freight?) six inch jointer. the knives were dull as dirt and I needed to change them.
Some newer jointers have special gizmos or inserts for setting the knives. -- Not ours; ours is old and cheap (read donated). So I needed a way to set the new knives to the height of the outfeed table.
Step 1: Materials & Tools
- piece of wood: 3/4" x 2-1/2" x ~10"
- Four rare earth magnets: 1/2" or 12mm x ±1/8" thick
- 1/2" or 12mm Forstner bit
- Drill Press with working depth stop
- 5 minute epoxy
- sharp pencil
- scratch awl or box cutter
Step 2: Construction
Find yourself a piece of wood that you can trim down to the appropriate dimensions.
- If your jointer is still serviceable, flatten one face and one edge of your stick.
- Plane the opposite face parallel.
- Rip the other edge parallel on the table saw, or run it through the planer.
- Square both ends on the table saw or chopsaw.
(Our jointer was in poor shape, so I found a stick that was "straight enough" and merely squared the ends.)
- Using a square, draw two lines across the face that will be against the jointer bed. One line about two inches from the leading edge and the second line about six or seven inches from the same edge.
Extend those lines up the sides of the stick.
- Drill a test hole or two to set the depth of the drill press so the magnets you have will sit just below the surface of the wood (0.01", 0.005"): Deep enough so that the knives will be pulled against the wood surface, but not so deep that the magnets won't pull the knives against the wood surface or won't "stick" to the outfeed table.
- Drill magnet holes in your stick as shown: centered on your lines and about 1-1/2" apart.
- Epoxy the magnets in place. DON'T USE A LOT! You want the magnets to set below surface and you don't want to have to clean up a bunch of epoxy off the face of the magnets or wood
- Have a cup of tea while it all cures. Or give it a day.
- At this point, your jig is done. Now let's see how to use it:
Step 3: Prepping for Changing the Knives
Make sure your jointer is unplugged. You'll probably need to remove the fence and blade guard too -- you want to have access to all the bolts holding the knives in place.
Remove the motor pulley belt so that the cutter head rotates freely.
If you have the manual for your jointer, this might be a good time to re-read it. If there are any contradictions between what I say and what your manual says, follow your manual.
- Lower your infeed table so it's out of the way (±1/2").
- Set your jig on the outfeed table, magnets down
- extend the jig so it's over the cutter head
- Rotate the cutter head to see if any of the knives hit the jig and lift it off the table.
Choose a knife that lifts the jig the least or not at all.
- Adjust your outfeed table up or down until the chosen knife, when rotated to it's highest point (apex?), just grazes the stick
- Set the jig so its front magnets' center line aligns with the apex of the knife. Jig should be square to the blade & table.
- Scribe a straight line across the outfeed table marking the location of the back end of the jig.
I used a box cutter to scratch the line into the table. If you don't want to do this, use a pencil or an ultra fine marker.
For each knife change, you'll want the jig set to this line and the knives set to the front jig line.
Step 4: Change Out Your Knives
Now it's just a matter of changing out all the knives.
I change one knife at a time. I number each knife, each gib and each slot in the cutter head they go into. Hopefully, that way I don't make a mistake.
- Loosen the screws holding the gib -- which holds the knife.
I wear well fitting gloves to protect from slicing up (open) my fingers when the wrench slips and I slide into the knife. (Been there. Done that!)
- If you're lucky, you have the wrench that came with your jointer; if not, you may have to buy an open-end wrench that is thin and narrow enough to fit in the small spaces of the cutter head slots.
I found the smallest wrench I could and still had to grind down the edges.
- Remove the knife and the gib.
- Clean them up. I use a wire brush to get all the crud & corruption off the gib and out of the slot in the cutterhead. (I discovered that not only is the old knife still sharp, but the leading edges of the cutterhead's slots are also sharp enough to cut! Keep those gloves on.)
- Replace the gib
- insert the new or sharpened knife. It should be a slip fit.
- Adjust the two outer gib screws so the gib is just snug against the knife.
Back them off slightly.
- Bring your knife setting jig over the cutterhead and align it to the scribe on the outfeed table.
- Align the knife to the front magnets. The knife should be held in place by the magnets.
- Now snug up the two outer gib screws again.
If you can reach any other screws with the jig in place, snug them too.
- Remove the jig; snug up the remaining screws.
- Tighten the screws: Work from the center out until all screws are of equal tightness.
- Do this for the remaining knives.
- Rotate the cutterhead to check that none of the knives are hitting the outfeed table. If any are, you will need to adjust the table &/or reset the knives.
- reassemble your jointer.
- Bring up your infeed table so it is about 1/32" below the level of the outfeed table. Again, make sure the knives clear the table.
- Plug the jointer in, and with the knife guard in place, turn in on.
Let it run for a bit -- to be sure nothing comes flying loose. Turn it off.
Step 5: Test You Machine
Get a scrap piece of solid wood (Never joint plywood!) with a relatively straight edge.
Set you jointer's fence square to the table.
Turn on the jointer and run the wood's edge over the knives as you normally would.
If the leading edge bumps the outfeed table's edge, the outfeed table is too high.
If the trailing edge drops into the knives when it leaves the end of the infeed table (you get a snipe), then the outfeed table is too low.
Adjust as needed.
If you get a pronounced scalloped edge, then probably one of the knives is higher than the rest and only that high knife is cutting. You will need to reset your knives.
If you don't get a straight edge -- it's concave or convex, then your tables are not parallel to each other (They are not Coplanar). And that's something I can't help you with. ;-(