Jointer Knife Life


A week ago, I bought a spare set of jointer knives because my original set needs sharpening. The old knives were leaving lines on the material and not cutting well. The new knives were just in time to straighten some soft maple for cabinet doors. The new knives are made by Amana.
Here's what surprised me...for the first 10 boards, the cuts were smooth as silk. I kept checking each one in amazement as to how perfect they were coming out. Then, without realizing it, I guess I passed a board with a small knot over the surface. After finishing the rest of the boards, the remaining few started to show signs of knife divots, etc. I barely used them for 30 minutes and now they're leaving knife marks (sandable but less than perfect) on the material.
How long should freshly-sharpened knives remain "perfect" until they start leaving some marks? Also, how many times can jointer knives (I have a 3-knife 8" jointer) be sharpened? Can a tougher knife be purchased for jointers, something with carbide, perhaps?
30 minutes of bliss and now it's back to reality.
-m
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Until they hit something tougher than the edge of the blade. Then they go down hill really fast. Watch that knot in the planer
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Until the firt knot? You can offset the knifes slightly.
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Mike Pio wrote:

When blades are sharpened the blades develop a "wire burr" on their edge. The marks you're seeing may be the result of this wire burr being removed by usage. Once the burr is fully removed the divots may stop.
FWIW, I don't expect a finished face/edge coming off my jointer. I only hope the board comes off with one face flat and one edge straight. The finish planer and sanding take care of the rest.
--
Jack Novak
Buffalo, NY - USA
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there should be no wire burr if they are sharpened right. and even if a burr was left the first pass would remove it. Knight-Toolworks http://www.knight-toolworks.com affordable handmade wooden planes
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wrote:

I can't say what actually caused your problem, but my experience has been that my jointer blades (HSS for an Inca) are quite a bit harder than knots. Of course they wear eventually, but not on a single pass. What does get them is to negligently pass a board through that has been sitting on the ground and collected "grit". This results in immediate feedback in the form of fine lines down the length of subsequent stock. I usually keep an old bench brush handy and give each new, rough, plank a thorough brushing before the first pass.
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Mike Pio wrote:

Depends on your definition of "perfect", I suppose. As someone else noted, the pristine edge will last until something harder comes along. I don't buy the wire edge theory.

Depends on how big they are to start w/ and how much one has to take off (obviously). I have only bought one set or replacement blades for the 8" Delta w/ the in over 30 yrs. I couldn't tell you how many times they've been resharpened, but I make sure they're done before large amounts of material must be removed. I also keep an old set if I know I'm going to have to clean up some old material and take the timet to swap them in to protect the "good" ones....

Solid carbide knives are available, for a price.
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this seems to happen to fresh knives. seems to be the way life kicks you in the pants (G) but shifting the knives can fix it. I have had this happen several times right after a knife change. Knight-Toolworks http://www.knight-toolworks.com affordable handmade wooden planes
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I wonder if the knives you got are not as tuff as what you were using? that could very well be the problem. putting a back bevel on them would make them stronger. Knight-Toolworks http://www.knight-toolworks.com affordable handmade wooden planes
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Steve knight wrote:

I don't know, Amana stuff is usually pretty darn good---unless they've started a "consumer" line I don't know of????
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What kind of blade material in the replacement blades you bought?
You really want HSS or Carbide if shooting for long life, HSS is better than most non-HSS speed steel, and Carbide at the top of the list both long lifewise as well as pricewise
John
wrote:

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On Tue, 18 Oct 2005 13:43:53 -0500, snipped-for-privacy@interoz.com wrote:

solid carbide is not always a good idea on jointer knives. it can be too brittle. the best of life is a helical carbide cutter head. most normal woodworkers may go a year or more before rotating the knives. Knight-Toolworks http://www.knight-toolworks.com affordable handmade wooden planes
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